Suspension Design [Archive]

11-02-2012, 05:06 AM

This thread is about "Suspension Design", so it might be worthwhile to consider some of the big-picture issues related to FSAE suspensions.

For instance, how much "ride comfort" is required? Well, FSAE is (sort of) racing, so comfort is not a priority. But even if it was, FSAE tracks are billiard table smooth, so there is really no need for suspension to provide "comfort".

Ah, but what about cornering? That is, isn't suspension supposed to be important for "handling and grip"? Well, it is well known that go-carts corner at lateral Gs at least as high as FSAE, if not higher, and they have NO suspension. And there are countless other examples of effectively suspensionless cars winning race series. So, again, suspension is NOT necessary to go fast around corners.

Really, the only reason FSAE cars need suspensions is that the Rules mandate them. Nevertheless, I have seen an FSAE competition that was won, very convincingly, and against high standard opposition, by a car that had its springs so highly preloaded that on track it had NO suspension movement whatsoever. The appropriate quote here, usually attributed to Colin Chapman, is "Any suspension will work, if you don't let it..."

Now I really enjoy suspension design. For this reason I have been suggesting, since 2005, that FSAE tracks should be laid out with a lot of real bumps, dips, corrugations, etc. This is in the slim hope that eventually some FSAEers would end up in the auto industry and actually start designing cars with good suspensions. But the bumpy tracks haven't happened, and are unlikely to for many reasons that have nothing to do with "educating young engineers".

So, instead of FSAE being a venue where young engineers start to learn about good suspension design, we have a situation where the teams spend a ridiculous amount of time designing and building the most complicated suspensions possible, but which are also (mostly) useless! There is not, and doesn't have to be, any real thinking involved in this process. Brainless mimicry suffices.

To put it bluntly, the vast majority of you FSAEers are simply pulling your dicks, suspension-wise. http://fsae.com/groupee_common/emoticons/icon_smile.gif
~~~o0o~~~

Errrrr, so what exactly is the grumpy old-fart getting at???

The Three-Step-Dance that will take us into the Next Dark Ages.
================================================== =

1. LAZINESS.
Today's students have grown up in an "Every kiddy gets a gold-star!" education system. Not all, but certainly some of said students have learnt that they can do just fine with next to no effort.

"Hey, doing background research, like homework, and reading, and trying to figure out stuff by myself is really hard. But that's what these forums are for, eh? I just have to ask something like,
'...it's really baffling me ... to get more funding from our university ... I just need a rough guide of where i need to start ... a set of calculations that can be worked with...'
and yahoo!, all the hard work gets done for me."

2. DON'T THINK, JUST FOLLOW.
The gist of above student's plan is simply to copy whatever the majority of the flock are doing. It is not necessary to understand anything. It is mostly just a matter of talking-the-talk. Fortunately, this requires no more intellectual capacity than a parrot.

"Squawk!... We've minimised our roll-centre migration... optimised the camber-compensation... kept the motion-ratio at a sensible value of 1:1, with slightly progressive rates... and next year we're fitting two pushrods-and-rockers in series, because if one can reduce unsprung mass, then two will obviously be even lighter! ..."

All of which, of course, is bulldust!

3. THIS IS THE REALLY SCARY BIT.
After spending a year or two mastering the above gibberish, and becoming firmly entrenched at the centre of the flock, our hero starts recruiting as many newbies as he can to boost the flock numbers. There is nothing more blissful for a sheep, than being at the centre of a humungous flock... (it's a fear-of-wolves thing...).

"... one day people can ask me for advice ... Once the project is done ... I will be able to help the following years ... "

And thus is established a self-perpetuating downward spiral of STUPIDITY...
~~~o0o~~~

For FSAE, the double-wishbone-with-push/pullrods-and-rockers suspension is a complete and utter wank. As noted above, no thought, just mimicry.

There are many alternative, much simpler, and much more suitable suspension types that can be used. Very briefly, and roughly chronologically, the major suspension types are,

1. Beam-axles (eg. on all vehicles ever, can be live or De-Dion at rear),
2. Sliding-pillar (eg. Morgan and Lancia, and most motorbikes and aeroplanes at front),
3. Lateral swing-axles (eg. Tatra (still on their trucks), can be high or low-pivot),
4. Leading and trailing-arms (eg. Citroen 2CV front and rear, and most motorbikes at rear),
5. Semi-leading/trailing-arms (eg. F100 at front, and many mid to late 1900s cars at rear),
6. Strut-wishbone (eg. McPherson at front, strut at rear),
7. Double-wishbone (eg. originally mostly at front to allow shorter wheelbase with front engine),
8. 5-link (eg. recent fashion, mainly for NVH reasons).

Of these, any of the first five are more than adequate for a winning FSAE car. They are also structurally much simpler than double wishbones, allowing quicker design and build of the whole car. So more time for the more important big-picture issues!

To repeat, FSAE CAN BE WON WITHOUT ANY SUSPENSION MOVEMENT AT ALL! However, about +/- 1cm of approximately vertical wheelprint movement can be advantageous. The requirements of FSAE suspension are simple. They can be provided by any of the simpler suspension types.

Rational criticism welcome. http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

Dash

11-02-2012, 06:46 AM

Originally posted by Z:

To repeat, FSAE CAN BE WON WITHOUT ANY SUSPENSION MOVEMENT AT ALL!
Z

I'd agree with this statement. After running low on funding we snatched the dampers off the 2010 car and fitted it with steel beams instead. Its a tad rough over the bumps, and the Torsen differential doesn't like taking hard corners ( tire deflection lets it lift). Still wicked fast though!

ben

11-02-2012, 07:55 AM

Z: FSAE is an engineering competition with the stated goal of creating a car for the weekend autocross racer. If the weekend autocross racer expects to see pushrod and rocker suspension because F1 cars have it, then giving the market what it wants is a sound engineering judgement for an engineer who wants to remain employed.

I think you're on the money with a lot of what you say, but the example of pushrod/rockers doesn't illustrate your case at all well in my opinion.

Ben

Owen Thomas

11-02-2012, 12:33 PM

Z,

While I agree with many of your sentiments about "gold stars" and "kids these days", I have to say that you may be harping at the wrong crowd. Generally (not absolutely), people who engage in FSAE are not the type of student you are describing. In most programs, it is entirely extra-curricular and very under supported. Keep in mind that most teams are all engineering students, and have little time which can be considered extra to their curriculum. There are many choice things I could say about the typical FSAE-er, but rarely would the words "lazy" or "stupid" be among them.

The learning curve is steep, and it is not always possible to understand the fine details behind WHY a certain roll center migration or other parameter would be good/bad in the ~2-4 months OR LESS in which the student has to design thier suspension system. Starting from a higher level and picking goals which may or may not have been "copied" is still a valid design decision. The engineering comes in when you validate your choices and continue to learn why they were a good or bad idea. Unfortunately, this cycle is often forced to repeat itself every few years as well, since students are very cynical of their former leaders' designs, or simply want to learn it themselves. Which I think is completely fair, since this is an educational competition.

On that note, I would also have to respectfully disagree about how the typical double A-arm, rocker style suspension is apparently the worst thing to happen since rear wings on hatchbacks. From a learning perspective, this type of suspension design almost forces the designer to have a basic understanding of vehicle dynamics. There are many choices one can make which allows the creation of your own, UNIQUE system that you think is appropriate. It also provides easy and effective adjustment of many parameters, allowing the would-be engineers to test and validate thier design. I am not a suspension guru, and will not pretend to know more than I do, but I see this as a massive benefit for this type of system over some of the others you suggested. I am not saying that they would not work, or that "status quo is best (F1 does it)", but that if the goal is to learn the most about engineering, motorsports and automotive trickery the typical FSAE suspension is not a bad place to start.

I would also like to make note that most of us recognize that less suspension travel can be advantageous, but there are compromises to be had. Rookie drivers are not used to lightning-quick, spine-wrenching vehicle characteristics. We also typically test in parking lots (or whatever can be found), so there is not always a smooth surface to run on. There is also the infamous bump at FSG, and last year at Lincoln one team hit the ~3/4" bump in the autocross and lost thier suspension. I suspect this was caused by a number of factors, but I think we can all agree that would be a devastating thing to have happen, especially if you were the suspension designer!

That was not supposed to be such a long post, but I think sometimes it is forgotten that us FSAE-ers have to live in a world of compromise, just like everyone else. The best choice for one reason is not always the best choice in the end, and sometimes the better decision lies with the easier path.

Claude Rouelle

11-02-2012, 02:28 PM

Ben,

Spot on comments about giving the customer what he wants!

More generally,

While I am less convinced about the efficiency of semi-trailing arms and even less about live axles (actually I call them dead axles, I always have been convinced that you can make a good car with a McPherson.... but if you if you miss the window of good mix between of tire exploitation and kinematics design it will be more difficult to make a change. Providing some suspension pick up points adjustability, with a 5 links or a double wishbone you can get and/or adapt the heave and roll camber variations, roll centers, pitch centers position and movement to what the tires (and good drivers) "need" much better than with a McPherson. And if for whatever reason you are forced to switch to another brand of tire, your design could not be adaptable...

As a student I will most probably go for a double wishbone or a 5 links. As a design judge I regret there is a lot of monkey see monkey do and often students cannot explain WHY they choose the suspension type they use.

Fantomas

11-02-2012, 03:03 PM

Now I really enjoy suspension design. For this reason I have been suggesting, since 2005, that FSAE tracks should be laid out with a lot of real bumps, dips, corrugations, etc. This is in the slim hope that eventually some FSAEers would end up in the auto industry and actually start designing cars with good suspensions. But the bumpy tracks haven't happened, and are unlikely to for many reasons that have nothing to do with "educating young engineers".

Z, as always you show your agnorance. A good mixture of ignorance and arrogance.

You obviously haven't been to many different FSAE/FS competitions, otherwise you would not make that statement. Something to remember, when reading Z's posts -> He is not a former FSAE team member.

Intended or not, many current FSAE/FS tracks have a lot of bumps.
The bumpiest of the bigger competitions probably being FSG, where the track layout is done in a way that quite some hard direction changes are directly "on/in" bumps by intention.

Fantomas

Warpspeed

11-02-2012, 03:05 PM

Double wishbones are an obvious natural choice at the front, but at the back, I am not so sure.

There can be a lot of problems with the potential for bump and compliance steer at the back with a double wishbone design, which seems to be a regularly occurring design problem area on many FSAE cars.

Good solid toe control at the back would have to be be pretty high on my wish list.

A single stiff rear trailing arm, with transverse upper and lower camber control links would give excellent toe and bump steer control while being simple.

Claude Rouelle

11-02-2012, 03:35 PM

Warspeed,

Speaking about compliance you obviously have not work with solid rear axle race cars (such as Nascar... but is Nascar racecar?) or Australian V8 Supercars

Yes I have seen a few solid rear axle race cars with less compliance than badly designed double wishbone rear suspension but I have designed, track tested and K&C tested enough race cars and seen many FSAE cars to know that you simply can design good front or rear independent suspensions and minimize compliance if you just make it a goal.

Don't tell a design judge that you choose solid rear axle over a double wishbone to minimize compliance; your argument will be less solid than your rear axle.

Claude Rouelle

11-02-2012, 03:40 PM

quote:
Now I really enjoy suspension design. For this reason I have been suggesting, since 2005, that FSAE tracks should be laid out with a lot of real bumps, dips, corrugations, etc. This is in the slim hope that eventually some FSAEers would end up in the auto industry and actually start designing cars with good suspensions. But the bumpy tracks haven't happened, and are unlikely to for many reasons that have nothing to do with "educating young engineers".

Z, Good to Baja, enjoy your bumps and keep your negativity for other chat rooms.

exFSAE

11-02-2012, 04:57 PM

I still vote to get an understanding of fundamental handling before diving into the dickathon of kinematics.

Or perhaps more generally, suspension design comes as a result of a handling spec. If you don't know what you want from your handling, you cannot know what kind of suspension topology accomplishes it. If you don't know what kind of conceptual kinematic characteristics you want, you cannot design control arms and such.

For as much attention as it gets, kinematics are in my personal and professional opinion and experience largely secondary. They take a back seat to the big hitters of tire properties and dynamic load distribution (fore/aft, side to side, and diagonal). This is particularly so on a stiffly sprung "autocross car" with minimal suspension travel.

Don't focus on the secondary things and minutiae before you understanding the primary factors.

Claude Rouelle

11-02-2012, 05:36 PM

Ex FSAE,

I wish you could be a bit more specific in your comments.

There is no "dickathon" of kinematics. Nor any "dickathon" of aero, stiffness or damping.

It is all about predicting (understanding, quantifying and simulating) and ideally measuring how much each N (or lb) of vertical load, each degree of slip angle, each % of slip ratio and each degree of camber influence each tire forced and moments and from there influence your car longitudinal and lateral grip, balance, control and stability. If the student can demonstrate what does what when and by how much and from there that kinematics is negligible so be it and I will be the first to admit it.

However, my experience with both race cars and passenger cars is that kinematics (and compliance)
1. Has a non negligible influence and sometimes an even bigger than expected influence than spring, ARB and damper. There is along list of screwed up passenger cars (starting with the Mercedes Class A and the first Smart) to prove it.
2. You cannot "patch" a wrong kinematics with "right" springs, dampers and ARBs.
3. Many automotive engineers underestimate the influence that kinematics has on TRANSIENT car handling.

Try to open a door with a force parallel to the door plane or a force perpendicular to the door plane but a few millimeters from the door hinge.
Try to get good traction and braking with 15 degrees of camber.
What is the point to have 1000 HP (or more) on a dragster if you have just a few 1/10 of a degree of toe out steer compliance (or toe out bump steer) on the rear wheels?

Archimedes, said: "Give me a lever long enough and a place to stand, and I can move the world."

A force is defined by a application point, a direction and an intensity. Worry about the first two before the third one.

exFSAE

11-02-2012, 07:01 PM

Those are all certainly valid points.

Perhaps a more accurate statement is that the first step is to understand the relative influence of all your tools and design options, and establish a spec and design intent. Once that's established, all the piece parts fall into place and "design themselves" as it were. For this specific series I do maintain my opinion that getting the right CG location, load transfer distribution, tires, and diff are the big ones to worry about... and kinematics are just not as big a factor. I agree that in general that relative proportioning of what contributes to your handling can vary depending on platform. Even then, when I think about starting a design from scratch I begin by conceptualizing what I want from a bicycle model.. then expanded into a 4-corner rigid body model.. then adding inertia, and kinematics.

I do maintain as well that there is a dickathon of kinematics in this series. It's a trap I fell into when I was doing it. It's easy to get caught up in questions of "What VSAL do I want?" or "How much lateral roll center migration is acceptable?" or whatever. You can get too focused on the piece parts without understanding WHAT it is you're trying to accomplish at the system level. In a way, you're not seeing the forest because of all the trees in the way.

It wasn't until after I graduated that the light switch flipped and I came to realize that engineering is a top-down approach. You come up with something at a high, conceptual level.. and then the kinematics and kinetics (force elements) to achieve it are more or less explicitly defined. It makes the design process much easier.

Claude Rouelle

11-02-2012, 07:37 PM

Ex Fsae,

1. "Even then, when I think about starting a design from scratch I begin by conceptualizing what I want from a bicycle model.." I can't agree more with you: that is why we created the (free of charge) OptimumLap simple and simplified mass point software

2. "For this specific series I do maintain my opinion that getting the right CG location, load transfer distribution, tires, and diff are the big ones to worry about... and kinematics are just not as big a factor" Yep except there is a logical and a chronological order: you cam always decide different springs and ARB and damping later while you test the car while it will be difficult to change the suspension type and majorly change the kinematics as it will influence your car chassis (and other parts) design. Where I agree with you is that weight distribution Vs front / rear tire cornering stiffness ratio should be decided very early on, even before your kinematics.

3. " It's easy to get caught up in questions of "What VSAL do I want?" or "How much lateral roll center migration is acceptable?" " How boy there I am 100 % with you. If I come with this topic in design I will sure be more demanding and specific in the how / why / how much parts of thee questions. Maybe I should organize a seminar for design judges also. I am sure I will also learn from other judges too but there will at least be some common approach in our questions.

4."It wasn't until after I graduated that the light switch flipped" Ahhh You too? http://fsae.com/groupee_common/emoticons/icon_smile.gif Welcome to the club. I graduate and flip a new switch practically every month.

11-02-2012, 08:06 PM

Some quick replies...
~~~o0o~~~

Dash, Thanks for "a real life testimonial"...
~~~o0o~~~

Ben,

"FSAE is an engineering competition with the stated goal of creating a car for the weekend autocross racer. If the weekend autocross racer expects to see pushrod and rocker suspension because F1 cars have it, then giving the market what it wants is a sound engineering judgement for an engineer who wants to remain employed." (My emphasis.)

That might be a good business decision, but it is not engineering. Essentially, any fool can make that decision... But it takes a good engineer to build a truly fast car, and I reckon "weekend autocrosser" would rather pay for that...

I note that another stated goal of FSAE is to encourage "creativity and imagination". Copying what every other fool is doing is in breach of the C&I goal. But no team loses points for that, because, despite my repeated requests, no official or judge has yet publically supported C&I!

Anyway, from the point of view of winning the competition, there are very few points available for "bling" and many more points for building a fast car.
~~~o0o~~~

Owen,

Firstly, I do agree with you that most FSAEers are from the right end of the "Effort" spectrum. The reality of all the hours that MUST be put in soon weeds out the slackers.

Unfortunately, "the system" is sending the students in the wrong direction. By this I mean senior design judges who clearly have a poor understanding of Classical Mechanics, but nevertheless pressure students to follow their faulty advise (eg. the much repeated "Pushrods and rockers are good because they can lower your unsprung mass"!!!, and other such codswallop...).

I cover this in a bit more detail below, but briefly, most FSAE suspensions are so poorly designed that they reduce the car's potential performance. Unfortunately, the students never learn of this, because no one tells them. In the short term future I would like to see at least some good engineers out there designing cars with good suspensions. (That's for my children's sake, I'm happy with cars that are older than me...).
~~~o0o~~~

exFSAE,

"I still vote to get an understanding of fundamental handling before diving into the dickathon of kinematics.
...
For as much attention as it gets, kinematics are in my personal and professional opinion and experience largely secondary. They take a back seat to the big hitters of tire properties and dynamic load distribution (fore/aft, side to side, and diagonal). This is particularly so on a stiffly sprung "autocross car" with minimal suspension travel."

Agreed!

In a bit more detail, on smooth track racing (=FSAE) the biggest influences on vehicle DYNAMICS are overall mass distribution, tyre types/sizes/pressures/cambers/toes (none of which require a "suspension"), then engine power, compliances, etc., with suspension spring rates, dampers, and kinematics some way back (depending on details). And should significant aero be allowed, as it is in FSAE, then aero can trump all!

Stressing one point again, the tyre camber and toe values have an initial "static" setting that DOES NOT REQUIRE A SUSPENSION. All that "kinematics" does is to vary these settings according to spring rates and applied loads. For smooth track, stiffly sprung cars, the variation possible is very small. In fact, variation from compliance effects can be much greater, and more adverse.

"Don't focus on the secondary things and minutiae before you understanding the primary factors."

Or more bluntly, "Don't try to make babies by pulling your dick." (Oops, apologies to exFSAE... http://fsae.com/groupee_common/emoticons/icon_smile.gif)
~~~o0o~~~

Claude,

"There is no "dickathon" of kinematics.
...
However, my experience with both race cars and passenger cars is that kinematics (and compliance)..."

"Kinematics" and "compliance" are two entirely different subjects. (FWIW, Kinematics is a field of Applied Mathematics where the idealised geometric bodies are perfectly rigid, frictionless, massless, etc...) By conflating the two subjects you are misleading the students.

And then there is the whole issue of the "2 x 2-D" version of kinematics that is almost universally taught to FSAE students, even though it is wrong!

(And BTW, in Mechanics it is very misleading to say "A force is defined by a application point...", because ANY point along the force's Line-of-Action is as good as any other point...)
~~~o0o~~~

Bottom line is that FSAE students spend an inordinate amount of time on suspension design, but come out of the whole process with a very poor understanding of how suspensions work.

Something is clearly wrong with this system of "education".

(Ooops, a bit longer than intended...)

Z

(Edit: Posts are coming thick and fast... Last two posts I missed, but agree with...)

Claude Rouelle

11-02-2012, 08:27 PM

Z,

1. "And then there is the whole issue of the "2 x 2-D" version of kinematics that is almost universally taught to FSAE students, even though it is wrong!" That is why we have OptimumKinematics which is real 3 D software contrarily to many others (or none in less than 500 $ (FS version) price range). You can have a trial version whenever you want and your comments will sure be analyzed in great details, I promise you. Email engineering@optimumg.com

2. "And BTW, in Mechanics it is very misleading to say "A force is defined by a application point...", because ANY point along the force's Line-of-Action is as good as any other point..." OK mister purist.

3. ""Kinematics" and "compliance" are two entirely different subjects. (FWIW, Kinematics is a field of Applied Mathematics where the idealized geometric bodies are perfectly rigid, friction-less, mass-less, etc...) By conflating the two subjects you are misleading the students." See above comment + Yeah then what... The term AND in Kinematics AND Compliance does not exclude or necessarily mix one with the other. Also K&C is a term that the automotive engineers commonly uses and I am fine with that. Are you in touch and do you speak a common language with these people on daily basis?

4. "Something is clearly wrong with this system of "education"." Yeah ... Yeah.. Yeah.... And what have YOU done or plan to do to change that? Time to change the song, Z.

11-03-2012, 04:36 AM

Originally posted by Claude Rouelle:
4. "Something is clearly wrong with this system of "education"." Yeah ... Yeah.. Yeah.... And what have YOU done or plan to do to change that? Time to change the song, Z.
Claude,

Well, in the last year, in my spare time (ie. unpaid), and just in the field of suspension design, I have;

1. Pointed out that the concept of a "Roll Centre" is very poorly defined in the automotive industry (see 3 below), and is frequently misused and abused.

2. Explained that, contrary to what many design judges preach, "RC migration" is not necessarily a bad thing. In fact, a great many suspensions have RCs that shoot off to infinity with the slightest body roll, yet there are no adverse effects at all! This is something that should be common knowledge, but isn't.

3. Reminded the students of the centuries old and very useful concept of "n-lines". Amongst other things this concept allows much better definitions of RCs to be made. Better than, say, the SAE definition, which describes a "point" but in such a way that only its height can be determined.

4. Explained that the conventional automotive kinematics that is taught in almost all textbooks is, in fact, only a dumbed down 2-D simplification of real world 3-D kinematics.

5. Introduced the students to the centuries old concept of the "motion screw", aka "Instantaneous Screw Axis", or "ISA". And explained that knowing a suspension ISA's position can be used to determine important characteristics "at a glance". (BTW, does your OptimumK software show the position of the suspension's ISA. If not, why not? You were made aware of it over ten years ago...)

6. Introduced the students to the equally old and useful concept of the "force screw", aka "action screw" or "wrench". This fundamental concept is also very useful outside of suspension design, and is also very easy to understand. But for some reason many students, and their teachers, have never heard of it. Why???

7. Briefly touched on the "cylindroid", another old and useful concept for understanding kinematics. Will provide more detailed explanation when time permits.

8. Pointed out some aspects of "Ackermann". In particular (and contrary to your advice Claude), how in FSAE the Ackermann requirements have very little to do with tyre properties, and are mostly determined by geometric factors.

9. Explained "jacking", and that the all too common argument that "the tyre Fy forces must be moved to some special point (IC? RC? FAP?)..." is nonsense.

And some other stuff. But going into double figures would make me look like a real dick... http://fsae.com/groupee_common/emoticons/icon_smile.gif
~~~~~~~~~~o0o~~~~~~~~~~

To any Newbie students interested in Suspension Design,

I have written out the above issues because they are either wrongly taught in the standard textbooks, or else never discussed. You should at least be aware of them now. More details are available scattered throughout this forum.

Of course, criticisms, questions, or counter-arguments to the above are welcome. But preferably rational.
~~~o0o~~~

To date I have mostly promoted the beam-axle as being well suited to FSAE. I have done this because, besides having many good characteristics, it is widespread in many racing series so I thought it would be readily accepted by FSAEers. Maybe not? Yet?

Anyway, IMO there is a very close second on that short list of five (in earlier post).

But what is your suggestion for a simple suspension that is "good enough" for FSAE?

Z

Claude Rouelle

11-03-2012, 10:22 AM

Z,

Your contribution to this forum is appreciable. But it could be way more useful. I simply wish you would use a less rude, negative and pessimistic language. I think of my self as a provocative guy but there you add 5 more layers of negativity (if not vulgarity) and use words in this forum which makes no one a winner. You could simply be excluded (again I think) from this forum. What would you gain from that? What would the community gain from that?

I have been teaching dozens of course in universities and over 300 seminars in big corporation and small race teams to close to 10.000 people in 15 years and 34 different countries. I have learned that is indispensable to be spectacular if you want to keep your audience awake and interested. But there are ego levels and aggressive statements lines that you do not want to cross if you want to gain reputation, respect and simply keep your business growing.

But that was not my question. My question is: who are you? You came to my seminar long time ago, you were much quieter than in this post but I still do not know who you are and what you have accomplished. What are your credentials and what success have you achieved in the industrial automotive (which race team or race car or passenger car manufacturer?) world which would support that the ideas you develop in this forum have effectively been tested and applied with success?

One more question: Have you ever have been wrong? Have you ever learned from mistakes? If so why don't you explain how and why and when a bit more often; it will be make us feel that you are more than a knowledgeable but grumpy man, it will make us fell that you are one of us.

Gruntguru

11-03-2012, 03:45 PM

Originally posted by Claude Rouelle:
Z,
. . . .
But that was not my question. My question is: who are you? You came to my seminar long time ago, you were much quieter than in this post but I still do not know who you are and what you have accomplished. What are your credentials and what success have you achieved in the industrial automotive (which race team or race car or passenger car manufacturer?) world which would support that the ideas you develop in this forum have effectively been tested and applied with success?

You may not have intended it this way Claude, but this line of argument is dubious because it smells like "You don't have success, peer acclaim, lots of experience etc therefore your views carry less weight than mine"

Claude Rouelle

11-03-2012, 05:45 PM

Gruntguru,

I DID intend it that way. But it is not about me. It is about PRACTICALLY demonstrate your point of view. You win race on the track not at the office.

Believe me in 35 years I had more failure than success. But that is how I learnt. I won race and championships but I was also fired twice in my career and I most probably deserved it. My career was a mix of intense satisfactions, lots of frustration, sometimes bitter humiliations but most of it is good memories of hard work, short nights, often decent sense of achievements and amazing spirit of camaraderie (even if in the intensity of the fight and the stress that is not the biggest feeling which emerges) but definitely no regrets.

I am very aware that one (but no all) of the reasons I had (so far) a successful career is that I often worked with the right team , the right team manager, the right mechanics and definitely the right drivers at the right time.

However I always like people who take risks, believe in their elaborated theories, car design and setup choices, who can objectively explain and share the reasons of their choice, have the guts to take risk in order to demonstrate these choices on race tracks. And if their theories do not work I like them even better if they understand and explain why, admit it, maybe suffer the consequences and move on. That is how I work.

You can be the best vehicle dynamicist in the world but if it is only on papers, in front of your computer, alone in an office you do not prove anything. That is why in our consulting business we never stop asking ourselves: does it work? did we prove it? and my approach is sure not different with FSAE students or anybody else.

This is FSAE, this is not a theoretical competition for the Nobel price of physics. Maybe you will find the world cruel but until you theories are validated on the track they won't have the same recognition.

Interestingly (and except some sad, heart breaking reliability issues of most of the time 1 or 2 $ car parts), FSAE / FS teams who can demonstrate solid arguments for their car design choices most of the time do well on the race track. You think there isn't a link?

exFSAE

11-03-2012, 07:29 PM

Originally posted by Claude Rouelle:
Yep except there is a logical and a chronological order: you cam always decide different springs and ARB and damping later while you test the car while it will be difficult to change the suspension type and majorly change the kinematics as it will influence your car chassis (and other parts) design.

This is a good point. Though as you're aware there are series in which there is an appreciable room for adjustment in kinematics even if the topology itself (5-link vs double control arm vs trailing arm) is stipulated by rule.

On a somewhat related note to suspension design but purely out of my own curiosity, is this Optimum Dynamics development item on the OptimumG site to be multi-body / kinematic solved... or lumped parameter / lookup table kinematics?

Claude Rouelle

11-03-2012, 09:22 PM

exFSAE,

Thank you for asking. We use OptimumDynamics with pretty good success with our consulting clients in ALMS, WEC, Indycar, Touring car in South America and Australia. We keep improving it and making it bug free (ahaha, well let's say better and better). It is not over complicated and it is very useful to find good setup and setup DOE. We plan to make a first version available to the public sometimes in 2013. Using OptimumKinematics could be but won't necessarily have to be used with it but there will be other features. It is clear that tire models and OptimumLap (which is "only" a mass point simulation) are part of the building puzzle.

Speaking of OptimumLap we have been amazed about the amount of users (close to 1000!). Some critics told us that it was too simple to be true but amazingly these are the same people who make their software so complicated that they have to "fudge" the on track recorded data to prove their usefulness. One of my motto has always been "make it useful before you make it complicated"

11-04-2012, 04:37 AM

Originally posted by Claude Rouelle:
I have been teaching dozens of course in universities and over 300 seminars in big corporation and small race teams to close to 10.000 people in 15 years and 34 different countries.
Claude,

You might have done all that teaching, but a lot of what you teach is plain wrong (yes, I was there...). In an earlier post I described the "Three-Step-Dance" that takes us nowhere good. Sadly, it seems that it is people like you (and some of the other judges) who are taking us there.

Step 2. Early in your career you learnt to talk-the-talk of motorsports. "Migrating roll-centres", "force application points", and so on.

Step 3. You then made a career of teaching this talk to many of the "newbies" of the industry.

Step 1. But then ten years ago it was pointed out to you that some of what you were teaching was plain wrong. There was a better, 3-D, explanation for suspension kinematics that you should adopt.

What did you do? Did you try to learn this quite simple, and at one time well known, body of knowledge? No! It seems you ignored that helpful advice. Why? Did you just want to maintain the "status quo" that exists at the centre of the motorsport flock? Was that the easiest option? Is that not laziness?

Anyway, you closed the above loop and kept teaching the same nonsense to another ten years worth of students. And you suggest that is a good thing???
~~~o0o~~~

You talk about my "negative and pessimistic language". Yet I am on this forum trying to encourage the students to actually build better cars. Not to keep building the same-old-same-old cars, because that is easier to do (no original thought required), and all the other sheep are doing it. Not to keep believing the same old cottage industry fairytales that are passed off as "racecar suspension theory". But actual PROGRESS!

And yet here you are doing your best to stop that... Why the "negativity"???
~~~o0o~~~

Finally, does it really matter what my "credentials" are? On this thread I have stated the empirical fact that suspensionless go-karts can go around corners fast. Elsewhere I have passed on theoretical knowledge that was discovered centuries ago by much smarter people than you or I.

How does my past work experience (or anybody's, for that matter) make any of that less or more true???

"Past experience", Claude, is something people must promote when they no longer have the "ability".
~~~~~~~~~~o0o~~~~~~~~~~~

Anyway...
Anyone want to have a rational discussion about simple suspensions suitable for FSAE? http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

(PS. Oh, yes... Claude, do you really think "pulling your dick" is so much more offensive than one of your favourite, and oft-repeated phrases, namely "intellectual masturbation"??? http://fsae.com/groupee_common/emoticons/icon_smile.gif)

exFSAE

11-04-2012, 04:46 AM

Eh, I'd be wary to say that 2-d kinematic analysis is "plain wrong." It is one method / model / approach. 3-d kinematics may be more complete, but also perhaps a bit less obvious to draw or wrap your head around. Could just as well make the argument that this approach too is "wrong" and that you should be doing everything as elastokinematics in ADAMS; the addition of compliance rates may have more impact on the final answer than going between a 2-d or 3-d approach.

All models or analyses are "wrong" to some extent. You use the right tool / method for the right job depending on what you're trying to achieve... and in my experience I prefer to use the most simple approximation that sufficiently answers a question presented.

11-04-2012, 05:28 AM

Originally posted by exFSAE:
I'd be wary to say that 2-d kinematic analysis is "plain wrong."
exFSAE,

Yes, I guess I should say "plane wrong". http://fsae.com/groupee_common/emoticons/icon_smile.gif

More seriously, the 3-D approach is really no more difficult to understand, or use, once you get your head around it. Overall, I find it easier (more answers from less effort).

But the thing that really irks me is that the whole auto industry (production and racing) has promoted the 2-D theory as being the "absolutely" correct kinematic theory. Additional refinements like compliance, to get closer to "reality", are supposedly just added on top of this "accurate" kinematics.

I'd like to see some "truth in advertising" here. Something like a "this 2-D theory may not work properly in 3-D worlds" type of warning. Of course, this will only happen when the promoters of 2-D kinematics realize that it is wrong, which many of them don't yet know.

It is a bit like a builder laying out a house using "flat-earth theory" (spherical geometry not being needed), while the builder's kndergarten children are taught that the world is a "globe", and "we live here, on the underneath part..." (which the kiddies can sort of understand...).

Oh, and BTW, the errors can be significant, 10++%...

Z

exFSAE

11-04-2012, 06:24 AM

Originally posted by Z:
But the thing that really irks me is that the whole auto industry (production and racing) has promoted the 2-D theory as being the "absolutely" correct kinematic theory.

Having worked in both the consumer and motorsport areas of the auto industry, I don't agree with this statement... specifically not agreeing that the 2D approach has promoted as "absolutely correct." I'd say it is viewed as one tool, of many.

Not to mention that in the real world where you are sometimes faced with kinematically overconstrained dependent suspensions (i.e. if rigid would be impossible to move), all the discussion of the pro's and con's of 2D vs 3D kinematic approach is pure masturbation.

BillCobb

11-04-2012, 07:31 AM

Well I worked in the industry for 40 years and with 4 top echelon racing teams and I don't recall any proponents of "2-D" theory. When you have a K&C machine to use as readily as a copy machine, a top notch Adams group to support the latest international theories, and an Instrumentstion and Test group to analyze and evaluate the latest legends, lore, and hocus-pocus, you can get to the bottom of of vehicle dynamics pretty quickly. Getting Purchasing, Manufacturing, Marketting, Safety, Rules and Regulations, Durability and Dealer Support to accept "optimum", "refined", or "acceptable" chassis, steering and tire specifications, now that takes some serious magic.

11-04-2012, 04:06 PM

Originally posted by BillCobb:
When you have a K&C machine to use as readily as a copy machine, a top notch Adams group to support the latest international theories, and an Instrumentstion and Test group to analyze and evaluate the latest legends, lore, and hocus-pocus, you can get to the bottom of of vehicle dynamics pretty quickly.
Bill,

Why do you need all that equipment? It is a very easy subject to understand.

And what are these "latest international theories"? Are they any better than the "hocus-pocus" developed by Euclid, Galileo, Newton, d'Alembert, Euler, Chasles, Ball, and a great many other very clever men, hundreds of years ago?
~~~o0o~~~

Anyway, it is perhaps pointless trying to reason with the angry mob...

"Heresy!!! It is heresy!!!!!!!
That blasphemer Z is saying that the world is NOT flat! He claims it is a globe, and it moves around the sun!!!!!
Bring the firewood and torches. He must be burnt at the stake...!!!!"

Bring on the next Dark Ages! http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

Z

exFSAE

11-04-2012, 04:35 PM

Originally posted by Z:
Why do you need all that equipment? It is a very easy subject to understand.

You seriously just said this?

http://www.burdandkeyz.com/wp-content/uploads/2010/11/son-300x224.jpg

But I will bow down to thee Z, who hath eclipsed us in professional motorsport experience and championship victories by thine screw axis method.

jlangholzj

11-04-2012, 05:11 PM

Just keep going, you only need to dig a little bit deeper to hit that magic 6ft mark

BillCobb

11-04-2012, 06:35 PM

A few very amusing anecdotes come to mind when I read your question back to me.

1). When GM got together with Saab, a very loud and self centered 'expurt' went to all the GM execs spouting about how rear lateral force oversteer was a key ingredient to the superb handling of Saab cars. (You know what 'Saab' stands for, right? Still Ain't A BMW). OK, so we test and analyze and guess what? No Saab car has lateral force deflection oversteer. Their Panhard bar twist axle has a little on one rear wheel and deflection understeer on the other. Just like the 3-D theory says it ought to be. In other words, Dr. Shitforbrains is an ignorant fool, but with a big mouth and a bigger lame audience. Saabs need very unusual tire sizes and constructions because their suspensions are sloppy and require high cornering stiffnesses to achieve just 'average' transient response characteristics.

2). Lotus lovers would be horrified if they could see the k&C measurements on ANY of their cars. Probably never has been a single one build the same. No wonder they all have their 'unique' British flavor and distinction. Use of 3-D suspension analysis allows you to perform build tolerance studies. You use Monte Carlo techniques to statistically build suspensions and identify what locations are critical and what tolerances you must have to maintain consistency, alignment, safety, forseeable misuse, aftermarket tire use and durabilty concerns. If the plant can't hold the tolerance, you can't use that design. Lotus uses the Monte Python technique (IMHO) to slap their cars together. Good grief Charlie Brown. Of course, the world adores their products because of all the inebriated exuberance of their own verbosity. (You know what Lotus stands for, right? Lots of Technically Useless Shit).

3) The Holden G8 turned into the Pontiac Grand Prix was another reason to go to testing and analysis. When turned over to GM N.A. as a Finished product (not to be confused with the Swedish products), the G8 was release on a tire that was absurd for stopping distance, wet traction and fuel economy requirements. Any other brand of tire (Bridgstone would never even have had enought to meet US production volumes) made the car extremely unmanageable, failed NHTSA stopping tests and STBed its fuel economy requirements. GM exeecs who drove them lost control of a few cars in normal driving maneuvers. Analysis and testing forces a redesign of the head and azz end of the car by N.A. to make it LEGAL to sell here, in spite of the Aussie 'ex-purr-tease'. So much for Erector Set vehicle dynamics. (You know what Pontiac stands for, right? Penniless Old Nigerians Think Its A Continental).

If BMW's are so great, why does it take so long to put out a new model? and why does every generation of BMW have a different roll steer gradient? Yes, they are very close per given model year (unlike Lotus). Can't they stick with a great recipe, or don't they have one in mind yet. Analysis would tell you this. Testing would validate the analysis. (You know... Bite My Wieney).

If Porsche could add just abit more roll understeer to their cars, it would steer more than it rolls. But wait, that not what I read about those Black Forest gnomes. Back to the very expensive tires and now we need two different sizes on the car. Buddy can you spare a spare tire? (You know what Porsche stands for, right? Proof Only Rich Suckers Can Have Everything).

Want to know who does the most supurb, consistent, fun to drive, technically brilliant, blah, blah, blah best in all classes? You already know, eh? (Historically Orientals Never Drove Automobiles). And I've still got the DATA to prove it. When I saw a K&C summary sheet or a road test set of plots and metrics, any Honda car hits you like a DaVinci viewed in a museum.

I could go on, but I'd like not to resurrect old nightmares about how the Lords of Vehicle Dynamics get away without the triad of synthesis/analysis, lab tests, and road tests.

ben

11-05-2012, 12:15 AM

Originally posted by Z:
It is a very easy subject to understand.

Z

Now your just being facile. Tennis is just hitting a ball over a net - can I be a Grand Slam winner please?

Understanding the underlying engineering concept is often a tiny part of the battle, particularly when you're working with partners from different companies and cultures.

You never have all the data, so you have to make assumptions. When those assumptions conflict with those made by another engineer with the same incomplete set of data you have a problem that's no longer about engineering. Every engineering situation I've been in is like that from aerospace to mainstream OEM automotive to motorsport.

The reality Z is that you're a smart guy, but you're also likely to be the guy in a meeting that people will want to ignore and marginalise simply because you're arrogant and annoying. You might be right (you might be right all the time!!), but that's often not the point.

Ben

Claude Rouelle

11-05-2012, 06:58 PM

Z,

1. You cannot compare the seminar you came to more than 10 years ago with the one taught today. You should come for a refresh.

2. Whether you speak about the heave, roll, pitch and steering induced chassis Vs the ground movement (force or displacement input) or one wheel movement Vs the chassis there will (or could) be track, camber, caster, steer, wheelbase variations so implicitly in a good 3D geometry you describe these movements as a "screw axis" whether you call it or demonstrate it this way or not.

3. "You then made a career of teaching this talk to many of the "newbies" of the industry"
First my company is not only about teaching. That is 1/3, the 2 other 1/3 are consulting and simulation.
Second, think again Z. Not later than last week I gave seminar #347 at Chrysler to 100 engineers: 30 engineers from the vehicle dynamics group, 9 tire engineers, 10 “chassis controls” engineers (think ESP, ABS, etc...), 5 engineers from the testing group (K&C and instrumented handling), 23 Chassis Virtual Analysis engineers (modeling & simulation), 11 steering & suspension engineers, 7 people from SRT (ALMS Vipers) and a handful of other engineers, supervisors & managers. Not bad for somebody who "kept teaching the same nonsense"

4. You can be provocative but you need to adapt your words to your audience chose and your expression mode (written or spoken). It is a fine border. You use good judgment. If you don't people will. At your expense. I am sure you know things I don't and many other people don't. I am sure the community would benefit from both your knowledge and perspectives. Write more article, give seminars, become a teacher or a faculty adviser. Make the world benefiting from you but for God sake Z show some respect for others' experience and yes.. credentials. Maybe the whole education system is wrong, maybe the whole world is wrong but you are not helping anything and surely not yourself by being unnecessarily so aggressive. Time to unscrew your own axis Z. If you want a face to face meeting, I will be in Australia Melbourne area from December 6th to 15th. Not tie required for the meeting http://fsae.com/groupee_common/emoticons/icon_smile.gif just respect and desire to improve both knowledge and communication.

Kirk Feldkamp

11-05-2012, 07:13 PM

Originally posted by Claude Rouelle:
If you want a face to face meeting, I will be in Australia Melbourne area from December 6th to 15th.

If this actually happens, please please please put a camera in the room to record the conversation!

-Kirk

dprmb

11-05-2012, 10:57 PM

Z,

It seems a little one-sided when you critique forum members and then don't give said members the opportunity to critique you... What's your claim to fame besides book smarts and an acid tongue?

As Claude implied, think about your audience... this is a forum for a student engineering competition, with the bulk of us being undergraduates, alumni, volunteers and judges.

What category do you fit in? Maybe you don't...

11-06-2012, 02:54 AM

What Has Been Established So Far.
=============================

1. Anyone who believes the world is a globe circling the sun is a lunatic, a risk to society, and should be burnt at the stake! As everyone knows, the world has hills and dales, which makes it truly 3-D. But other than that it is a flat 2-D plane. Err..., supported at its corners by four huge elephants... that stand on a giant tortoise... and below that it is turtles all the way down! http://fsae.com/groupee_common/emoticons/icon_smile.gif

2. If the Bible of RCVD says "In true three-dimensional space, instant centers are replaced by instant axes..." (p612), then it MUST be true! And, of course, the 250+ year old concept of a "screw axis" is the work of the devil!!! If a computer Suspension Program claims on the packaging that it is "fully 3-D" (like the one Z trialled a few years ago), then that MUST also be true. Even if the results it gives for "anti-pitch/roll" are calculated according to RCVD type "instant axis theory", so often have errors of 10++%. http://fsae.com/groupee_common/emoticons/icon_frown.gif

3. A US manufacturer of pick-up trucks makes the goddamn best handl'n vee-hikels on the whole gosh-darn planet! Apparently, they do this by spending a lot of time in casinos rolling dice... Hey, that's gotta be more fun than doing geometry, especially during Happy Hour! http://fsae.com/groupee_common/emoticons/icon_smile.gif Oh, and Porkers handle like... well..., porkers... http://fsae.com/groupee_common/emoticons/icon_frown.gif

4. Z will never, ever, ever be welcome in a Product Development Design Review Committee Meeting. Err... ever! ("Yahoo!", thinks Z, "They're sooo boring..."). And the game of Tennis, like so many other fields of endeavour, really does benefit from KISS. Winning Tennis players do, indeed, just keep hitting the ball over the net. By analogy, winning FSAE cars just keep going fast around corners. Which they can do without suspension... http://fsae.com/groupee_common/emoticons/icon_smile.gif

5. Finally, for those slackers who are too lazy to do their homework (try Find!), Z is a nobody from nowhere, who has done nothing, no-wise, ever... Err..., other than mindlessly ramble on about some long lost bunkum called "Classical Mechanics". And, err..., endlessly ask for some rational discussion on suspension design. But, geez... "reason" ain't gonna educate engineers, is it? http://fsae.com/groupee_common/emoticons/icon_confused.gif

~~~o0o~~~

Moving on..., (or back?) to FSAE suspension design.

So far nobody has argued against the notion that "suspensionless cars" can go fast around smooth circuits. Likewise, no compelling arguments that the most complicated suspension types are necessary for a FSAE car to be fast.

Is this agreed? Any "Nays"???

So, on the assumption that FSAE suspensions could be much simpler, I will, in later posts, go through the pros and cons of the top five on this list.

... the major suspension types are,

1. Beam-axles (eg. on all vehicles ever, can be live or De-Dion at rear),
2. Sliding-pillar (eg. Morgan and Lancia, and most motorbikes and aeroplanes at front),
3. Lateral swing-axles (eg. Tatra (still on their trucks), can be high or low-pivot),
4. Leading and trailing-arms (eg. Citroen 2CV front and rear, and most motorbikes at rear),
5. Semi-leading/trailing-arms (eg. F100 at front, and many mid to late 1900s cars at rear),
6. Strut-wishbone (eg. McPherson at front, strut at rear),
7. Double-wishbone (eg. originally mostly at front to allow shorter wheelbase with front engine),
8. 5-link (eg. recent fashion, mainly for NVH reasons).

Of these, any of the first five are more than adequate for a winning FSAE car.
But would anyone else like to give their thoughts first?

For example, Claude says,

While I am less convinced about the efficiency of semi-trailing arms and even less about live axles (actually I call them dead axles, I always have been convinced that you can make a good car with a McPherson.... but if you if you miss the window of good mix between of tire exploitation and kinematics design it will be more difficult to make a change. Providing some suspension pick up points adjustability, with a 5 links or a double wishbone you can get and/or adapt the heave and roll camber variations, roll centers, pitch centers position and movement to what the tires (and good drivers) "need" much better than with a McPherson. And if for whatever reason you are forced to switch to another brand of tire, your design could not be adaptable...

As a student I will most probably go for a double wishbone or a 5 links.
And Tony says,

Double wishbones are an obvious natural choice at the front, but at the back, I am not so sure.

There can be a lot of problems with the potential for bump and compliance steer at the back with a double wishbone design, which seems to be a regularly occurring design problem area on many FSAE cars.

Good solid toe control at the back would have to be be pretty high on my wish list.

A single stiff rear trailing arm, with transverse upper and lower camber control links would give excellent toe and bump steer control while being simple.
Now I don't like any of the above options, and have rational critcisms of them which I will give later (and, of course, which is why I'm not welcome in any meetings! http://fsae.com/groupee_common/emoticons/icon_smile.gif).

But in the meantime, Claude and Tony, would you like to "defend your decisions" any further, in a Design Tent sort of way?

It might be helpful to the "education of young engineers".

Z

Jay Lawrence

11-06-2012, 08:33 PM

Z,

there is the distinct possibility that some of these "young engineers" will be lucky enough to end up in the automotive industry, from which they will be promptly expelled should they suggest beam axles on a sports/race car. My current car has a rear beam, and yes it is fantastic on a nice smooth road (in Australia? Where?) but off that it can give some frightening and unexpected tail movement.

Some input from one of the ADFA guys would be interesting here, as would someone from Monash. Independent to beams on the former, direct actuation to push/pull rod actuation on the latter.

Warpspeed

11-06-2012, 10:42 PM

Ah !

All working professional engineers have to function under constraints, sometimes very severe and unreasonable constraints, be they political, managerial, or financial.
And so do student teams.
And sometimes, just sometimes, a really clever SIMPLE design can really triumph in cost, performance, and reliability.
And that can take you far.

One day your boss will demand you and your team come up with a superior design of mouse trap. And do it in two weeks.......
And then JUSTIFY all your decisions.

That is what FSAE is really all about.
Boot camp for real practical engineering in the real world.
And pie in the sky ain't really a part of that.

And neither is noxious aggressive hubris.
Be kind, be patient, be humble, and help others whenever you can, or remain silent.

Have fun, and creative engineering can truly be a most fulfilling vocation.

oz_olly

11-07-2012, 01:24 AM

The ADFA beam axle car was designed using a clean sheet approach and a fresh look at what the FSAE rules were all about. After learning FSAE the monkey see monkey do way over 4 years as an undergraduate I was lucky enough to have another go as a masters student after a year to reflect on my undergraduate experiences. I should caveat that Z was consulted during the conceptual design of the car. He might come across a bit gruff and blunt at times on this forum but I cold called him from the telephone book and went on to chat to him for about 3 or 4 hours discussing different concepts. Z was instrumental in helping me to muster up the courage to do something very different to the accepted norm.

The first question I asked myself was what is the job of the suspension. I came up with the following answers:
- To control wheel attitude (camber, toe, steering angle, wheel base and track)
- To allow adjustment to roll stiffness distribution (elastic and geometric)
- To try and minimise contact patch load variation over undulating and rough ground
- Allow for +- 25mm suspension travel to be compliant with the rules

The team I was designing the car for had next to no construction or design experience due to a break in the program at our uni so it was basically made up of 1st and 2nd year students who had never built an FSAE car. I very quickly ruled out double A arm suspension because of the number of jig points required on the chassis, number of components required for fabrication and the jigging required to make the A arms.

The beam axles gave me the ability to do all the things I listed above and only required 10 accurate points on the chassis. I think I talked about this more in the beam axle thread I started. Now that the car has been running for 4 years and competed in 4 competitions (FSAE-A x 3 and FSUK x 1) I think we have a good feel for the weaknesses of the design. The first main weakness I discovered was excessive camber compliance. I designed a camber compliance rig and quantified it, we then welded in some gussets which made a significant improvement and was very noticeable when viewing the car on the track and improved tyre wear. The next one is toe compliance in the steering mechanism. To avoid or minimise bump steer I came up with an R&P mounted to the beam, this combined with trying to fit steering linkages inside a 10in rim meant I had to use a bell crank to transfer the rack motion to the steering arm. The steering arm design, number of joints in the linkage and large linkage angles (close to straight) on the inside wheel at full lock all contribute to make the mechanism more flexible than I would like.

The car has a large amount of steady state understeer and requires much more roll stiffness on the rear to try and tune it out. I think this is partly due to the roll axis being too close to level and the toe compliance in the front. We also have very short trailing arms on the rear axle which seems to promote brake hop. It’s hard to say if it’s all the suspension as the engine tune isn’t great, especially on the over run and not many drivers have mastered down shifting properly.

I have no regrets in choosing that design and would love the opportunity to develop the concept and take it further through better understanding and detailed mechanical design. One of the biggest issues with choosing the design is that as new people have come to the team it has been very hard to get them onboard with the concept and explain it to them. They can’t pick up any of the popular suspension or vehicle dynamics books and learn much about it. They should read the Olley book put together by the Millikens and a few good FSAE papers on twist axles but they are not what I would call Tune to Win level references. The car was received very sceptically at its’ first competition and design event. We took the feedback on board and then managed to make it into design finals at FSAE-A and FSUK turning quite a few heads along the way.

I have recently been talking to the current ADFA team as they approach the design of their next car and the most common thread of conversation is ‘we don’t know enough’. Any first year team and most undergraduates (even post graduates) are in the same boat. FSAE is an opportunity to learn and learn through mistakes in a relatively benign environment (compared to real racing, mining or aerospace). So my advice to new or inexperienced teams is take a reasoned stab if you don’t know about a parameter and design in some adjustment. If you stop worrying about design decisions and make some you could even have the time to redesign and iterate to improve towards the end of the project. I think it takes years to learn enough about FSAE and vehicle dynamics to be able to properly design a race car system with high level design targets and seek to predict the performance through modelling and simulation and then verify through testing. I am learning more and more in my career in industry that coming up with a solution you are confident will work through reason and experience with a little analysis to prove it is a highly sought after skill. Analysis paralysis will not take you far in a career. A good engineer will use a variety of design methods to achieve results i.e. it looks right from experience method, big and strong can’t go wrong and finely optimised and refined. They all have their time and place and it’s about knowing when to use them.

Cheers

js10coastr

11-07-2012, 08:16 AM

wah wah wah wah Look at me I'm the smartest man in the room! wah wah wah wah

Drew Price

11-07-2012, 10:21 AM

http://www.saabrally.com/forums/download/file.php?avatar=5_1217271771.gif

OspreysGoSWOOP

11-07-2012, 05:26 PM

Originally posted by BillCobb:
You know what 'Saab' stands for, right? Still Ain't A BMW

You know what Lotus stands for, right? Lots of Technically Useless Shit

You know what Pontiac stands for, right? Penniless Old Nigerians Think Its A Continental

You know what Porsche stands for, right? Proof Only Rich Suckers Can Have Everything).

Historically Orientals Never Drove Automobiles

What does Volkswagen stand for? http://fsae.com/groupee_common/emoticons/icon_razz.gif

Jay Lawrence

11-07-2012, 06:08 PM

Thanks for the response Olly

Interesting to note that even a much simpler design had its share of problems. I always saw the ADFA car as an interesting concept that could have gotten some much better results with some gun drivers

Chapo

11-07-2012, 11:23 PM

I dont know if I should be happy because you complimented our car, or sad because you insulted my driving (though I admit it is quite average).

One of the major issues that affects ADFA in that sense is the university only has a total population of around 1000 people, you dont have much of a pool to draw on so it can be difficult to find a team let alone drivers of any skill.

When you take into account the time burden as well, ADFA does a lot of military training instead of uni holidays and also seems to really enjoy scheduling "compulsory fun" activities in fairly regularly, there isnt much time left to do driver training.

The above points are some of the other reasons why we went down the design path that we did. I manufactured the suspension for the last double A-arm car in 2007 and was also involved in the manufacture of the beams. The beams were signifigantly easier to make, they required less skill from the people working on them (the majority of the sheet metal work was done by first years, if they screwed it up they just did it again as sheet steel isnt that expensive). The majority of the components on the car were designed to be done this way, granted some items on it are rather complicated (rear bulkhead, sump plate etc) but these were manufactured as part of the team sponsorship deals.

Designing with the ease of manufacture in mind allowed a small team to build a car around the constraints and still get testing and driver training done so that we didnt look too bad on track.

Having said that, I would love to see that car go round the SAE track with some good and experienced drivers in it, just to see what it is actually capable of.

exFSAE

11-08-2012, 03:58 PM

Originally posted by Z:
As everyone knows, the world has hills and dales, which makes it truly 3-D. But other than that it is a flat 2-D plane. Err..., supported at its corners by four huge elephants... that stand on a giant tortoise... and below that it is turtles all the way down! http://fsae.com/groupee_common/emoticons/icon_smile.gif

When I want to pull up a map and figure out the distance to get from my home to the local deli, to be precise I should be considering the curvature of the earth and its effect on the trajectory I shall take.

Or, for the sake of time and my own sanity, I can approximate it as a 2D planar problem and get an answer quite sufficient to my needs.

I would make the argument that in professional motorsport engineering you are much more up against the clock and needing to provide someone with a quick and simple answer than one which is of 99.99% accuracy.

Point I've been trying to make all along is that there is a level of precision you need, above which is a waste of time. Or as Mr Cobb points out, if your process already involves a K&C rig or ADAMS model with elastokinematics, the whole process of working out all this stuff in your head is a waste of time. If you have any sort of multibody analysis then you get straight to the answer. Or any kinematic solver really. I've never, ever heard any race engineers sitting and debating the pros and cons of 2d vs 3d analytical solutions because it's all a waste of time and already wrapped into your analysis package.

Leave the programming to the programmers and the physical modelling to MSC. The team engineers have a list of 1000 items to do before a race, of which they might accomplish 10. No time to waste.

2. If the Bible of RCVD

Not at all a knock on RCVD, but it is not the end-all-be-all of racecar engineering.

11-08-2012, 08:36 PM

Jay,

"There is the distinct possibility that some of these "young engineers" will be lucky enough to end up in the automotive industry, from which they will be promptly expelled should they suggest beam axles on a sports/race car. My current car has a rear beam, and yes it is fantastic on a nice smooth road..."

Doesn't this worry you? It worries me, although more for my kids' sake than for myself.

As you say, beams work fine on smooth roads. Yet a young engineer will be ridiculed for suggesting one on a car intended for smooth roads (probably not "expelled", what with IR laws, etc...). Why? Simply because of the mindless following of fashion by his peers and superiors.

The really bizarre (humourous?) thing about this, is that all these passionate, almost religously zealous views, held by supposedly sober, rational engineers, are about nothing more than a "bunch of pipes". Different suspension = different bunch of pipes. If any arts, law, or med students, or for that matter, butchers, bakers, or candlestick makers, read these posts, I very much doubt they would understand why all the fuss. Why all the drama queens? Why are they arguing about different bunches of pipes??? http://fsae.com/groupee_common/emoticons/icon_smile.gif

(Answer to "Why?" at bottom...)
~~~o0o~~~

Tony,

In an earlier post you wrote.

"For a first effort, keep it all very simple and very conventional."

I agree entirely with the intent of that comment, namely KISS.

However, "very conventional" in FSAE is about as complicated as you can get. Namely, Double-Wishbone-With-Push/Pullrod&Rocker (let's call it DWWPP&R). Sadly, peer group pressure keeps forcing new students down this complicated path.

I am trying to help the new students by pointing out that there are, in fact, much simpler suspension types that are suitable for FSAE. Might get around to discussing that one day...
~~~o0o~~~

Olly,

"The ADFA beam axle car was designed using a clean sheet approach ...
... to muster up the courage to do something very different to the accepted norm."

Firstly, you deserve a medal for "extreme courage under fire" for taking that route. Honestly, and unfortunately, it seems there are very few people left in this world who are prepared to make such tough decisions.

Secondly, I wish you would have called me a few more times. I might have been able to help you get over those first few hurdles (camber compliance, steering, etc.) a bit more quickly.

Thirdly, the reason I came back to this forum is that I saw that there was at least one student in tens of thousands who was actually prepared to think things through for themself, rather than simply follow the flock. There is hope!

(Ooops..., perhaps I shouldn't have written that third one. Now the lynch mob might be coming down your street. Sorry! http://fsae.com/groupee_common/emoticons/icon_smile.gif )
~~~o0o~~~

js10coastr,

"wah wah wah wah Look at me I'm the smartest man in the room! wah wah wah wah"

Yep, that's straight out of the movie "Idiocracy". Well done.
~~~o0o~~~

Matthew,

"Designing with the ease of manufacture in mind allowed a small team to build a car around the constraints and still get testing and driver training done so that we didnt look too bad on track."

Thanks for further "real life" confirmation that KISS works. If a large enough "critical mass" of teams keeps proving that KISS is really good, then eventually the rest of the students might believe it.
~~~o0o~~~

exFSAE,

"When I want to pull up a map and figure out the distance to get from my home to the local deli, ...
... I can approximate it as a 2D planar problem and get an answer quite sufficient to my needs."

I have covered this before, but once again...

The key thing is that YOU KNOW that the world is round. So do kindergarten kiddies, because it is a simple concept. So if you have to build a really big bridge, or shoot rocketships into space, then YOU KNOW you will have to move from the 2-D approximation to something 3-D.

Here are your words from another thread (re: TLV).

"... there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns – there are things we do not know we don't know.
It's that last one that will get ya."

I very much agree with that! I have simply been pointing out (mostly on other threads) that many students of automotive kinematics "do not know what they don't know" (ie. that there is a whole world of interesting, and more accurate, and useful, kinematic theory out there).

"... working out all this stuff in your head is a waste of time...
... No time to waste."

This is a bit off-topic (covered elsewhere), but knowing the approximate location of a suspension's ISA requires negligible "working out", and at most can be done with simple software. Importantly, once you know the ISA's location, you can tell "at a glance" many of the behavioural properties of the suspension (eg. anti-roll/pitch, jacking, toe/camber/castor changes with bump, etc.). Briefly, there are very good places, and very bad places for it to be. And an inbetween area where you have to check the details...

It is a very useful tool, is easy to use, and can save a lot of time. Why the reluctance to use it?
~~~o0o~~~

To students interested in winning FSAE by using simpler suspensions.

As should be obvious from the above, you will get a lot of flak from both your peers and the Design Judges, if you go down this path. As Olly said above, "The [beam-axle] car was received very sceptically at its first competition and design event."

You should expect something like;

Design Judge, "So, Suspension Boy, what do you have here???"

Suspension Boy, "Well, Sir, due to our limited resources we decided to avoid all the complexity of double-wishbones, especially with their seemingly unnecessary push or pullrods and rockers. We couldn't find any reasons why we needed ..."

Design Judge, interrupting, "What the!!! How the hell do you expect to score points in this Design event, with such a NEGATIVE and PESSIMISTIC attitude! Geez ... http://fsae.com/groupee_common/emoticons/icon_mad.gif "

Hopefully by discussing this heresy here the Design Judges will be somewhat "softened up" to these concepts when you bring your cars. It won't be such a shock to their systems. Also, going through the various pros and cons of different suspensions, in a rational way, might help when you are "defending your decisions".
~~~o0o~~~

Finally, why the extreme resistance?

Well, IMO it is indirectly due to the success of previous generations of engineers, particulary with regard to mechanised farming. Put simply, full stomachs = empty heads. With an abundant food supply, effort is no longer needed.

If "necessity is the mother of invention", then with no necessity, there is no invention. Everybody gets by just fine these days, simply doing the same-old-same-old...

Z

Warpspeed

11-08-2012, 11:16 PM

It's like the engineer that was once asked by a fireworks company to design a rocket.

He said, three stages, liquid fuel, full telemetry, GPS guidance, and we can do it for a million dollars.

Hell son, we were thinking more like fifty cents, stick it in a bottle and light it with a match............
WTF are you going on about...

Max Trenkle

11-09-2012, 01:22 AM

FSAE:

Step 1 - Organize your team.
Step 2 - Figure out what you want to do.
Step 3 - Design what you want to do.
Step 4 - Optimize what you've done.

It took me one year just to accomplish Step 1.
For step 2, I read this forum and many, many books. The majority of racing history (important word here) led me to deciding to go for double wishbone suspension. Was it because everyone else used it? Well no, but I understood it at first glance and it seemed relatively simple to me. I've gone over almost all of Z's work and I think he's spot on, but the problem is that there's little history involved (in comparison to double wishbone history). Now that's not a bad thing, but if I know two different systems work, and one has 1000 people behind it, the other 10, I'm going to to with the 1000 people, because now there is a larger population of resources of information and experience available.

As much as I would love to innovate and try out some of these elegant suspension solutions and theories you've suggested Z, the problem is reliability. To me, if doing the same-old stuff gets me around the track 100%, I'm fine with that. Seeing as scoring points in all event at the Michigan event will net me at least top 40, that's what I'm shooting for. Putting the components together for double wishbone suspension is rather simple to me. There's nothing wrong with it, and everybody knows it works, whether they know the reasons behind it or not. Not saying that other designs would be less reliable, but I don't have 1000+ examples to look at and examine in racing history related to these alternative suspension designs. Then again, maybe I'm just ignorant of a vast wealth of history. I am ignorant of many things. Pretty sure my team is still going to do well at competition.

Z, I think you need to review your expectations of this competition. There are many upfront costs: large amounts of money, large amounts of time, and large amounts of mistakes. You're right about so many things Z, but at this point no one gives a shit because you aren't helping hardly anyone in this competition with your theoretical nitpicking.

"You've got to do it once before you can do it right."

This is what I tell everyone on my team. Z, I did not understand a word of anything you said until I figured out how double wishbone suspension worked. Once I figured out how it worked, it then became clear to me how it could be simplified, and eureka, it all made sense. Funny that every single other person I know that has tried to figure this out has gone through the same process.

It's an awfully tough decision to even decide to do Formula SAE. I chose to do it because it has been my life goal to design cars, and to become a first class engineer/designer/R&D guy, etc. If I didn't have to:
1 - Actually attend classes
2 - Pay for classes by working
3 - Pay for living expenses by working
4 - Enjoy the company of other human beings in order to retain sanity
then I'd sure as hell design a Grade A suspension system. Unfortunately, I'm rather caught up in optimizing the little time I have, which is what FSAE is all about.

Z, I'm sorry that you're so right, yet hardly any of us can do anything about it. It's rather a shame. :'(

Maybe if enough people "critical mass" keep telling that you have a bad attitude, are hostile, and unhelpful, perhaps you might believe it.

Do us a favor and write a book of your thoughts and theories on suspension design. I'll buy the first copy.

exFSAE

11-09-2012, 08:37 AM

Originally posted by Z:
The key thing is that YOU KNOW that the world is round. So do kindergarten kiddies, because it is a simple concept. So if you have to build a really big bridge, or shoot rocketships into space, then YOU KNOW you will have to move from the 2-D approximation to something 3-D.

Indeed. And likewise, every professional automotive or motorsport engineer I've worked with are quite well aware that drawing your linkage (if you have one) out in 2D isn't the end-all-be-all of kinematic solutions.

This goes back to your previous assertion that "everyone thinks 2D is the best / the answer." I don't agree with that. In FSAE? Maybe. But FSAE students are rife with misconceptions and BS anyway.

To Max's point.. you're not gonna get it all down pat in this series. It's an opportunity to know the difference between caster and kingpin, camber and toe, and do some very rudimentary vehicular engineering work.

Drew Price

11-09-2012, 11:00 AM

This thread isn't about suspension design anymore....

Owen Thomas

11-09-2012, 11:10 AM

I dunno Drew, seems like half of "suspension design" is painfully wading through opinions and data, picking what you want out of it, and slapping it all together. The other half is dealing with skeptics and project managers, then being told at the end that you were just plain wrong anyways.

I think this thread prepares suspension designers for what they're going up against. http://fsae.com/groupee_common/emoticons/icon_rolleyes.gif

js10coastr

11-09-2012, 01:03 PM

Originally posted by Drew Price:
This thread isn't about suspension design anymore....

I think it's now a cross between "LALALALA I CAN YELL LOUDER THAN YOU!!! LALALA" and

http://imgs.xkcd.com/comics/duty_calls.png

js10coastr

11-09-2012, 01:05 PM

Originally posted by Owen Thomas:
I dunno Drew, seems like half of "suspension design" is painfully wading through opinions and data, picking what you want out of it, and slapping it all together. The other half is dealing with skeptics and project managers, then being told at the end that you were just plain wrong anyways.

I think this thread prepares suspension designers for what they're going up against. http://fsae.com/groupee_common/emoticons/icon_rolleyes.gif

I think this needs to be included in Chapter 1 of RCVD "The Problem Imposed By Racing"

exFSAE

11-09-2012, 01:51 PM

Biggest / only takeaway point I have from all of this is the following:

Start by know what you're trying to achieve. What do you want the car to do? There is no "optimum" linkage or VSAL or FLLTD whatever. It's all a function of what you want to try to achieve. Start at a high level and the subsystems and piece parts will fall into place.

Once you know that, use the most appropriate and available design and analysis tool(s) to meet your objective - realizing that "most complicated" or "most detailed" is not equal to "most appropriate," and that perfect is the enemy of good.

Stupidly simple when written out, but easy to overlook.

Drew Price

11-09-2012, 02:17 PM

Our first year car it was a miracle that the car kept goiung in a straight line when you let go of the wheel, and the left and right sides were just about symmetric to each other.

Packaging, load paths, and nothing failing during brake test should most of what beginning teams focus on.

After that start playing with kinematics.

M. Nader

11-09-2012, 08:07 PM

Originally posted by Drew Price:
Our first year car it was a miracle that the car kept goiung in a straight line when you let go of the wheel, and the left and right sides were just about symmetric to each other.

Packaging, load paths, and nothing failing during brake test should most of what beginning teams focus on.

After that start playing with kinematics.

That's about it

Gruntguru

11-09-2012, 10:22 PM

After reading a good chunk of this thread, I am amazed by one thing. A couple of posters have stated that beam axles are OK - on smooth roads only - and no-one, not even the proponents of beam axles, has challenged this view. The reason for the above fallacy is that most people's experience of a beam axle is the traditional live-rear-axle on RWD cars. For starters, most of these have LEAF SPRINGS, however the real problem is UNSPRUNG MASS. A live-rear-axle is typically a cast iron or fabricated steel housing, wheels, hubs, brake assemblies, solid steel drive axles, crownwheel & pinion, differential assembly, rear universal joint and a good chunk of the tailshaft - ALL UNSPRUNG MASS.

At the front, a beam axle on a road car is a packaging nightmare - much harder to justify and implement correctly than strut or double wishbone especially on FWD which has become the standard for small and medium size cars.

For FSAE purposes, a well designed and implemented beam axle system (front, rear or both) could match the best cars out there for handling while offering improvements in weight, cost, construction complexity and reliability.

Warpspeed

11-10-2012, 12:02 AM

If you add up all the absolutely unavoidable unsprung weight of, tire, rim, hub, upright, brake disc, and caliper, it can all add up to a surprising amount.

And the unsprung fraction of outboard suspension springs and linkages themselves will probably not be all that significant in the final total unsprung weight.

Another factor at the front is camber gain and scrub.
When you are going straight ahead, you don't really want either, in fact both can be a positive detriment.

You only need some slight additional front camber gain when cornering, and you can get that when the front wheels steer by being clever with caster and KPI, even with a beam axle...

A beam axle does not have to be a massive rigid single beam, it could be a fairly light spidery flat truss, to locate the widely spaced upper and lower ball joints laterally.

Gruntguru

11-10-2012, 12:29 AM

Agree with all that Tony, and even the spidery truss fron beam will have better camber compliance than a double wishbone.

MileyCyrus

11-10-2012, 02:28 AM

Originally posted by js10coastr:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Drew Price:
This thread isn't about suspension design anymore....

I think it's now a cross between "LALALALA I CAN YELL LOUDER THAN YOU!!! LALALA" and

http://imgs.xkcd.com/comics/duty_calls.png </div></BLOCKQUOTE>

How many other people hovered over the image for 10 seconds waiting for the alt-text to come up :/

Jay Lawrence

11-11-2012, 12:06 AM

Sorry Chapo, didn't mean any offence. I just never saw the ADFA car go as fast as it needed to to win. I assumed it was a driver training thing rather than a car thing, and I fully appreciate the time constraints you guys have that stops you from doing things above and beyond design/build.

Loz

11-11-2012, 09:39 PM

Z

If it makes you feel better, the "current" ADFA design has retained beam axle suspension. Not as a direct result of the previous car, but as a concept that fell out of a requirements analysis. I was originally a bit cynical of the beam axle and how it was implemented, but always have appreciated the rationale behind design of the ADFA car. As I have become more familiar with the nuances of the car, it has become more apparent that the beam concept can be competitive with the podium finishers both on and off the track. After all, we took the beam axles to design finals at FSAE-A and FSUK.

There was also a lot of test and development capability left in WS11 (the last of the original ADFA beam axle iterations) but unfortunately for the moment the full capability of the platform remains unexplored. However, the potential that was evident in WS11 and the fact that we now have an operational beam axle car to use as an R&D platform is an additional reason for us to consider future beam axle concepts.

For all we know at this point, a new requirements analysis could have the beam concept iterated and attached to an RHS space frame with 8" wheels and a 250cc engine...

Loz

11-14-2012, 02:51 AM

So..., with the polite, pre-dinner social chit-chat now quietening down, I would like to start the "main course" of discussing the pros and cons of these different suspension types. http://fsae.com/groupee_common/emoticons/icon_smile.gif

... the major suspension types are,

1. Beam-axles (eg. on all vehicles ever, can be live or De-Dion at rear),
2. Sliding-pillar (eg. Morgan and Lancia, and most motorbikes and aeroplanes at front),
3. Lateral swing-axles (eg. Tatra (still on their trucks), can be high or low-pivot),
4. Leading and trailing-arms (eg. Citroen 2CV front and rear, and most motorbikes at rear),
5. Semi-leading/trailing-arms (eg. F100 at front, and many mid to late 1900s cars at rear),
6. Strut-wishbone (eg. McPherson at front, strut at rear),
7. Double-wishbone (eg. originally mostly at front to allow shorter wheelbase with front engine),
8. 5-link (eg. recent fashion, mainly for NVH reasons).

Of these, any of the first five are more than adequate for a winning FSAE car.
It is good to see that most posts above regarding beam-axles have been positive. I might add some more later, but most of my views are already on Olly's "Beam Axles - Front, Rear, or Both" thread.

Instead, here I would like to get FSAEers thinking about the other suspension types on that list. I will cover the others in the "top five" later, but first a very brief, and hopefully also rational (which means critical http://fsae.com/groupee_common/emoticons/icon_smile.gif) assessment of the suspension types I see as LESS suitable for FSAE.
~~~o0o~~~

Back on page 2 (before the hors d'oeuvres http://fsae.com/groupee_common/emoticons/icon_smile.gif) Claude suggested "I always have been convinced that you can make a good car with a McPherson...".

Tony suggested "Good solid toe control at the back would have to be be pretty high on my wish list. A single stiff rear trailing arm, with transverse upper and lower camber control links would give excellent toe and bump steer control while being simple."

Both also supported double-wishbones or 5-links, at least at the front (corrections welcome).

And I said "I don't like any of the above options, and have rational critcisms of them which I will give later..."

So...
~~~o0o~~~

Strut-Type (aka McPherson).
====================
I see this as a cross between a Sliding-Pillar and Wishbone/5-Link suspension.

Its main advantage, as argued by Ford's Mr McPherson (late 1940s?), is that it replaces the upper-wishbone-and-its-outer-BJ-and-chassis-mounts, plus the damper, with a single beefed up damper assembly, for a small cost saving.

Its main disadvantage IMO, which it shares with the Sliding-Pillar, is "stiction" in the sliding parts. This is especially so when the strut is loaded in bending, as during racing when there are significant horizontal loads at the wheelprint. I will cover this more under the Sliding-Pillar, but this is a significant and ongoing problem for racecars with these type suspensions. In fact, many race series ban "exotic" low friction bearings on the struts for "cost" reasons (even though they are available over the counter at your local bearing shop...).

Another disadvantage is that strut front suspensions intermingle the suspension kinematics with those of the steering. Very briefly, you either have a lot of camber-compensation (= short FVSA, = good for FSAE) and large SAI (large KPI, = bad), or else small camber-compensation (= bad, depending...) and small SAI (= good). However, as noted earlier, FSAE cars can do just fine with very little suspension movement, so such kinematic issues can be largely ignored.

In FSAE there would also have to be some chassis structure reaching out towards the top of the wheel to support the top of the strut. This stucture is already there on production cars, so comes at no extra cost. But on FSAE the whole assembly ends up looking very similar to a double-wishbone, except that it has a few less BJs, but the additional stiction.

Bottom line, IMO, this option combines the disadvantages of Sliding-Pillar with those of Wishbone, with no real advantages over either.
~~~o0o~~~

Single-Stiff-Trailing-Link plus two Transverse-Camber-Control-Links.
================================================== =====
For advantages, I agree entirely with Tony that "solid toe control at the back" is very important, and can be achieved with this type of suspension. However, the same toe and camber control can also be achieved with any of the Swing-Arm types (3, 4, or 5, on the list). For example, any amount of camber-compensation (ie. FVSA length) can be had by selecting between Lateral, Longitudinal, or Semi-Trailing/Leading arms.

One disadvantage is that poor choice of Camber-Link geometry can give a motion screw (ISA) that moves to poor locations during suspension movement (ie. the kinematics can easily give bump toe-out, = bad). By comparison, the Swing-Arm ISAs always stay where you put them. But, of course, moving ISAs can be cured by stiffening up the springs. "Any suspension will work, if you don't let it.".

A bigger disadvantage is that the two Camber-Links require four BJs, plus their mounting brackets, chassis nodes, etc., together with the main Trailing-Link BJ. Any of the Swing-Arm options only require two BJs, or a single "revolute" joint.

Bottom line, IMO this option is half-way between the Swing-Arm and Wishbone types, so it has some of the Wishbone's complexity (lots of BJs and brackets) which is unnecessary in FSAE.
~~~o0o~~~

Double-Wishone or 5-Link.
====================
Clearly, these types have the most components. The DW has 8 x BJs, plus 2 more on the spring-damper (Push/Pullrods covered below). The 5-link has 10 x BJs, plus the spring-damper. Each BJ also requires a mounting bracket, and the complex load path through these can lead to a lot of compliance. At the chassis each BJ/bracket should be at a node of a spaceframe chassis (often with ~6 tubes converging on it...), or requires local strengthening of a monocoque. All of these attachment points, at chassis and upright, should be accurately positioned. Inevitably, there is a lot of fiddly, time-consuming, and thus expensive work required.

Claude claims these advantages,
"Providing some suspension pick up points adjustability, with a 5 links or a double wishbone you can get and/or adapt the heave and roll camber variations, roll centers, pitch centers position and movement to what the tires (and good drivers) "need" ..."

Firstly, how many teams provide multiple mounting points for each wishbone? Most teams seem to decide on a kinematic arrangement at the beginning of the year, and then build the one car, which they race at the one competition at the end of the year.

Secondly, most of the important tuning adjustments are also easily done on the simpler suspensions. Again, briefly, tyre-type/pressure/toe/camber/etc., and steer-axis adjustments, can be done on a suspensionless car. Spring-rate and damper adjustments (say for LLTD) can be done on any suspension that allows a bit of up-down movement. Lateral and longitudinal n-line slopes (= roll and pitch center heights) can be done on any of the suspension types, usually with fewer adjustable brakets than with wishbones. Which leaves only camber-compensation that would require some "cutting and butting".

Lastly, "what the tyres (and good drivers) REALLY need" is a shed-load of aero downforce (check the lap-sims! http://fsae.com/groupee_common/emoticons/icon_smile.gif). Building unnecessarily complicated suspensions, and dicking around with n-th degree refinements to "roll/pitch centers position and movement" is a very poor use of scarce resources, IMO.
~~~o0o~~~

Push/Pullrod-and-Rocker actuated Spring-Dampers.
======================================
Of all the features on an FSAE car, this is the one I consider the least justifiable.

I could repeat the criticisms I have given elsewhere for these "fashion accessories", but this post is already too long. However, if anyone wants to provide supporting arguments (eg. "they help lower the unsprung mass"), then I will happily discuss...
~~~o0o~~~

Comments, criticism, or vitriolic bile (preferably of an educational nature), all welcome... http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

Brent J. Butler

11-14-2012, 09:33 AM

Z,

I certainly understand why you consider push/pull rods and rockers to be "fashion accessories", but I think they can provide some practical advantages.

-Typical FSAE dampers are quite small and typical FSAE chassis are quite narrow, so I think many teams would have to go to a lot of trouble to get an acceptable place on the chassis to mount the dampers with a direct actuation arrangement.

-Rockers allow for easy setup changes. Ride height and corner weight adjustments can be made with the push/pull rods instead of spring preload so that damper travel is maintained (damper is not extended or compressed due to changes).

-Although this is not the case for some, my team found it far more economical to manufacture new rockers with a different motion ratio than purchase new springs to achieve a wheel rate change. The motion ratio changes also affect damping, while a simple spring change does not.

-I believe that ARB's would be heavier and harder to package with direct actuation (if you use them). Maybe I am wrong.

-The additional cost of using rods and rockers is minimal when your team can get bearings and rod ends for free.

----------

Beam axles sure do seem like a good idea for FSAE/FS, but perhaps it really is the "monkey see, monkey do" mentality that explains why we don't see more of them. I spent this summer designing a nice, simple 4-link deDion rear suspension and our chassis guy designed a really neat, compact chassis for it. When the fall semester started, we gave a detailed presentation to the team which clearly described the pros and cons of switching to our beam axle design. Some of our team fell asleep or left the room until it was time to vote. The team voted against it. The primary reasons cited included "No one else does it, so it can't be good", "Double wishbones didn't break last year", and "Carroll Smith said double wishbones are the best and he knows more than you" (I am the only "suspension guy" on our team). Some of our teammates were actually quite angry at us for "wasting the summer working on a different design" (even though most of these team members did zero work themselves). Frustrating, but I try to look at it as another way that FSAE is preparing me for the real world.

Drew Price

11-14-2012, 11:13 AM

Brent, I think you hit it on the head. Same applied for us, it was cheap and quick to drastically change motion ratios and progressive/regressive behavior with just a rocker change.

I believe Carrol Smith makes a comment in one of his books that if you're really clever, you could have a 'dry' set of rockers and a 'rain' set of rockers, so if the track gets damp or wet you can just swap the rockers to simultaneously raise the ride height and soften the wheel rate without having to corner-weight the car again like you would have to with a spring change.

Price

11-14-2012, 12:19 PM

I believe Carrol Smith makes a comment in one of his books that if you're really clever, you could have a 'dry' set of rockers and a 'rain' set of rockers, so if the track gets damp or wet you can just swap the rockers to simultaneously raise the ride height and soften the wheel rate without having to corner-weight the car again like you would have to with a spring change.

Drew,

I was under the impression that this was the historic reason that rockers were developed and used in the first place. Also, I'm pretty sure you could have both "dry" and "setup" positions with the same rocker with two different mounting holes for either the push/pull rod or the damper. http://fsae.com/groupee_common/emoticons/icon_wink.gif

I also have not seen too many examples of FSAE cars with outboard mounted direct actuated dampers that did not have significantly low motion ratios(spring travel/wheel travel), or a drastically(in my opinion) digressive nature. Both of those reasons would push my choice to rockers.

Warpspeed

11-14-2012, 12:53 PM

Changing over an entire outboard coil shock is both fast and easy.
And what you replace it with can have been previously set up during testing for correct corner weight, wheel rate, ride height, and correct damping.
This is a vastly more flexible and much more easily understood system to have.

The late Carroll Smith had quite a bit to say about rising rate suspensions, and the evils of over complexity.

js10coastr

11-15-2012, 01:36 PM

http://www.gagtics.com/wp-content/uploads/Abraham-Lincoln.jpg

I also love the fact that the site says "thank you for contributing to this forum"

woodsy96

11-15-2012, 03:44 PM

I agree with Brent and drew, designing with rockers and pushrods/pullrods defintely makes life easier and acheiving more/less linear motion rations without inducing massive bending loads in a lower wishbone, especially the shocks generally used in FSAE. Trying to design a chassis to reach out to a node for a Kaz 7800 to directly actuate against sounds troublesome.

Other advantages:

-your shock is less susceptibe to damage inside the chassis than hanging in free space if/when something breaks.
-Pushrod shocks are way easier to access to make changes

wagemd

11-16-2012, 04:42 AM

Originally posted by Brent J. Butler:
The team voted against it. The primary reasons cited included "No one else does it, so it can't be good", "Double wishbones didn't break last year", and "Carroll Smith said double wishbones are the best and he knows more than you" (I am the only "suspension guy" on our team). Some of our teammates were actually quite angry at us for "wasting the summer working on a different design" (even though most of these team members did zero work themselves). Frustrating, but I try to look at it as another way that FSAE is preparing me for the real world.

Ah yes, the good ol' design by democracy... Sad to hear you had a cool suspension concept that got shot down by someone who was designing a dash and someone else that cobbled a muffler together. Some decisions are better left to the people who know what they are doing. - Learned that the hard way.

Warpspeed

11-16-2012, 01:46 PM

Ah,
But this whole "thing" is a DESIGN COMPETITION.

I can imagine the look on a design judges face when you answer by telling him you did it that way because everyone else does it that way.

If you can come up with a strikingly practical, original and clever idea, and can justify your decisions, and DARE to be different, a jaded design judge may become truly fascinated.

Pete Marsh

11-16-2012, 05:39 PM

11-16-2012, 07:05 PM

Brent,

I agree with Pete above. The "real world" part is true, and probably why some old codgers around here are so grumpy...
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~o0o~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~

Before moving on (back?) to suspensions, better get this one out of the way first... (please read with lots of http://fsae.com/groupee_common/emoticons/icon_smile.gif).

The Many Advantages of Push/PullRods and Rockers (and other fairy tales).
================================================== ==========
The following compares P/PR&Rs with "direct acting" Spring-Dampers that go from chassis to outer-end-of-whatever-suspension. All the "reasons" below appear somewhere on this forum.
~~~o0o~~~

1. "Direct acting SDs are too short to reach the end of the wishbone, so require complicated structure going out to the wheel."
--------------------------------------------------------------------------------------------------------------------------------------------------------------------
Piffle. Unscrew the spring-base/BJ-eye on the end of the damper piston rod, and make up a new, longer one. So now your modified SD is as long as your old pushrod. (See next...)
~~~o0o~~~

2. "Push/pullrods make it easier to adjust ride height without changing spring pre-load, or damper stroke position."
---------------------------------------------------------------------------------------------------------------------------------------------------------
Yawn. Add an adjustment screw in your extended damper rod (1. above). This would look like the adjustment you currently build into one end of your push/pullrods, and serve the same purpose.
~~~o0o~~~

3. "P/PR&Rs allow a Motion Ratio close to the magical 1:1."
--------------------------------------------------------------------------------
So what? What's so special about 1:1? Well, apparently...

3a. "MR=1:1 gives lighter springs."
-----------------------------------------------
Crap. A short travel, thick wire, stiff spring that, due to a different MR, gives the same wheel-rate as a long travel, thin wire, softer spring, will have very similar mass. The strain energy (= force x distance) that the two springs must absorb is the same. So if they are both made from the same quality spring steel, with same energy/mass capability, then they have the same mass.

3b. "MR=1:1 gives more effective damping."
-----------------------------------------------------------
Groan. These are very small cars, racing on very smooth roads. They do NOT need a lot of damping. (Geez, they don't really need any suspension at all... http://fsae.com/groupee_common/emoticons/icon_smile.gif) Who runs their dampers at their stiffest settings, and feels they need even more stiffness? Worst case, just fit slightly larger diameter dampers with shorter stroke.
~~~o0o~~~

4. "The rockers allow a constant, or slightly rising wheel-rate."
------------------------------------------------------------------------------------
Well, to keep Claude happy, I'll just call this one "pure, unadulterated, intellectual masturbation"!

Firstly, it is easy to get similar, approximately constant (= good), wheel-rates with direct acting SDs.

Secondly, the rapidly rising rate suspensions that work great for absorbing bumps on dirt bikes are NOT at all suitable for circuit racing cars. Which is why no such racecars use them. Great off-road, but not on track. Also, if your FSAE car has to cope with bumps (perhaps at the old-parking-lot test track), then good bump rubbers are the best option.

Thirdly, this is a big subject, but read this thread about droop-limited suspensions (http://fsae.com/eve/forums/a/tpc/f/125607348/m/18910968321?r=18910968321#18910968321) to see that often severely "digressive" (= falling rate) springing can be desirable (I start at the bottom of page 1 http://fsae.com/groupee_common/emoticons/icon_smile.gif).
~~~o0o~~~

5. "They can lower the car's CG."
----------------------------------------------
Yes, agreed! Pullrods certainly do allow the SDs to be mounted lower, for a slightly lower CG. But for some reason the majority of cars (from pics I've seen) use pushrods with the SDs mounted as HIGH as is physically possible! Ie., on the very top of the nose, or well up above the final drive. See next...
~~~o0o~~~

6. "The dampers are easier to adjust."
----------------------------------------------------
Ah, yes, that makes sense now. Mount them as high as is physically possible...
I can picture it...

"Sure, all our talk about low CGs is just blarney to keep the judges happy. But when our driver dived into the pits during Enduro, and I gave it that lightening quick two-clicks of extra hi-speed bump... (didn't even have to bend over!)... well, that won us the comp!!!"

Yes. Sure.
~~~o0o~~~

7. "Unsprung weight may be decreased..." (<- This one from a senior Design Judge!)
-------------------------------------------------------------------------------------------------------------------
So, clearly, the more push/pullrods-and-rockers you use, err ... perhaps in series and/or parallel ... , then ... err ... the lower the mass becomes... http://fsae.com/groupee_common/emoticons/icon_confused.gif

"Err..., thinking ... thinking ... YES!!! THAT'S IT!!!!
Geez, forget about FSAE!! We've just invented the fabled ANTI-GRAVITY DRIVE!!!
Yippeeeee!!!!! Now off to the stars, and beyond!!!"
~~~o0o~~~

8. "They can save your damper in an accident."
----------------------------------------------------------------
So, something like...

"Wow ... that was a bad one! Totally wiped out the corner. And that bone sticking out of the driver's leg doesn't look good.
But HAPPY DAYS! The damper doesn't have a scratch!"

Or, taking this thinking a bit further...

"Ha! Those other teams may scoff at our "tungsten-cobalt ballistic umbrella" (patent pending). But we'll have the last laugh! [insert maniacal laugh]
Oh, yeah... When the sky starts to fall and all the other teams are struck down in the middle of Enduro... [add much more maniacal laughter]!!!"

This sort of "design for the worst, most improbable, case..." is quite common in industry.
~~~o0o~~~

9. "They make it easier to do fully interconnected springing of all wheels..."
----------------------------------------------------------------------------------------------------
I agree with this one (in fact, I think I'm the only one pushing it...).

But, even though the one and only (?) team using fully interconnected springing for the last decade has been very successful with it, no other team seems to want to take the plunge. http://fsae.com/groupee_common/emoticons/icon_confused.gif I would suggest cable "pullrods", possibly running over pulleys rather than via rockers, as a good way to interconnect front and rear suspensions.
~~~o0o~~~

Probably missed some, but methinks that people using some of the above "reasoning" are mainly kidding themselves...

You will find a lot more of that sort of reasoning when you get real jobs... http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

Warpspeed

11-16-2012, 07:33 PM

On the interconnected spring thing...

Has anyone ever tried fitting longitudinal "antiroll bars" down each side, linked between front and rear suspension, to increase wheel rate in pitch ?

I have never seen this done, but it would be a rather different approach to the anti dive and anti squat problem.

Gruntguru

11-17-2012, 01:23 AM

Tony
Quite the opposite of what Z proposes.

GSpeedR

11-17-2012, 08:33 AM

10. "They allow you to move the spring/damper assembly out of the airstream and (potentially) under the bodywork..."
-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Replace with push/pull rod which can be designed with a much more aero-efficient shape to reduce drag and improve flow management.

I'm not arguing this advantage in an FSAE setting (though someone might like to), but this is why we see rockers in other motorsports series. So, this fairy tale is true. http://fsae.com/groupee_common/emoticons/icon_smile.gif

Markus

11-18-2012, 02:07 AM

Originally posted by Z:
8. "They can save your damper in an accident."
----------------------------------------------------------------
So, something like...

"Wow ... that was a bad one! Totally wiped out the corner. And that bone sticking out of the driver's leg doesn't look good.
But HAPPY DAYS! The damper doesn't have a scratch!"

Or, taking this thinking a bit further...

"Ha! Those other teams may scoff at our "tungsten-cobalt ballistic umbrella" (patent pending). But we'll have the last laugh! [insert maniacal laugh]
Oh, yeah... When the sky starts to fall and all the other teams are struck down in the middle of Enduro... [add much more maniacal laughter]!!!"

This sort of "design for the worst, most improbable, case..." is quite common in industry.
~~~o0o~~~
I'm inclined to comment on this, as we have first hand experience. Wiping a corner off the car isn't that uncommon in FSAE cars and it usually doesn't result in broken legs or other injuries. Even the bodywork might be completely fine...

Things would be different if we had unlimited time and money but we have neither. Pushrods and a-arms are free and fast to make, dampers cost us money and time.

So a hypothetical example: 4 weeks of intensive testing before comp and we lose a corner.

Option 1 /w rockers: Slap new rods, adjust, back on track the same day. Cost ~$0.

Option 2 /w DA dampers: Order a shock, hang around and drink beer for 3 weeks. Cost ~$600 + beer + testing time.

Which one is better? You can't put a high enough price on time...

From hypothetical to real: we had an interesting problem with a drive shaft during initial testing. Rocker actuation saved us 3 dampers. What do you say Z, was it worth it?

11-18-2012, 04:35 PM

Tony,

I'm pretty sure longitudinal "anti-pitch" U-bars were tried in various Formula back in the 1960's (forgotten who?). But as GG suggests, they are NOT a good idea.

To recap, (I have posted extensively on other threads)...

U-bars, like typical ARBs, are "interconnecting springs" that resist different movement at their ends (one wheel up, other down), but offer no resistance to similar movement (both up, or both down). When interconnecting end pairs of wheels (F or R) they stiffen roll and twist (aka warp) modes. When interconnecting side pairs they stiffen pitch and twist modes. Either way the stiffened twist mode is a big disadvantage. (BTW, when interconnecting diagonal pairs they stiffen pitch and roll only, which is not too bad.)

Z-bars, like the so called "third springs" on many Formula cars, resist similar movements at their ends, but allow different movements. As such, when interconnecting end pairs they stiffen heave and pitch, and when interconnecting side pairs they stiffen heave and roll. And this is pretty much all you need, ie. end and side pair Z-bars. Even if these springs are very "stiff", the suspension is still very supple over bumps (because of soft twist mode). And LLTD is easily adjusted geometrically, rather than through spring-rates, thus making it independent of twist in the road.

This "interconnecting springing" is another suspension subject that, although extremely useful in practice, is either never taught in the textbooks, or else is poorly, or wrongly, explained. Perhaps students going to Claude's seminar during upcoming FSAE-Oz can ask him to explain it? http://fsae.com/groupee_common/emoticons/icon_smile.gif
~~~~~~~~~~o0o~~~~~~~~~~

GSpeedR,

Yes, I agree entirely that improved aero flow is the primary reason for push/pullrods in most modern Formula racing. Funnily enough, that reason is hardly ever mentioned on this forum.

It follows that,

11. "Fashion! All real racing teams use them, so they must be good..."
------------------------------------------------------------------------------------------
"And they look so hot, and sexxxxyyy! Especially when the rockers are laser cut so they're all lacy looking, like... err... girls' ... err... undergarments..." http://fsae.com/groupee_common/emoticons/icon_biggrin.gif
~~~~~~~~~~o0o~~~~~~~~~~

Markus,

You pay $600 for ONE damper!!!? Why???

Do you realize you can win FSAE with next to no suspension at all?

IMHO, if you really want to dominate this competition, then build a simple car, with simple suspension, with simple (50hp?) engine, and then keep adding more and more aero downforce (in a simple way http://fsae.com/groupee_common/emoticons/icon_smile.gif).

But please note, since you should aim to spend at least half your track time at 3+G lateral, your suspension must be rugged enough to cope with the loads (eg. tapered-roller wheel bearings, on quality steel axles, etc.).

"Wiping a corner off the car isn't that uncommon..." (because you hit a little bump?) should NOT be a goal.

Z

MCoach

11-18-2012, 09:01 PM

Z,

What is your opinion on the car produced by Oxford Brookes?

11-18-2012, 10:04 PM

MCoach,

Well, from a quick look around this interweb thingy, I see a car that is far too complicated, and hence far too fragile (soft rubber cone took out suspension at Michigan?), and has no aero. Apparently, next year's car gets aero, but also bespoke 12,500 rpm V-twin with electro-pneumatic 4-speed box, and active torque vectoring diff...

I might have been looking at the wrong car (corrections welcome http://fsae.com/groupee_common/emoticons/icon_smile.gif), but I didn't see a "brown go-kart with aero undertray...".

IMO, aim for "drive on the ceiling aero at ~15m/s" (giving ~3G lateral on most corners) and you should have a ~300+ point jump on the opposition in Dynamic events alone. This sort of performance comes only from aero, not from any amount of fancy suspension.

Z

MCoach

11-19-2012, 12:03 AM

Z,

Let's start off with this:
Every single one of these cars are designed by amateurs (read college students) where the goal is to make it around a track with the highest V_avg.
The cars are compromised by being designed to a reasonable price to the "customer", and not eating fuel at an astronomical rate, while still being reliable enough to drive several days of events (never mind that that isn't a huge feat).

Now, then, on to the fun part:
I think asking for 3G lateral aero packages is an unreasonable quest.
In fact, if you look at the requirements to achieve such a feat with such a small vehicle on these speed limited courses, it may be damn near impossible.
If this methodology would be followed, something would need to be sacrificed in pursuit of aero driven performance. I will use Maryland (Terps Racing) as an example of the closet thing I can think of to achieving this. Sorry for putting you guys on the spot, but your batshit-insane-aero car makes a pretty good benchmark in this case.

Maryland typically designs a car for SCCA solo nationals entries and then just so happens to meet the FSAE rules, or at least so it seems. The car is designed for, most likely, the highest level of downforce achieved in an FSAE car. They destroyed 9" wide Hoosier slicks during a less than one minute autocross run. They took FTD, but even then, much of the time was spent at WOT. Unfortunately, Terps has compromised their packaging of all components, especially the engine, which looks extremely difficult to access after installed. The car itself, is quite heavy compared to several top cars currently competing. Most of any area available on the car has been filled with some sort of device to generate download. Wings, undertray, endplates, it's all there.

Unfortunately, on top of that, the aero package rules has just shrunk back from the tires slightly. Several other cars such as UWA may have also been responsible for this change.
To add more downforce, more area must be used. Even more unfortunately, aero download is effective with reference to the velocity of the vehicle squared. So, to even assume that Maryland makes enough downforce to generate 3Gs with that package at 60mph, only the root of that should be make at 30mph (~15m/s). Are the wings sounding big enough, yet?

I believe the problem that one might run into if they attempt to build and compete with a car will be the insane amount of drag that is bound to follow with all that downforce.
...Which requires a bigger motor to overcome, which requires, more wing to carry the increased mass, which requires more chassis to support more load, which requires a harder tire compound to support all of this, which requires...etc. Even if you spend all of your time drag limited at 30mph at WOT, pulling 3 Gs, you will lose quite a bit in fuel efficiency, and still not be the fastest.

Basically, I believe there are many problems that are faced when the design theory follows these goals.
You might drop off the face of competition with a DNF before you achieve that 300+ point jump.

TMichaels

11-19-2012, 12:09 AM

Markus,

You pay $600 for ONE damper!!!? Why???

Do you realize you can win FSAE with next to no suspension at all?

IMHO, if you really want to dominate this competition, then build a simple car, with simple suspension, with simple (50hp?) engine, and then keep adding more and more aero downforce (in a simple way ).

But please note, since you should aim to spend at least half your track time at 3+G lateral, your suspension must be rugged enough to cope with the loads (eg. tapered-roller wheel bearings, on quality steel axles, etc.).

"Wiping a corner off the car isn't that uncommon..." (because you hit a little bump?) should NOT be a goal.

Z

Z, you are changing the direction here to avoid having to respond to the issue that Markus raised.

We also have been in the exact same situation and our rockers saved us 4 dampers, making it possible to attend the comp that directly followed the accident. The price tag to be put on missing an entire comp does not exist.

This is not negligible from a project management point of view.

And good luck by the way attending FS Germany with a "next-to-nothing" suspension. A video of the final five (fastest five cars from AutoX) combustion endurance of FSG2012 can be seen here:
FSG2012 Final Five Combustion Endurance (http://bnerd.de/2012/07/live-stream-formula-student-germany-2012-hockenheimring/)

Lap times are shown in the lower left corner.

I am not in any way a suspension guy, but Maryland is jumping around all the time. It seems they still manage not too lose to much time though, but it does not look "healthy". Maybe I am also mixing up a stiff suspension with a non-existing suspension.

11-19-2012, 04:55 AM

MCoach,

"The cars are compromised by being designed to a reasonable price to the "customer", and not eating fuel at an astronomical rate, while still being reliable enough to drive several days of events"
IMO, best solution for above is a very simple "brown go-kart" (ie. in FSAE a simple chassis, single cylinder engine, single speed box, and simplest suspension that passes scrutineering).

"I think asking for 3G lateral aero packages is an unreasonable quest.
In fact, ... it may be damn near impossible."
If you believe it is impossible, then, for you, it will be impossible. However, history suggests it is easily achievable.

"Maryland ... destroyed 9" wide Hoosier slicks ...
The car itself, is quite heavy ..."
Aero and heavy don't go well together.

"To add more downforce, more area must be used."
No. Better design must be used (ie. suck harder! http://fsae.com/groupee_common/emoticons/icon_smile.gif).

"I believe the problem ... will be the insane amount of drag that is bound to follow with all that downforce."
Absolutely not! A well designed, 3-G aero car should have less drag than current non-aero cars.

I have covered most of the above elsewhere, and will go into more detail later on one of the wings threads. However, a key point is that the aero you see on most racecars today is the LEAST effective aero possible. This is because the really effective aero (ie. low drag, high downforce, and simple) always gets banned. The organising bodies prefer high drag, low downforce aero, because it is safer, because the cars are slower.
~~~~~~~~~~~o0o~~~~~~~~~~~~~~

Tobias,

"Z, you are changing the direction here ...
We also have been in the exact same situation and our rockers saved us 4 dampers, making it possible to attend the comp that directly followed the accident."

I guess you are saying that you lost four corners (or at least the wishbones), and you could replace them, but somehow you could not replace the dampers? Why not?

In brief, IMO FSAE suspension control-arms should be much simpler and much more rugged (so they don't fail whenever hitting little bumps or rubber cones), and the dampers should be much cheaper, so carrying spares is not a problem.
~~~o0o~~~

"And good luck by the way attending FS Germany with a "next-to-nothing" suspension."

I see a track with a few "undulations", but otherwise billiard table smooth. This track would be very well suited to a beam-axle car, or perhaps a simple independent suspension car with soft twist mode, good camber compensation, but otherwise quite stiff springing (especially heave and pitch, for reasons given elsewhere). Oh, and lots of aero downforce.

If the track had lots of severe, high frequency "corrugations" everywhere, as well as many more, and larger, undulations, then I would use leading and trailing arms (similar to 2CV) with fully interconnected springing, but still with rather simple dampers.

Neither of the above track conditions needs double-wishbones, and certainly not push/pullrods and rockers. FWIW, double-wishbones are a solution to a packaging problem (trying to squeeze a long virtual-swing-arm into a small space in a passenger car). Likewise 5-links, although they have been developed further to solve an NVH problem (low profile tyres "thumping" over cracks/steps in the road).

Z

TMichaels

11-19-2012, 05:19 AM

I guess you are saying that you lost four corners (or at least the wishbones), and you could replace them, but somehow you could not replace the dampers? Why not?

In brief, IMO FSAE suspension control-arms should be much simpler and much more rugged (so they don't fail whenever hitting little bumps or rubber cones), and the dampers should be much cheaper, so carrying spares is not a problem.

We lost all four corners, because the car nearly flipped and when landing back on its tires, it destroyed the rockers and not the dampers.

We did not have spares for the dampers, because we could not afford them.
I can not judge whether we needed such expensive dampers or not. I am only saying that "expensive part protection" is a smart thing to do.

I see a track with a few "undulations", but otherwise billiard table smooth. This track would be very well suited to a beam-axle car, or perhaps a simple independent suspension car with soft twist mode, good camber compensation, but otherwise quite stiff springing (especially heave and pitch, for reasons given elsewhere). Oh, and lots of aero downforce.
I have driven the track several times myself and it is quite bumpy. Look at this picture, the rear wheels are completely up in the air. I don't know, if this is a setup issue, but no team managed to get over this bump without lifting both rear wheels off the ground.
http://media.formulastudent.de/FSG11/Hockenheim2011/20110807-Sunday/i-JQxkDX3/0/L/20110807_16-29-13_0999_reichmann-L.jpg

Let me suggest something:
You are talking a lot about how every team that would dare to follow your advice e.g. building a car with a simple suspension and 3G cornering ability by using aero right could rock any comp, but no team does.

So why not offer a team nearby, I guess in Australia, to coach them and prove everybody wrong by building a car according to your theories.
With that you could really change something, because even if "Monkey see, Monkey do" happens and all other teams start copying the winning concept, they will start to consider other solutions than the already proven ones and when they become professionals, they will carry your thoughts into industry.
So go ahead and do it, no excuses!

Luke Phersson

11-19-2012, 07:07 AM

For what it's worth, direct acting dampers are back on the table for us (Monash)in 2013. We ran them from 2006-2010, and managed to win 2 events and 1 design event with them. There are some legitimate pros and cons discussed so far, however people claiming it's impossible/or even difficult to achieve decent motion ratios with direct acting dampers clearly haven't tried too hard. From memory our 2010 car had linear (in the working range) motion ratios of 0.7. The problem you get with smaller motion ratios isn't really from the springs, but the damping force needs to be much higher and if you've got a small diameter piston you can overheat the dampers and run into hysteresis problems.

Since the chassis is mandated to be quite wide due to template rules, and the large advantages of running a small track-width they are a good simple solution for a team. Not to mention reducing part count and stiction in the system. Other advantages from my experience are much easier/faster spring and damper adjustment, and corner weights are far less sensitive/fiddly. Furthermore, they provide better load paths into the chassis and reduce suspension compliance by reducing the force multiplications in the suspension system due to acute pull rod angles.

The major negative about direct acting is the difficulty in packaging an effective anti-roll bar. You need to get pretty creative to get something that actually works, see our 'shuttle bar' rear ARB from '09 and '10, and our integrated front wing mount ARB from '10. Then again, I'm not entirely convinced ARB's are needed, as we've tested the same roll gradient (and balance) through a number of different bar and spring combinations (from soft to hard wheel rates) and seen little difference in lap time. And this is on a bumpy test track. Any aerodynamic drag from direct acting dampers at FSAE speeds is seriously negligible, its under 1 point out of 1000.

Since people keep claiming push/pull rod suspensions reduce unsprung mass - what effect does this have on your point score at competition? Have you actually tested the sensitivity of these cars to unsprung mass? Unsprung mass isn't even really the correct term, unsprung inertia is probably more appropriate. What happens when you hypothetically add a heavy large diameter bellcrank to the system when you go over a bump?

Z, I've been meaning to reply to the WINGS thread for a long time, hopefully I can get a free moment soon.

Luke.

Markus

11-19-2012, 09:02 AM

Originally posted by Z:
Markus,

You pay $600 for ONE damper!!!? Why???

Do you realize you can win FSAE with next to no suspension at all?

"Wiping a corner off the car isn't that uncommon..." (because you hit a little bump?) should NOT be a goal.

Z

Because stealing dampers would be criminal. We pay money for dampers because we don't have enough team members to make them. I won't make any guesses how the company comes up with that price but I'd believe it has something to do with the law of demand and supply.

And we don't do all this just to win. There's more to this competition than winning, at least for the majority of the teams.

So should the goal be that the car withstands everything? While being light and not made to withstand everything? You're talking against yourself...

Did I mention anything about bumps? These cars are also run on venues that have a chance of crashing (tire walls etc.). Everything will break at some point, a good engineer will design when and where. It might be beneficial to have (CF) a-arms that shatter in a crash to save the frame and damper. Otherwise you might end up here with your "much more rugged" control arms:

"Ha! Those other teams may scoff at our "tungsten-cobalt ballistic umbrella" (patent pending). But we'll have the last laugh! [insert maniacal laugh]
Oh, yeah... When the sky starts to fall and all the other teams are struck down in the middle of Enduro... [add much more maniacal laughter]!!!"
So pick your direction, you can't have both.

PS. You didn't comment anything on saving the dampers in the driveshaft problem. Running out of arguments?

Francis Gagné

11-19-2012, 01:33 PM

Since people keep claiming push/pull rod suspensions reduce unsprung mass - what effect does this have on your point score at competition? Have you actually tested the sensitivity of these cars to unsprung mass? Unsprung mass isn't even really the correct term, unsprung inertia is probably more appropriate. What happens when you hypothetically add a heavy large diameter bellcrank to the system when you go over a bump?

This summer we tested our '12 car with 6kg mass added per corner randomly mounted either inside the car or attached to the uprights. Made multiple runs with two experienced drivers. Our conclusion was there were no statistically significance between mean lap time and position of the mass. There was a significant difference between the two drivers though. The test protocol did had its flaws (Suspension parameters was not adjusted for both mass setups, etc.) but it still shows a bit of insight about the relative influence of sprung mass.

Just food for thought.

M. Nader

11-19-2012, 01:50 PM

Originally posted by Francis Gagné:

This summer we tested our '12 car with 6kg mass added per corner randomly mounted either inside the car or attached to the uprights. Made multiple runs with two experienced drivers. Our conclusion was there were no statistically significance between mean lap time and position of the mass. There was a significant difference between the two drivers though. The test protocol did had its flaws (Suspension parameters was not adjusted for both mass setups, etc.) but it still shows a bit of insight about the relative influence of sprung mass.

Just food for thought.

please tell me more about this test, i would like to know more details if that is OK

Warpspeed

11-19-2012, 03:22 PM

I suppose it comes down to the fact that the cars are not that sensitive, and the rookie drivers not world champions.

Yeah, we ran back to back tests, and the titanium wheel nuts consistently improved lap times by 34.86 microseconds.

JT A.

11-19-2012, 05:01 PM

Originally posted by Warpspeed:
I suppose it comes down to the fact that the cars are not that sensitive, and the rookie drivers not world champions.

Yeah, we ran back to back tests, and the titanium wheel nuts consistently improved lap times by 34.86 microseconds.

Ever since I've started looking into tire data I've started to think that if they really wanted to make this an engineering competition and not a driving competition, they'd force a really finnicky spec tire (with TTC data available for it, of course). One that only works well in a very small range of slip angles, is very sensitive to camber, and friction coefficient drops off rapidly with increased load.

When I was designing kinematics for our 2011 car, I really had no idea what I was doing. I made many of the mistakes that have been discussed on these forums- focusing too much on minimizing roll center migration without any understanding of why/if that is even important, no thought whatsoever into tire selection (other than "goodyear gives them to us for free"). Instead of looking for the camber that our tires like to operate at from the TTC data and designing the kinematics to achieve it, I just made sure camber gain was in the same ballpark as our previous cars. We had no idea how the ideal slip angle for our tires varied with load & what Ackermann % to use...the list goes on and on.

Looking back at how little I knew about suspension, how our wings didn't really make that much downforce considering their size, the car was ~480 lbs etc, I've always wondered how the hell that car won autocross in Michigan. After working with tire data for a while, all I can come up with is that FSAE tires just aren't very sensitive to weight and poorly designed kinematics, and we have good drivers. I honestly think any team can get top 5-10 in dynamic events by building a dead simple & reliable car (as long as it has no extreme design flaws and a decent running engine), finding some kid at your school with kart racing experience, and giving him 3 months to practice driving it.

Warpspeed

11-19-2012, 05:40 PM

Rex Chan

11-20-2012, 04:57 AM

Re: TMichaels - I do beleive there are a group of guys at Uni of Tas that want to build a car (they've been hanging around the past few FSAE-A comps). Why not get in touch with them and see where they're at - they'll probably be very receptive to a simple car design.

M. Nader

11-20-2012, 07:06 AM

Originally posted by JT A.:
I honestly think any team can get top 5-10 in dynamic events by building a dead simple & reliable car (as long as it has no extreme design flaws and a decent running engine), finding some kid at your school with kart racing experience, and giving him 3 months to practice driving it.

I'll just take that and show it to our schools new team if you don't mind, sums it up perfectly.

Once i attended Formula Student (first and last year unfortunately) i really did find that out, a simple car with good, trained drivers will easily beat a much better and more sophisticated car that was just finished in time. the track layouts just don't allow car difference to be measured.

The competition is not really that tough, top 20 is possible to nearly everyone the only thing stopping each team from doing that is the team shooting themselves in the feet trying to "optimize" something or giving any item more importance than it deserves. but that does get you points in the design event if you can show for it.

11-21-2012, 02:26 AM

Tobias,

"I have driven the [FSG] track several times myself and it is quite bumpy. Look at this picture, the rear wheels are completely up in the air. I don't know, if this is a setup issue, but no team managed to get over this bump without lifting both rear wheels off the ground."

I touched on the above issue (http://fsae.com/eve/forums/a/tpc/f/125607348/m/781103883?r=27120204151#27120204151) on the Beam-Axle thread. Specifically, here is the section regarding the "Bump Map" (I might try to post such a map one day).

It is instructive to draw a "map" of the type of bumps that ground vehicles drive over. On the the horizontal axis plot "Frequency" (Hz) as a log scale. On the vertical axis plot "Amplitude" (say, in metres) again as a log scale. Now, assuming sinusoidal bumps, the maximum vertical velocities and accelerations of the wheelprints are shown as two series of diagonal lines on the map (different slopes for V and A). FSAE is at the bottom-left of this map (along with forklifts, etc.), while desert racers are at the top-right.
The key lesson learnt from such a map is that suspensions only work (and thus are only required) in a narrow range of the map. At the left of the map (very low vertical accelerations) the whole vehicle moves as one, with no suspension movement (other than perhaps twist mode). At the bottom right (high frequency, but low amplitude bumps) the body might travel quite smoothly, but even the tyreprints cannot accelerate fast enough to maintain contact with the road, so no grip. At top right (big bumps at high speed) the whole car necessarily leaves the road.

Note that the maximum downward acceleration of tyreprint, wheel-assembly, or whole car (so as to maintain contact with the road), is simply = (downward force)/(mass of object). Clearly, tyreprints can accelerate fastest, the whole car slowest, at 1G, and the wheel-assembly somewhere inbetween at 5-10G (= sprung/unsprung-mass x Gs).

The FSG cars you mention (medium amplitude bump at medium frequency) might maintain road contact with a better suspension (eg. longitudinally connected springing, but still with very simple control-arms). However, a better all round solution is more aero downforce, since it increases the car's downward acceleration to better follow the road.
~~~o0o~~~

"Let me suggest something:
You are talking a lot about how every team that would dare to follow your advice...
...
why not offer a team nearby, I guess in Australia, to coach them ..."

Hmmm... So I have to find a very brave team, that is nearby, and is also prepared to mindlessly follow my advice...

Then...

"... even if "Monkey see, Monkey do" happens and all other teams start copying the winning concept...
... they will carry your thoughts into industry..."

Without them having the foggiest idea of what they are doing!!!

Tobias, how would the above improve the "education of young engineers"? (Please, no excuses... http://fsae.com/groupee_common/emoticons/icon_smile.gif).

Rather than giving them a fish, I would prefer they learnt how to fish...
~~~~~~~~~~~o0o~~~~~~~~~~~

Luke,

Thanks for some rational analysis of direct acting spring-dampers. BTW, best way to prevent damper overheating is to have a soft spring-rate in the middle of the suspension's working range, so only low damping forces required, and use bump rubbers for the occasional overshoots.
~~~~~~~~~~~o0o~~~~~~~~~~~

Markus,

"So should the goal be that the car withstands everything? While being light and not made to withstand everything?
... you can't have both."

Well, actually you can. The Model T Ford (~700kg), VW Beetle (~720kg), and Citroen 2 CV (~530KG) were all designed to be as simple as possible, while still being "mid-size four-passenger cars". These cars are all very light (weights dependent on spec), but they also have the rugged off-road capabilities that very few 2+ ton SUVs have today.

KISS really works.
~~~o0o~~~

"PS. You didn't comment anything on saving the dampers in the driveshaft problem..."

I have no idea what happened to your dampers/driveshafts. Honest ... I was not there ... I didn't do it ... http://fsae.com/groupee_common/emoticons/icon_smile.gif

But I am sure that spending excessive money on dampers is a poor use of resources. And spending even more time and money building deliberately fragile wishbones/pushrods&rockers, just to "save the dampers", is also a waste of resources.

Focus on the "big picture"...
~~~~~~~~~~o0o~~~~~~~~~~

Francis,

"This summer we tested our '12 car with 6kg mass added per corner randomly mounted either inside the car or attached to the uprights...
... there were no statistically significance between mean lap time and position of the mass...
Just food for thought."

This is confirmation that FSAE tracks are at the bottom left of the "Bump Map". The vertical accelerations of the wheels over the "bumps" (???) is small enough that large variations in unsprung-mass don't really matter.
~~~~~~~~~o0o~~~~~~~~~~~

Hmmm ... half-way down page 6 already...

[Note to self] Better start (get back to?) discussing KISS suspensions soon... http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

Markus

11-21-2012, 06:55 AM

Originally posted by Z:
Markus,

"So should the goal be that the car withstands everything? While being light and not made to withstand everything?
... you can't have both."

Well, actually you can. The Model T Ford (~700kg), VW Beetle (~720kg), and Citroen 2 CV (~530KG) were all designed to be as simple as possible, while still being "mid-size four-passenger cars". These cars are all very light (weights dependent on spec), but they also have the rugged off-road capabilities that very few 2+ ton SUVs have today.

KISS really works.
None of the mentioned cars are "very light" especially compared to their spec. I'd say medium weight, or you could give them light if you where looking through heavily shaded KISS-glasses while being almost pope-drunk.

I've personally driven off road with a VW beetle and the front suspension got damaged badly. The speed was low, there was no big bumbs or impacts to front wheels yet the suspension was bent enough to be undriveable. I don't remember if the frame stayed intact or needed repairing too. This was actually closer to just driving off-road than crashing.

So at least the beetle is not very light nor very withstanding to harsh use...

"PS. You didn't comment anything on saving the dampers in the driveshaft problem..."

I have no idea what happened to your dampers/driveshafts. Honest ... I was not there ... I didn't do it ... Smile

But I am sure that spending excessive money on dampers is a poor use of resources. And spending even more time and money building deliberately fragile wishbones/pushrods&rockers, just to "save the dampers", is also a waste of resources.

Focus on the "big picture"...
With only a very limited mechanical understanding you should be able to visualize a couple of possible cases of what could have happened. There is not many ways how a driveshaft can jeopardize a DA-damper or push-/pullrod.

But if you weren't there and didn't see it, does it mean that it didn't happen? And in no way could there be a benefit in that situation from using rockers compared to DA? Of course there can't, after losing a damper we just fly to Australia and pick a new one from the damper-growing tree...

Wishbones are fast to make and cost very little money. Even "cheap" dampers will need more time and money, and waste more resources.

Teams can of course decide where they want the weak link or convince themselves there isn't one - false sense of reliability is a really good character for an engineer, especially in the future career. We had some good examples of that in history with trades like bridge and ship building.

We chose a-arms, not only because they save the damper, but because they save the frame, the wheel height sensor and the corner assembly too. And the shattered CF arms can't injure the driver and even though Finland is known to raise a lot of good drivers we still can't afford to lose any of them...

By the way, I love how they use the soft materials in 2CV to absorb impact energy. Namely the face of the driver.
http://crash.citroenz.biz/2CV/2cvcrash.jpg

MCoach

11-21-2012, 07:48 AM

Look guys, this really isn't that hard. http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z is just advocating very simple systems for a very simple competition. The cars should posses these systems and no others (to keep it simple):

A single cylinder engine

A simple frame (preferably painted brown)

A single speed transmission

Longitudinally connected springs (help with bump capability)

Diagonally connected springs (to stiffen in pitch and roll)

Don't forget interconnectedly sprung left to right

NO FANCY SUSPENSION, beam axles front and rear

DIRECT actuating dampers that are cheap as dirt and don't work because you don't use them. The courses are smooth enough that you don't need to worry about wheel movement, but they are just bumpy enough that you need to spring all of the wheels together.

Pull/push rods are unnecessary

Suspension should be robust enough to enter the Baja SAE competition with

An aero package capable of 3G on an autocross couse but still is rules compliant with less drag than a non aero car. (but aero also isn't considered a big enough deal to try to move dampers and springs out of the air stream).

Remember guys, keep it simple. http://fsae.com/groupee_common/emoticons/icon_razz.gif

Max Trenkle

11-21-2012, 09:14 PM

@Z
I've read a very large amount of the material you have posted on this forum and find nearly all of it useful and enlightening. However, you still haven't convinced me that push/pull rod suspension is 'complicated'. It's really quite simple! Just a couple of triangles with little joints on the ends. Put your dampers in your favorite spot, make a line from one of your triangles to somewhere in front of the damper, find the vector normal to the rod axis and the damper axis, and make one more triangle to rule them all! http://fsae.com/groupee_common/emoticons/icon_wink.gif
Plus, all of that math is something college freshmen can actually accomplish (with some guidance). In order to prove the beam axle setup you're designing is going to work @ +/- 1 in of travel before you build it, you're going to need some material classes under your belt, and you're looking at at least 2+ years into undergrad before than can even really be considered. Is a beam axle setup more simple than double wishbone... maybe. Is designing a beam axle that you can prove will work more simple than double wishbone? I do not think so, but perhaps in other schools they teach Mechanics of Materials before Calc III.

@Markus
Hey, we all know that in FSAE, the legs of the driver are included in the crush structure of the car. As long as your head and torso survive intact, you ought to be ok!

@MCoach
You know, Z knows a lot of things that are accurate and useful. His opinions on how to design as FSAE car... well they are opinions. Grain of salt: taken. http://fsae.com/groupee_common/emoticons/icon_wink.gif

js10coastr

11-24-2012, 05:15 PM

why not offer a team nearby, I guess in Australia, to coach them ..."[/i]

Hmmm... So I have to find a very brave team, that is nearby, and is also prepared to mindlessly follow my advice...

Then...

"... even if "Monkey see, Monkey do" happens and all other teams start copying the winning concept...
... they will carry your thoughts into industry..."

Without them having the foggiest idea of what they are doing!!!

Tobias, how would the above improve the "education of young engineers"? (Please, no excuses... http://fsae.com/groupee_common/emoticons/icon_smile.gif).

Rather than giving them a fish, I would prefer they learnt how to fish...

Z

There is a lot to be said about suspension design, and one's approach to fsae. However the only thing that I will add is that Tobias is improving the education of young engineers by volunteering his time as an organizer and a design judge. He is helping to create an opportunity for students to learn, understand and gain valuable experience. Tobias has (to my recollection) never instilled a singular path/solution to the fsae competition (and honestly, I don't know of any design judge who has). In my entire 12 years of involvement with the fsae community, I have never met any judge who has forced the status quo, or would not be interested in an alternative given sound engineering decisions and justification. I believe all the judges would and do encourage critical thinking over following the status quo.

11-24-2012, 05:38 PM

MCoach,

You are overcomplicating it! http://fsae.com/groupee_common/emoticons/icon_rolleyes.gif

Interconnected suspension is ONLY needed if the FSAE organisers ever get around to putting some real bumps on the track. You know, to encourage good suspension design...

And diagonal U-bars are NOT recommended. Lateral and longitudinal Z-bars are enough. No more is needed (ie. no corner springs). In fact, it can all be done with one spring, but baby steps first...

Otherwise, you seem to have grasped the concept. http://fsae.com/groupee_common/emoticons/icon_smile.gif
~~~o0o~~~

Still on the KISS theme...

A picture is worth a thousand words, so here is one that was in most drawing offices back when I started out.

https://lh4.googleusercontent.com/-SPkR9U8a6-o/ULF0hbYtjnI/AAAAAAAAAL0/ppEqpxUw7rY/s800/tree_swing_70s.jpg

(I reckon Markus's team should build something like the mid-bottom picture, to protect Markus from himself. He does so love to crash into things! http://fsae.com/groupee_common/emoticons/icon_smile.gif)

A bit of background here. (http://www.businessballs.com/treeswing.htm) Short story, all engineering companies back then knew they were going to cock things up along the way, but at least they tried to keep the end goal in sight (even if it was just on a cartoon). I'm not sure that happens these days? Also interesting is that variations on the above cartoon mostly appear in software companies these days...

My strong recommendation is that FSAEers should focus on the bottom-right picture, from first concept meeting all the way through to final testing and competition. I honestly reckon that way is the most fun. But that's just my opinion... http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

11-25-2012, 05:54 PM

Here is a picture of the "Bump Map" I mentioned earlier.

As the figure number (1.1.1) suggests, this sort of map is the first thing suspension engineers should consider when laying out their design. The different types of vehicles (in italics) are shown roughly at the top right of the range of bumps they might expect to meet on typical tracks. They will likely all drive over hills (at top-left of map), but these don't effect the suspension. It is the bumps at the top-right of the vehicle's range (and how often they are encountered) that most determines the suspension requirements.

The diagonal, dashed lines for maximum Velocity and Acceleration are based on hypothetical bumps of sinusoidal profile. Maximum velocity occurs halfway up or down the bump, and might correspond with damper velocity (depending on MR). Maximum acceleration is at the top and bottom of the bump. If the tyreprint, wheel, or whole car are not pushed down hard enough to match the required acceleration, then they leave the road at the top of the bumps.
https://lh6.googleusercontent.com/-EiEbneR92WM/ULK9_tVBRbI/AAAAAAAAAME/MHxqTVEjCBs/s800/BumpMap.jpg
The "FSG bump" shown in Tobias's post (bottom page 5) is an "outlier" in FSAE terms (ie. rare, with possibly only one on the planet). It is probably halfway up the map on the "A=1G" line. As should be obvious, a suspension can only absorb a bump when the suspension has more travel than the bump has height. So a bump height of 10 cm requires at least 10 cm of suspension travel (double that is better). Hence an FSAE car with +/- 2.5 cm travel will necessarily get airborne over a 10 cm bump at the A=1G line. Well, unless it has aero downforce equal to its weight, in which case it stays on the ground up to the A=2G line.

So, IMO, the above FSG bump is not a particularly good test of suspension (other than that the car should land without bouncing too much on its tyres...). A better test would be many bumps about 5 cm high, and between the A=1G and 10G lines, ie. between 3 to 10 Hz (where it says "Kerbs"). For an average FSAE speed of, say, 15 m/s (~54 kph), this means bump "wavelengths" of 5 m (gentler) to 1.5 m (harsher). Well designed, soft suspensions will drive over these bumps unaffected, while the stiffly sprung "real racecars" will launch ... (and crash???). http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

11-30-2012, 03:00 AM

Back on page 1 I tried to start a rational, big-picture, discussion of suspension design.

There are many alternative, much simpler, and much more suitable suspension types that can be used [other than double-wishbones]. Very briefly, and roughly chronologically, the major suspension types are,

1. Beam-axles (eg. on all vehicles ever, can be live or De-Dion at rear),
2. Sliding-pillar (eg. Morgan and Lancia, and most motorbikes and aeroplanes at front),
3. Lateral swing-axles (eg. Tatra (still on their trucks), can be high or low-pivot),
4. Leading and trailing-arms (eg. Citroen 2CV front and rear, and most motorbikes at rear),
5. Semi-leading/trailing-arms (eg. F100 at front, and many mid to late 1900s cars at rear),
6. Strut-wishbone (eg. McPherson at front, strut at rear),
7. Double-wishbone (eg. originally mostly at front to allow shorter wheelbase with front engine),
8. 5-link (eg. recent fashion, mainly for NVH reasons).

Of these, any of the first five are more than adequate for a winning FSAE car.
...
The requirements of FSAE suspension are simple. They can be provided by any of the simpler suspension types.

Rational criticism welcome.

So far, IMO, the rational arguments have come mostly from those who have already tried the simpler approaches (eg. beam-axles and direct acting spring-dampers), and have been mostly favourable. Unsurprisingly, the negative arguments are mostly from those who haven't tried the alternative approaches. And rather than being rational, these arguments have a strong religious flavour, IMO. http://fsae.com/groupee_common/emoticons/icon_smile.gif

So, in the slim hope that there are at least some FSAEers out there who believe in reason over religion (and having covered Beam-Axles on another thread), let's move on...
~~~o0o~~~

The SLIDING-PILLAR Suspension.
===========================
This was the first independent suspension type widely used. Morgan used it on the front of its three-wheelers from before 1910, up until a few years ago (?). Lancia used it from the early 1920s (Lambda) to late 1950s (Aurelia). In both these cases the "evolution" from a beam-axle is quite obvious. The king-pin was simply lengthened and "sprung", thus allowing the wheel to both slide up-and-down, as well as steer. This suspension type is still common today (see below).

I should stress that I would NOT use this type in FSAE. However, it is potentially a very minimalist design, so worth considering.

Here are my (hopefully rational and objective) views of its pros and cons.

Advantages.
==========
1. Potentially the most compact type. All the suspension can be "inside the wheel".

2. Puts minimal constraints on chassis design. A spaceframe would look similar to current cars, but the "triangles" going out to the wheels (= current wishbones) would be laid out to best suit the load paths to other important nodes, rather than having to suit the kinematics (eg. RC height, RC movement, etc.). And they would be welded to the nodes rather than requiring adjustable BJs, brackets, etc. A monocoque could be combined with a structural undertray that best suits aero requirements, and also happens to have four hardpoints at the corners (to which are bolted the sliders).

3. The kinematic wheelprint n-lines have constant slope wrt chassis, giving predictable handling. These slopes can be adjusted by "shimming" the slider mounts. Vertical sliders, and hence horizontal lateral and longitudinal n-lines (= ground level "roll and pitch centres"), is a good place to start.

4. Can easily be arranged to have zero bump-steer (see below).

Disadvantages.
============
1. The sliding surfaces necessarily have much higher relative velocities than those seen in the joints of wishbones or swing-arms. This means that any "stiction" at the sliders is more acutely felt as harshness in the suspension. This is the main reason I don't like these types, and probably why they are so rare in nature. However, the stiction problem can be overcome by "linear roller bearings", readily available at your friendly local bearing shop.

2. A straight slider gives zero camber gain/compensation. This means that any body roll gives an equal, and adverse, amount of undesirable wheel inclination. About half of all FSAE cars currently have similar kinematics (ie. very long front-view-virtual-swing-arms), so clearly not a big problem (ie. roll countered by stiff ARBs). A curved slider can be used to solve this problem, but that can get expensive. Incidentally, the Morgan and Lancia kinematics have a vertical ISA at the centreline of the cylindric slider, with the steering-rod giving a very long screw pitch, and hence small bump-steer.
~~~o0o~~~

The above probably makes more sense if you have some idea how this could be done in FSAE.

So, perhaps a 16mm diameter shaft, ~200+mm long, fixed top and bottom to the corner of your chassis. The ~150mm tall upright both steers about this "kingpin", and can also slide up-and-down it, giving the +/- 25mm travel. Bronze bushes (cheap and compact, but with stiction) or linear bearings (low friction, but bulkier) can be used for the sliding surfaces. A conventional steering/toe-link from the centreline of the car can be used to control toe-angle, with negligible effect on handling from the small bump-steer. Incidentally, hardened and ground round shafts, optionally hard chrome plated, are readily and cheaply available from bearing shops or hydraulic suppliers.

Alternatively, two vertical shafts through the upright could be used (one in front, and one behind the wheel axle). These would be connected top and bottom with motorcycle style "triple-clamps". These clamps would be bolted to the chassis for the rear wheels, but would have their own steer-axis at the front (= easy zero bump-steer, since suspension is "outboard" of steering).

Or an "inside-out" arrangement would be a suitable match for the Amberg-Weiden car (ie. odd wheel bearings with similarly odd suspension). The vertical stiffener inside AM's ringlike wheel-hub could be replaced by the above round bar slider, and this would slide and steer inside a bracket bolted to the chassis.

Springing and damping can be added in any number of ways. But remembering that "zero-suspension-movement" cars have won FSAE before, perhaps just some rubber bungy straps, assisted by PU-foam bump-rubbers, could be used. Or you could complicate it... http://fsae.com/groupee_common/emoticons/icon_smile.gif
~~~o0o~~~

So, has Z totally lost it on this one? Is any of this nonsense even feasible???

Well, almost all motorcycles today use "sliding-pillar" front suspension. Most pre-WWII bikes with rear suspension started with these. Interestingly, most pre-WWII motorcycles had a form of double-wishbone front suspension (= 4-bar-linkage "girder forks"), but those were eventually ditched...

Almost all nose wheels on aeroplanes, from small Cessnas to 500+ ton super-jumbos, use a sliding-pillar. Many of the main wheels on these aeroplanes are similarly sprung.

Many ~500+ ton mining trucks use these at the front, again for both suspension and steering. Typically they use a ~0.4+ metre diameter shaft, several metres long. The stiction breakout force used to be measured in TENS of tons, but nowadays hydrostatic bearings reduce that to, oh..., only about a ton. One of my tractors also has them, again at front. So far completely reliable, and quite effective.

So for the very small number of FSAEers who hope to get jobs in motorsport, perhaps you should stick with wishbones. For rest of you who might get jobs working on motorbikes, aeroplanes, mining trucks, tractors, or whatever, you just might have to learn about this suspension type eventually. http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

DougMilliken

11-30-2012, 03:54 AM

Originally posted by Z:
The SLIDING-PILLAR Suspension.

Here's a technology demonstrator (trade show attention-grabber) with updated sliding pillar front suspension, inside large diameter wheels:
http://autos.sympatico.ca/feat...gy-meet-pre-war-ford (http://autos.sympatico.ca/features/14156/racecar-technology-meet-pre-war-ford)
As noted in the video, a hot rod was chosen because the suspension is exposed. A more practical application mentioned in the video could be the rear axle on a minivan...very low floor height.

Warpspeed

11-30-2012, 12:24 PM

Very interesting Doug, especially how the bump steer problem has been solved, but where are the springs ??

Zac

11-30-2012, 12:31 PM

Originally posted by Warpspeed:
Very interesting Doug, especially how the bump steer problem has been solved, but where are the springs ??

They were moved to the firewall using a hydraulic system.

AxelRipper

11-30-2012, 12:44 PM

From the depths of the forum (aka, 2004)

http://dot.etec.wwu.edu/fsae/James/Detroit%202004/Other%20Teams/Concordia%20suspensionsmall.jpg

And Z, you make some good points, but at 3-10 hz, you're basically just saying use rumble strips instead of cones. If you want anything above that height, there's another competition for that. It's called Baja SAE.

M. Nader

11-30-2012, 03:01 PM

I am currently working on designing the suspension of a powerful offroader, i decided to start from scratch and have a look at most suspension types.

I was very impressed with the sliding pillar for such an application, it is kind of like an active suspension but more simple. i decided however to against it because of:

-for me and where i am located manufacturing and assembling its components will be quite difficult
- roll angle influence on camber, i can't use a too stiff anti roll bar as with an offroader (susceptible to many single wheel bumps) it will greatly influence ride comfort.
- to design correctly will take a lot of time which i don't really have.

I would say this type of suspension is quite ideal for long term development and the potential gains from it are there to be found.

In the end i went with a configuration of double wishbones in the front and a De-dion in the rear (which does have its negatives)

for Z:

you were previously saying a De-dion is suitable for FSAE, while it is quite good and has benefits but i don't agree and to a limit i also don't agree with the sliding pillar. all vs using double wishbone.

- with a De-dion you should have superior traction but why? because the wheels will be perpendicular? that can be arranged with a SLA as well so what is the advantage here?
-simplicity, maybe in design but not in cost, weight or packaging. you will save some weight on the upright and some with the links and instead will replace them with a very long beam that could be just as heavy + watt linkage and other control links, and will need extra space behind the suspension. teams will mostly use rockers to activate the spring and damper and perhaps attach an ARB (why not use the de-dion as ARB?) for tuneability so not much saving when coming to links.
- Given the above the only thing a De-dion will save on is perhaps rod ends costs, and that won't be much really and even them most teams get them for free

Sliding Pillar:

-Potential wise it is a very impressive system as i said above.
- the design of a successfully operating system is not easy, and i believe should be much harder than a SLA system.
- Tolerances and fits will be critical
- the system is not popular and thus trouble shooting won't be easy.
- with this complexity in design and manufacturing, it will most certainly need more time to be made, for sure more than the already known SLA setup.
- I think it will be more expensive than SLA as well.

Theoretically, a lot of suspension systems can work on an FSAE car, but you are overlooking an important fact. these teams are amateurs who have very limited time, resources and need something adjustable (for testing on rough roads and sponsor days) and tuneable so that when they screw up they can adjust themselves and give their drivers decent practice time.

the SLA's beauty is not that it is the one which is used frequently (and i do admit that most will use it simply because it is the most common) but more that even though it's design process and optimization is not easy, the beauty of this system is that it will work no matter how bad you screw it up, and even if the team screws it up beyond repair they will just brake the Arms and make something new on the spot with no design and see how it goes. with most other systems the packaging process should be very neat and major problems will have to wait till next year to be fixed, oh and if they do fail they will fail miserably and repair won't be easy.

Please do correct me if i am wrong.

11-30-2012, 07:20 PM

Axel,

"... at 3-10 hz, you're basically just saying use rumble strips instead of cones."

Well, longish wavelength rumble strips that are the full width of the track, with cones either side so the cars MUST drive over the bumps.

Imagine a large radius, sweeping bend with many bumps/corrugations about 2 to 5 cm high, with 2 to 5 metre wavelength (like the roads around here... http://fsae.com/groupee_common/emoticons/icon_smile.gif ). A stiffly sprung car will have its wheels lift off the ground at relatively low speed, and will thus lose grip and drift outwards, knocking over cones. So it has to slow down. A well suspended car (ie. softly sprung and correctly damped) takes the corner at whatever Gs its tyres would give on a flat course. So it wins. http://fsae.com/groupee_common/emoticons/icon_cool.gif

BTW, that yellow car (Concordia) has 8" rims, so the sliding pillars couldn't quite fit inside the wheels. If you look closely, the suspension is "outboard" of the steering, so no bump-steer.
~~~~~~~~~~o0o~~~~~~~~~~

M. Nader,

I will (briefly) cover what I think is the most suitable suspension type for off-road in a later post.
~o0o~

Regarding De-Dions:

"- with a De-dion you should have superior traction but why? because the wheels will be perpendicular? that can be arranged with a SLA as well so what is the advantage here?"

With ANY independent suspension you always have to compromise on the wheel inclination angle that you get with roll, versus that with pitch/heave of the body.

So, with 0% camber compensation (= very long Front-View-Swing-Arm) the wheels remain perpendicular to the road during pitch and heave (eg. accelerating and braking), but lean outwards during cornering to the same extent as the body-roll. With 100% camber compensation (FVSA = half track) the wheels remain perpendicular during cornering (regardless of amount of body roll), but lean in or out during body pitch and heave.

So with independent suspensions you either have to stiffen the roll-mode (with ARBs = bad), or stiffen the pitch/heave-modes (with "third-springs" = less bad). With beams you can have all-round soft springs, and no problems with adverse wheel camber.

Over the years there have been many attempts to solve the camber problem with independent suspensions (eg. the Dax thread (http://fsae.com/eve/forums/a/tpc/f/125607348/m/83710394621?r=83710394621#83710394621)), but really all they do is reconnect the "independent' suspensions so they behave like a beam-axle (but with a much more complicated linkage!). There are some more comments on beam-axles, etc., on the Dax thread.
~o0o~

"-simplicity, maybe in design but not in cost, weight or packaging."

Oz Olly and Chapo (ADFA team) reckoned the whole build process was a lot easier with their twin-beam car, than with their earlier wishbone cars (see earlier posts this thread).

A key point that they mentioned, and I will restress, is THE CHASSIS BECOMES MUCH SIMPLER. There are far fewer chassis "hardpoints" required with beams. The sketch I posted on the Beam-Axle thread only requires 4 hardpoints per beam. When I get around to posting a De-Dion sketch on that thread (gotta buy right computer cables first, grrrr...), it only requires 3 hardpoints.
~o0o~

"Sliding Pillar:
-Potential wise it is a very impressive system as i said above.
... [harder design, tolerances are critical, more time...] ...
- I think it will be more expensive than SLA as well."

I agree with all your comments there. As I said, it is not my favourite. But perhaps for a team like Amberg-Weiden, who use wheel bearings that probably cost more than many other FSAEer's whole cars... http://fsae.com/groupee_common/emoticons/icon_smile.gif
~o0o~

"... these teams are amateurs who have very limited time, resources and need something adjustable ... so that when they screw up they can adjust ...
... the beauty of [double-wishbones] is that it will work no matter how bad you screw it up..."

Well, not really.

I have seen a great many amateur, albeit very wealthy, Gentleman racers, who invariably "screw it up" so bad that the final adjustment that they have to make is to fit springs so stiff that the suspension "doesn't work" (ie. "Any suspension will work, if you don't let it.").

The main problem is all the adjustability. This is a great toy that the amateurs love playing with, but more often than not it leads them to places that don't work at all well (rear bump toe-out being a common "bad place"). So the universal solution is "lock her up!".

A much simpler suspension, with just some approximately vertical up-down movement of the wheelprint, minimal (perhaps unadjustable) bump-steer, and some adjustability of static-toe and camber, and spring rates, is more likely to lead to a suspension setup that actually works (ie. it moves!, it soaks up the bumps, and the car is fast).

Z

M. Nader

11-30-2012, 07:57 PM

"With ANY independent suspension you always have to compromise on the wheel inclination angle that you get with roll, versus that with pitch/heave of the body."

I agree with that 100% in fact the equations clearly say so, but with a SLA you can do the adjustments in many ways using Kinematics and/or anti-roll bars (which are not that bad for FSAE considering the lack of single wheel bumps). so still the de-dion s not that much better than SLA.

+ the De-dion will also pose the single wheel bump problem, and camber change on the other wheel.

for me the De-dion i am designing will have 6 pickups. 2 for the rocker/damper (i MUST have them inboard, or adjustable to a 15cm length!) and 1 for the Watt on each side. 3+3.

A much simpler suspension, with just some approximately vertical up-down movement of the wheelprint, minimal (perhaps unadjustable) bump-steer, and some adjustability of static-toe and camber, and spring rates, is more likely to lead to a suspension setup that actually works (ie. it moves!, it soaks up the bumps, and the car is fast).

but isn't that too adjustable?, i mean this is basically what every team is aiming for and that is why they use the SLA, for ease of adjust-ability of such parameters. I think no teams want to vary track length or pickup points, but only focus on toe, camber and ARB. (+ perhaps some Motion ratios for the wheel rates). and would you agree that a bunch of amateurs playing around with a SLA is potentially safer (for the car's performance) than them playing around with a live axle for example?

I think you know now that i am not bashing down any of these systems, but in no way are they superior to a well designed SLA setup as an overall package, as each of them will have his merits and faults. i think for FSAE the team should choose what "they think" is the easiest to make and which works with their tuning goals in the end. It won't really matter as they can make any of these setups work.

But yes, it is very much worthwhile to open the eyes of some that there are other systems out there other than the FSAE standard SLA (i didn't have that when starting the project) as new entrants will only look to the SLA and not even know what is their because they will think they are in beyond themselves so will just choose what others use. while we are at it, why not use the 4 bar system in RCVD as well, it kinda takes the best of both worlds!

Now hurry up with that offroad suspensions, i am looking forward to what you can bring to the table on that one. http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

Will M

11-30-2012, 08:03 PM

Z,

That Dax thread is from 7 years ago...
I had no idea you had been pushing the beam axle concept for so long!
Your persistence does you credit.

-William

Markus

12-01-2012, 03:56 AM

Originally posted by Z:
But perhaps for a team like Amberg-Weiden, who use wheel bearings that probably cost more than many other FSAEer's whole cars... http://fsae.com/groupee_common/emoticons/icon_smile.gif
Small correction: Amberg-Weidens wheel bearings are probably cheaper than any of the top teams and at the same level as most of the mid-class teams ball bearings.

Their solution for bearings is actually very clever but I don't know how much compliance / friction it has.

M. Nader

12-01-2012, 05:35 AM

Originally posted by Markus:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Z:
But perhaps for a team like Amberg-Weiden, who use wheel bearings that probably cost more than many other FSAEer's whole cars... http://fsae.com/groupee_common/emoticons/icon_smile.gif
Small correction: Amberg-Weidens wheel bearings are probably cheaper than any of the top teams and at the same level as most of the mid-class teams ball bearings.

Their solution for bearings is actually very clever but I don't know how much compliance / friction it has. </div></BLOCKQUOTE>

didn't they say it was 30% more expensive than the wheel/hub/spindle/tapered bearings combo?

Markus

12-01-2012, 09:11 AM

Something along those lines but I think that might be the cost event price for the corner i.e. != real price.

I would say the hub / upright is the expensive part in their design, not the bearing.

12-02-2012, 06:49 PM

M. Nader,

"... use the SLA, for ease of adjust-ability of such parameters [as] toe, camber and ARB. (+ perhaps some Motion ratios for the wheel rates)"

The thing is, "toe and camber" are two of the main adjustments you use when you have NO suspension at all! Throw in tyre pressures, steering geometry, and chassis stiffness, and you can control LLTD and pretty much all of handling balance, still with NO SUSPENSION.

"... would you agree that a bunch of amateurs playing around with a SLA is potentially safer (for the car's performance) than them playing around with a live axle for example?"

No. Again, the problem is the SLA has far more "adjustments" that can be made, and the amateurs (and a great many professionals) get lost.

This applies during design where a huge amount of time is wasted (What RC heights? RC migrations? Pitch centres/migrations? Long or short VSALs? Toe-changes? Rising or falling spring-rates???). And then again when setting the car-up, because the designers cleverly (!) built-in a lot of optional chassis attachment points, "just in case".

"So, if we move these brackets to here, ummm, ... that'll give us more anti-dive..., which should cure the excessive neg-camber during braking... err..., but then what happens to our RCs? Ummmn... and the toe-curves...???"

Like I said, they make great toys...
~~~o0o~~~

"Now hurry up with that offroad suspensions, i am looking forward to what you can bring to the table on that one. http://fsae.com/groupee_common/emoticons/icon_biggrin.gif "

Well, this could fill a book, but briefly (and roughly in order of importance).

REQUIREMENTS of OFF-ROAD RACING SUSPENSIONS.
=================================================

1. Huge vertical wheel travel.
This trumps just about all else. It is why front-engine+rear-live-axle "truggies" (truck/buggies) are still competitive off-road. The combination of a longish (~2 m) prop-shaft plus roll motion of the axle means that one metre wheel travel (or more) is possible to the driven wheels using only two off-the-shelf UJs. Even with very wide tracks the rear-engine+transaxle cars only get about 0.6m travel with conventional CV'd halfshafts. (There is a simple solution for 1+m rear independent suspension, though... http://fsae.com/groupee_common/emoticons/icon_wink.gif )

2. Relatively soft springing.
The above travel is useless if not used. Best is to have a very soft twist-mode. The truggies typically have axle-roll softer than axle-bounce (by mounting springs closer together on beam), which helps soften the twist-mode. Rock-Crawlers also do this, with live beams front and rear, each with highish RC and soft roll springing. ARBs stiffen the twist-mode as much as they stiffen roll-mode, so they are almost never seen off-road (I have NEVER seen them, but...?). One day off-roaders might rediscover interconnected suspension (like 1930s 2CV design), but I'm not holding my breath...

3. No wheel camber change in bounce.
Heavy, large diameter, and rapidly spinning wheels exert huge gyroscopic couples when forced to change their direction of rotation (calc the numbers, they are big!). So Pure-Trailing-Arms at rear are good (and very common). VW Beetle style Twin-Trailing-Arms (a type of double-wishbone) at front are also good (but put a lot of mass out front, and hit banks because of bad "entry" angle). Currently fashionable Lateral-Double-Wishbones are (almost?) invariably equal length and parallel to give zero camber change. Unfortunately, these give hugely varying "jacking" forces in corners, and many drivers hate them, but still use them "because that's what real racecars have...".

4. No, or small, lateral wheelprint movement (= "scrub") in bounce.
This means close to horizontal front-view n-lines (ie. ground level RC, wheelprint moves vertically). Any lateral wheelprint movement over bumps causes lateral forces between wheelprint and body, hence either breaks tyre grip, or shakes car, or a bit of both. Also, lots of lateral movement (= high RC, = steep n-lines) implies lots of jacking forces for independent suspension, so lots of vertical motion of body when using soft springs, and greater possibility of rollover.

5. Wheels move backwards in bounce.
This lessens forces as wheels climb over bumps (note, this also has a lot to do with changing rotational speed of the wheel, rather than just the rearward road-to-wheel forces). This wheel "recession" comes automatically with rear Trailing-Arms, and Beetle-style front TTAs. With the Lateral-DWs the wishbone chassis mounts are tilted up-at-front to allow this backwards movement. This gives significant pro-dive under braking, but real off-roaders don't brake! Well, the front brakes are tiny... http://fsae.com/groupee_common/emoticons/icon_smile.gif

Oh..., and everything is BIG, and STRONG! Definitely, NO FSAE 5mm BJs!!! http://fsae.com/groupee_common/emoticons/icon_smile.gif
~~~~~~~~~~o0o~~~~~~~~~~

Will,

"That Dax thread is from 7 years ago...
I had no idea you had been pushing the beam axle concept for so long!"

I should point out that FSAE is one of the few places I would use beams. I, personally, would NOT use them for a top-speed off-roader (maybe for Trials or Rock-Crawling), and they take up too much room for passenger vehicles (ie. bad packaging). However, for FSAE they are a very good "fit" (quick and easy design and build), especially if you want to do aero-direct-to-wheels.

The main reason I keep pushing beams here is that I know that quite a few students agree with me, and would like to go that way, but they are dissuaded by completely irrational (and thoughtless, moronic, etc., etc...) arguments from their peers (bottom of Brent Butler post (http://fsae.com/eve/forums/a/tpc/f/125607348/m/73320357151?r=11020477151#11020477151)) and the judges (bottom of Clausen post (http://fsae.com/eve/forums/a/tpc/f/125607348/m/83710394621?r=17510576621#17510576621)).

I have no problem with people wanting to follow the flock, but I don't think it at all good for society when the braindead insist that everyone else do as they do.

Much of this thread shows how obstinately some people resist change, and how fiercely they condemn others who want to try something different. And, for some reason, in this fight to maintain the status quo, never is a rational argument used... http://fsae.com/groupee_common/emoticons/icon_confused.gif

It's just not engineering! http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

M. Nader

12-03-2012, 08:07 AM

With regards to off roaders:

1. Huge vertical wheel travel.

Yup agree, but the exact value of that is dictated by the roads used, if used on soft or moderate desert roads you won't need more than 0.3-0.4m of travel. for huge offroad cliff jumps or mountain riding that number can go up to 0.6-1m.

2. Relatively soft springing.

Yes again, i would think a ride frequency around the 1Hz range (more or less) would be sufficient?

3. No wheel camber change in bounce.

Here is one the problems. for an offroader the aim will always be less camber change in bumps, but then when turning ( i wouldn't think a normal/amateur will be able to get more that 0.4G in the desert roads) and with the relatively soft roll gradient required for driver comfort (nothing less than 5deg/g?) the camber gain would be around 2 degrees and that would yield substantial loss of traction on both ends. so i wouldn't say full parallel A-arms are a good solution, maybe a a wide FVSAL but with a small consideration for roll would be better with a static camber of around 1 degree.

4. No, or small, lateral wheelprint movement (= "scrub") in bounce. This means close to horizontal front-view n-lines (ie. ground level RC, wheelprint moves vertically)

A ground level RC/IC is nearly impossible to get in offroaders, the chassis is quite a distance off the ground (let's say 0.5m) so basically you will need a 20 inch wheel so that the upper wishbone can just be at the same level as the lowest point in the chassis, I don't think this will change with any independent suspension, only independent ones will get a horizontal IC/RC close to the ground but will still be around 0.4-0.5 higher. with any independent suspension the IC will be pretty high since the lower wishbone will have a very steep upward direction and so would the upper one (for less camber variation in bump+ limited by rim diameter), resulting in a high and wide IC, resulting in a high RC/n line slopes. in general i would say the RC/n-lines/FAP will be higher than the CG, but the designer should make it as close as possible to have less rolling torque and thus less roll and camber variation in roll.

5. Wheels move backwards in bounce

I never really thought of that, would you care to explain why this would lessen the forces? wheelbase variations will be very small in either case and i didn't really think they will do much to reduce forces sine the force paths through the wishbones and n-lines won't really change significantly as the contact patch's position changes.

Trailing arms + (semi)dependent rear suspensions i would say have a clear advantage in the rear of offroaders (a consideration for drive shaft length change should also be made here if it is rear wheel driver), the front will depend on many factors and really most independent systems should work well there.
ARB most negative side in offroaders would be single wheel bumps, so either no ARB should be used or a very soft one with contribution no more than 10-20% of springs anti roll torque just for driver adjust-ability (most won't really need this, as time can be gained elsewhere).

and yes i know, FSAE sizes/suspension won't really work when a heavy (a single seat offroader with a strong engine i would say weigh around 300-450 kg to say the least) car is jumping 1 meter off the ground http://fsae.com/groupee_common/emoticons/icon_smile.gif

Jay Lawrence

12-03-2012, 06:50 PM

The wheels moving backwards in bounce reduces loading because you are reducing the vertical acceleration required of the assembly as it goes over a bump. Think about it: if your wheel hits a bump and the geometry tells it to move forward as it does so, you are causing the wheel to accelerate up the bump at a greater rate than if it moved backwards. Hard to explain, relatively easy to picture.

murpia

12-04-2012, 07:00 AM

Originally posted by MCoach:
Z is just advocating very simple systems for a very simple competition. The cars should posses these systems and no others (to keep it simple):
<UL TYPE=SQUARE><LI>A single cylinder engine
<LI>A simple frame (preferably painted brown)
<LI>A single speed transmission
<LI>Longitudinally connected springs (help with bump capability)
<LI>Diagonally connected springs (to stiffen in pitch and roll)
<LI>Don't forget interconnectedly sprung left to right
<LI>NO FANCY SUSPENSION, beam axles front and rear
<LI>DIRECT actuating dampers that are cheap as dirt and don't work because you don't use them. The courses are smooth enough that you don't need to worry about wheel movement, but they are just bumpy enough that you need to spring all of the wheels together.
<LI>Pull/push rods are unnecessary
<LI>Suspension should be robust enough to enter the Baja SAE competition with
<LI>An aero package capable of 3G on an autocross couse but still is rules compliant with less drag than a non aero car. (but aero also isn't considered a big enough deal to try to move dampers and springs out of the air stream).[/list]
Remember guys, keep it simple. http://fsae.com/groupee_common/emoticons/icon_razz.gif
Z,

Assuming you agree with (at least some of) the above, where do you stand on:
<UL TYPE=SQUARE><LI>active yaw control?
<LI>four wheel steer?[/list]
To me, many of the manoeuvres required for FSAE are not best achieved by conventional front wheel steer vehicle dynamics:
<UL TYPE=SQUARE><LI>why yaw the car when driving a slalom?
<LI>why create yaw moments via axle translation forces?
<LI>why pivot the car around a turn centre within the wheelbase
<LI>(etc.)[/list]
With beam axles, many common parts could be used to easily achieve 4 wheel steer.
Additional actively controlled brake systems are not banned, as long as there's a simple passive braking system in place as well.

I can envisage a 'sidewinder' car with a very short chassis structure and the engine beside the driver. A beam axle front and rear, mounting off the front and back bulkheads. A 2nd brake caliper for each wheel linked to a control ECU. Mechanical linkage rear steering (no steer-by-wire) using cams or similar to program the rear steer characteristics for each event (accel, skidpan, autocross). If necessary, a selector lever, a bit like a gear shifter / ARB adjuster / weight jacker, to select different rear steer characteristics around the autocross lap.

Regards, Ian

AxelRipper

12-04-2012, 07:43 AM

Originally posted by Jay Lawrence:
The wheels moving backwards in bounce reduces loading because you are reducing the vertical acceleration required of the assembly as it goes over a bump. Think about it: if your wheel hits a bump and the geometry tells it to move forward as it does so, you are causing the wheel to accelerate up the bump at a greater rate than if it moved backwards. Hard to explain, relatively easy to picture.

Also, ride quality increases as you're using your suspension to absorb the wheel deflection over the front of the bump. Slow moving constant sinusoidal bumps, this is not as big of an issue since your wheel can follow the profile of the road vertically quite well, but with a sudden step bump (real life bump, rock, curb, etc.) your wheel needs to absorb a bit of the impact as it goes over the bump. If you're pushing straight up on the front of your tire, you're also pushing straight back in X. Allowing your wheels to move backwards a bit helps take up this force.

Probably talking in circles here and a diagram would surely help, but hopefully you get the gist. Bump forces aren't purely in the Z direction.

12-04-2012, 06:35 PM

M. Nader,

"... a ride frequency around the 1Hz range (more or less) would be sufficient?"

Off-roaders usually have highly non-linear rate springing (which is not good for circuit racing). For example, using a really soft rate around ride height (as close to 0 Hz as possible), which then gets progressively stiffer at each end of the travel is good. This is typically achieved with conventional (linear but soft) main springs, backed up by quite long, rapidly rising-rate, hydro-pneumatic bump stops, themselves backed up by heavy duty bump rubbers. There are also rising-rate droop stops (often using "springy" seat-belt webbing) to lessen the shock when the wheels drop down. And multi-rate dampers with adjustable bypass channels for softer damping at mid-stroke, stiffer at the ends.
~o0o~

"Here is one the problems. for an offroader the aim will always be less camber change in bumps, but then when turning ...
... the relatively soft roll gradient ... would yield substantial loss of traction on both ends."

I see this as an unjustified bias, or prejudice, that comes from circuit racing (ie. "real racing" http://fsae.com/groupee_common/emoticons/icon_rolleyes.gif). Namely, that for good cornering grip the wheel must be upright, or with a few degrees of negative camber. IMO this is not so off-road.

The best way to objectively decide this is to do TTC type testing "in the dirt". This is unlikely to happen in a laboratory, so I reckon the off-road tyre companies just ask the off-road racers "How does it feel?". The massive roll-angles of the winning cars suggests "adverse tyre camber angle" is NOT a problem.

From my observations, the rounded profile, cross-ply tyres grip just as well when the knobs on their outer shoulder are digging into the ground, as when the knobs on their centreline or inner shoulder are digging in. The square section shoulders of radials often (not always) "bite" better with considerable positive camber. With negative camber they seem to "surf", or "skip", over the sand and gravel. Depends on the surface...

More importantly, off-road corners are never really flat enough to measure any sort of "inclination angle". Invariably, they are ruts-on-top-of-more-ruts, and good drivers try to drop their wheels into the best ruts that get them around the corner like a train. Furthermore, a lot (most?) of off-roading is simply about going as fast as the suspension will allow in a relatively straight line. Time spent, or lost, in corners is relatively small compared with time gained by going fast over bumps.
~o0o~

"A ground level RC/IC is nearly impossible to get in offroaders...,"

Not at all. Very easy! http://fsae.com/groupee_common/emoticons/icon_smile.gif

To repeat, I strongly suggest you aim for zero camber change and horizontal front-view n-lines (= ground level ICs a long way away). Also be very wary of following the fashion to lateral double-wishbones. As I said, these are now common on the top-level $500,000 cars "Because that's what real racecars use...". Unfortunately, the undeniable benefits of zero camber change result in equal length, parallel wishbones, which then result in massive jacking forces... Which many drivers hate, but have to tolerate because "That's they way everyone is going... It's progress...". http://fsae.com/groupee_common/emoticons/icon_frown.gif
~o0o~

".. would you care to explain why [wheels moving backward over bumps] would lessen the forces?"

As Jay and Axel pointed out, the upward acceleration of the wheel, as it hits the rising face of a steep bump, also results in a considerable rearward force acting on the wheel, and then to the body.

Furthermore, if the wheel is rolling at, say, 1000 rpm on the level ground, and it suddenly must roll up a 45 degree face of a bump, then for constant forward car speed the wheel must now roll at ~1400 rpm (= sqrt(2) x original speed). This sudden increase in rotational speed required of the heavy, large MoI, wheel, requires a couple of forces acting on the wheel, one of which has a rearward reaction on the body (do the FBD http://fsae.com/groupee_common/emoticons/icon_smile.gif). Allowing the wheel to move backwards, in effect slowing it down, lessens these forces.

BTW, the above "recession" issue (I think that's what they call it) is currently a big deal with production car suspension designers. It is mainly an NVH issue caused by low profile, wide and heavy belted radials, hitting lateral cracks/steps/seams in the pavement. The proliferation of 5-link suspensions is a response to this NVH problem. Namely, with a 5-link it is quite easy to fit the longitudinal links with soft rubber bushes (allowing the recession movement), while the lateral links are fitted with stiffer bushes (maybe even BJs) to better control toe change.
~o0o~

Oh, and an extra "Requirement for Off-Road...".

6. At each front wheel, have the CG of the wheel assembly as close as possible to the steer-axis.
Ie. aim for a "centre-plane" steer-axis, passing through the middle of the wheel. When the wheel CG is away from the steer-axis, then any inertial forces at the CG (due to wheel assembly accelerations) can result is severe steering kickback. This kickback is minimised when the CG is on the steer-axis, although the gyroscopic couples (eg. from camber change) still get through.
~~~~~~~~~~~~~~~~~~~~~~o0o~~~~~~~~~~~~~~~~~~~~~~~~

Ian,

Assuming you agree with (at least some of) the above, where do you stand on:

* active yaw control?
* four wheel steer?

To me, many of the manoeuvres required for FSAE are not best achieved by conventional front wheel steer vehicle dynamics:

* why yaw the car when driving a slalom?
* why create yaw moments via axle translation forces?
* why pivot the car around a turn centre within the wheelbase...

Yes, I agree with MCoach's accurate summary (except that interconnected springing is only needed if the tracks get a lot bumpier.)

Regarding "active yaw control" (eg. by differential longitudinal forces at the left and right wheels), and "four wheel steer", I agree that these can be done relatively easily, and can be extremely powerful. However, I believe that for FSAE type courses they might be MUCH TOO POWERFUL!

I accept that FSAE is meant to be "tight and twisty". But, from what I have seen, it requires nowhere near as much agility as, say, "Motorkhana" (or whatever they call it in other places). So nowhere in FSAE must the car "turn on a dime", do "snap spins", manoueuvre through impossible zig-zags, etc. For Motorkhana that sort of extreme yawing power is good. Also for some Trails, Rock-Crawling, etc. (though I note that some Rock-Crawling comps ban "torque-vectoring", because it is too effective!).

But I think too much yawing power might be counter-productive in FSAE, especially considering the amateur drivers. The slaloms and hairpins are not all that tight. Note that "differential longitudinal forces" can be utilised by having as much positive Ackermann as possible in the steering. This is good for the hairpins. Also some rear steer is possible by having some rear static toe-out, or adjusting the toe-links to give a bit of rear roll oversteer. In both cases you only need a very small amount for a big result. Best investigated with a lot of testing...
~o0o~

"If necessary, a selector lever, a bit like a gear shifter / ARB adjuster / weight jacker, to select different rear steer characteristics around the autocross lap."

I note that "telehandlers" (an evolution of farm tractors) nowadays have selectable steering. Either normal front wheel only. Or rear wheels steering in same direction as the fronts, for a "crabbing" motion (useful in slalom). Or rear wheels opposite to fronts, for extremely tight radius turns (useful in hairpins). I guess this is what you are getting at.

Again I wonder if the amateur FSAE drivers are good enough to benefit by switching between different steering modes between every slalom and hairpin? If a team had the resources to investigate this, I would love to see the results. Worst case, they just set the "steering knob" to "Normal" (front only).
~~~o0o~~~

Err..., better get back to work...

Z

Francis Gagné

12-05-2012, 08:13 AM

Originally posted by AxelRipper:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Jay Lawrence:
The wheels moving backwards in bounce reduces loading because you are reducing the vertical acceleration required of the assembly as it goes over a bump. Think about it: if your wheel hits a bump and the geometry tells it to move forward as it does so, you are causing the wheel to accelerate up the bump at a greater rate than if it moved backwards. Hard to explain, relatively easy to picture.

Also, ride quality increases as you're using your suspension to absorb the wheel deflection over the front of the bump. Slow moving constant sinusoidal bumps, this is not as big of an issue since your wheel can follow the profile of the road vertically quite well, but with a sudden step bump (real life bump, rock, curb, etc.) your wheel needs to absorb a bit of the impact as it goes over the bump. If you're pushing straight up on the front of your tire, you're also pushing straight back in X. Allowing your wheels to move backwards a bit helps take up this force.

Probably talking in circles here and a diagram would surely help, but hopefully you get the gist. Bump forces aren't purely in the Z direction. </div></BLOCKQUOTE>

The best example to see it is to look at mountain bikes (Especially downhill bikes or all-mountain, not freeride which are made more to absorbs huge shocks and not road bumps). There suspension are not directly applicable to cars/truck because they have more constraints : very flat longitudinally, and need to reduce chain length change (though that might be good depending how you transfer motor torque). But the latest mtb suspension are quite good at minimizing impacts from bumps. A bump that hits high on the wheel slows you a lot on a hardtail bike, and also tends to throw you over because of your high CG. So the systems do absorb Up-Down and Front-rear bumps. And with suspension like the DW link minimize squat when applying torque to the wheel. There might be some inspiration there.

MCoach

12-05-2012, 04:16 PM

Z, there is a national competition here in the United States for high schools known as FIRST Robotics. It's meant to introduce kids to science and technology with control systems, autonomous controls, sometimes extremely basic AI, and overall, a goal oriented teamwork based competition. It looks flashy because it's partial goal is to gain the attention of high schools and be more like a sports match (footy games).

Anyway, down to the point. The robotics are all remote controlled, typically through joysticks. It's very common for drivers to be able to switch back and forth between different drive styles on the field. Tank drive? Click the left button. 4 wheel steer (crab drive)? Click the middle button. Ackermann steering? Click the right button. This also all happens on the fly. The speeds are nowhere near as high as FSAE, but the reaction speeds I would estimate to be on par with several other robots are maniacally flailing around in the same fashion and ability to adjust direction just as quickly.

Video for reference:
http://www.youtube.com/watch?v=JqJK0E9Jpmk

http://www.youtube.com/watch?v=kZHaTGiakZM

It's simple, no suspension, but control scheme dense.

I've done it, I've driven them, and it takes some thought to keep up with the available power you have in those styles of controls. However, one quickly adapts to these movements that would be very foreign to car and being able to readily and capably control them.

12-05-2012, 05:39 PM

Ian, MCoach,

Here is an earlier thread about 4 Wheel Steer. (http://fsae.com/eve/forums/a/tpc/f/125607348/m/81720309051?r=81720309051#81720309051)

Thinking about it a bit more now, I still reckon that with the current Rules the biggest gains are available from better aero. But if the Rules were changed to greatly lessen aero, then I would probably go for greater "agility" from something like 4WS, before chasing ultimate horsepower, 4WD, etc. The mechanical cost/complexity of 4WS is quite small. It can be done with any of the simple suspension types, probably using very similar suspension components at each corner.

Ergonomically, the easiest for the driver might be two big thumb-actuated buttons on the steering wheel. When neither button is pushed the steering is normal "front-only". Pushing and holding the green button gives stable "same-steer" at the rear wheels, for low-yaw side-stepping through the slaloms. Very briefly pushing and holding the red (danger!) button gives small rear counter-steer, to get around the super-sharp hairpin.

The only way to know for sure is to build it, and then extensively test it. This would make a great "parallel design project" (using last year's car) for a well resourced team, and should be a lot of fun! http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

rjwoods77

12-06-2012, 05:52 AM

Z,

Look at what UWA is up to...

http://sphotos-a.xx.fbcdn.net/...8445_214229793_n.jpg (http://sphotos-a.xx.fbcdn.net/hphotos-snc7/484379_10151187091878445_214229793_n.jpg)

http://sphotos-d.ak.fbcdn.net/...810_1823465174_o.jpg (http://sphotos-d.ak.fbcdn.net/hphotos-ak-ash4/192603_10151039477760810_1823465174_o.jpg)

murpia

12-06-2012, 05:56 AM

Originally posted by Z:
... I still reckon that with the current Rules the biggest gains are available from better aero. But if the Rules were changed to greatly lessen aero, then I would probably go for greater "agility" from something like 4WS, before chasing ultimate horsepower, 4WD, etc. The mechanical cost/complexity of 4WS is quite small. It can be done with any of the simple suspension types, probably using very similar suspension components at each corner.
I think you may be right, but there's a gain to be made from torque vectoring as well and it should be additive to the aero gain.

Practically, I think getting the aero right is not easy without independent facilities (wind tunnel, CFD cluster) while the torque vectoring is retro-fit to an existing car. Having developed a couple of hydraulic control systems recently I was pleasantly surprised as to how straightforward it can be.

Anyway, down to the point. The robotics are all remote controlled, typically through joysticks. It's very common for drivers to be able to switch back and forth between different drive styles on the field. Tank drive? Click the left button. 4 wheel steer (crab drive)? Click the middle button. Ackermann steering? Click the right button. This also all happens on the fly. The speeds are nowhere near as high as FSAE, but the reaction speeds I would estimate to be on par with several other robots are maniacally flailing around in the same fashion and ability to adjust direction just as quickly.

Ergonomically, the easiest for the driver might be two big thumb-actuated buttons on the steering wheel. When neither button is pushed the steering is normal "front-only". Pushing and holding the green button gives stable "same-steer" at the rear wheels, for low-yaw side-stepping through the slaloms. Very briefly pushing and holding the red (danger!) button gives small rear counter-steer, to get around the super-sharp hairpin.
That 'robot control' description was something I had in the back of my mind. Kind of like a first-person-shooter game where sideslip and yaw control have equal priority and tactical use (Google 'circle strafing').

I think a 4WS electronic interface might prove difficult or impossible to get declared legal. But what about another degree of freedom in the steering wheel:

Rotating the wheel is conventional Ackermann steering. Pulling the LH of the wheel toward the driver, and the RH away indicates a requirement for left side-slip for the slalom. This would be linked to the rear axle in the 'same steer' sense, generating the low-yaw slalom.

For 'counter-steer' hairpins the driver would need to learn to apply the opposite steer input, in this situation, with significant lock, the driver would pull the top of the wheel towards them.

Of course, the torque vectoring system would have sensors to detect both wheel steer and wheel 'push / pull' and use these plus speed and throttle / brake to determine what torque vectoring actuations to add.

Regards, Ian

Rex Chan

12-06-2012, 06:08 AM

http://motorsport.mech.uwa.edu...2012/day-1-to-4.html (http://motorsport.mech.uwa.edu.au/news/the-competition/australia-2012/day-1-to-4.html)

Xavier ABRAM

12-06-2012, 08:19 AM

I have no problem with people wanting to follow the flock, but I don't think it at all good for society when the braindead insist that everyone else do as they do.

Much of this thread shows how obstinately some people resist change, and how fiercely they condemn others who want to try something different. And, for some reason, in this fight to maintain the status quo, never is a rational argument used... Confused

It's just not engineering! Smile

Z

Hello "Z",

You miss a point there... engineering is not solely based on the idea to have the best concept ever... Engineering is to MAKE THINGS WORK ! And one of the aspect of making something work is risk management. Your ideas are perhaps great but they are a HUGE gamble. With the numbers of years we put in every FSAE years it's a huge gamble.

You also miss another point : gain.

What is the gain Z? We want laptimes comparaison, we want compliances study and weight study, we want drag and downforce figures, we want inertia and CG comparaison...

Give us figures! Science is based on facts. Be an engineer. The day you prove your point, well all FSAE will be rocking those beam axled side-engined mutli-steered cars on whatever tire you used with the frame type you used. And there is no problem with that.

You might have the best ever concept but a concept is just a concept. History is written on what is done not what is thought.

Max Trenkle

12-06-2012, 11:44 AM

Hello "Z",

You miss a point there... engineering is not solely based on the idea to have the best concept ever... Engineering is to MAKE THINGS WORK ! And one of the aspect of making something work is risk management. Your ideas are perhaps great but they are a HUGE gamble. With the numbers of years we put in every FSAE years it's a huge gamble.

You also miss another point : gain.

What is the gain Z? We want laptimes comparaison, we want compliances study and weight study, we want drag and downforce figures, we want inertia and CG comparaison...

Give us figures! Science is based on facts. Be an engineer. The day you prove your point, well all FSAE will be rocking those beam axled side-engined mutli-steered cars on whatever tire you used with the frame type you used. And there is no problem with that.

You might have the best ever concept but a concept is just a concept. History is written on what is done not what is thought.

You should explain WHY you think it's such a gamble to try one of many Z's suggestions. Earlier I explained that sure, there is very little out there on FSAE cars that isn't double wishbone suspension, but there have been cars that use 'other' suspensions that are very competitive. Single cylinders, v-twins, and four cylinders have all managed to win at different points and times, why not different types of suspension as well?

As far as facts and figures go, there's a lot you can figure out without building anything. In face, a LOT of money is spent on engineers who say they can figure stuff out before they build it, bc it costs a lot less money to do it that way.

Car companies release concepts that usually are better than the final product. What's wrong with Z's concepts? After all, that may be his strategy: hoping some of us will pull the wool from our eyes and try something new and potentially more competitive.
_____________________________

4WS should be totally allowed in the current rules:
Steering
T6.5.1
The steering wheel must be mechanically connected to the wheels, i.e. “steer-by-wire” is prohibited or
electrically actuated steering, is prohibited.
T6.5.2
The steering system must have positive steering stops that prevent the steering linkages from locking
up (the inversion of a four-bar linkage at one of the pivots). The stops may be placed on the uprights
or on the rack and must prevent the tires from contacting suspension, body, or frame members during
the track events.
T6.5.3
Allowable steering system free play is limited to seven degrees (7°) total measured at the steering
wheel.
T6.5.4
The steering wheel must be attached to the column with a quick disconnect. The driver must be able
to operate the quick disconnect while in the normal driving position with gloves on.
T6.5.5
The steering wheel must have a continuous perimeter that is near circular or near oval, i.e. the outer
perimeter profile can have some straight sections, but no concave sections. “H”, “Figure 8”, or cutout
wheels are not allowed.
T6.5.6
In any angular position, the top of the steering wheel must be no higher than the top-most surface of
the Front Hoop. See Figure 3.
T6.5.7
Steering systems using cables for actuation are not prohibited by T6.5.1 but additional documentation
must be submitted. The team must submit a failure modes and effects analysis report with design
details of the proposed system as part of the structural equivalency spreadsheet (SES) or structural
requirements certification form (SRCF). The report must outline the analysis that was done to show
the steering system will function properly, potential failure modes and the effects of each failure mode
and finally failure mitigation strategies used by the team. The organizing committee will review the
submission and advise the team if the design is approved. If not approved, a non-cable based steering system must be used.

I wonder how much extra development time and driver training having 4WS would take. I think it's most definitely a try before you buy idea on an old car. Lots of testing, lots of decisions to make.

Ian, if the steering is all mechanically attached, but you have electronics that say 'switch gears' in the rear steering to change the steer direction (turn left wheels left to turn left wheels right) then I believe you would be in the clear. You aren't electronically controlling the steering, just what will happen when you steer.

Xavier ABRAM

12-06-2012, 12:09 PM

You should explain WHY you think it's such a gamble to try one of many Z's suggestions. Earlier I explained that sure, there is very little out there on FSAE cars that isn't double wishbone suspension, but there have been cars that use 'other' suspensions that are very competitive. Single cylinders, v-twins, and four cylinders have all managed to win at different points and times, why not different types of suspension as well?

As far as facts and figures go, there's a lot you can figure out without building anything. In face, a LOT of money is spent on engineers who say they can figure stuff out before they build it, bc it costs a lot less money to do it that way.

Max, you got it right entirely! My point isn't making a whole car. My point is using simulation to prove your point. It's like in design event, explain WITH NUMBERS what is backing your choices! Laptime simulation and FEA are a very good arguments when done right.

And there is no wool to pull from any eye... just some time to spent analysing on a system that nobody in FSAE has real experience on.

Will M

12-06-2012, 12:11 PM

Xavier,

As I understand it Z’s complaint is not that you aren’t using his ‘killer-app’ design, but rather that you/we are not doing preforming the full analysis you just described.
Z’s concept is sound and has been well explained, and critiqued.
That is to say it could be used to design and build a car that meets FSAE rules and has certain positives and negatives.
Ultimately everything important about the design will come from the individual team’s implementation.

There are many concepts that can win FSAE.
If you want to know the real gains (or losses) that Z’s design will realize you will have to do that work yourself.

-William

AxelRipper

12-06-2012, 12:19 PM

Well with the new cable steering rule, it would make rear steer a lot easier to accomplish. You wouldn't be able to select your steering option, however. And without steer-by-wire, the entire topic is pretty much moot.

Am I wrong in thinking there was a rule limiting the amount of rear toe compliance (steer) allowed in the car? I can't find it in the rules.

Zac

12-06-2012, 12:44 PM

Didn't TU Graz run rear steer a couple years ago?

Warpspeed

12-06-2012, 12:56 PM

I am not so sure about this whole rear steer idea.

While it might be possible to come up with some numbers with a suspension simulator that imply some advantage under certain conditions, that may not be the entire story.

The idea of crabbing around a slalom turn may feel decidedly strange and unnatural to the driver in such a way that it detracts from the drivers intuitive feel of what the car is doing.

It is possible to crab an aircraft by applying rudder while keeping the wings level, and it is a most peculiar and unsettling feeling.

Don't underestimate the importance of being able to see, feel, and sense what the car is doing within the traction circle, and have all the drivers senses agree.

Really odd effects such as banking the opposite way, or crossed over steering direction, or opposite yaw to real directional change can really confuse the human brain.

If the car does not feel a natural extension of the human body, there is very likely to be a much longer learning curve which some drivers may never be able to come to terms with, of feel truly at ease with.

It would be fun to try though, especially after a few beers.
But will it make the car consistently faster ?

Will M

12-06-2012, 12:58 PM

AxelRipper

There is a rule about 'slop' in the steering system.
I think it is about 5 degrees at the wheel, or something like that.

-William

12-06-2012, 06:38 PM

Originally posted by rjwoods77:
Look at what UWA is up to...

http://sphotos-a.xx.fbcdn.net/...8445_214229793_n.jpg (http://sphotos-a.xx.fbcdn.net/hphotos-snc7/484379_10151187091878445_214229793_n.jpg)
Rob,

Hmmm..., I'm seeing a beam-axle-ish Lotus 88?

I think front is at bottom-right. Are those "E" shaped steel pieces the lateral-location-links-cum-heave/pitch-springs? Hope they did enough testing...

Should be interesting!
~~~~~~~~~~~o0o~~~~~~~~~~~

Ian,

"I think a 4WS electronic interface might prove difficult or impossible to get declared legal."

My "red and green steering wheel buttons" would simply move a linkage at the back of the car to one of three fixed positions (eg. changing the length of a "Pitman" arm there). If electronics not allowed, then do it all pneumatically (ie. via pneumatic push buttons on wheel). If this not allowed, then two "bicycle handbrake type levers" (or "paddles") on the wheel, and all mechanical back to the rear Pitman arm (cables or whatever). This last one is ergonomically very similar to your "pull/pushing" of the steering-wheel rim.

As also suggested by others (Max, Tony), this would be best done as a parallel research project by modifying a previous year's car. I think it would be VERY educational from a VD perspective. So much so that following years' cars might be better even with conventional front-only steer (ie. from better understanding of ackermann, rear roll-steer, compliance-steer, etc.).

In fact, IMO, all bigger than average teams should spend half their resources modifying old cars, and combining "blue-sky" research with driver training. Cut-and-butt Monday to Friday, burn rubber every Saturday and Sunday...

Also note that good ergonomics is a tricky problem. Very often the "most obvious way" turns out to be awkward and clumsy, while the "most comfortable way" is not at all obvious at the start. Computer gaming software writers figured this out ages ago. Engineering software writers still haven't got a clue how this works! http://fsae.com/groupee_common/emoticons/icon_rolleyes.gif It takes an open mind, and a lot of trail and error... http://fsae.com/groupee_common/emoticons/icon_smile.gif
~~~~~~~~~~~o0o~~~~~~~~~~~

Xavier,

"Your ideas are perhaps great but they are a HUGE gamble."

As I have argued before, the reason FSAEers keep copying the same-old-same-old is that they have nothing to lose by losing. THERE IS NO GAMBLE! (Ie. no necks on chopping blocks.)

If your school implements a policy something like,
"As positive inspiration to future FSAE teams, all current team members who fail to win their events outright ... will have their rotting carcasses nailed to the workshop wall!!!",
then I am quite sure you would consider at least some of the alternative suspension types on this thread. After all, you can't get in front of the others, by following them... http://fsae.com/groupee_common/emoticons/icon_smile.gif
~o0o~

"Give us figures! ..."

Read my posts. I give plenty of figures. I like figures!

Or, better yet, (as Max asks) give us your figures. Come on, "Be an engineer!". http://fsae.com/groupee_common/emoticons/icon_smile.gif
~~~~~~~~~~o0o~~~~~~~~~~

William,

"Z’s concept ..."

So far I have pushed the Twin Beam-Axle concept because I reckon it gives the most benefits, for the least effort, in FSAE. The "radicalness" of it may work against it. (Why? They are common in Speedway, Nascar++, US Autocross, many types of production car racing, Off-road, etc., etc...)

Anyway, coming soon, the "very close second" from that list of alternative suspension types (hint: it is structurally very simple, is perhaps an easier step for FSAEers used to double-wishbones, but has minor kinematic disadvantages, which don't matter much, because "Any suspension will work, if...").

Z

Jay Lawrence

12-06-2012, 06:48 PM

^^ What Tony said

MCoach

12-07-2012, 01:51 AM

Originally posted by Warpspeed:
I am not so sure about this whole rear steer idea.

While it might be possible to come up with some numbers with a suspension simulator that imply some advantage under certain conditions, that may not be the entire story.

The idea of crabbing around a slalom turn may feel decidedly strange and unnatural to the driver in such a way that it detracts from the drivers intuitive feel of what the car is doing.

It is possible to crab an aircraft by applying rudder while keeping the wings level, and it is a most peculiar and unsettling feeling.

But will it make the car consistently faster ?

As I've mentioned very recently, these type of actions are unfamiliar to cars, but common in my background. They key is practice.
Assuming a car is set up properly for a course...going a bit out on a limb here:

What three makes a fast driver fast?

Talent, Practice, Practice.

Practice, as everyone should know, makes perfect. But, the second practice is different than the first. The second practice I list to illustrate a point that there is more that comes with it. A driver must be able to anticipate the movements and actions to be taken in the near future or react on instinct to adjust for errors. I would say that after driving something with a mecanum style drive system, even at only a top speed of 20mph, that you get used to it. You are already turning your head before you hit that button to strafe a vehicle (feels weird to say that on these forums without quotes. HA!) completely sideways around obstacles. Practice the controls and you get past that sick feeling that the vehicle is doing something that it shouldn't be, you just anticipate it.

On a note of rear steer (bump steer):

Rear steer can be considered to come into play in two different ways, either it steers in phase or out of phase with the vehicles steered direction. In the OEMs such as Mazda or Nissan with the RX-7 or Skyline, they either use compliance bushings or hydraulics, respectively. In our application I've seen geometry used to achieve this (bump steer aligned toe link). When used in phase, the car tends to push itself out of the corner, creating less of a yaw moment from the rear, and leading to a stable understeer condition. The other direction leads to out of phase steering that rotates the car more quickly around a corner and can lead to an unstable oversteer condition.

This can be all fine and dandy depending on what design theory you are following about cornering, but it must also be noted that a straight line is the most awkward path to follow. Bumps cause the car to dart around like a Chihuahua shitting razor blades and transience plays hell on the system. It's widely documented that cars that incorporate these design features into the OEM vehicles are typically overridden by whatever means it takes for track use because it causes the car to become unpredictable and unstable as the driver doesn't know when or how much the system is active at higher speeds. Things like that become dangerous. But again, it's about anticipation, and a system that you can anticipate is useless.

Z,

My thoughts going through suspension design this year:

With double wishbones basically mimicking a swing axle, why not use one? We did end up using double wishbone this year (with some relatively big changes from previous), but a swing axle doesn't seem to be all that negative for these uses. Travel is +1/-1 inch of travel which leaves little camber gain, and design steps are nearly the same as wishbones. It is a little inferior to double wishbone kinematicly, but could potentially save weight and give a mounting link to attach a direct actuating damper.

12-07-2012, 04:37 AM

Look at what UWA is up to...

http://sphotos-a.xx.fbcdn.net/...8445_214229793_n.jpg (http://sphotos-a.xx.fbcdn.net/hphotos-snc7/484379_10151187091878445_214229793_n.jpg)

http://sphotos-d.ak.fbcdn.net/...810_1823465174_o.jpg (http://sphotos-d.ak.fbcdn.net/hphotos-ak-ash4/192603_10151039477760810_1823465174_o.jpg)
Some comments regarding UWA's suspension (after a closer look at all three pics - more please! http://fsae.com/groupee_common/emoticons/icon_smile.gif).

Firstly, I have no connection with UWA (they are on the other side of the island!), so this is just speculation. Also, about the time I posted the Twin Beam-Axle sketches, and waffled on about "soft twist-modes", I heard that UWA had already proposed a similar idea to their advisors. Their design is entirely theirs.

The distinguishing feature of UWA's car is its "Twin-Beam+Structural-Aero-Undertray" (or whatever they call it). The front and rear pairs of wheels are each connected as with beam-axles, so camber is always constant wrt a line through the two adjacent wheelprints. The two "beams" are then connected longitudinally by the undertray, with the two tunnels providing most of the strength/stiffness.

The undertray prevents the two beams from "parallelograming" in plan-view (ie. getting out-of-square with car centreline). However, the undertray has relatively low torsional stiffness, allowing a very soft twist-mode suspension. Note that many teams struggle to make a spaceframe chassis torsionally stiff enough, so making a flattish sheet of "plastic" torsionally soft is not hard.

The "E" shaped brackets at front and rear locate the (separate spaceframe) chassis to the undertray, both laterally and longitudinally. I now think these are probably made of carbonfibre (not steel like I said in earlier post). These probably provide some, or all of, the pitch and heave springing.

About half-way front-to-rear on each tunnel is a small vertical bracket. I think this is where the roll-mode springs attach. These might be like conventional, vertical, coil springs, in which case they also work in heave-mode. Or else a single lateral U-bar (like a conventional ARB) might be used, thus only working in roll-mode. Importantly, LLTD is determined soley by the fore-aft position of the tunnel brackets. Spring-rates, or twisting motions of the four wheels, do not affect LLTD.

Anyway, I hope I haven't slandered their concept, or given any secrets away. Also, I am sure there are many other interesting features (eg. Is the steering via a bevel gearbox and Pitman arm? How far back is that engine tilted? Etc...).

Awaiting results and more pictures with great interest!!! http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

(PS. MCoach, Yes! More soon...)

AxelRipper

12-07-2012, 07:15 AM

Originally posted by MCoach:
Z,

My thoughts going through suspension design this year:

With double wishbones basically mimicking a swing axle, why not use one? We did end up using double wishbone this year (with some relatively big changes from previous), but a swing axle doesn't seem to be all that negative for these uses. Travel is +1/-1 inch of travel which leaves little camber gain, and design steps are nearly the same as wishbones. It is a little inferior to double wishbone kinematicly, but could potentially save weight and give a mounting link to attach a direct actuating damper.

Well this all depends on your desired tire kinematics. The advantage of a double wishbone is that you can get a wide instant center (and thus little camber gain) with a relatively light setup.

Take this for example: The Ford twin I-beam.

http://www.hemmings.com/story_image/119312-500-0.jpg

What you have here is a swing arm with a rather wide instant center. If you were to design double wishbone to hit these same instant centers on the same chassis, your arms, while there would be 2x the number, would be about .25 the length. I would say that the size and packaging would be a tip towards the wishbone, unless (on a typical FSAE car) you only want your instant centers at about 1 foot, assuming mounting to the outside of a chassis.

Now, one thing that I'm going to throw out there for consideration since it doesn't get brought up here much is the "Jacob's Ladder":

http://pitstopusa.com/images/F14581178.jpg

These are used quite commonly in circle track applications to replace a panhard bar (that thing in the back that holds the solid axle in place laterally). Since you really don't want your axle moving laterally, you want a rather long panhard bar. However, when you get into sprint cars, which aren't much bigger than your standard FSAE car, you don't have as much room for that length like you would in a stock car. The solution:

http://img.photobucket.com/albums/v707/gbarnes/JacobsLadder2.jpg

Now, it seems to me that this could potentially be used to make a swing axle, with a much smaller camber gain than you could reasonably package on a FSAE car, and likely much less weight.

Just some more stoking of the fire...

MCoach

12-07-2012, 09:37 AM

What you have here is a swing arm with a rather wide instant center. If you were to design double wishbone to hit these same instant centers on the same chassis, your arms, while there would be 2x the number, would be about .25 the length. I would say that the size and packaging would be a tip towards the wishbone, unless (on a typical FSAE car) you only want your instant centers at about 1 foot, assuming mounting to the outside of a chassis.

Just some more stoking of the fire...

I would imagine that camber gain isn't really an issue after working with these tires. Camber seems to be 'add to taste' and relatively used to evening out wear and temperatures on these tires. Everyone has a different quoted number and these seem to vary widely, with little ill effect.

Now, to aim for the same goals as a double wishbone, yes, you would end up with something similar to the Twin I-beam, but there can be a compromise to allow more camber than desired to shorten the track to something more reasonable.

A car that I would look to for reasonable comparison would be this little guy that's been up in the past:

http://www.hyperracer.com/hype...oracer/pictures.html (http://www.hyperracer.com/hyper-racer/proracer/pictures.html)

It's swing arm front, beam axle rear. It sports monoshocks front and rear.

Discuss.

Warpspeed

12-07-2012, 11:43 AM

I am still trying to get my head around the concept of how steering the rear wheels will get you around a skid pad any faster.

Crossed I beams and Jacobs ladders at the front also raise the issue of bump steer.
At least with double wishbones or a Chapman strut, placing the tie rod exactly at wishbone height gives a nice neat simple solution.

With many (most?) other front suspension designs, the bump steer problem may be far from simple to solve satisfactorily.

Owen Thomas

12-07-2012, 12:47 PM

I am still trying to get my head around the concept of how steering the rear wheels will get you around a skid pad any faster.
Not sure if rhetorical question. Regardless, steering the rear wheels in the opposite direction of the fronts generates an opposite slip angle, which generates (outwards) lateral force. Inward lateral force at front + outward lateral force at rear = larger yaw couple (than without 4WS). Larger yaw couple = larger yaw rate = faster turning. Faster turning means you can have a larger tangential velocity (aka speed) to get around the same radius corner.

Getting the tires to stay on the road and managing your understeer balance with that setup is something else, however...

Warpspeed

12-07-2012, 01:14 PM

Not a rhetorical question.

Suppose you have already set up your car to perfection on the skid pad by adjusting relative front/rear roll stiffness right up to the limit of the traction circle at both ends.

Now you deliberately steer the rear wheels to increase the rear slip angles even further.
What do you suppose is going to happen at the back if the rear tires are already at their limit ?

Edward M. Kasprzak

12-07-2012, 01:27 PM

Zac

12-07-2012, 01:28 PM

Owen Thomas

12-07-2012, 02:19 PM

Warpspeed: I was under the assumption that this theoretical rear steer was not an add-on to a tuned car, but set up for. You're definitely right, introducing that to a car already at the limit would make it pretty tail happy (I think?).

Dr. Kasprzak: No, I cannot. Definitely got ahead of myself. I also guess I did not mean to say that the resultant force on the rear tires would be outwards, but it would be smaller, allowing you to go faster and have a larger buildup of force to achieve the same resultant as you would without rear steer. Put more simply, the opposing (steered)slip angle force would allow a larger reaction from the road to reach the same tire limit. This could certainly be very wrong, but hopefully I'm straightened out by the end of this discussion.

Zac: Yaw rate is probably not a limiting factor, but I think it is a large contributor. I have not examined its impact myself, I was just throwing my ideas out there.

MCoach

12-07-2012, 02:45 PM

Originally posted by Zac:

How often is yaw rate the limiting factor for a FSAE car getting around a medium radius and presumably steady-state turn like a skid pad? I think translation is a much bigger problem than rotation.

Yaw may be a contributing factor, but if it was the biggest thing to be concerned with, I would say our Japanese friends are ahead of us with the sidewinder style cars. I would also say that if it was the biggest part of it, that those style cars would be significantly faster. This is probably more of a concern for transience, but not so much for steady state. Feel free to compare data as I will admit I have not.

I have my prejudices against skid pads for measuring anything other than steady state balance and traction. It's easy to set up and doesn't tell you anything about other phases of cornering, such as turn in or out.

Warpspeed

12-07-2012, 03:06 PM

@ Warpspeed: How would steering the rear wheels change the rear slip angles on a skidpad?

Now that is a really interesting question.

Individual wheel steer angles should probably not be thought of as being just relative to the chassis, but referenced to the steer angles of all the other wheels on the car....

Seat of the pants experience suggests that any sudden changes to steer angles at the back, even significant lateral compliance at the back, can produce some very unstable dynamic handling. Even though steady state skid pad testing may seem fine.

Having the rear wheels always pointing rigidly dead ahead seems to be a fundamental requirement for fast stable transient response to steering input, as well as straight line stability.

Once you have that, a bit of tweaking of the roll axis inclination can put the icing on the cake.

Every time I have had rear wheels flapping about, there have been huge wide eyed pants wetting problems for the driver that don't go away.

nowhere fast

12-07-2012, 07:53 PM

Originally posted by Owen Thomas:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content"> I am still trying to get my head around the concept of how steering the rear wheels will get you around a skid pad any faster.
Not sure if rhetorical question. Regardless, steering the rear wheels in the opposite direction of the fronts generates an opposite slip angle, which generates (outwards) lateral force. Inward lateral force at front + outward lateral force at rear = larger yaw couple (than without 4WS). Larger yaw couple = larger yaw rate = faster turning. Faster turning means you can have a larger tangential velocity (aka speed) to get around the same radius corner.
</div></BLOCKQUOTE>

A correction here: larger yaw couple = larger yaw acceleration (alpha) = faster turn in. There is a discussion on how greater yaw acceleration can reduce lap times in this thread:
http://fsae.com/eve/forums/a/t...7348/m/824105905/p/9 (http://fsae.com/eve/forums/a/tpc/f/125607348/m/824105905/p/9)

I can think of three ways that 4 wheel steer may improve skid-pad times:

1. The skid pad is taken below the tangent speed (correct me if I’m wrong), so the car will take the corner with the front axle further from the inside of the corner than the rear axle. In other words the car will corner with a nose out attitude below the tangent speed, and a tail out attitude above it. This means the centre of mass must travel on a larger radius to avoid clipping cones with the inside rear wheel than it would need to if it could travel at the tangent speed.

Steering the rear wheels in the opposite direction to the fronts can reduce or eliminate the nose out attitude of the car, allowing the centre of mass take a tighter radius without clipping cones. For the same lateral acceleration a smaller radius will give a higher yaw rate (omega) than a larger one, meaning the car will complete a lap of the skid pad quicker (skid pad lap is 360 degrees).

2. Skid pad should be close to pure steady-state cornering because the first lap in each direction (where the yaw rate will be established) is not timed. This means the car is undergoing lateral acceleration only; and as such requires the resultant force acting at the centre of mass to point towards the centre of the turn, with no resultant yaw couple.

When the front wheels are steered a component of the force from the front tyres now acts longitudinally, and does not contribute to the lateral acceleration.
By steering the rear wheels in the opposite direction to the fronts; the required steer angle on the fronts will be reduced. The component of the force at each tyre that acts laterally is related to the cosine of the steered angle; which means that the amount of lateral force lost by steering the rear wheels is less than the amount gained by reduced steering angle of the fronts. The result is the car will be able to achieve higher steady-state lateral acceleration with four wheel steering than without.

3. Lastly steering the rear wheels allows for dynamic toe adjustment (aka Ackerman) to be applied to the rear wheels as well as the fronts. This may enable a closer to ideal slip angle to be achieved on the inside rear tyre.

This is mainly me thinking out loud, so corrections/criticism is welcome.

Edward M. Kasprzak

12-08-2012, 07:00 AM

Excellent reply nowhere fast. If we agree that skidpad is close to being a steady-state event then, to summarize what you said, the primary effect of rear steer is to adjust the vehicle sideslip angle. You have mentioned several follow-on effects.

Warpspeed, this is a good observation and the reason design judges frown upon (uncontrolled) rear toe compliance:

Seat of the pants experience suggests that any sudden changes to steer angles at the back, even significant lateral compliance at the back, can produce some very unstable dynamic handling. Even though steady state skid pad testing may seem fine.

Once we're talking about transients the use of rear steer becomes very "interesting". "Good" or "bad" would depend on how it is implemented.

Owen Thomas: One way to think about the interaction between tires making lateral force and tires making yaw moment is to study the Milliken Moment Method diagrams in Chapter 8 of RCVD. Keep in mind that at steady-state the yaw moment on the vehicle must be zero. Thus, the relative amount of lateral force each end of the vehicle must contribute is dominated by the yaw moment requirement. Of course there are other contributors to yaw moment besides lateral force, but the relationship between the lateral forces, the fore/aft CG location and the yaw moment is a great starting point.

12-09-2012, 05:18 PM

Axel,

"Take this for example: The Ford twin I-beam.
...
What you have here is a swing arm with a rather wide [front-view] instant center. If you were to design double wishbone to hit these same instant centers on the same chassis, your arms, while there would be 2x the number, would be about .25 the length. I would say that the size and packaging would be a tip towards the wishbone... "

It is indeed the "packaging" advantage that makes the double-wishbone so common on production cars. They were first developed so that the mid-front engine could be pushed further forwards between the two front wheels, thus making the whole car more space efficient (bigger passenger space in smaller envelope).

BTW, the Ford twin I-beam is what I call a Semi-Leading Swing-Arm. Its pivot axis is about 30 degrees off lateral, passing through the ends of the I-beam and torque-reaction-arm. I will try to cover these this week (but silly season now in full swing!).
~o0o~

"Now, one thing that I'm going to throw out there for consideration ... is the "Jacob's Ladder""

The Jacob's Ladder, Watt's Linkage, and even the SLA Linkage (in front-view), are all variations of similar "planar 4-bar" linkages. That is, they all have a "frame" (link 0) connected to two straight links (links 1 and 3), which in turn are connected to the centre link (link 2). These linkages are useful because they can all give approximately straight line motion at a certain point on their centre link.

Move to 3-D linkages and it is possible to get exact straight line motion in a given plane. An example is the "scissor" linkage that controls the steering of the sliding-pillar suspension on aeroplane nose wheels. This "scissor" linkage is a good option for controlling lateral motion of beam-axles.
~~~~~~~~~~o0o~~~~~~~~~~

Tony,

"With many (most?) other front suspension designs, the bump steer problem may be far from simple to solve satisfactorily."

IMO, bump-steer, front or rear, is an easily solved problem with most supension types (in FSAE). Assuming you are using a normal toe-link (rather than something more exotic), the simple solution is to start with a long toe-link (preferably starting near car centreline, so ~60 cm long), then attach to a long steer-arm (ie. distance on upright from steer-axis to toe-link BJ at least ~10 cm long). If the ends of the toe-link are at the right heights (which should be adjustable), then the bump-steer for the +/- 2.5 cm suspension travel should be small enough to be unnoticeable.

BTW, the above Ford twin I-beams have criminally negligent amounts of bump-steer, all through attrociously bad design that could have been easily fixed! Many rollovers!!! http://fsae.com/groupee_common/emoticons/icon_rolleyes.gif
~o0o~

"I am still trying to get my head around the concept of how steering the rear wheels will get you around a skid pad any faster."

This already covered by Ed and Nathan. I will add to Nathan's list that rear counter-steer will give larger body side-slip angle, which could give a significant aero sideforce, especially if huge wing endplates are used.

To restress the main point of rear counter-steer, it is to increase YAW ACCELERATION. With normal front-only steer it is only the front wheels that exert a force that yaws the car. Counter-steering the rear wheels allows them to exert a similar yawing force, doubling the potential yaw acceleration. The tricky part is to control that yaw acceleration so it doesn't run away on you. Below is how I described it on the 4 Wheel Steer (http://fsae.com/eve/forums/a/tpc/f/125607348/m/81720309051?r=29020429051#29020429051) thread.

"If you watch nature documentaries you see the big cats using "active rear steer" when they chase their dinner. The rear briefly steers out [counter-steers] on corner entry (all in one big step), then immediately steers (or pushes) inward [= same steer]. (The tail also swings around for "active yaw and roll control".) Downhill ski racers also "hop" the rear of the skis outward on corner entry. But both of these have extremely capable active control systems!"

That is, a very brief period of rear counter-steer to get the right yaw velocity, then immediately switch to rear-straight-ahead (or even same-steer) so the rear exerts the necessary centripetal cornering force.

Z

Warpspeed

12-09-2012, 06:23 PM

Z, I disagree with bump steer being a simple problem.

With crossed I beams, both tie rods would need to be made the same lengths as the I beams, and these long tie rods would also need to cross over.
Not impossible, but not exactly a neat solution.

While I agree, longer steering arms would tend to make the steering less sensitive to any bump steer errors arising from the front view SLA motions.
it also magnifies the bump steer errors created by caster change introduced by any anti dive.

As the front upright tilts with changing caster, the steering arm and relative outer steering ball joint height changes, and it does not then follow the simple arc the tie rod end is constrained to follow.

Many very simple suspensions look good, until you start seriously thinking about bump steer.
In many cases that simple suspension may need a much more complex steering linkage which kind of defeats the initial purpose.

The best practical steering solution for a beam front axle is probably to bolt the steering rack direct onto the beam. That has a lot going for it for both simplicity and pretty good geometry, and should not be underestimated.

If I had do design and build a very simple car ultra quickly, with extremely limited resources, that is how I would very likely go about it.

By the time you add up all the unsprung weight that MUST be there, the added extra weight of a light weight steering rack would be negligible.

12-10-2012, 02:34 AM

Originally posted by Warpspeed:
I disagree with bump steer being a simple problem.

With crossed I beams, both tie rods would need to be made the same lengths as the I beams, and these long tie rods would also need to cross over..
Tony,

No, no, no...

In plan-view each I-beam has a longitudinal "torque-arm" going backward from just inboard of the wheel to the chassis rail (ie. to about 0.6 m behind axle line). The screw-axis for the wheel passes through the I-beam and torque-arm chassis attachment bushes, so about 30 degrees back from lateral. Being a "Swing-Arm", the I-beam's screw pitch is zero (ie. it is a revolute joint = simple hinge).

For zero bump steer the steering tie-rod's inner BJ can lie anywhere on the screw-axis. A convenient place is close to the intersection point of the two front wheel's screw-axes, namely close to the centreline of the vehicle and about 0.3 m behind axle line. A steering linkage similar to most latter 1900 US double-wishbone cars would be suitable (ie. no more complicated than any other suspension type).

BTW, Ford used a slightly simpler steering linkage that steered both wheels towards the driver's side whenever one wheel moved up and the other down. Lots of rollovers of short wheelbase F100's on righthand corners here in Oz. Mine had a long wheelbase, but even so it managed to fall over one night (though it was "tired and emotional"...).

In FSAE, I reckon a team has to make a spectacular design cock-up to get their bump-steer wrong. However, I accept that this does happen. http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

(PS. R&P-on-the-beam gives a small amount of roll understeer. For the minimal roll angles in FSAE this is not a problem.)

GSpeedR

12-10-2012, 05:38 AM

Originally posted by nowhere fast:

2. Skid pad should be close to pure steady-state cornering because the first lap in each direction (where the yaw rate will be established) is not timed. This means the car is undergoing lateral acceleration only; and as such requires the resultant force acting at the centre of mass to point towards the centre of the turn, with no resultant yaw couple.

When the front wheels are steered a component of the force from the front tyres now acts longitudinally, and does not contribute to the lateral acceleration.
By steering the rear wheels in the opposite direction to the fronts; the required steer angle on the fronts will be reduced. The component of the force at each tyre that acts laterally is related to the cosine of the steered angle; which means that the amount of lateral force lost by steering the rear wheels is less than the amount gained by reduced steering angle of the fronts. The result is the car will be able to achieve higher steady-state lateral acceleration with four wheel steering than without.

This makes sense regarding tire force vectors relative to the chassis-fixed coordinate system. However, once we have added (opposite) rear steer and the vehicle again reaches a steady-state condition, it will have a larger magnitude of body yaw relative to the CG heading. Have the tire force vectors changed relative to the turn center (or, equally, relative to the vehicle CG heading) compared to the front-steer less-yawed vehicle?

AxelRipper

12-10-2012, 06:03 AM

Originally posted by Z:In plan-view each I-beam has a longitudinal "torque-arm" going backward from just inboard of the wheel to the chassis rail (ie. to about 0.6 m behind axle line). The screw-axis for the wheel passes through the I-beam and torque-arm chassis attachment bushes, so about 30 degrees back from lateral. Being a "Swing-Arm", the I-beam's screw pitch is zero (ie. it is a revolute joint = simple hinge).

For zero bump steer the steering tie-rod's inner BJ can lie anywhere on the screw-axis. A convenient place is close to the intersection point of the two front wheel's screw-axes, namely close to the centreline of the vehicle and about 0.3 m behind axle line. A steering linkage similar to most latter 1900 US double-wishbone cars would be suitable (ie. no more complicated than any other suspension type).

BTW, Ford used a slightly simpler steering linkage that steered both wheels towards the driver's side whenever one wheel moved up and the other down. Lots of rollovers of short wheelbase F100's on righthand corners here in Oz. Mine had a long wheelbase, but even so it managed to fall over one night (though it was "tired and emotional"...).

In FSAE, I reckon a team has to make a spectacular design cock-up to get their bump-steer wrong. However, I accept that this does happen. http://fsae.com/groupee_common/emoticons/icon_smile.gif

To have a substantial amount of bump steer in an FSAE car's amount of wheel travel, you're right there. Getting it as low as possible however is a fun thing to keep the suspension designers busy and out of the manufacturing guys hair...

As for the twin I-beam, as my avatar shows, I have the earliest iteration of said suspension (1966 F100) and I never honestly noticed the bump steer until I got to college (never really thought a whole lot about suspensions until then) and then I only ever notice it when I first jump back into it for the summer. However, you're correct, and not just on bump steer. The camber change of these is also criminal (same qualm with swing arms, but that discussion is upcoming still...)

For something that was made for 30+ years with no design changes to really speak of, there aren't many good pictures out there of this setup (as of 84, still no swaybar. As of mid 90's, still criminal bump steer induced roll over). Here's a shot for those unfamiliar:

http://1.bp.blogspot.com/_UeLVY2Znz-Q/THcnUkrlQqI/AAAAAAAAAIk/izS6maluCdM/s1600/IMG_3190.JPG

Now, lets add in the screw axes:

http://img.gawkerassets.com/img/187x5p17h4je1jpg/original.jpg

Here, you can see the point that Z is referring to (If I'm understanding correctly) and you can see where the bump steer comes from. However, this is pretty significantly behind the centerline of the axle, and it is in the center of the chassis. Not sure how exactly you'd package your steering box in the center of the chassis, especially with an engine there.

Warpspeed

12-10-2012, 11:49 AM

That is exactly what I was referring to AR.

For it to work properly, a central steering box would have to be located about in the middle of the sump where your two red lines cross.

To move the inner ends of the steering tie rods forward to a much more practical location, the tie rods must be both much longer and cross over.

BillCobb

12-10-2012, 06:53 PM

Just for clarification, rear wheel steer (and 4 wheel steer has NO effect on max lateral capability. It (obviously) thus has NO effect on the vehicle's understeer gradient. Simply stated: Only effects directly or indirectly related to cornering force dependency affect understeer, hence no effect on lateral capability. Knowledgeable readers should recall that you remove the Ackerman gradient from the net lateral acceleration gain to calculate understeer. If you have rear steer (only) or in addition too, its effect must be factored into this calculation. Since its driver activated (an input) and not a response (output) it's not a playa by definition.

12-10-2012, 10:26 PM

Originally posted by AxelRipper:
http://img.gawkerassets.com/img/187x5p17h4je1jpg/original.jpg
... Not sure how exactly you'd package your steering box in the center of the chassis, especially with an engine there.
(Note, front-of-vehicle = bottom-of-pic, and driver's-side = right-of-pic.)

Axel and Tony,

Once again...

Almost all US wishbone cars of the same era as the F100 had a "recirculating-ball" steering box located in front of the driver, as does the F100 (at right of pic). At the bottom of the steering box is a rotating "Pitman Arm". On the wishbone cars this PA connects to a "Lateral-Link" that passes under the engine, and in turn connects to a rotating "Idler Arm". The PA and IA have the same geometry, so when switching from LHD to RHD the manufacturer only has to swap steering-box-PA and IA (well, sort of...).

When steering, the LL moves laterally back and forth under the engine. At two positions on this LL are bolted the inner BJs of the two steering tie-rods. Now picture this common steering arrangement sitting slightly behind the I-beams of the F100 (ie. slightly above I-beams, in above pic). Putting the tie-rod inner BJs near the centre of the LL puts them close to the two suspension's pivot axes, giving small bump-steer, or maybe none at all.

Note that when the tie-rod inner BJ is EXACTLY ON the pivot axis, then there is NO bump-steer (well, except for some other effects I'll ignore for now). When the BJ is OFF the axis, but the line through inner and outer tie-rod BJs points directly at the axis, then there is no bump-steer at the instant, although some small bump-steer develops towards full bump and droop (because the tie-rod line no longer intersects the pivot axis).

Anyway, if they used the standard steering linkage of the time, then the F100 would have had minimal bump-steer, with a simpler all-round suspension than double-wishbones. Even if the tie-rod inner BJs were about a foot away from the swing-arm axes (ie. closer to the I-beams and wheels), but pointing at the axes at ride height, then there would only be small bump-toe-out at full bump/droop, which is not too bad (it gives small roll-understeer)

Instead, the F100 has one long tie-rod from the steering-box-PA out to the opposite-side wheel. Some distance along this "major" tie-rod is a BJ attached to the "minor" tie-rod that connects back to the driver's-side wheel. I'll let you figure it out, but bottom line is that bump steer of BOTH wheels is (mostly) towards the driver's side. Worst is when one wheel bounces up and the other droops down.

Z

Warpspeed

12-10-2012, 10:51 PM

That is all true Z.

Except the swing axis for each swing arm is located on the opposite side of the vehicle.
The further forward the tie rods are located, the closer to the opposite side they will need to reach.

That still means the tie rods need to cross over.

MCoach

12-11-2012, 11:15 PM

There quite a few different conversations going on at once here, but I feel the one that should be paid most attention to is the original one about suspension design and not active toe.
I'd like to request that this topic be moved to it's own thread to avoid further clutter.

Z,

I'm still very curious on your input on swing arm suspension.

12-12-2012, 04:16 AM

Tony,

No! You're overcomplicating it. KISS! http://fsae.com/groupee_common/emoticons/icon_smile.gif
~~~~~o0o~~~~~

MCoach,

Sorry for delay. 'Tis the season for family to come a'knockin.

Will post by Friday night...

Z

Rex Chan

12-12-2012, 04:33 AM

Z: some pics I took at FSAE-A 2012.

http://www.facebook.com/media/...3272531135703&type=3 (http://www.facebook.com/media/set/?set=a.256889624440660.61213.223272531135703&type=3)

12-14-2012, 03:09 AM

Rex,

Thanks for the pics! Lots of interesting stuff there (on UWA's car).

Bevel-gear and Pitman arm (?) steering. Face-spline centrelock wheel attachment. And, of course, the suspension with roll-mode controlled by a single lateral U-bar...

Z

12-14-2012, 03:10 AM

... the major suspension types are,

1. Beam-axles (eg. on all vehicles ever, can be live or De-Dion at rear),
2. Sliding-pillar (eg. Morgan and Lancia, and most motorbikes and aeroplanes at front),
3. Lateral swing-axles (eg. Tatra (still on their trucks), can be high or low-pivot),
4. Leading and trailing-arms (eg. Citroen 2CV front and rear, and most motorbikes at rear),
5. Semi-leading/trailing-arms (eg. F100 at front, and many mid to late 1900s cars at rear),
6. Strut-wishbone (eg. McPherson at front, strut at rear),
7. Double-wishbone (eg. originally mostly at front to allow shorter wheelbase with front engine),
8. 5-link (eg. recent fashion, mainly for NVH reasons).

Of these, any of the first five are more than adequate for a winning FSAE car.
Apologies for this very long post, but I will cover all three of the above bold suspension types under the same general heading.

The SWING-ARM Suspensions.
===========================
These all have a single pivot joint at the chassis (ie. kinematically a single "revolute joint", but physically it may consist of two bushes or BJs). From this joint a single structural "arm" extends out to the wheel. Front wheels have a steer-axis, or "kingpin", at the wheel end of the arm, together with a separate means of controlling this steering DoF (typically via a tie-rod). Rear wheels can have the axle housing fixed rigidly to the end of the arm.

The main difference between the three types is the orientation of their pivot axis wrt the body.

"Pure Lateral Swing Arm (or Axle)" has its pivot axis close to longitudinal wrt the body (ie. parallel to X axis). Historically, Tatra was the first to use these Lateral SAs in the early 1920s (first at rear with their "jointless" diff, then a few years later both front and rear). They still use them on their off-road trucks.

"Pure Leading or Trailing Arm" has its pivot axis close to lateral (ie. parallel to Y axis). The Citroen 2CV, designed in the late 1930s, but only sold post WWII (late 1940s), was the first, and possibly only car to use these at both ends. Pure Trailing Arms at the rear became very common post WWII, mainly on French and some English cars. PTAs are almost universal on motorbikes today.

"Semi-Leading or Trailing-Arm" has its pivot axis somewhere inbetween lateral and longitudinal. The Ford F100 Twin-I-Beams in earlier posts are Semi-Leading-Arms. Semi-Trailing-Arms at the rear were extremely common from about 1960s to 1990s, mainly on German designed cars (VW/Porsche, Audi, BMW, Mercedes, NSU, German Fords and GMs, etc...). Funny how cultural fashions can so influence engineering decisions, eh? http://fsae.com/groupee_common/emoticons/icon_smile.gif
~~~~~o0o~~~~~

Here are the major pros and cons, IMO.

ADVANTAGES.
=============
1. Significantly simpler than Double-Wishbones or 5-Links.
The Swing-Arm typically only requires two hard points on the chassis (ie. for the 2 bushes/BJs forming the arm's pivot-axis), plus one more for a direct-acting Spring-Damper. The left and right suspensions can share at least one of these hard points, or even all three (giving only 3 x HPs at each end of car, though benefits here depend on other details).

IMO, one of the biggest costs of DWs is the multiple chassis mounts required for them. For example, each upper wishbone's two chassis mounts are relatively lightly loaded, but on FSAE cars they often have 6, or even 8 tubes converging on each node. This multiplied by the four corners = a lot of work. Furthermore, these nodes usually don't match neatly with the various Rules mandated roll-hoops and side structures. The end result, IMO, is a horrible dog's breakfast of tubes running everywhere. In fact, I can't recall seeing a single FSAE spaceframe that looks at all efficient, let alone elegant.

2. Significantly stiffer than DWs or 5-Ls.
At the rear, the wheel's "upright", namely the axle's bearing housing, can be one continous piece with the Swing-Arm. The ball-joints between DWs and their upright inevitably result in tortuous, high stress load paths through the bolts, brackets, balls, etc. This inevitably results in increased compliance for a given amount of material. And then there is the issue of the BJs developing freeplay! http://fsae.com/groupee_common/emoticons/icon_frown.gif A direct, continuous load path from Swing-Arm to bearing housing is always going to be lighter and stiffer. And also cheaper and quicker to build.

However, at the rear it can be beneficial to provide some camber and toe adjustment (at least during testing). This can be done with different SAs (swap the whole assembly), adjustments at the chassis mounts (up-down, in-out, etc.), or an adjustable joint in the SA near the wheel. This last type of joint requires no movement during suspension travel, so, IMO, it can be made much stiffer for given mass than a typical DW-to-upright joint.

At the front the Swing-Arm's steer-axis can be made with two ball-joints, like a DW. But it can also be made as a simple revolute joint, which can provide significant advantages. Specifically, low friction rolling element bearings can be used, giving lighter steering (important when the aero downforce gets interesting... http://fsae.com/groupee_common/emoticons/icon_smile.gif). Might post a sketch of such a steer-axis one day...

3. Lower CG than DWs or 5-Ls.
Most, or all, of the Swing-Arm's structure can be well below axle height. This can also be done with DWs, but the above complicance issues likely then give too much camber/castor change under load. You can try estimating this compliance with FEA, or better yet, just build it and physically test.

4. Easier to find the "Instantaneous Screw Axis", which is well-behaved.
As discussed at length elsewhere, any suspension's wheel assembly "screws" about an ISA, wrt the car body. In the non-bold suspensions above the ISA usually moves about a lot as the suspension moves through its travel. Knowing the position of the ISA is very useful to the suspension designer, although I accept that this is rarely done.

With Swing-Arms the ISA is easily found. It is the physical pivot-axis! An exception is when the steering linkage gives significant bump-steer, which then requires a knowledge of the "cylindroid" (maybe later...). Nevertheless, even with bump-steer the Swing-Arm ISA is usually very close to the physical pivot axis.

Once the ISA is found, then so also are all of its n-lines (or vice versa). Because the Swing-Arm's ISA and n-lines are so well behaved (ie. relatively immobile), the suspension's behaviour is also very predictable. More below under "Kinematics".
~o0o~

DISADVANTAGES.
===============
1. Packaging.
It is often beneficial for the suspension's ISA to be a long way away from the wheel. For a given amount of suspension travel a distant ISA gives smaller rotations of the wheel-assembly (in camber, castor, or steer) than a nearby ISA. DWs and 5-Ls can give distant ISAs with a compact linkage, which is their main advantage.

However, the open-wheel style of FSAE cars allows relatively long physical Swing-Arms to be easily packaged (easier than under passenger cars). Also the smooth tracks and very short mandated suspension travel of +/- 2.5 cm means that rotations due to nearby ISAs are not too big a problem (eg. negligible gyroscopic effects from too much camber change over bunmps).

2. Highly unusual in FSAE.
Not much to copy, so you have to work it out for yourselves. Worse yet, you can expect lots of irrational bias and prejudice against your ideas from FSAE peers, and, yes, officials too. See last section below.
~~~~~o0o~~~~~

KINEMATICS.
=============
Some brief points here.

If a suspension's ISA has any vertical component (ie. it is NOT horizontal), then any motion about the ISA necessarily results in some steer motion (ie. rotation about the vertical component of the ISA). The amount of this bump-steer depends on the distance of the ISA from the wheel, its steepness, and the travel of the suspension (eg. size of bump, or amount of body-heave/pitch/roll).

With Swing-Arms on FSAE cars you would really have to cock things up big time for this to be a problem. Nevertheless, let's assume the ISA (= pivot axis) is horizontal wrt body.

Pure Lateral Swing Arm.
====================
Historically, to allow easier drive from the diff, the ISA was at axle height. This gives large jacking forces. So only use if some sort of anti-axle-bounce springing is provided (eg. "third spring", lateral-Z-bar, or see Formula Vees). Better to have a low-pivot swing-arm, pivoting from floor level. This gives a small (acceptable) slope to the lateral n-lines (ie. lowish RC).

ISAs close to the centreline of the car give ~100% camber compensation (ie. short Front-View-Virtual-Swing-Arm). So you can run very soft springs and the body-roll doesn't change wheel camber, so good grip is maintained. However, heave and pitch motion will change camber. Again, this best fixed with anti-axle-bounce springing (as above).

Side-View-Virtual-Swing-Arm is infinitely long, giving zero castor change wrt body. Horizontal ISA means horizontal longitudinal (side-view) n-lines, so 0% anti-dive/squat, which is actually quite good.

Pure Leading or Trailing Arms.
=========================
Front-View VSA is now infinitely long, so 0% camber compensation and any body-roll adversely affects wheel camber (but no gyro forces over bumps, so good off-road). Body-roll best fixed with side-pair longitudinal Z-bars, NOT end-pair lateral U-bars like normal ARBs (= bad). Lateral n-lines are always horizontal wrt body, so very predictable handling, even though RCs shoot of to infinity with slightest body-roll!

Side-View VSA now very short, so lots of castor change, but this has surprisingly small adverse effects. Longitudinal n-lines dependent on whether drive or braking is inboard or outboard. Generally, with outboard brakes (eg. at front), the ISA should be at floor level. With inboard drive/braking (eg. at rear), the ISA should be at axle height, or slightly higher. This keeps all side-view n-lines close to horizontal, or sloping slightly up-to-centre-of-car for some front anti-dive and rear anti-squat.

Semi-Leading or Trailing Arms.
=========================
These are somewhere between the above two types. They allow the option of varying front and side-view VSA lengths, although not so that both are very long at the same time.

If the wheel is at a non-zero camber angle, then this suspension will give toe-change (= bump-steer) even if the axis is horizontal (it is a castor/camber coupling effect). This can be a problem for long travel suspensions (such as passenger cars), but should be no problem at all in FSAE.

And a lot of other stuff... But moving on...
~~~~~o0o~~~~~

HOW TO DO IT.
===============
Although Semi-Leading/Trailing-Arms have possibly the worst (most compromised) kinematics of the various suspension types, they probably fit in best with the rest of an FSAE car, so are the simplest overall solution. So these are my "second favourite" solution after Beam-Axles. They are structurally the simplest, so potentially the cheapest, quickest to build, and most rugged. And the dodgy kinematics don't really matter because of the very short suspension travel.

So I would have the arms attaching to the chassis close to floor level (because already strong structure down there). Each arm would have one BJ on the centreline of the car just under its axle line, giving 100% camber compensation. The front arms would have their second BJs attach to the sides of the chassis close to the front roll hoop. Similarly, the rear arms would attach either side of the seat-back/main-roll-hoop. The kinematics would thus be closer to that of the Lateral-Swing-Arms (ie. pivot axis close to longitudinal).

I would fabricate the Swing-Arms out of sheet-steel (1 to 2 mm thick) as a hollow structure of roughly overall "V" or "Y" shape (picture wheel at bottom of V/Y, chassis at top). The hollow shell would be roughly 80 mm diameter at the wheel, tapering to maybe 20-30 mm at each chassis BJ. See Twin-I-beams in earlier posts as rough idea, but hollow, and attached to centreline of chassis rather than "crossed".

Note that for a given wall thickness and length, the five, say, 16 mm diameter tubes of a DW or 5-Link weigh exactly the same as a single 80 mm diameter tube. And this larger tube doesn't buckle whenever it hits a squishy little rubber cone. A shell-like structure would also suit carbonfibre, but not my choice. I also think all four arms could be identical, with an adjustable, three-bolt connection to the front-kingpin, or rear-bearing-housing (might post pic later...).
~o0o~

Or the Swing-Arm could be an evolutionary simplification of a DW. Just weld the inner ends of the upper-wishbone to the tops of the lower-wishbone. Toss the upper-wishbone-inner-BJs, and all that messy chassis structure they connected to. Also toss all the outer BJs at the rear of the car (the front uprights can stay same). The kinematics are the same as with any short FVVSA double-wishbone suspension.

Would that work?

Well, the HyperProRacer (http://www.hyperracer.com) has just this sort of suspension at the front, and is at least as fast as the faster FSAE cars (search their website). And similar Swing-Arms have already been tried in FSAE, as seen on the "Lancaster Link" (http://fsae.com/eve/forums/a/tpc/f/125607348/m/190100615/p/1) thread, although I'm not sure of their performance.

Most interesting on the Lancaster thread is the almost universal dismissal of the idea, including by officials who should know better! (The only positive comments come from Pete Marsh, who makes similar suggestions to above.) Much of the negativity is spurious, such as the many comments regarding RCE magazine's claim of "the greatest innovation in 40 years".

Unfortunately, even more of the criticisms are completely wrong (frankly, plain stupid!), such as the many claims that the pushrod will be under very high load. Honestly, the simplest of FBDs, or even just a quick inspection, shows that the pushrods are under very similar loads to typical DW suspensions.

When I see that sort of fallacious codswallop being peddled as the main argument against something new, then I really start to worry about YOUR futures!
~~~~~o0o~~~~~

Much more to say, but maybe later...

As always, comments and (FBD'd!) criticisms welcome... http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

Will M

12-14-2012, 09:47 AM

Z,

Any chance we could get some of your excellent drawings of some of these designs?
I don't know how long they take you, but they are always impressive http://fsae.com/groupee_common/emoticons/icon_smile.gif

-William

Sormaz

12-14-2012, 03:25 PM

Originally posted by Z:
Ian, MCoach,

Here is an earlier thread about 4 Wheel Steer. (http://fsae.com/eve/forums/a/tpc/f/125607348/m/81720309051?r=81720309051#81720309051)

Thinking about it a bit more now, I still reckon that with the current Rules the biggest gains are available from better aero. But if the Rules were changed to greatly lessen aero, then I would probably go for greater "agility" from something like 4WS, before chasing ultimate horsepower, 4WD, etc. The mechanical cost/complexity of 4WS is quite small. It can be done with any of the simple suspension types, probably using very similar suspension components at each corner.

Ergonomically, the easiest for the driver might be two big thumb-actuated buttons on the steering wheel. When neither button is pushed the steering is normal "front-only". Pushing and holding the green button gives stable "same-steer" at the rear wheels, for low-yaw side-stepping through the slaloms. Very briefly pushing and holding the red (danger!) button gives small rear counter-steer, to get around the super-sharp hairpin.

The only way to know for sure is to build it, and then extensively test it. This would make a great "parallel design project" (using last year's car) for a well resourced team, and should be a lot of fun! http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

Instead of buttons, how about a steering wheel that is free to translate in the y direction. push the wheel to left points the rear wheels to the left, pushing it to the right points the rear wheels to the right. This would actually be a pretty simple linkage and gives the driver full independent control of the front and rear wheels for any combination and any type of 4WS or 2 wheel steering

MCoach

12-24-2012, 10:23 AM

The one thing that still has me hesitant about swing arm suspensions is the high roll center that comes with it and the jacking forces that follow that. Because of the high roll center, and the Corvair comes to mind on, the suspension may have a tendency to tuck under due to the very short swing arm length.

I may be missing a key part here...

Mounting them low as possible, and with the mentioned anti-bounce, then it might be effective.

01-02-2013, 03:24 AM

MCoach,

Any of the Swing-Arm suspensions (ie. Lateral, Longitudinal, or Semi-) can have the "RC" at pretty much any height (though this might require a non-horizontal pivot axis).

Considering specifically the Pure Lateral Swing-Arm, or "Swing-Axle" (with pivot axis parallel to car centreline), these initially had the pivot axis at axle height for ease of drive from the differential (ie. Tatra "jointless" drive, early 1920s). Tatra's off-road trucks still use this design.

The Auto Union V16 Grand Prix cars of mid-1930s also had these, and won many races at 300+kph, travelling on roads similar to current World Rally Racing. Likewise the very successful early 1950s Mercedes 300SLs (W198).

However, as you say, the high RC with conventional long travel springing suffers from jacking problems. So Auto Union in the late 1930s switched to a De Dion rear axle. Mercedes in late 1950s and 60s modified their swing-axles to "low-pivot" (about floor level, so RC still ~0.2m above ground), together with a "compensating spring" (= "axle-bounce" spring).

Note again, much of the racing in those times was more Baja or Dakar than modern circuit racing. Modern Formula Vees, with their mandated axle height RCs, seem to manage just fine on modern smooth circuits with or without "compensating springs". Short travel conventional springs can work, although a single axle-bounce spring with zero roll stiffness works much better (it gives a very soft twist mode).

However, all things considered, I prefer a floor level low-pivot RC. Mainly because there is already good structure down there to take the Swing-Arm forces. Also good for low CG. The Lancaster Link (below) is reasonable, though a wider base to the arms would be better (ie. move front BJs to seat-back/main roll hoop, and rear BJs to under diff). And use direct acting spring-dampers. And a few other things... http://fsae.com/groupee_common/emoticons/icon_smile.gif

http://www.noots.org.uk/fs/images/lancasterlinks.jpg

Z

(PS. Note that the LL doesn't seem to have, or need, an outer CV. Possibly just a spline?)

MCoach

01-08-2013, 03:26 PM

At this point, still hesitant on considering swing arm as a viable design, because of several key issues in my mind. I know most people over think their roll centers and blabber on about the horrors of roll center migration, when they talk about suspension design, but some parts of this stands out to me in being able to take care of an outstanding issue with almost every car design put forth so far. It is an issue that is not present in any other series (that I am aware of).

I'll go into my concern in more detail here. The tires that are used at this level are very load insensitive compared to most other styles and competitions, which may make it a lot easier to get things closer to 'right'. This means that CG height does not play that huge of a role in determining which car will be competitive. There is only a slight difference between a car built with a CG of 12" and 9". Lateral load capability doesn't change all that much. Unsprung masses also don't change the effect that much. So, optimization of weight there by removing a ball joint and maybe a link become irrelevant. By far, I would rather have a 402 lb car that is able to put the wheels right where I want them, how I want them, compared to a 390lb car that I know is making sacrifices with camber rolling back and fort as the car is thrashed about in.

The high roll center of ~0.2m (7.8 in) is still well above what I would consider for these cars. This bothers me because of the further unequal loading that is caused by cornering.
This also means that the cars may respond too quickly for the driver, considering the CG height:RC ratio.

Yes, many of the kinematic issues of suspensions that have been used before the near unanimous double wishbone can be solved through additions here and there, but they seem to be not much more than patches.

My overall concern with these cars is the fact that the cars seem to be too wide for the tracks. This becomes more notable the more chicanes are added. Things slow down very quickly. I don't want to run an increased track width because my camber gain and roll center values say otherwise. Transience is the best way to see these things, and typically some of the best performers in skidpad are the best overall. I don't think this is a coincidence, but more so an overlap of who is making the best choices of what is 'right' for these courses. Anyone who has taken a formula SAE car, go-kart, or similar vehicle around a typical SCCA course can admit the difference of a narrower wheelbase than everyone else makes. Those chicanes? Point straight and continue about your day. It takes a hell of a lot of time off of the time it takes to negotiate left, right, slow down, etc.

So, that is my main concern. This is why you see teams like GFR, Washington, several of the Germans, Austrians, and Australians as well lifting two wheels coming around corners. The track width is kept to a minimum to keep these things nimble as possible, even if the prospect of roll over may be a twitch of the steering wheel away.

01-15-2013, 05:39 PM

MCoach,

"The high roll center of ~0.2m (7.8 in) is still well above what I would consider for these cars. This bothers me because of the further unequal loading that is caused by cornering.
This also means that the cars may respond too quickly for the driver, considering the CG height:RC ratio."

I assume you are referring to the "high RC" of a diff-height pivotted Swing-Axle. Honestly, this is NOT A PROBLEM.

The proof can be found anywhere where Formula Vees race. F-Vees are only slightly bigger than FSAEs. About 1.2m track and ~2m? wheelbase. Similar CG height. 1.5-2 x mass. Similar power. And their team members know about as much (or as little?) Vehicle Dynamics, etc., as FSAEers. Yet the cars themselves corner as fast as, and handle as well as, any other formula style car on similar tyres.

If an FSAE team thought it could do a really simple, light, rugged F-Vee style rear Swing-Axle, then I have no doubts whatsoever that they could extract just as much performance from their tyres, and the car as a whole, as any other FSAE team. (BTW, the best approach, IMO, is a single axle-bounce spring with zero elastic roll stiffness at rear. So all LLT at front is through the springs, and all (or most) LLT at rear is through the high RC. This coincidentally also gives a soft twist mode, which makes handling more predictable on undulating circuits. So it is BETTER than conventional suspensions!)
~o0o~

"Yes, many of the kinematic issues of suspensions that have been used before the near unanimous double wishbone can be solved through additions here and there, but they seem to be not much more than patches."

The more you will learn about conventional double-wishbone suspensions, the more you will see that they are riddled with compromises. These are usually inadequately addressed by "additions here and there" and "patches".

One example is the complicated hydraulically interconnected ARBs used to soften the twist mode ... err ... which has been stiffened by ... errrrr ... the ARBs which ... err ... where added to stiffen the roll mode ... http://fsae.com/groupee_common/emoticons/icon_confused.gif

Another example is the various complicated attempts at "camber correction" aimed at getting good camber for an axle in both roll and ride. These work ... err .. sort of like ... errrrr ... the much simpler beam-axles! And many more examples... http://fsae.com/groupee_common/emoticons/icon_smile.gif
~o0o~

"My overall concern with these cars is the fact that the cars seem to be too wide for the [race] tracks. This becomes more notable the more chicanes are added.
...
The [car] track width is kept to a minimum to keep these things nimble as possible, even if the prospect of roll over may be a twitch of the steering wheel away."

I agree that having a narrow car is a benefit in FSAE. But this really has little direct influence on the type of suspension you might choose. Remember, "all suspensions will work (the same), if you don't let them...".

The two main things you should aim for if building a narrow track car are;

1. Low CG.
Half-Track/CG-height should be greater than Max-Tyre-CF, or else rollover! A suspension type with as many of its parts low down helps here. The ubiquitous high-mounted, pushrod-operated spring-dampers are a wank!

2. Shed-loads of aero downforce!
For the same Max-Tyre-CF you can keep the same "distance" away from rollover, while going much faster around corners (ie. higher lateral Gs). Combined with 1 above, this suggests an aero undertray.
~o0o~

BTW, the Swing-Axled Tatra trucks (http://www.tatratrucks.com/why-tatra/tatra-vehicle-design/tatra-vehicle-design-1/) are doing well in the currently running Dakar. (http://www.dakar.com/) Pivot height of SA, and so also RC of these independently suspended trucks is about 0.6m at ride height.

Z

(Edit: Here is a pic of above.)
http://www.tatratrucks.com/cache/images/galleryPreviewBig/02_centralni-nosna-roura.png

murpia

01-16-2013, 05:08 AM

I have a question somewhat related to the swing-axle concepts:

Does anyone know of a commercial source of driveshaft joints (Cardan or otherwise) capable of transmitting suspension loads axially?

Think Jaguar IRS: http://en.wikipedia.org/wiki/Jaguar_IRS or Formula Vee (Google Images for 'formula vee rear suspension').

Something light enough for FSAE, obviously.

Thanks, Ian

Warpspeed

01-16-2013, 12:08 PM

Both Jaguar and some Lotus cars used the driveshaft as an active upper suspension link.
Both used ordinary Cardan type joints.

http://t3.gstatic.com/images?q=tbn:ANd9GcROZnxvxnvDYZ95XtZCfRx8CuKmxU3Bm Clo2HHkZbvC-jxcSe-rTiuxWmg

Zac

01-18-2013, 06:06 AM

Originally posted by Warpspeed:
Both Jaguar and some Lotus cars used the driveshaft as an active upper suspension link.
Both used ordinary Cardan type joints.

http://t3.gstatic.com/images?q=tbn:ANd9GcROZnxvxnvDYZ95XtZCfRx8CuKmxU3Bm Clo2HHkZbvC-jxcSe-rTiuxWmg

GM used the same arrangement on the Corvette from 63-82.

murpia

01-18-2013, 06:12 AM

Clearly it's a technique that can work well.

In those 3 applications, Lotus, Jag & Corvette, I assume a Cardan joint was specifically engineered for the job.

For FSAE, it might be possible to find something off the shelf. Does anyone have any suggestions? Maybe there's an ATV, golf buggy, shaft drive motorcycle or something else that has a part like this?

Regards, Ian

rjwoods77

01-18-2013, 06:49 AM

University of Wisconsin Madison's Mini Baja team did this many years (2000?-2006?) and very successfully. It had an upper a-arm then then lower a-arm was the driveshaft that used what looked like over the counter universal joints. The shaft was maybe 2" OD. To be honest it looked like a shortened driveshaft from a 1980's RWD Toyota. Garner what you can from this...

http://www.youtube.com/watch?v=tre5YUEcBFE

Warpspeed

01-18-2013, 11:46 AM

Originally posted by murpia:
Clearly it's a technique that can work well.

In those 3 applications, Lotus, Jag & Corvette, I assume a Cardan joint was specifically engineered for the job.

The Cardan joints themselves are all simple off the shelf standard automotive "tailshaft" universal joint components.
The driveshafts themselves are either of standard ordinary tubular tailshaft construction, or special solid steel shafts with forged eyes to hold the needle roller cups.

Nothing at all special. The inboard end can be shimmed to adjust wheel camber, and look just like any bolt on rear tailshaft flange.

I originally owned a Lotus Europa with a system like this, it is very simple, and a local driveshaft shop even made me up a pair of new driveshafts for me at one stage.

Some of the Cardan joints used on small European and Japanese cars are not all that large, and a pair of slim fairly light weight shafts could be put together at very low cost, with very easily obtained new or secondhand parts.

If you plan to do it this way, IMHO Lotus have some of the cleverest ideas of how to go about it.
It is probably one of the simplest and easiest to make IRS systems possible, with the fewest parts, and it can have excellent wheel location and geometry.

01-18-2013, 04:53 PM

Originally posted by murpia:
Does anyone know of a commercial source of driveshaft joints (Cardan or otherwise) capable of transmitting suspension loads axially?
...
Something light enough for FSAE, obviously.
Ian,

Think about it like this. The maximum horizontal force at the tyreprint, say Fh, is pretty much the same laterally and longitudinally. All the cars considered (Jag -> FSAE) should be able to spin their rear wheels under power, and so generate Fh at the tyre. So, if the Hookes joint (= UJ or Cardan shaft joint) has its two bushes about 5 cm apart, and wheel radius is about 30cm, then the force at each UJ bush during hard acceleration is about 6 x Fh.

Now consider cornering with the driveshaft+UJs as the upper lateral link, and the lower wishbone about half way down (at 15cm height). Now the driveshaft has a tension load on it (for outside wheel) of about 1 x Fh. Furthermore, this load is shared between two UJ bushes, so each only carries about 1/2 x Fh.

This is only a rough calc, but it shows that because of the various leverages involved it is the "driveshaft-under-accelerating-torque" that puts by far the biggest loads onto the UJs. So for FSAE, any smallish, off-the-shelf UJ should do. http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

(PS: Make sure both UJs at the ends of the shaft are oriented the same way, so they cancel out their "non-CV" behaviour.)

Markus

01-19-2013, 12:18 AM

In driveshaft-as-a-suspension-link configuration I would be more worried about achieving the load carrying capacity and stiffness onboard - i.e. differential / spool axle bearings and carriers.

Beefier mountings and bearings quickly bring the weight up (and over?) the level of seperate driveshafts and suspension links for the same stiffness.

Warpspeed

01-19-2013, 12:59 AM

Ah ! But you need driveshafts anyway, so you gain a free zero weight suspension link for nothing.

Locating the diff properly, and providing some decent diff carrier bearings that will accept some thrust loading, should not present any huge obstacle.

An adequate driveshaft will very likely have more lateral strength and stiffness than many of the frail spidery suspension links we are used to seeing.

Markus

01-19-2013, 02:27 AM

Dig deeper Tony.

I can weight and calculate you some exact numbers later but our suspension links weight around 100g per link. Beefier bearings, diff mounts and tripod -> UJ change could easily mean +250g per side...

Warpspeed

01-19-2013, 12:31 PM

Why should designing laterally stiff differential bearings have to weigh any more ?
It's just a matter of straight load paths to the frame.

And how much heavier are a pair tapered roller bearings over a pair of deep groove ball bearing for the diff carrier?

Your driveshafts still need to have articulated and sliding joints, why should a pair of Cardan joints be significantly heavier than CV or tripod joints ?

As Z has already pointed out, if the driveshafts are up to scratch anyway, they will be more than sufficient to locate the wheel without needing to be any heavier.

01-19-2013, 05:04 PM

Agree with Tony.

And there is a little bit less chassis to build (ie. the part that the lateral link attached to).

And a little bit less upright (ie. don't need the top part anymore - see Jag).

Z

mech5496

01-20-2013, 02:15 AM

Random (possibly stupid) thought: building an "upside down" Jag-style IRS allows you to ditch lower Aarm and therefore cleaning up some space in between the rear wheels (think aero). Wonder if camber compliance could be treated somehow though...

Markus

01-20-2013, 04:35 AM

Tony, Z, forgive me for posting the information bit by bit as I have a lot on my hands right now.

I did a quick scaling and our a-arms come at an average of 130 grams per a-arm. Modified upright's would remove some 30-50 grams per side so we're looking at around 350 grams of reduced weight.

Depending on the rulebook there is more or less benefits on the frame. Considering FSG rules (standard for European teams?) pretty much all that chassis is already there - with regular FSAE rules you can shave some tubes / 'coq to further reduce weight.

Our diff bearings weight a total of 140 grams with a hefty FoS. I didn't do any calculations but I'd estimate double row angular bearings good for dynamic axial loading could be found for around 250 grams total. Also single row angular contact bearings would be an option (wheel bearing style solution). Tapered roller bearings seemed to be out of the question or I just couldn't find any of reasonable dimensions.

In seperate driveshafts / suspension links configuration the differential is quite insensitive to lateral stiffness so the mountings can be fairly light. Then again lateral support can be taken into account in quite a reasonable manner so the added weight could stay around 50 grams per side.

I have no time to calculate differences between UJ's and tripods but my quick assumption was based on tripod being lateral stiffness insensitive and having one more contact per joint than UJ's.

So we have 25 grams per UJ to use to break even and any compliance in the UJ would mean suspension compliance... Hard to say where that would end up.

There's potential. And also for disaster.

mech5496

01-20-2013, 05:11 AM

Markus,

I like your analysis. The thing is I cannot see any advantage over a more traditional layout, except maybe from my "stupid" aero advantage as described above.IMO it is extra risk and complication (possibly).

Markus

01-20-2013, 05:25 AM

I forgot to add I'm eager to see Delfts '13 car as they did something I'd consider more beneficial than using driveshafts as suspension links: they designed a car without driveshafts. http://fsae.com/groupee_common/emoticons/icon_smile.gif

AxelRipper

01-20-2013, 07:30 AM

Originally posted by Markus:
I forgot to add I'm eager to see Delfts '13 car as they did something I'd consider more beneficial than using driveshafts as suspension links: they designed a car without driveshafts. http://fsae.com/groupee_common/emoticons/icon_smile.gif

Well that would be a fun design, but the intake sealing to get the 4 gas hub motors through one restrictor while still allowing for suspension travel seems like a nightmare...

murpia

01-20-2013, 09:02 AM

Originally posted by Warpspeed:
Why should designing laterally stiff differential bearings have to weigh any more ?
It's just a matter of straight load paths to the frame.

And how much heavier are a pair tapered roller bearings over a pair of deep groove ball bearing for the diff carrier?

Your driveshafts still need to have articulated and sliding joints, why should a pair of Cardan joints be significantly heavier than CV or tripod joints ?

As Z has already pointed out, if the driveshafts are up to scratch anyway, they will be more than sufficient to locate the wheel without needing to be any heavier.
It may be helpful to put my suggestions into the right context...

For some time my vision of an ideal FSAE package has been fully stressed engine & transaxle, so the idea of feeding lateral suspension loads through the driveshafts is simply a repositioning of existing loads within that structure.

I would be very tempted to go (dual) inboard brakes with this setup, to minimise the upright design. Also this would remove the suspension loads from the torque reaction of the brakes.

Probably I'd compare a swing axle with some form of 5-link design (well, 4-link plus driveshaft). I like the warp-soft / heave-stiff Formula Vee idea quite a lot.

The crunch will be toe & camber compliance, there can be no compromise there. I'd add 5kg to a car if necessary to ensure those...

Regards, Ian

Warpspeed

01-20-2013, 12:40 PM

Originally posted by mech5496:
Random (possibly stupid) thought: building an "upside down" Jag-style IRS allows you to ditch lower Aarm and therefore cleaning up some space in between the rear wheels (think aero). Wonder if camber compliance could be treated somehow though...
Many possible ways to do it...
Almost any suspension layout could be adapted to replace one link with the driveshaft, even the ubiquitous double wishbone suspension.

Warpspeed

01-20-2013, 06:11 PM

Originally posted by murpia:
I would be very tempted to go (dual) inboard brakes with this setup, to minimise the upright design. Also this would remove the suspension loads from the torque reaction of the brakes.
Ian
Inboard rear brakes will also dramatically reduce unsprung weight.
I am working on an inboard rear brake project right now, but I am also mindful of losing all rear antidive doing it this way.
The back is going to come up a fair way under very hard braking, but I think I can live with that...

MCoach

01-21-2013, 08:26 AM

Originally posted by Warpspeed:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by murpia:
I would be very tempted to go (dual) inboard brakes with this setup, to minimise the upright design. Also this would remove the suspension loads from the torque reaction of the brakes.
Ian
Inboard rear brakes will also dramatically reduce unsprung weight.
I am working on an inboard rear brake project right now, but I am also mindful of losing all rear antidive doing it this way.
The back is going to come up a fair way under very hard braking, but I think I can live with that... </div></BLOCKQUOTE>

Dramatically reduce unsprung weight how?
I think proper upright design is worth more than moving the brakes inboard.

our rotor and caliper package weighs 1.375lbs (625g avg) per corner.

Including fluids, valves, pedals, and lines system weighs 7.25 lbs on the entire car.

EDIT: did my unit conversion wrong. 625g per corner.

mech5496

01-26-2013, 03:13 AM

Regarding Tatra's swing axle design, today was the first time I saw a Tatra truck up close; a military vehicle of the 60-70's I suppose. The first thing to note (even before I noticed it was a Tatra truck) was the excessive jacking which resulted in really funny camber angles on the tires, when the driver decided to do a U-turn right in fron of my eyes, at a really low speed. I shall note though that the RC is about a meter high on that thing.... http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

01-28-2013, 03:23 AM

Originally posted by mech5496:
Regarding Tatra's swing axle design...
The first thing to note (even before I noticed it was a Tatra truck) was the excessive jacking which resulted in really funny camber angles on the tires, when the driver decided to do a U-turn right in front of my eyes, at a really low speed.
Harry,

I'm only guessing, but the above might have been a good example of not enough Ackermann in the steering (ie. not enough dynamic toe-out). That is, the front wheels might have been effectively toed-in during the tight turn, and this squeezed their lateral n-lines together, jacking the body up. You saw the normally dynamic jacking effect in slow motion.
~~~o0o~~~

Regardless of jacking, the Swing-Axle Tatra's did reasonably well in the recent 2013 Dakar (5th and 6th IIRC). Their times were only a few percent behind the fastest cars/buggies.

Interestingly, the Tatra's with their swing-axles are the only independently suspended trucks in the race. The others, including the winning Kamaz's, all have beam-axles. Not a double-wishbone in sight! And, as mentioned, these 10+ ton monsters are very nearly as fast as the fastest cars. http://fsae.com/groupee_common/emoticons/icon_smile.gif
~~~o0o~~~

Some social commentary:

The Russian Kamaz's took 1st, 2nd, and 3rd. Not surprising given that heavy duty off-road truck racing seems to be Russia's favourite motorsport. The fact these commies were sponsored by Red Bull, the archetypical capitalist company, probably helped.

The private, but very big bucks, Dutch DeRooy/Iveco team of 5 trucks managed to get their lead driver home in 4th place.

Even though they now have significant American shareholding (Terex?, %?), Tatra has only run fairly low budget teams in recent years. They still do well, but are not the regular winners that they were in the pre-US investment days.

Makes me wonder what is the difference between capitalism and communism these days? http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

Xavier ABRAM

01-28-2013, 10:44 AM

Z,

for your information Russia is not communist since 1991. See wikipedia and get ramped up on history.

"The others, including the winning Kamaz's, all have beam-axles. Not a double-wishbone in sight! And, as mentioned, these 10+ ton monsters are very nearly as fast as the fastest cars."

And if you want to compare, at least compare vehicules that operates under the same rulebook. It will limit a lot of variables to enable a true comparaison.

FYI :

... Chagin, Kabirov, and Ilgizar Mardeev are busy shifting the 16-speed ZF transmissions of their race trucks to keep the 730hp, V-8 diesel engine in its powerband ...

Read more: http://www.dieselpowermag.com/...l.html#ixzz2JIRAoUvw (http://www.dieselpowermag.com/features/1008dp_dakar_rally_kamaz_dakar_t4_race/viewall.html#ixzz2JIRAoUvw)

A Mini All4 has about 315 hp...

MCoach

01-28-2013, 10:57 AM

I have a feeling Kamaz may have won even without Red Bulls support.

They do have 11 wins over the past few years to their name, essentially dominating the race.

Back to the swing axle. The problem I'm seeing here with the swing axle is there are two things fighting each other. On one side, you need to mount the inboard points low enough to get a reasonable roll center. On the other side, the further you move the pivots away from the axle, the more plunge depth that is needed.

Thus, the problem you run into is that once the roll center is reasonable, the plunge depth is extreme.

Thefore, for this competition, with the amount of control available from the double wishbone suspension and freedom of design, I may have to once again side against the swing axle.

Max Trenkle

01-28-2013, 11:28 PM

Motion to sticky!

01-29-2013, 09:09 PM

Originally posted by Xavier ABRAM:
... Russia is not communist since 1991. See wikipedia and get ramped up on history.
Xavier,

I should use the interweb to learn about politics/history? That's funny! http://fsae.com/groupee_common/emoticons/icon_biggrin.gif
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~oooooo0ooooooo~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~

Originally posted by MCoach:
I have a feeling Kamaz may have won even without Red Bulls support.
MCoach,

I am also quite sure they would have won. Red Bull is simply good at picking winners, like backing F1 teams that have Adrian Newey in them.

I have great admiration for Russian engineering. Their approach seems to be very results focussed, with little regard for current fashions. Instead they seem to identify the most important factors for success, do extensive testing (usually to destruction), then improve and repeat...

IMO this is exactly the approach FSAE teams should take. See below for an example.
~o0o~

"Back to the swing axle...
... the problem you run into is that once the roll center is reasonable [low], the plunge depth is extreme."

Plunge-Depth-of-Inner-CV = ~ Suspension-travel x RC-to-CV/Half-Track.

For a Lancaster type suspension, RC-to-CV = ~20cm, and Half-Track = ~60cm, so for 6cm suspension travel (~2.5") the Inner-CV only needs about 2cm plunge (less than 1").

I don't see any problem at all? If you make your own tripod-CV outer cages, then even 5cm plunge should not be too hard.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~oooooo0oooooo~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~

An example of how FSAE design might (should?) be approached: (Note: Some facts below may be false due to laziness in checking...)

A BRIEF HISTORY of the WWII RUSSIAN T-34 TANK.
=============================================

This tank is widely regarded as the best tank of WWII, and as such, was a dominant factor in the final result. See this amusing 5 minute video (http://www.liveleak.com/view?i=dce_1271264917) for one take on it.

This story should start with American engineer/inventor John Walter Christie. In 1907 Christie entered a car in the French Grand Prix with a transversely mounted 20 litre V4 engine driving the front wheels, and Sliding-Pillar front suspension. This may have inspired another competitor, a Mr Vincenzo Lancia, to use something similar on his 1920s Lancia Lambda. The similarity of Christie's car to most modern passenger vehicles (transverse engines and McP-Struts) suggests Christie was a little ahead of his times.

Some unpleasant accidents discouraged Christie from further motor racing. So in the 1920s he offered the US military his ideas on better tank designs. The first prototypes with conventional leaf-spring suspensions were too slow for Christie's tastes, so he developed the "Christie Suspension". (http://en.wikipedia.org/wiki/Christie_suspension) This was a coil-sprung, Trailing-Arm suspension, carrying large diameter wheels that spanned the gap from top to bottom of the tracks.

This new suspension gave Christie's tanks great speed over rough ground, which suited his philosophy of tanks as high speed, long distance, strike weapons. Unfortunately, the US Top Brass had the opposite philosophy. Like so many "experts" everywhere, they were stuck in the past. They believed tanks should be really big, and heavy, and slow, ... just like in WWI! So Christie's prototypes were rejected.

Heavily in debt, Christie looked to recoup some of his money. Fortunately, in 1930 a Russian delegation from some sort of "Bureau of Agricultural" arrived and bought two of the prototypes at a generous price. Without the turrets of course, because "Uz only vor ur gloriooos farmers!".
~o0o~

Through several generations of prototypes and models, such as the mid 1930s BT "Fast Tank", the Russians improved and ruggedized the Christie design. They also fitted much thicker armour plating than usual at the time, and also a considerably bigger gun. For an engine, they were, ahem... "inspired" to blend the prototype's original Hispano Suiza V12 aero engine, and a more modern Fiat L6 direct-injection, diesel aero engine. The 1939 Battle of Nomonhan (River Khalkin) against Japan provided an excellent testing ground for some of these developments.

So, come "The Big One" (WWII), and the Russians were well equipped, tank-wise. The T-34 had a big gun and was well protected. It could travel at high speed over very rough ground. And, very importantly, it was of such a simple overall design that it could be manufactured very cheaply, in the tens of thousands, and by unskilled labour. Indeed, all the contemporary Allied and German tank designers criticised it for its extreme crudeness. It needed a mallet just to change gears! (Some say (imagine J. Clarkson voice) that the whole tank could be disassembled and reassembled using same said mallet! http://fsae.com/groupee_common/emoticons/icon_smile.gif)

But most Allied and German Generals considered it a "very good tank". Perhaps its greatest strength was its diesel engine, very unusual for the time. Not only did this fuel NOT blow up at the slightest provocation, but it gave the T-34 a range of 300+km over rough ground. The best the other tanks could manage was about 200km on good roads. When you find yourself surrounded by dozens of enemy tanks, and the fuel gauge reads empty, ... well, ... that's game over. http://fsae.com/groupee_common/emoticons/icon_frown.gif

If you read up on it, you will find that a great many T-34s were lost due to mechanical failure. That was the price of dumbing down the design to extreme crudeness. But then again, this crudeness allowed them to be produced in far greater numbers than their opposition. The end result was that the T-34s won the war (according to some...).
~o0o~

What's all this got to do with FSAE?

Well, the T-34 design decisions, like extreme simplicity, heavy armour, high-speed suspension, and diesel engine, were all "big picture" decisions that were quite unusual at the time. They were innovative decisions, despite the fact that some of them were "borrowed". They were the Level 4&3 decisions in Geoff's (Big Bird's) "Reasoning..." thread. And they were the decisions that made the biggest difference.

There was very little "detail optimisation" of the T-34, at least not in the FSAE sense. Instead, just an early identification of what is most important to performance, and then endless testing and iteration to achieve those goals.

And that should also work in FSAE! http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

Warpspeed

01-29-2013, 10:10 PM

Haha Z, how true.

http://4.bp.blogspot.com/-OtvptC8k1QI/T6b2ilEWttI/AAAAAAAADyg/_Y7uj1zfb8g/s1600/924mousetrap.jpg

An international design competition to better this simple device was run several months ago.

The judging panel, composed entirely of mice still cannot stop laughing....

ben

01-30-2013, 04:22 AM

Z's last post:

Mock someone for suggesting Wikipedia as a reference.

Link to Wikipedia article to support own view... ;-)

Ben

Max Trenkle

01-30-2013, 07:46 AM

You know, the only people that cry foul at new and innovative suspension designs seem to be those who either don't understand the subject at all, or understand one type of setup and have no desire to learn about the other setups.

(I used to be one of these people... sorry)

Z thank you for covering so much history in suspension design. I have no clue why or how universities expect engineers to learn ANYTHING without history on what's already been done. http://fsae.com/groupee_common/emoticons/icon_frown.gif

The pattern seems to be that someone with a new suspension/chassis/packaging design gets berated by the world around them, and then eventually when they're old or dead someone figures out they were on the right track.

DougMilliken

01-31-2013, 06:18 AM

...The pattern seems to be that someone with a new suspension/chassis/packaging design gets berated by the world around them, and then eventually when they're old or dead someone figures out they were on the right track.
Maurice Olley was kind enough to leave behind his Notes on Suspension and other related topics (written after his retirement). Unfortunately, the Corvair litigation was going on at the same time and General Motors embargoed his Notes. It took some time before Bill/Dad and I were able to finally get permission to publish them--which had always been Olley's wish. This became "Chassis Design". While Olley doesn't cover every conceivable type of suspension, he hits many of the popular ones with a lot of details. Our page includes a link to the SAE bookstore,
http://www.millikenresearch.com/olley.html

Max Trenkle

01-31-2013, 09:34 PM

I literally read what you just told me in "Chassis Design" a week ago. I got the book for Christmas. http://fsae.com/groupee_common/emoticons/icon_smile.gif

I'm really enjoying the heck out of the book, and I really think it's fantastic the way you took his work and put in updated graphs and drawings as well as your own words. Also, the clarifications are quite nice where he quotes rules of thumb that are outdated.

I have read hundreds of books in my life, fiction and nonfiction, and this one is one of the highlights. Well done sir!

MCoach

01-31-2013, 09:37 PM

Doug,

When I run into a topic from RCVD that I feel should be filled out a little more on the background I turn to that book. Lately, I find myself falling back on Chassis Design to fill out the history. The work that Bill, Maurice, and the rest of the research group accomplished still quite surprises me. It does help give a much larger picture to reference when the two books are put together. And a much better perspective when someone comes in saying random stuff like, "Guys! We should run 4WD, leaf springs, and drum brakes! It worked it in the 50's!"

As a reader of Chassis Design, I would say it is one of the most comprehensive books on the history of suspension design for those interested. On top of that Olley notes the problems faced with the applications of each suspension type and how a solution came about (such as wheel shimmy in beam axles and excessive jacking in swing axles).

mech5496

01-31-2013, 09:53 PM

What MCoach said. It is still quite impressive how well-structured and defined is their work, and it helped me understanding many things RCVD left out. In a word, read it! http://fsae.com/groupee_common/emoticons/icon_wink.gif

Max Trenkle

01-31-2013, 10:01 PM

...it helped me understanding many things RCVD left out. In a word, read it!

+1

They are surely meant to compliment each other, as suggested. http://fsae.com/groupee_common/emoticons/icon_smile.gif

02-06-2013, 06:06 PM

Regarding Low-Pivot Lateral Swing-Arms.
===============================

Here is a picture of a 2004 double-wishbone car that recently appeared on another thread.

(Courtesy of James Waltman... "Univ Wisconsin-Madison, Another one of my favorites...")
http://dot.etec.wwu.edu/fsae/James/Detroit%202004/Other%20Teams/U%20Wisc%20Mad%20rearsmall.jpg

I imagine that this was described by the team, and agreed upon by the judges and officials, as a sensible choice of "a short FVVSA that gives 100% camber compensation in roll", because this suits the FSAE conditions of predominantly lateral roll forces, with lesser occurance of longitudinal pitch, or vertical heave forces. That is, it gives the right wheel camber angles in the many corners, but less so when accelerating/braking hard, or over the very rare "bumps".

The kinematic properties of this car are all but IDENTICAL to the Lancaster Link car shown previously. The above layout also has very similar mass and CG height as the LL (ie. very similar layout of suspension tubes). However, the above car's cost and build time are significantly greater than the LL due to the extra 5 BJs required per corner, and their 5x attachments to chassis and upright. Plus it needs extra chassis nodes and tubes... Structurally (ie. strength and stiffness), this layout is considerably inferior to the LL because of the inherent play in the many BJs, and the strains due to the tortuous load paths through them.

Nevertheless, the Lancaster Link was almost universally condemned on its thread. A one time official (?) described it as a "disaster"!

WHERE IS THE REASON??? http://fsae.com/groupee_common/emoticons/icon_confused.gif

Are there any engineers on this forum? Does anybody think things through these days? Mutter, ... grumble ..... http://fsae.com/groupee_common/emoticons/icon_rolleyes.gif

Z

Big Bird

02-22-2013, 09:06 PM

Food for thought...

http://www.fsae.rmit.edu.au/rmit_racing_website/images/history/fsae_a_2005.bmp

Comments coming

Big Bird

02-23-2013, 06:36 AM

The above photo is of RMIT R05 at the Oz 2005 comp, which went on to win FSAE Detroit in 2006. I have posted it as it is a good example of swing axle geometry - without the simplicity.

Reasoning? The short VSA's were an attempt to achieve full camber compensation in roll, noting that our simulations were showing that lateral accelerations were around 8 to 15 times more important than longitudinal accelerations. We were happy to compromise our contact patches in pitch a little to keep the patch flat on the road in cornering.

We had noted that with the track map we had to work with, 10 of the 14 cornering manouevres were entered with no pitch attitude - i.e. no braking. Therefore we wanted camber compensation right from when the car would first start to roll.

The fault? The car would lift the inside rear right from corner entry, and would then transfer all the torque the inside rear when under power - no drive. The team then over a few years progressively dropped roll centres and tightened up the diff (torsen then moving onto clutch packs) to get the power down on exit.

Think of where the c of g of the unsprung mass is - it is at axle height. Now think of where the instantaneous centre is for the VSA - below axle height. Now imagine the unsprung mass inertial force under cornering, and what sort of a moment it will create around the IC of the VSA, and how little moment of inertia the unsprung mass has with such a short VSA...

Instant lifting of the inside rear right from turn in. Terrible when you are trying to get a torsen to work on corner exit. So you can either try to fix the geometry to suit the final drive you have - OR thinking a little laterally, could you find a final drive to suit that kind of geometry?

Hmmm...

Big Bird

02-23-2013, 07:09 AM

p.s. Z asked me to post the above explanation of our geometry (see fantasy car thread) as he has a few of you expressing concern to him that you wont do this sort of geometry because the design judges "hate" short VSAs.

My responses:
- The design judges pointed out to us the design flaws of the short VSAs on our car. If you worried about such things, you might go home and tell your mum that the design judges hated it
- The above car got 3rd in Design at FSAE Cali, and and 5th in Design at Detroit
- If we could have managed to slip through some pinhole in the time/space continuum, it could have been fun to go back in time and redesign our suspension just the way the judges would have "liked". Nice long VSAs. With this newfound knowledge, and a vehicle specifically tailored to thir "liking", I have no doubt we could have achieved 3rd in Design at FSAE Cali, and and 5th in Design at Detroit.

Seriously, don't design for the judges. It is your car, design it for yourself. Trying to second guess what a stranger "likes" will give you a life lesson in futility. And you are being assessed on your knowledge, not how like-able your car is.

Cheers all

Adambomb

02-23-2013, 01:36 PM

Originally posted by Big Bird:
Seriously, don't design for the judges. It is your car, design it for yourself. Trying to second guess what a stranger "likes" will give you a life lesson in futility. And you are being assessed on your knowledge, not how like-able your car is.

I nominate this as quote of the decade!

03-06-2013, 04:13 AM

Originally posted by murpia:
... a swing axle ... I like the warp-soft / heave-stiff Formula Vee idea quite a lot.

The crunch will be toe & camber compliance, there can be no compromise there...
Ian,

You got me thinking (always dangerous http://fsae.com/groupee_common/emoticons/icon_smile.gif), and I reckon a Swing-Axle could possibly be the simplest way to do Independent Rear Suspension for FSAE. Here is one possibility.

(Semantic note: I refer to a "Swing-Axle" as a suspension that moves, both structurally and kinematically, about an axis (= the Motion Screw, ISA) that intersects the wheel's actual AXLE. So, typically, the wheel is fixed rigidly to its axle, with this axle swinging about a joint near the car centreline. By contrast, a "Swing-Arm" moves about an ISA that isn't necessarily close to the wheel's axle. Just my usage, though...)

https://lh5.googleusercontent.com/-rxd0F_HK7u8/UTLZs_xTLqI/AAAAAAAAAM0/ft1GFw4J5Wk/s800/Swing-Axle.jpg

STRUCTURE.
============
1. Only two major components required per wheel, the Swing-Axle and Diagonal-Link.

1a. The Swing-Axle itself is a strong tubular structure that has the wheel fixed rigidly to its outer-end, and has a ball-joint at its inner-end that allows it to "swing" wrt the diff. Also at its inner-end is a "rubber donut" CV that transmits driving and braking torque (more below). The SA thus carries the driving/braking torques, as well as most of the cornering loads.

The sketched SA is a tapered, large diameter, thin-walled tube, since this is the best shape to carry the loads. The large diameter at the outer end minimises camber and toe compliance, negating Ian's (rightful) concerns. The wheel is attached via a splined joint that also locates the outer bearing (use bearing size for scale). I would probably fabricate the axle out of 4130/4140 steel. A constant diameter steel tube, as commonly used on car "prop-shafts", could also be used. A similar sized axle, but with thicker wall in aluminium or CF, is also an option. Or a smaller OD, thicker and tapered wall, one-piece, machined 4140 (or 4340+) hollow bar, with suitable heat-treatment, etc., would also work.

1b. The Diagonal-Link carries the major longitudinal loads from the tyre->wheel->axle->DL->chassis via a 6010 Deep-Groove-Ball-Bearing. (I have done no calcs at all on these sizes, just what feels right. http://fsae.com/groupee_common/emoticons/icon_smile.gif) The DL also carries the vertical loads from the springing (see below). The DGBB is mounted to the DL via a vertical axis "fixed" joint (ie. top and bottom bolts) that allows adjustment of the angle between DL and SA. This is to cater for manufacturing tolerances of the chassis, and adjustments of toe-angle, etc.

I have again sketched the DL as a tapered, sheet steel fabrication, because this is easy to make. It also resembles trees and bones, which know a thing or two about good design. http://fsae.com/groupee_common/emoticons/icon_smile.gif Two tubes in a narrow "V" would also work. Or a CF shell, similar to the sketch.

2. Drive and braking torque must pass from the sprocket/brake-disc, via the "spool" diff, to the SA via some sort of CV joint. I have shown a "rubber donut" because it can smooth a big-single's torque pulses, which may be helpful given that the SA itself will be torsionally very stiff (compared with normal driveshafts). Lateral loads from the wheel are passed to chassis via the SA-inner-Ball-Joint, which is made from a modified OEM steering linkage BJ. The donut+BJ might be replaced with a conventional UJ (= Hookes joint), although this will be torsionally stiffer and doesn't give a "constant" angular velocity. Build-and-test ...?

3. As sketched, the chassis only needs 3 strong-points for the whole rear suspension. The two DLs attach at two forward points near the main-roll-hoop/seat-back/side-impact-structure (perhaps the strongest part of the car). The only other strong-point needed is the differential area, which should already be strong to take the chain loads.

4. All parts are identical on left and right sides, which makes manufacturing and carrying spares easier.
~o0o~

KINEMATICS.
============
1. The suspension pivot-axis (= ISA), passing through SA-inner-BJ and DL-front-BJ, should be close to horizontal to minimise bump-steer (given small FSAE suspension travel, this doesn't matter too much). A small slope here can give designed-in roll-steer, but that is polishing.

2. More important is setting static Toe and Camber.
2a. Toe is set by adjusting the rod-ends at the front of the DLs. Shorten for toe-in, lengthen for toe-out.
2b. Camber is set by moving the diff up or down wrt chassis. This could be combined with (ie. added to) the common diff back-and-forth chain adjustment. Note that this sort of chain adjustment requires resetting the Toe.

3. As noted in earlier posts, this suspension has a relatively high "Roll Centre" (= steep lateral n-lines). The longitudinal n-lines are horizontal, giving 0% anti-squat/lift, which is generally a good thing. The implications of these kinematics can all be taken care of by appropriate springing.
~o0o~

SPRINGING.
===========
1. The main sketch shows a single Anti-Axle-Bounce spring-damper only. This connects to the DLs via a smallish Longitudinal-Link and two pullrods. The pullrods might look to be under high loads, but as noted before the torsional loads through the axles are the really big ones, and I don't see any problems here. Moving the AAB spring, LngLnk, and pullrods, forward into the engine bay would be beneficial, if there is space there.

2. Importantly, the end of the LngLnk is free to move sideways. This arrangement gives zero elastic roll stiffness for the rear suspension. In turn, this implies that all Lateral-Load-Transfer at the rear is "kinematic" (= "geometric" LLT) via the high RC. It also means that the four-wheel suspension is very soft in twist, which is an advantage on undulating and bumpy tracks.

3. If more rear LLT is required, say to unstick the inner tyre and improve performance of the spool-diff in tight corners, then Anti-Axle-Roll springing can be added by constraining sideways movement of the LngLnk (as shown, centre of pic). Many ways to do this, including rubber bungees in tension either side of the LngLnk (just add more loops to increase roll stiffness!). Or overcomplicate to your taste... http://fsae.com/groupee_common/emoticons/icon_smile.gif

4. The advantage of separating the Axle-Bounce and Axle-Roll modes is that the respective spring curves can be tailored to suit your requirements. Here the high RC implies high jacking forces, so the AABounce spring should be droop limited (ie. only extends ~5-10 mm from ride height). This prevents the body jacking up during cornering.

Having short travel of the AAB spring also minimises camber change during acceleration/braking (= pitch couple), and minimises ride height change if you have sprung aero (= heave force). The short travel of the AAB should also be soft for grip over small bumps, with rising rate bump rubbers catering for the big bumps. Meanwhile this suspension can still have very long and very soft travel in Roll, because there is zero camber change in roll. This softness is also good for absorbing big single-wheel bumps.

5. Conventional "independent" springing is also shown at top-left of sketch. If you are in doubt, then you can always revert to this approach. Remember that "any suspension will work, if you don't let it". And this, in fact, is the approach used by many winning FSAE teams!
~~~~~o0o~~~~~

As always, comments and criticisms welcome. http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

(PS. [Mini-Rant] The above only possible because I gave up on cable & OS incompatabilities and bought a new scanner! What a farce!!! The ~15 year old scanner+computer worked fine, and had a reasonable editing facility that allowed me to tidy up sketches, compress files, save in multiple formats, etc. The new scanner comes with atrocious software, with one of the very few editing options being "Try Your Luck"!!!!! http://fsae.com/groupee_common/emoticons/icon_eek.gif Idiocrasy, here we come! [Rant Over]

I will try in a few weeks to post a sketch of Semi-Leading/Trailing-Swing-Arms, a very good option for "independent" suspension for current FSAE conditions (off-road is different again).)

rjwoods77

03-06-2013, 07:49 AM

Z,

Would you car to comment about combining your longitudinal z bars and the twin beam concepts? I got into a conversation with someone about it and we were trying to figure out if it would be worth doing in two cases, one with a aero package and one without.

DougMilliken

03-06-2013, 07:52 AM

Originally posted by Z:
The wheel is attached via a splined joint that also locates the outer bearing (use bearing size for scale).

Z, you are slipping, shouldn't the wheel be attached by one of those fancy tapered face torque couplings<grin>?

Jay Lawrence

03-06-2013, 07:36 PM

This sort of stuff makes me want to go back to uni.

I was thinking perhaps you could make the longitudinal link have only 1 DOF so you could turn it into an antiroll bar (if deemed necessary) whilst still having it swing up/down for pitch/heave motion. Probably be a compliance hassle though

murpia

03-07-2013, 08:36 AM

Z,

Fantastic, thank you for this.

I wish I had the time & skill to put together concept sketches like that. In the absence of both, I'll just let you know where my mind had wandered recently...

Considering a'sidewinder' engine location, I had the idea to use a beam axle with the differential way over towards one wheel. The driveshaft from the engine to the diff could form a longitudinal suspension link. Optionally, the beam axle could have a single joint at the car centerline to instead form two swing arms.

I still worry that the true weight saving over dedicated suspension links and dedicated driveshafts & joints gets lost in the implementation unless the structural engineering is done really well.

Regards, Ian

03-07-2013, 06:20 PM

Rob,

"Would you care to comment about combining your longitudinal z bars and the twin beam concepts?'

I think I covered that in the "Beam Axles" thread. Briefly, IMO, a combination of the "Z-bars" and "Twin Beam" sketches would work very well. A spring between chassis and centre of each beam (acting as lateral Z-bars) would control heave and pitch modes. A longitudinal Z-bar down each side (in any of many practical implementations) would give additional heave and roll control. Twist (= warp) mode thus remains soft.

Importantly, the side-pair Z-bars allow LLTD (actually Elastic Roll Moment Distribution) to be set independently of any heave, pitch, or roll motions of the chassis. And, especially importantly, independently of any twist in the road. A conventional suspension's ERMD, and thus over/understeer behaviour, is greatly affected by the road profile. FSAE has relatively flat tracks, so I showed a soft spring-at-each-corner on the Twin Beam sketch, because no camber change during heave/pitch/roll, and I didn't want to overcomplicate the message.

UWA's 2012 car has very cleverly taken this a step further. Their "W" springs do the job of the centre-of-beam springs controlling heave and pitch. They also kinematically locate the undertray and wheels wrt the chassis, doing the job of my peg-and-slots and BJs. The UWA car then controls roll, entirely independently of the other modes, by a mid-chassis, lateral U-bar connected to the two undertray tunnels, which in turn act as longitudinal "balance-beams".

A very well integrated, minimal parts count design. How they only managed 3 points (out of 200!) in Design is beyond me. Something very wrong there... It is clear to me that the Design score does NOT reflect the car's design, or the student's knowledge... Some more openness and transparency in these decisions would be welcome (might have to rant on the other thread http://fsae.com/groupee_common/emoticons/icon_smile.gif).
~o0o~

"...we were trying to figure out if it would be worth doing in two cases, one with a aero package and one without."

With unsprung (= beam mounted) aero, on FSAE type tracks, I think a simple SOFT spring-at-each-corner would be the best place to start. Next, add the beam centre springs (or just bump rubbers) to tighten up heave and pitch.

With sprung (= body mounted) aero, then short travel beam centre springs, with good bump rubbers, are essential. If excess body-roll is now leading to front-wing or bodywork scraping, or if you are racing on very undulating or bumpy tracks, then short travel, side-pair (longitudinal) Z-bars would be good. You can now remove the corner springs and the car has full control of heave, pitch, and roll, with a fully soft twist mode to cope with the uneven road.

With no aero, but an undulating/bumpy track, then having a soft twist mode is very beneficial. So lateral and longitudinal Z-bars are definitely good. Or just the lateral Z-bars (beam centre springs), together with an anti-roll arrangement like UWA's (which does require some sort of side-pair balance beams).
~~~~~o0o~~~~~

Doug,

"... shouldn't the wheel be attached by one of those fancy tapered face torque couplings<grin>?"

But then some other means would be required to locate the outer ball bearing. http://fsae.com/groupee_common/emoticons/icon_smile.gif (Perhaps a "crush spacer", or second ring-nut...)

FWIW, the axle-to-wheel joint shown is fairly common. Well, the ~1980s 4WD Subarus I having rusting away out back have it on all four corners.

An important trick is the spacer between big-nut-on-end-of-axle and the wheel-mounting-flange (this appears in the sketch as a small triangle). This spacer looks like a fat washer with one conical side, but importantly it has a slit in it so that it is "C" shaped. The nut pushes on the flat face of the spacer, which in turn is pushed into the female cone of the wheel-flange, which in turn reduces the diameter of the spacer, clamping it onto the axle.

Bottom line is that all axial and radial play is eliminated between axle and wheel, so no "fretting" in those directions. Rotational fretting is still possible between inner and outer parallel splines, but this doesn't have the high frequency of wheel load induced fretting. There are ways of positively clamping all degrees of freedom, and still allow the ball bearing to be clamped up during assembly (similar to above, but +++), but that would require a whole lot more words... http://fsae.com/groupee_common/emoticons/icon_smile.gif
~~~~~o0o~~~~~

Jay,

"I was thinking perhaps you could make the longitudinal link have only 1 DOF so you could turn it into an antiroll bar (if deemed necessary) whilst still having it swing up/down for pitch/heave motion."

Yes, that would be similar to many current "monoshock" linkages, though upside down. These have a "T-bar" that pivots wrt chassis on a lateral axis. For roll control the leg of the T-bar could flex side-to-side, or the whole T-bar could slide along its pivot axis against springs (as is currently done).

Note that if the AAB spring, as shown in the sketch, has its top mount significantly above axle height, then it exerts a small anti-roll force during body roll (err,... do a FBD http://fsae.com/groupee_common/emoticons/icon_smile.gif). If tension springs are fitted either side of the LngLnk, for roll control, then these will add some rising rate force to the AAB spring, which, in general, is a good thing.
~~~~~o0o~~~~~

Ian,

"... the beam axle could have a single joint at the car centerline to instead form two swing arms."

Ah, yes, this was used very widely by Mercedes around the 1950s. They won everything from Grand Prix's to the East Africa Safari (= today's Dakar) with that sort of suspension. Essentially, take a live axle with diff offset a bit to one side, cut it in half next to the diff, put a single UJ there, and presto!, two Swing-Axles. Their next step was to allow axial plunge of one of the half-shafts (via a spline), which allowed a "low-pivot Swing-Arm" with lesser jacking forces. There must be pics on the web?
~o0o~

"I still worry that the true weight saving over dedicated suspension links and dedicated driveshafts & joints gets lost in the implementation unless the structural engineering is done really well."

Yes. I hope I haven't overcomplicated it by showing the tapered axles. I think using the ~2" diameter 7075(?) aluminium axles used on Rob's previous car, and currently by U of Berkeley(?), together with a lot of the off-the-shelf fittings for these axles, might be a good way to start.

Z

rjwoods77

03-07-2013, 08:24 PM

We used a 1.75 OD but they make 2.00 now as well...

http://www.hyperracing.com/pag...tore.aspx?class=1100 (http://www.hyperracing.com/pages/home/products/online_store.aspx?class=1100)

Since I am linking a sprint car shop I should take a moment and point out that is was 1994 University of Illinois Urbana #83 car (Finished 7th overall) that was a FSAE legal 600cc spring car that had a sidewinder engine and a 4 link sprint axle that got me thinking that maybe using a solid axle was a competitive direction to go. I wish I had a picture but UIUC took down the pictures of their old cars. This was further reinforced by the 1996 Akron team for showing that a Briggs V Twin with CVT and solid axle could be competitive as well...

http://forums.bajasae.net/foru...f-fury_topic637.html (http://forums.bajasae.net/forum/bajamula-vtwin-of-fury_topic637.html)

I was dumbstruck at what Akron pulled off in 1996. There hasn't been anything like it sense unfortunately. Maybe someone will do a Bajamula again one day and use Z's trailing/leading link with interconnection and unsprung aero and show how really good engineers they are. At the very least it would have amazing f*&k you factor since many teams would finish below it. These two cars and their high placement along with my love of F500 cars sealed the deal on my decision to push for the direction we went for the car that eventually ran in 2008 and finished 18th.

Kevin Hayward

03-08-2013, 04:55 AM

Cole,

It is not such a weird mechanism. It is just a four link rear end with triangulated lower links. Way easier to get a lower roll centre than using a panhard bar or a watts linkage.

Difficult to keep the engine low with the setup and from the look of it the links aren't orientated too much in the lateral direction which would increase the load in the a-arm. pretty ordinary attachment of the rod end as well. There would have been better ways to build that adjustable roll centre in.

With these systems you need to watch longitudinal movement of the axle. This can be pretty noticeable in roll as it will angle the beam to the chassis, however you can control this by the upper link angles. Good toe control will lead to some amount of anti in the suspension however.

Not a bad approach to keeping structure away in the rear. Also like the simple side mounted shocks at the front. setup that way the rockers have very little load, everything operates in the same plane and the parts get about as simple as they can for a rocker based double a-arm setup. 4 of the cars (2 as student) that I have been involved with have had that style setup and it is amazing how much time can be saved not worrying about making sure all the movements are in the right plane etc.

Kev

Kevin Hayward

03-08-2013, 05:09 AM

Z,

More great drawings, thank you!

These are great ideas for teams to investigate especially for teams further down on performance. Not because they are inferior, rather because a team of 100 people with a current super fast car are less likely to make changes to their concept and take the risks. That leaves an opening.

One of the great thing about the trailing links (and some four link designs) is you design them quite effectively with much easier and cheaper bearings (or bushes) rather than just using ball joints and rod ends at every opportunity. By reducing the accuracy required in manufacturing you save yourself time and money, both of which are far more important than most teams realise.

Again these concepts very much suit a single (or small) engine with a narrow sump such that you can still keep the engine low.

I wonder if the real question for teams to ask is how do you setup a team such that you do not immediately reject these ideas. For example if you break a team up into roles based on components and allocate a person to a diff then you are much more likely to end up with a diff of some sort (most likely an expensive drexler) than a spool or beam axle.

These sorts of suspension systems require a complete game change of the vehicle with people working very closely on suspension, chassis, engine, aero etc. and having a lot of people accept compromises for the better vehicles that at first look horrible.

How many teams share and discuss these sorts of concepts, with a reasonable amount of discussion and preliminary design, before rejecting them. Or are teams just jumping into double arms, rockers, etc. just because they want to save time and arguments?

Kev

mech5496

03-08-2013, 06:12 AM

Kev I bet 95%of the teams do the latter; we all tend to forget the big picture and focus to details.

rjwoods77

03-08-2013, 06:18 AM

Cole,

That design is fairly typical of a F500 car just a different variation of it. It was most likely copied over from that series. For example...

http://www.apexspeed.com/forum...?p=191266#post191266 (http://www.apexspeed.com/forums/showthread.php?p=191266#post191266)

...our rear suspension was based very much off the Novakar but with dampers and a satchell 4-link instead of the trapazoid link...

http://www.youtube.com/watch?v=PviU2A_uOo4 (bunch of pics in there that's why i posted the vid instead of link to photo dump)

Kevin,

I think the single biggest factor in making radical decisions is to have a healthy "f*&k it...lets go for it" attitude amongst your peers. Geoff said it best in his previous post about designing the car for yourself. While I am sure that there were lapsims, data and discussions about the competitiveness of the RMIT single at some point there was belief in a concept/dream, whether founded in reality or not, followed by a "f*&k it...lets go for it". That leap of faith is something that many engineers have a hard time doing because of lack of creative thought, hum drum personalities and anti-rebelliousness. Engineering is rife with personalities that are the antithesis of someone like Neil Degrasse Tyson. In our case we knew there wasn't a chance in hell of beating RIT (example team because of proximity)at their own game(double irs with 600cc 4 cylinder) so we decided to take a lateral direction and held onto that concept of doing things different. When I came on to that team I sold the concept to them based on passion and exuberance followed up with some rough comparative analysis. It turned out well given all the factors we dealt with and could have been a real contender in following years of turd polishing iterations but alas it didn't happen.

I think the second biggest factor is it appears that these radical changes or directions are carried out by tight small teams. I think in general the bigger and more established a FSAE program is the more the likely they are to not rock the boat. This leads to the much discussed stagnation of only doing tiny changes versus great leaps. Obviously there are reasons for doing this in those big programs so I can't be entirely critical. The small team structure allows for easier subsystem integration since people spend more time talking about concept integration versus going off into respective corners and obsessing over the individual components. Holistic vs big medicine if you will. Once you get through that part of it the questions of how to actually carry it all out are pretty easy to be honest. For instance myself and Harry Bikas from U.o.P. Racing Team just go nuts and riff on cool ideas which from those talks and him working with his team have come up with an astonishingly cool design that I think will turn heads and rack up a good amount of points. Being from a small team allows him the freedom to pursue such things because there isn't a culture of "we have to make sure we gain another position this year for the dean to be happy" or "Chauncey needs to make this happen on the car so he can get a job with Stuffy Engineering Inc. after bragging about his boring design to people at the yacht club"...

http://www.youtube.com/watch?v=MWolPnWF3m0

Big Bird

03-08-2013, 06:34 AM

I wonder if the real question for teams to ask is how do you setup a team such that you do not immediately reject these ideas.

Alternative question, if I may Mr H

I wonder if the real question for us to ask is how do you setup a competition such that teams do not immediately reject these ideas...

Rex Chan

03-08-2013, 07:46 AM

I'm going to leave all the suspension talk to the suspension guys, but I just wanted to say something about the idea that "we'll never beat Team X at their own game":

I suppose I should say that I interpret "beat/better/etc" as being having equal laptimes, or points in events.

Anyway, I think teams are much more fragile than we think, and that personnel changes happen year to year. Just because a team did really well last year, doesn't mean it will be a repeat the following year.

I think too many things go wrong when decisions *aren't* supported my some sound logic (of which the best/most convincing is probably a points sim).

Also, having lots of similar cars means sharing ideas on how to make those cars go fast works better. This is all about learning, so having lots of similar systems out there means a lot of info to share for the new/slow guys.

Kevin: while some of these systems are *simpler* to build, teams with experience can build the standard car quite *easily*. I know Z goes on about less a-arms and hardpoints, but I don't think those things take up an unreasonable amount of time. Considering that most failures in FSAE stem from a lack of basic engineering (with known designs), there's a lot to be said about getting the basics right.

I'd be happy to be part of a team that produces a car that finishes top 3-5 (in AUS) {3-5% in the big comps} for many years in a row. That would be greater achievement than winning for a year or two, then dropping off the map. Or never bringing a fast car at all (all those teams running 1-2 seconds (over a 60s lap) behind the fastest car are seriously off the ball, in my opinion).

DougMilliken

03-08-2013, 09:50 AM

Originally posted by Z:
"... shouldn't the wheel be attached by one of those fancy tapered face torque couplings<grin>?"
An important trick is the spacer ... (this appears in the sketch as a small triangle).

Thanks for clarification--a good trick to use the split tapered washer.

"... the beam axle could have a single joint at the car centerline to instead form two swing arms."
Ah, yes, this was used very widely by Mercedes around the 1950s. ... Essentially, take a live axle with diff offset a bit to one side, cut it in half next to the diff, put a single UJ there, and presto!, two Swing-Axles.

Also used by Archie Butterworth on the front of the AJB Special. He started with a Jeep front axle (offset diff) and split it to make a front swing axle. After being wrecked and rebuilt, it became the Butterball that my father raced. Just one "little" problem, on hard acceleration, the propshaft torque reaction would lift that front wheel off the ground! Some details at http://en.wikipedia.org/wiki/Archie_Butterworth Built in 1948, I wonder if Mercedes copied Butterworth?

03-09-2013, 01:57 AM

Originally posted by Big Bird:
I wonder if the real question for us to ask is how do you setup a competition such that teams do not immediately reject these ideas...
Geoff,

Originally posted by Z:
... How [UWA] only managed 3 points (out of 200!) in Design is beyond me. Something very wrong there... It is clear to me that the Design score does NOT reflect the car's design, or the student's knowledge... Some more openness and transparency in these decisions would be welcome...
Yep, more openness and transparency. For example;

Full public release of Design Judges scoresheets and opinions on each car. Why did this DJ give that team a high/low score? Etc.

Similar release of scoresheets from the Teams, rating the Design Judges performance during Design event. Did the DJs seem to understand what the Team was trying to achieve? Did the Team learn anything from the DJs, either during or after Design event?

Perhaps the Teams could also rate each others cars (not their own) with this contributing to the overall Design score for each car.
~o0o~

Currently, the 200 point Design score in FSAE-A reminds me a lot of that editing option on my new scanner - "Try Your Luck!" http://fsae.com/groupee_common/emoticons/icon_eek.gif

Z

Kevin Hayward

03-09-2013, 03:04 AM

Z,

I am right behind publishing design feedback for all cars with the design reports. This can only aid the educational aspect of the competition. It would be great to see how other teams have approached the same problem.

Would be considered unacceptable in the competitive world of professional motorsport, but we shouldn't be following the same rules.

Share and learn together!

Kev

mech5496

03-09-2013, 11:45 PM

Z thats a great proposal. Kev, wow mate!Sure thee have been teams sending out their DRs in the past (ADFA had theirs on their website and I have sent ours out to someone who asked) bur having access to such info from a top team like UWA is really cool. Also hats off to any team who dares to be different, think outside the box and often change the game. UWA especially despite not being a small or unsuccessful team and still being brave enough to try novel ideas. I will also have my eyes open for Cal Berkeley team this uear...

05-31-2013, 08:30 PM

Way back in December 2012 (page 9) I wrote a rather long post describing the various types of Swing-Arm suspensions (http://fsae.com/eve/forums/a/tpc/f/125607348/m/73320357151?r=34120987151#34120987151). There I said that Semi-Leading&Trailing Swing-Arms are my second favourite type of suspension for FSAE. They don't have quite as good overall properties as Beam-Axles, but they are perhaps the structurally simplest solution, so in that way are well suited to FSAE.

So here is a sketch of how I would do Semi-Leading&Trailing Swing-Arms.

https://lh5.googleusercontent.com/-F0xdq59r6fE/UabqSvQEMvI/AAAAAAAAANY/1NCjkQZr-T8/s800/SLT-Swing-Arms.jpg

Some comments (much of this also covered in the above link).
~~~~~o0o~~~~~

1. KINEMATICS - These Swing-Arms have a short "Front-View-Swing-Arm" that gives "100% Camber Compensation". This means there is no wheel camber change during cornering, regardless of any amount of body roll. The penalty of short FVSAs (not seen on Beam-Axles! http://fsae.com/groupee_common/emoticons/icon_smile.gif) is that the wheels will have camber change when the body pitches due to accelerating or braking forces, or heaves due to vertical forces from sprung aero or big bumps. FSAE is mostly about lateral cornering forces, with lesser amounts of longitudinal or vertical forces, so this type of front-view kinematics (for independent suspensions) is preferable (ie. better than very long FVSAs). See also Geoff's and other posts on previous pages.

The side-view kinematics have the front wheels' longitudinal n-lines rising slightly up-to-rear, so giving a small amount of anti-dive. At the rear the longitudinal n-lines are such that there is some anti-lift under braking for outboard brakes, but with inboard brakes there is a little pro-lift, and with inboard drive there is a little pro-squat. (Hint: Draw side-view n-lines in the plane of the wheel, and see that they intersect a little above ground near the front wheel. Note that the n-lines for a Swing-Arm are ALL the straight lines in 3-D space that intersect the SA's pivot-axis (= its ISA).)

This side-view behaviour is generally OK for FSAE type racing. The pro-squat is not suited to drag-racing (where large anti-squat is better for good launch), but in FSAE such issues are adequately fixed with Anti-Axle-Bounce springs (see below).

At the front wheels bump-steer is mainly determined by steering tie-rod position (its centreline should pass through, or close to, the SA pivot-axis). Note that a little bump-steer at the front is not too bad, because the driver can correct for it. At the rear the Swing-Arm pivot-axes (ie. their ISAs) should be close to horizontal to minimise bump-steer.

Sloping the rear pivot-axes up-to-front gives bump-toe-out, and so roll-oversteer. This is generally bad for passenger cars (car spins out of corner), but may be good for an FSAE car that understeers through slaloms and hairpins. Sloping the pivot-axes down-to-front gives bump-toe-in and roll-understeer. I suggest it best to keep the pivot-axes horizontal, and then fine tune with static-toe adjustments (see below).
~~~~~o0o~~~~~

2. STRUCTURE - The chassis structure required for these S-L/T Swing-Arms is about as simple as possible. I see this as their biggest advantage. All the SA-to-chassis attachments (BJs or bushes) are at the perimeter of the floor. These attachments are at the centreline of the front bulkhead (where strong structure is required for the pedals and IA), at either side of the front and main roll hoops (which are required by the Rules), and under the drivetrain/diff (which must also be strong, least the engine fall out).

A flat floor can join all these points, and this is well suited to carrying the mainly horizontal loads from the SAs. Having all these attachments in the flat floor plane also simplifies jigging and manufacture of the chassis. All this major structure is at the lowest possible part of the chassis, so giving a lowest possible CG. The vertical loads from the wheelprints, which are carried mainly by the spring/dampers, can be fed from the Swing-Arms to the chassis by any convenient path (more below).

In the sketch the structure of the Swing-Arms themselves is a hollow sheet-steel fabrication. IMO this is easy technology to learn, can be done with simple tools, and gives the best strength and stiffness-to-mass properties. Thicker aluminium sheet could also be used, or carbonfibre if you really like the smell, sticky fingers, and extra time and cost.

Importantly, ALL FOUR SWING-ARMS ARE IDENTICAL. This means only one jig is required and fewer spares needed (eg. only have one spare, and it can replace any corner). There are important "Production" advantages here. Namely, once you figure out how to make one good one (ie. which welds to do first, where to rest your elbow while welding, etc.), you can then produce high quality items at high speed.

The two rear "uprights" are also identical, as are the front "upright/king-pins", and all axles and bearings are very similar (for lower spares count), but some parts of the front steering are different (too many funny angles! http://fsae.com/groupee_common/emoticons/icon_smile.gif).
~~~~~o0o~~~~~

3. SPRINGING - Given that suspension is NOT a very important performance factor in FSAE (rigidly sprung cars have won), a very simple spring-at-each-corner is shown, because it will do. So a conventional spring-damper is drawn at each corner, which feeds its loads into any convenient point on the chassis. These dampers have a Motion Ratio of about 0.6+ (damper/wheelprint movement). This is more than enough for lightweight FSAE cars, and the supposedly magical MR=1 is NOT necessary. In fact, I have shown the spring-dampers neatly tucked away because there is more to be gained from good aero flows than silly MRs! http://fsae.com/groupee_common/emoticons/icon_smile.gif

As noted above, the main disadvantage of these types of Swing-Arms is excessive wheel camber change during heave or pitch motions of the car. The easiest solution is to fit Anti-Axle-Bounce springs (these covered extensively in other posts). These "lateral Z-bars" are essentially the same as the "third-springs" used on many modern aero racecars.

I would possibly implement AAB springs as lateral centre-pivot-leaf-springs (see "Z-Bar" sketch somewhere), mainly for neat packaging and low CG. Perhaps go to an Archery store for inspiration (ie. a lightweight and stiff centre section with the pivot, and lightweight flexible fibreglass leafs at the ends). Zero-droop+rising-rate-in-bounce is also good, and easily done with this type of spring. Other options also possible...

If you use AAB springs as above, then these should carry most of the weight of the car (ie. they control pitch and heave). This way the corner springs really only have to carry the roll forces, and since body roll doesn't affect wheel camber angle, the corner springs can be quite soft. This, in turn, means that the chassis is subject to only low torsional loads, and thus high torsional stiffness is less important. Furthermore, the soft corner springs give the whole car a softish Twist-mode, which is a good thing if the track has any undulations.

Of course, longitudinal Z-bars can also be fitted, giving a completely soft Twist-mode, and thus very predictable and easily adjusted handling balance. (Might have to do another sketch of these one day to show various practical implementations...)
~~~~~o0o~~~~~

4. REAR CAMBER & TOE ADJUSTMENT - This is shown at bottom-left of the sketch. A one-piece Swing-Arm-From-Chassis-Out-To-Hub-Bearings would be simpler, stiffer, lighter, etc., but then camber and toe might be harder to adjust, and different SAs would be required for each corner.

The joint shown is essentially a three bolt flange, but with capability to vary the spacing between the three corners of the equilateral triangular flanges. The two forward bolts use shims (eg. washers) to vary the top and bottom spacing and thus adjust camber-angles. The rearmost bolt uses a threaded adjuster in the SA to give finer adjustment for toe-angles.

The "upright" has spherical surfaces machined into it, and these are clamped by the bolts and cup-like washers. This is necessary to accomodate the misalignment when making adjustments. Note that the three bolts are always parallel and with constant alignment to the SA, but the upright moves "out of square" wrt the bolts.

The bolt sizes, etc., shown in the sketched joint would have similar strength and stiffness to a one-piece SA+Hub, although it is slightly heavier. However, this joint should be considerably stronger and stiffer than many FSAE wishbone+uprights because the load paths are more direct, and the "balls" are clamped tight and thus don't have any of the usual freeplay.

For the record, the sketched joint only positively constrains 5 DoFs between SA and upright. The sixth DoF, a rotation about the rearmost bolt, is only constrained by friction. It is quite easy to positively constrain all 6 DoFs (yes, this has been covered before http://fsae.com/groupee_common/emoticons/icon_smile.gif) but that is harder to sketch, would require a bit more machining, and is most likely not necessary. Other variations are possible, but the above joint should do...
~~~~~o0o~~~~~

5. CENTRE-PLANE STEER-AXIS - The "upright" at the front wheels might seem rather novel (err, unless you play around with tractors http://fsae.com/groupee_common/emoticons/icon_smile.gif). It uses a three-bolt joint as described above to attatch the Swing-Arm to the king-pin. Camber adjustment is done through this joint, with toe being done through the steering linkage. Castor adjustments would require the piece between SA and king-pin, which is different L and R, to be a "change part" (you should NOT need to change castor in mid-comp).

The main differences in the sketch to normal FSAE cars is the use of Tapered-Roller-Bearings for both king-pin and axle, and the very compact overall packaging. These bearings (30ID x 55OD) are more than adequate for the loads, and IMO much better than the 68xx Deep-Groove-Ball-Bearings commonly used for FSAE axles. FWIW, for a given mass TRBs carry much greater loads than DGBBs, and can have better installation stiffness. The main disadvantages of TRBs is slightly higher friction (Mu=0.002 vs Mu=0.001 for BBs), and consequently lower MaxRPM.

One of the main advantages of this design is the low steering friction under very high loads. Here I am thinking of serious aero downforce and the resulting 3+G cornering loads ... minimum! As such, the "upright" (ie. the "king-pin" shaft + outer-housing of the axle bearings) should be made of reasonably good quality steel. A 3" square bar of 4140 would do for a start, hollowed out with "speed holes" as far as you dare.

A similar but different design for the front king-pin/steer-axis can be found on the Citroen 2CV, and is a good alternative.

Close to centre-plane Steer-Axes are quite common in FSAE these days, which is good because they work better than the massive Offset + SAI (= KPI) that used to be common. However, if tyre distortion due to low pressures or excessive negative camber start to give funny steering feel during mid-corner braking, then adding a small amount of Offset (= scrub radius) can help. This is most easily done with wheel spacers (between wheel and hub), or using a wheel with different "offset".

The sketch was done primarily to show that centre-plane steering is feasible in a 6" wide, 10" diameter wheel, with the brake-disc outboard of the Steer-Axis, and relatively large steer-angles are possible (30 degree outer, 45 degree inner).
~~~~~o0o~~~~~

As always, comments and criticism welcome. http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

Jay Lawrence

06-02-2013, 08:45 PM

I may be missing something, but I would imagine that the rear camber/toe adjust system would be quite difficult to operate in practice. I would also think that the tolerancing and matching of the bolted components (I'm assuming those washers around the 'upright' are domed to accomodate the misalignment) would be very critical and possibly prone to becoming misaligned if the bolts aren't torqued perfectly.

murpia

06-03-2013, 06:54 AM

Hi Z,

Nice concept and fantastic drawings as usual.

My concern with this is the weight of the swing arms compared to a 5-link version with very similar geometry. I agree with the geometry justification & the sensible load paths into the chassis. The swing arms as drawn need to resist bending forces from the springs, dampers & lateral Z-bars.

The 5-link version would use simple tubes with rod-ends at each end, creating triangulated structures. Co-locating the inboard pickups reduces the chassis compexity compared to double-wishbone.

So at the rear we have 2 roughly longitudinal links from a common chassis pickup out to the top & bottom of the upright. Then 2 roughly lateral links from a common chassis pickup out to the top & bottom of the upright. Plus a toe control link and a push or pullrod. Camber & toe are adjusted with LH & RH rod end threads slightly altering the link lengths.

Without a detailed design & stress analysis I can't answer the question which is lighter for equal camber & toe stiffness. I do know I'd rather fabricate the simple tube suspension links than the swing arms.

The front is more complex depending on steering requirements. An Audi A4 style virtual steering axis could be used, or a separate steering upright and suspension upright (think Ford RevoKnuckle, GM HiPer Strut or see RC cars). Again, a detailed design & stress analysis would be required to determine the solution with the best stiffness vs. weight.

My preference is for push or pullrods & rockers rather than direct acting suspension. In fact I think the lateral Z-bars would be easier with rockers (more a gut feeling than a rigorous analysis...) Or, to implement axle heave springs gaining much of the benefit of the lateral Z-bars with less packaging concerns.

Regards, Ian

Warpspeed

06-03-2013, 03:45 PM

On anything I have ever built, the wheels, tires, hubs, discs, calipers, driveshafts, and uprights all end up weighing vastly more than the unsprung portion of the actual suspension linkages.

Unsprung weight may appear to be a disadvantage of some suspension configurations, but in practice the actual real final numbers may not differ by as much as might be imagined.

06-04-2013, 08:08 PM

Jay,

The rear camber & toe adjustment joint works with "agricultural" tolerances. I have made similar joints (different application) using an ancient Chinese lathe, V-belt driven from a washing machine motor, and with only manual feed of cross-slide and saddle. Most really old machinery that was hand made (eg. from castings or blacksmith's hammer followed by hand filing) required similar consideration of DoFs, loose tolerances, etc. Studying such old machinery is very educational. Anyway, as long as the clearances are right it will work.

Note that all three bolts have to be loosened before making any adjustment.
~~~~~o0o~~~~~

Ian,

I agree that most students would be more comfortable making the arms from 5 tubes. Conventional rod-ends could be used for camber & toe adjustment at the upright, but I would prefer something similar to the sketch. I say this because normal rod-ends invariably have more flex (more complex load paths), and can develop free-play, which all adds up to more camber and toe compliance. The "home-made rod-ends" in the sketch have a more direct load path (no shear strain of the bolt-through-ball), and the clamping always eliminates free-play.

Nevertheless, the LL style 5-tubes+rod-ends-at-upright at least eliminates 2 rod-ends at the chassis, so is a good start...
~o0o~

With regard to weight, I think of it this way:

1. Five tubes of (say) 1.6 cm diameter, by 0.16 cm wall thickness, by 50 cm long, weigh,
5 x Pi x 1.6 x 0.16 x 50 x 7.85 = ~1600 gm = 1.6 kg

2. One big (!) tube of 8 cm diameter, same wall thickness, same length, weighs,
1 x Pi x 8 x ....... = 1.6 kg = exactly the same (because 5 x 1.6 = 8) !

3. Two tapered tubes of 8 cm diameter at one end, zero at other, same thickness and length, weigh,
1.6 kg = exactly the same!

All the above examples use exactly the same amount of sheet metal, just wrapped up in different ways.

Anyway, as sketched the Swing-Arms would be slighly more than 2 kg each (assuming 1.6 mm sheet, = 1/16", or 16 gauge), because they don't taper to zero as per the third example above.

More importantly, reducing the 5-tubes to 1 mm wall thickness reduces their weight to ~1 kg, and my SAs would be about 1.3 kg. BUT! 1 mm thick tubes of 16 mm diameter are easily buckled when hit by the rubber cones, or even by rough handling (eg. picking car up to put in trailer). I reckon the sketched SAs, in 1 mm thick 4130, would survive super-student jumping on them with his work boots! http://fsae.com/groupee_common/emoticons/icon_smile.gif

Furthermore, the Y-shape of the front SA, which gives it the large possible steer-angles, would be a bit more complicated as a tubular spaceframe (ie. would require two extra, shorter tubes). And as Tony pointed out most of the mass at the corners is in the tyres, wheels, hubs, etc. Typical FSAE "unsprung" corner weight is about 15 kg, with 10 kg being very lightweight. The 0.3 kg extra above, for much stonger suspension, is worth it, IMO.

Bottom line is that I discovered a long time ago, through trail-and-error and a bit of laziness, that folded sheet-steel structures work very well indeed. It is a worthwhile technology to learn, and very easy.

Or put another way, triangulated spaceframe structures are good for students who are still learning (don't let the triangles get too narrow!). But Nature seems to prefer doing everything in bending, and in single-shear, and with all those other FSAE taboos. I'll stick with Nature. http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

Chevalier

07-26-2013, 04:56 AM

Hi all,

The other night I was thinking about my time leading an FS team and found a thread (linked on page 9 of this thread) discussing Lancaster Links. Lo and behold there was a picture of "my" car.

The guys that commented on that thread gave it a bit of a panning. Some of which I don't think was entirely justified (obviously! everyone will defend their car!).

I am interested in what is being discussed here as I have always (perhaps stupidly?!?) thought that the LL system was a great, simple solution to FS suspension that was only really let down by our lack of resources and thus ability to execute it properly. Anyway, I posted the below on the other thread and will repeat here for peoples information/discussion.

Note: That pic of our car doing the rounds doesn't really do things favours. I don't know when it was taken but the ride height looks sky high so the geometry of the suspension and driveshafts and all that just looks a bit rubbish to begin with. That said we did have to run it quite high for none suspension reasons I don't really have time to go into right at this moment!

--------------------------

Long story short, I was the team leader of the 2009 Lancaster team and whilst this evening thinking back to my uni days I found this thread and was surprised to see a picture of "my" car. I know the original post was three years ago and the original posters are probably long gone but I felt compelled to respond and defending things. I would be interested in discussing things further if people are interested too.

Firstly that year we had a team of just five masters students (two of which were useless and just made more work for the other three!) a bit of help from some 3rd years, and a budget of about £10k for EVERYTHING including events costs etc. We also did a lot of the fabrication ourselves. I cut/welded most the chassis and did a lot of machining of components myself. It was a car the team built, not a car a sub-contractor or technician built. So with such finite resources a lot of things just got built after five minutes of back of fag packet design.

Anyway to answer a few questions/comments:

- There are no rod ends in bending. Also the LL system reduces the number of rod ends required as the inner pivots only require bushes rather than rod ends.

- The LL system has the advantage that it has excellent camber control in roll but sacrifices camber control in bump. This I think is obvious to all.

- For FS at Silverstone the track is flat so bump isn't really a problem and Braking/accelerating is controlled with a good dollop of anti-squat/anti-dive.

- In its "pure" form the links would join at the middle. By moving them slightly apart you encourage a small amount of camber gain in roll to compensate for compliance/tyre deflection that pushes the wheel into positive camber.

- The car can be set up with very small static cambers to improve traction.

- In ordinary double wishbone set ups the top and bottom bones are in tension/compression respectively. By joining them up at a single node these forces into the chassis are "cancelled out" to some extent.

- Yeah ok the rockers and push rods don't look great (in fact the car looks rather un-set-up in that pic). The judges panned us for the angles of these. But they were just put there cos we needed somewhere to put them (built not designed!). Another possible disadvantage, you must use push rods.

- The hyperbole of "best suspension in 40 years" or whatever I think is just the journalism. They have to make a magazine interesting to read. It's not a technical paper after all!

- Driveshaft plunge was a bit of an issue, in fact it caused an unforseen problem on the test pan at the event so we missed the early dynamic events whilst this was repaired.

- Brake lines at the back is criticism for the sake of criticism. Utter rubbish! When has any FS car suffered a rear end shunt?! A shunt from what? In the very very chance event that it did happen the fronts would still do plenty enough braking to pull you up safe anyway.

Anyway, it finished the endurance event. Better than many other could achieve (including, I remember the Red Bull team (Graz?!)). And survived a spin and backwards trip into the gravel! Zero driver training and not having time to sort the traction control plus a damp track meant we were slow and sideways; but the concept worked!

If the LL system was developed to a level that double arms are by every team every year I can't see why they wouldn't work. Maybe I am just blinkered in defending my car!

Interesting to enter into discussion on this if people want.

---------------------

Cheers all,
Malcolm

07-28-2013, 06:32 PM

Malcolm,

Thanks for sharing your experiences with the LLs.

Over the last half year or so (since page 9) I have had a few students PM'ing me and intending to go with LLs next year (for all the good reasons previously mentioned). Unfortunately, so far it seems they have all been outvoted by other team members who insist the car be built with "a real racing suspension". This coming from team members who know nothing about suspensions! And, worst of all, these are teams who have failed in previous years to build a running car! Ah well, young people and peer group pressure ... http://fsae.com/groupee_common/emoticons/icon_smile.gif

Anyway, some questions:

Q1. Is there a CV joint at the outer end of the driveshaft, or is it just a splined coupling?

Q2. Given your experience with the LLs so far, what would you do differently next time???

Z

Chevalier

07-29-2013, 05:10 AM

Hey Z,

To answer your questions, plus a few extra thoughts. I will try to be brief but it doesn't always work out as I like to be sure I am understood!

Q1. There are tripod style cv joints at both ends of the drive shaft to (obviously) transmit drive and accommodate the "extreme" drive shaft plunge. The CV at outer end was moved out as far as possible and became housed in the upright. This was to make the shafts as long as possible in order to reduce drive shaft angles.

Q2. That is tricky one... The car that was pictured on page 9 was the car the team and I built for FS 2009. That was four years ago and my final year at uni. Since then whilst occasionally thinking about such things I have never really sat down to seriously give it thought. Real life and other such things tend to get in the way! But a few thoughts I have had...

1. I have pondered the idea of how to mount the spring/damper units low down, preferably on the floor. But the obvious problem being that the pivot is effectively at floor level and that leads you to using an upwardly angled push rod.

The key benefit to this in my eyes (other than the obvious CoG etc.) would be that you could really easily build what would basically be a ladder frame chassis that all the suspension would work off. This would be easy to make with excellent precision and you would have a rolling chassis in no time. The rest of the structure could then be made as light and as minimal as possible just to comply to the regs.

2. Mono-shocks and air springs. Kind of conflicting again. I have long pondered air springs as a cheaper and lighter option. Also, we used cane creek shocks/springs and there was basically too much adjustment. Most teams I think struggle for testing so a more basic shock system set up well I think would be better than a fancy system set up badly.

A mono-shock system would also reduce cost. As you are basically de-coupling roll/bump with the LL I think a mono-shock system would work well, although I admit I haven't actually given this more than 2 minutes thought at the moment. If you ran a mono though I doubt an air shock would be beefy enough to cope with the weight of two axles.

3. The whole thing just needs building with a bit more finesse and precision than we managed! But we did the best with the limited time/resources we had.

4. More recent Lancaster teams split the top and bottom wishbones. They still pivot along the same axis but they are separate arms. Our systems the top and bottom arms were welded together at the pivot point. I guess this just makes fabrication easier for them but I'm not sure.

Cheers,
Malcolm

03-10-2014, 06:18 AM

INTERCONNECTED SPRINGING - TWO WHEELS AT A TIME.
=================================================
The subject of interconnected springing is currently being discussed on this "Roll Rates in RCVD" (http://www.fsae.com/forums/showthread.php?11652-Roll-rates-in-RCVD) thread. I am adding these notes here, mainly to keep as much of my "Suspension Design" ramblings on the one thread.

The simplest way to "spring" a car is to put one spring in control of each wheel, namely a "spring-at-each-corner" suspension. At the other extreme a single spring can be connected to, and be in control of, ALL the wheels via some sort of linkage. Search with keywords "Balanced Suspension", "Kinetic suspension" +++.

For now, let's consider the simplest types of interconnection, where a single spring interconnects the chassis and TWO wheels only. The only clear explanation of this particular subject that I have ever seen was an article by Mark Ortiz in RaceCar Engineering magazine in the late 1990s (maybe '97?). There may be other sources for this information, but it is definitely NOT mainstream VD.

If you students apply enough pressure now, then maybe one day Claude will start teaching it. Claude??? :)
~~~~~o0o~~~~~

DEFINITIONS - These covered in more depth elsewhere, but a brief recap here:

* U-BARS and Z-BARS - These are descriptive names given to torsion-bar versions of interconnecting springs. The names refer to their appearance when seen in plan-view and fitted to a car in the "usual" manner. The difference between these two types of spring is that the lever-arms on U-bars both point in the same direction, while the lever-arms on Z-bars point in opposite directions.

The functional behaviour of these springs can be implemented in many different ways (eg. with leaf, coil, or other spring types, and via mechanical, hydraulic, or other linkages...). The important functional differences are;

* U-BARS - RESIST DIFFERENT movements of their ends/wheels (ie. when one wheel moves up and the other wheel down, or vice versa), but ALLOW SAME movements of their controlled wheels (ie. both wheels move up, or both down) with no resistance, other than maybe some friction.

* Z-BARS - RESIST SAME movements (= both up, or both down), but ALLOW DIFFERENT movements (= one up and the other down).

So functionally U and Z-bars are complementary to each other.
~~~o0o~~~

* ALL-WHEEL-MODES (of a four-wheeled, rectangular pattern, vehicle) - These are different ways of describing the motion of all the wheels as they move wrt the car-body. There are many different ways to define these (in fact, four infinities!), so the following is only a taste.

HEAVE-MODE (aka Bounce-mode) - All four wheels move in same direction (either up, or down) by the same amount. So +1 cm of Heave-mode might have all wheels moving up, wrt car-body, by 1 cm.

PITCH-MODE - Both front wheels move up by the same amount, and both rear wheels move down by the same amount. A different, and equally valid, definition for a car with 40F:60R weight split might have "+1 cm Pitch" as front wheels up by 1.5 cm, and rears down by 1 cm. This corresponds to the car pitching about its CG.

ROLL-MODE - Both right-side wheels move up, and both left-side wheels move down, by the same amount.

TWIST-MODE (aka Warp-mode) - One diagonal-pair of wheels move up, and the other diagonal-pair move down, by the same amount. Again, a different definition might have the front wheels moving 50% more than the rears, to better describe a Twist motion about a 40F:60R car's CG.

Note that early work on these concepts (1930s to "active suspension" era in F1) usually had the definitions rather rigidly defined to be "all equal" movements of the wheels, up or down. This can then introduce calculational difficulties when the CG is not at 50% wheelbase (ie. the researchers talk of "mode-coupling", and insist that the Warp-mode must exert a force to help balance the handling).

IMO these difficulties are most easily overcome by REDEFINING the modes so that they better suit your particular problem (and you are free to define anything in any way you want!). This allows the Warp/Twist mode stiffness to always be zero.
~~~~~o0o~~~~~

So far, so simple.

Now what happens when we start fitting U and Z-bars to cars with four wheels? Here is a summary of all the possible interconnections when using either two U-bars, or two Z-bars, between the various possible pairs of wheels. We are looking for the effect these springs have on the various All-Wheel-Modes described above.

U-BARS.
=======
1. Between End-Pairs (ie. one U-bar connects the front-pair of wheels, another U-bar connects the rear-pair).
Stiffens Roll and Twist (Heave and Pitch free).

2. Between Side-Pairs.
Stiffens Pitch and Twist (Heave and Roll free).

3. Between Diagonal-Pairs.
Stiffens Pitch and Roll (Heave and Twist free).

Z-BARS.
=======
1. Between End-Pairs.
Stiffens Heave and Pitch (Roll and Twist free).

2. Between Side-Pairs.
Stiffens Heave and Roll (Pitch and Twist free).

3. Between Diagonal-Pairs.
Stiffens Heave and Twist (Pitch and Roll free).

Again, quite simple. These symmetrical two-wheel interconnections always stiffen up two of the all-wheel-modes, and leave the other two free. But what does all this mean in terms of overall car behaviour?
~~~~~o0o~~~~~

CONCLUSIONS (briefly).
====================
* Importantly, the above description is only for "linear" behaviour. That is, when the lengths of the lever-arms at the end of a given (U or Z) torsion-bar always stay in the SAME RATIO throughout the range of wheel travel. Typically (by design, or by accident) the effective lever-arm lengths will change by different amounts, so giving a rising-rate, or falling-rate, behaviour at each end, and a different force ratio between the ends. This non-linear behaviour can then "add stiffness" to the above "free" modes. This can make a real mess of your best laid plans, or it can be used to considerable advantage (see below).

* In general, anything that stiffens the Twist-mode is, at best, unnecessary, and at worst, VERY BAD (I will explain why in a later post). There are exceptions, but if your suspension layout starts with a zero-rate Twist-mode, then it is usually VERY EASY TO ADD more Twist stiffness. But if your suspension starts with a lot of stiffness in its Twist-mode, then it is all but IMPOSSIBLE TO SUBTRACT that stiffness (ie. it requires a total redesign).

* So of the U-bars, we can cross out the end-pair (#1) and side-pair (#2), because they add Twist stiffness. Note that end-pair U-bars are the all too common ARBs (which are considered mandatory in FSAE, by some Design Judges!). Likewise, we can cross out the diagonal-pair Z-bars (#3).

* Diagonal-pair U-bars (#3) are potentially useful, but have disadvantages. The diagonal interconnection can be difficult to package with a mechanical linkage. Also this arrangement does not control Heave, so some other springing must be used to hold the car up (quite important!). Perhaps worst, the Pitch and Roll stiffening are inextricably linked. So, if you want a stiff Roll-mode, say for flat cornering, then you MUST also have a stiff Pitch-mode. This might be acceptable for a racecar, but a soft(ish) Pitch-mode contributes greatly to ride comfort of passenger cars. The Spanish "Crueat" (spelling?, and maybe Portugese?) hydraulically interconnected suspension uses a variation of this type.

* This leaves end-pair and side-pair Z-bars. Both these control Heave, which is very useful as it is the mode that you MUST have (lest the car drag its belly along the ground).

* End-pair (or lateral) Z-bars are common these days in motorsport, and are usually called "third-springs" (more accurate would be "seventh and eighth-springs", given that they came after the 4 x corner-springs + 2 x ARBs). IMO these have evolved almost entirely by random trail-and-error selection, with next to no deep theoretical understanding. Nevertheless, they work well for big-aero cars because they can greatly (and non-linearly) stiffen Heave and Pitch, and thus provide a stable aero platform while leaving Roll and Twist UNAFFECTED. UWA 2013 car's "W-springs" are end-pair Z-bars.

* Finally, side-pair Z-bars are perhaps the best of all the above options (though the rarest!). Heave and Roll generally have to be the stiffest of the four modes (passenger or racecar), and they carry the largest, and similar, loads (ie. statically each bar carries half the car's H weight, and at high-G cornering the oustide bar carries close to the total car weight = H/2+RollLLT). Also, for given Gs, Pitch-longitudinal-load-transfer during acceleration or braking is less than Roll-lateral-load-transfer during cornering by the ratio of Track/Wheelbase.

Bottom line, side-pair-(longitudinal)-Z-bars have a lot going for them, and are very easy to implement (see next post). And by arranging the linkages to be rising-rate at each end of the bar, they can also control Pitch. (I call this "pendulum springing" and it comes almost for free, which may be why BL-Austin-Morris used it in the 1960s+). However, this rising-rate method has limitations, and smaller, lighter, dedicated end-pair Z-bars (one or two) can be used for more precise control of Pitch, with no stiffening of the Twist mode.

More coming...

Z

03-10-2014, 06:26 AM

"THE NEW PACKARD TORSION LEVEL SUSPENSION".
==============================================
(by F. R. McFarland, presented June 15 1955, and in SAE Transactions Vol 64, p284 1956, 560026.)

I made some comments about this paper on the "Roll rates..." thread (linked on previous post, page 25). Below is Figure 1 from the paper, and my interpretation of the system.

In the image the "Main Load Torsion Bars" are longitudinal-(side-pair)-Z-bars. The "Z" shape is quite apparent. IMO, all things considered, this particular layout is possibly the best way to do longitudinal-Z-bar suspension, both on production cars and many types of racecars. So also quite suitable for FSAE.

The entire torsion-bar+lever-arms lies in a horizontal plane at the bottom of the car, so is easy to package and gives a low CG (ie. all the "spring mass" is at the lowest possible position, and big springs, ie. for off-road or luxury cars, can be quite heavy). The front lever-arm cranks outward, allowing easy connection to the wheel, while giving room for the front-wheel to steer. This lever-arm can be in unit with the lower wishbone, or else it can be connected to the suspension by a flexible link (Packard tried both).

The rear lever-arm must now crank inward (to form the "Z"!). So the main body of the torsion-bar angles out towards the rear of the car. This allows the rear lever-arm to connect to any convenient point on the rear suspension. In this case the connection is by a short "pullrod" to the live-axle control-arm (= "Rear Axle Torque Arm"). Similar pullrod-like connection could be used on any independent suspension.

https://lh4.googleusercontent.com/-d7yirHvFrK8/Ux1ImjoEBsI/AAAAAAAAAPs/5kKGPu0_vwE/s800/PackardFig1.jpg

Other comments:
* The "Compensator Bars" at the rear act as simple corner springs (ie. no interconnections), but both can be simultaneously adjusted to reset rear-ride-height (hence "Levelizers"). This is important for a very softly-sprung luxury car, and is described as "an answer to the stylist's prayer"! IMO an adjustable lateral-Z-bar would be much better here.

* The "Rear Stabilizer Link" is a Watts-linkage for lateral control of the live-axle. This was deliberately made quite soft laterally to reduce "harshness". IMO this is poor design for too many reasons to cover here. Well, just one being rear-axle-(over)steer during cornering! Many ways to fix this, but they didn't...

* A "Front Stabilizer" (ie. lateral-U-bar) is also fitted. This, and the "Rear Axle Torque Arms", both act as ARBs, thus stiffening both the Roll AND Twist modes. IMO this shows a lack of deep understanding of the whole system...

* IMO the Packard engineers didn't seem to grasp this whole concept nearly as well as the French, who were doing this sort of thing twenty years earlier. The whole paper seems to be focussed on the side-view, 2-D behaviour of the car in "bounce and pitch". Admittedly, they had better understanding of these motions than is currently the case with the modern concepts of "front and rear ride frequencies" (see extensive ranting elsewhere! :)). But nowhere in the paper is any Twist or Warp-mode behaviour mentioned. Well, except, and only (!), the last Summary point "8. Reduced torsional stresses in frame.").

* Note that to understand Twist-mode motions, you have to think in 3-D, with the four wheelprints starting in a horizontal plane, and their vertical motions taking them out of this plane. By comparison, the "bounce and pitch" motions discussed in the paper are contained entirely in a 2-D, side-view plane. This limited 2-D thinking seems to have prevented the Packard engineers from fully appreciating the advantages of a soft Twist-mode.

* Finally, the paper starts by noting that "automobile developments seem to appear in cycles", such as manual to automatic gearboxes, straight-8 to V-8 engines, and so on. It ends with "The time is ripe for a cycle of development in automotive suspensions ... it would seem that within the next 2 to 5 years, we should see some radical changes in suspension design.". (My emphasis.)

Well, that didn't happen! And a good indicator of why not is in the subsequent discussion to the paper. The other manufacturer's engineers quite clearly did NOT understand the Packard system (not even its dumbed-down, 2-D, side-view version), as they gave some completely false criticisms of it. Having led with this bulldust, they then launched into a marketing spiel about how great their suspensions were!

Ahh, nothing changes... :)

More in a few days...

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