Beam Axles - Front, Rear or both.

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02-09-2012, 09:31 PM
Z

Harry,

You ask, "So how a [centreline] Z-bar connecting [centres of] front and rear axles contributes to heave/pitch stiffness?"

I think we have to start with a clearer description of what these "four-wheel-modes" mean. The following is not a rigorous definition (not enough space here) but hopefully it is of some help.
~~~~~o0o~~~~~~

Firstly, think of the car's body fixed in space (as the reference frame) and the four wheelprints moving in a generally vertical direction wrt the body. Importantly, we are not concerned with the horizontal (x,y) or rotational (Wx, Wy, Wz) constraints of the wheels or their uprights, which are a function of the control arms (beams, wishbones, whatever). We are only concerned with the approximately vertical (z) motion of the wheelprints, which is the function of the spring-dampers.

To specify the positions (heights) of the four wheelprints, wrt body, we need four numbers. The most obvious choice is LFz, RFz, LRz, and RRz (ie. the wheelprint "z" positions, or perhaps "altitudes" wrt car floorplane). A conventional "spring-at-each-corner" suspension follows this obvious approach and provides a single spring to control each of these numbers.

Now, the fact is that we have four infinities of choices as to how we specify these "four degrees of freedom" (wheelprint heights). I won't go through all these choices http://fsae.com/groupee_common/emoticons/icon_smile.gif , just the following "all-four-wheels" method.

Here we specify our four numbers thus:
1. Bounce mode, Bz = (LFz+RFz+LRz+RRz)/4. This is also called the "Heave" mode. As the equation suggests, this is the average height of all four wheels. We can picture this mode in motion as all four wheels moving up by an equal amount (or down for negative motion).
2. Pitch mode, Pz = ((LFz+RFz)/2-(LRz+RRz)/2)/2. In words, half the difference between the average front-pair and rear-pair heights. We picture this as the two front wheels moving up, and two rear wheels moving down, an equal amount.
3. Roll mode, Rz = ((LFz+LRz)/2-(RFz+RRz)/2)/2. Similar to Pitch mode, but turned 90 degrees.
4. Twist mode, Tz = ((LFz+RRz)/2-(RFz+LRz)/2)/2 (aka "Warp" mode). This time, half the difference between the average heights of diagonal pairs.

So, as an example consider a "single wheel bounce" of left-front wheel up four units (LFz = 4 inches, 4 cm, 4 whatever), and all other wheels at "zero".
1. Bz = (4+0+0+0)/4 = 1 unit.
2. Pz = ((4+0)/2-(0+0)/2)/2 =1 unit.
3. Rz = (likewise) = 1 unit.
4. Tz = (") = 1 unit.

This is saying that a single wheel bounce of LF up 4 units (with all other wheels at zero) is equal to (Bounce = all wheels up 1 unit) + (Pitch = fronts up 1, and rears down 1) + (Roll = lefts up 1, and rights down 1) + (Twist = LF and RR up 1, and RF and LR down 1).

I hope I'm not boring you, but a useful aspect of the above is that all the modes can be measured with simple linear dimensions (inches, metres), and angular measures for P, R, and T are not needed.
~~~~~o0o~~~~~

Anyway, back to your original question. I will redisplay the Z-bar sketch below because 1) it's free, 2) it is too much of a hassle to display another sketch (the sketching is easy, but then scanners, file xfers, Picassa web wanks, http://fsae.com/groupee_common/emoticons/icon_frown.gif ), and 3) hopefully it helps understanding.

Looking at the top-left of the sketch, picture only one "centre-pivot-leafspring" (= a "Z-bar") on the car centreline. Picture the ends of this leafspring as ball-jointed to the centres of the F & R beams. So now the chassis sits ONLY on the centre-pivot of this single centreline leafspring.

It should be apparent that NO Pitch, Roll, or Twist motions (as described above) can be transmitted FROM the wheelprints TO the chassis. So, the chassis responds ONLY to Bounce (=Heave) motions of the four wheelprints. So a centreline Z-bar is purely a Bounce mode spring. It does NOTHING MORE.

The above can be seen by noting that the height of the BJ at the centre of each beam provides an average of the heights of the beam's two wheelprints. The height of the centre-pivot of the leafspring then provides an average of these two averages. Thus the whole linkage (2 beams + CPL) provides an average height of all four wheelprint heights, and nothing more.

So, by INTERCONNECTING all four wheels, this linkage has SEPARATED the all-wheel Bounce mode from the other all-wheel modes (P, R, T). Similar (but a bit different http://fsae.com/groupee_common/emoticons/icon_smile.gif) linkages connecting all wheels can give separate, or "independent", control of each of the other modes.

See SAE paper 2000-01-3572 "Balanced Suspension" , or US Patent No. 6,702,265 (lapsed), for neat ways of doing this.

http://i16.photobucket.com/albums/b3...RFSAEsmall.jpg
~~~~~o0o~~~~~

You ask, "As I see it we have two separate issues to be resolved:
1st is axle location both longitudinal and lateral."

The Mumford Link does lateral control of a beam. My comments in earlier posts about "springing the rockers" would add axle-bounce control (but not axle-roll control). Personally, I do not think the ML is necessary for FSAE (it has too many parts for my taste), but it would work.
~~~~~o0o~~~~~

"2nd is control body movements....
The real issue to me right now is which modes you want to control, why and how."

As you say, with beam-axles it doesn't really matter how much the body moves, because the wheels always maintain the same camber. So Bounce, Pitch, and Roll can all be soft. For independent suspensions with little "camber recovery" (ie. long "virtual swing arms") it is beneficial to stiffen the Roll mode, but keep Bounce and Pitch soft (at least between limits, ie. bump stops). For sprung-aero cars the Bounce mode has to be stiff (to maintain constant ride height), but the Pitch mode can still be soft.

VERY IMPORTANTLY, in all cases there are huge advantages in having a completely soft twist mode (again, between bump stops that limit the range of twist).

It is truly astonishing that motorsports is the only sub-section of the "land vehicle" community that doesn't realise this! Even more incredible is that many of the people involved have been made aware of this, yet couldn't be bothered even thinking about it. Truly brain-dead!!! (Or just as likely, they have no need, or desire, to win!)
~~~~~o0o~~~~~

"Actually the only movement I would like to limit somehow is single wheel bump. Any thoughts on this?"

Why?

The big advantage of a soft Twist mode is that it allows the car to easily drive over large single wheel bumps.

Looking again at the example at the top of this post, if a car has rigid Bounce, Pitch, and Roll modes, and a completely soft Twist mode, then it can drive, even very slowly, over a 4" high single wheel bump with the car's CG only moving up 1", and the body only pitching 1" and rolling 1" (as defined earlier). (Picture top-left of Z-bar sketch with stiff leafs and coils, then look at top-right for the effect of a soft Twist mode.)

Most importantly, the soft Twist mode means no changes to the vertical wheel loads over bumps (not considering inertia). That is why almost everyone else bar the motorpsorts community uses it. (See Appendix of above-referenced SAE paper for examples.)

Z
02-10-2012, 03:37 AM
mech5496

Z,

First of all, thanks about all the info. Everything is put much much better than vehicle dynamics books I know and far easier to understand, so thank you (and I believe I speak for every member on the forum). I have already gone through the "balanced suspension" patent numerous times, and I have to admit it is more than interesting! (Actually I have a quite funny story behind that exact patent; have you ever heard about a Greek magazine called R&D? I bet you didn't, but they had a presentation on the "Zapletal suspension" back in 2006, so the first time I actually read it was when I was at highschool....)

Anyway, the reason I thought a stiffer single wheel bump will be beneficial is the camber change at both wheels, as well as gyroscopic phenomena of the axle while during this...but I assume the latter can be cured with fairly soft "springing" and relatively high damping. Plus, as you said, "the soft Twist mode means no changes to the vertical wheel loads over bumps". A quick calculation says that I would have a total of 2.3deg camber change for 2" single wheel bump (improbable in a FSAE track), so I suppose I could trade that off for all the above gains.
02-10-2012, 05:19 AM
DougMilliken

Quote:

Originally posted by Z:
Here we specify our four numbers thus:
1. Bounce mode,...
2. Pitch mode,...
3. Roll mode,...
4. Twist mode,...(aka "Warp" mode).

This is how the control system software for the Lotus Active Suspension was configured, starting in the early 1980's. By the time they got done, they built something over 80 prototype vehicles with variants of the system--from F1 cars to large single-unit trucks (for Volvo, among others). I drove a light tank fitted with a variant of the system that greatly reduced pitching, when compared to normal tracked vehicles.
A search will turn up a number of pages with various descriptions. In typical racing (and "trade secret") style they did not do a lot of technical publishing. Most of the written documentation went to specific customers.
02-21-2012, 12:51 AM
Z

Quote:

Originally posted by mech5496:
Anyway, the reason I thought a stiffer single wheel bump will be beneficial is the camber change at both wheels, as well as gyroscopic phenomena of the axle...
...
A quick calculation says that I would have a total of 2.3deg camber change for 2" single wheel bump (improbable in a FSAE track)...

Harry,

It is worth considering how these "bumps" affect the suspension.
~~~o0o~~~

First of all, bumps can come as "mountains", or as "molehills", ie. big or small.

1. Let's say that "big" bumps have a smooth sinusoidal shape in horizontal directions (x and y), with a wavelength significantly longer than the car (so, say, 3++ metres). Sort of like longish waves, or "swells", on the ocean. Given the relatively low speeds of FSAE, a car driving over these "undulations" will not feel significant vertical accelerations of its wheelprints wrt body, so discussions about "unsprung weight" are frankly irrelevant. Likewise, gyroscopic forces are minimal, so changes in wheel inclination angle are not a problem.

However, these undulations will twist the four wheelprints out of a flat plain, and thus very adversely affect the wheel loads of a stiff twist-mode suspension (ie. most racecars). So, a soft twist-mode is a big benefit here. Furthermore, in this case the best camber angles for the tyres is relative to the road surface as drawn as a straight line through laterally paired wheelprints. So the camber change of a beam-axle is just what you want. http://fsae.com/groupee_common/emoticons/icon_smile.gif

2. On the other hand, what about short wavelength bumps (say, <~1m wavelength)? I call these "corrugations". Now the accelerations are more severe, and, together with gyroscopic effects, suggest a suspension with lightweight wheel assemblies and NO camber change during the frequent single wheel bumps.

Is the resulting "zero camber recovery" bad for cornering grip? Not really. The fact is that in this situation the wheels are rolling over ground that is constantly changing its relative "inclination" angle to the wheel. So for about half the time there is an effective positive camber angle, and the rest of the time there is an effective negative camber angle. If the bumps, or ruts, are short enough the wheel might have both positive and negative "camber" at the same time (ie. on either side)!

In this case I think worrying about tenths of a degree of "camber recovery" is pointless. However, a soft twist-mode (as well as soft all-other modes) is beneficial because it gives more constant vertical wheel load, and thus better all round grip.
~~~o0o~~~

To sum up:

1. For smooth "undulating" roads, like circuit racing and FSAE, the car benefits from a soft twist-mode, and wheels that are kept at a constant camber angle relative to a line drawn through lateral pairs of wheelprints. So, for example, use beam-axles, or lateral swing arms with a lateral Z-bar that minimizes "axle-bounce", but allows "axle-roll".

2. For harsh "corrugated" roads, like rallying and off-road racing, the car benefits from a soft twist-mode, and wheels that are kept at a constant camber angle relative to the body. So, for example, use leading and trailing arms, or, if you must use lateral wishbones, make them long, equal length, parallel, and horizontal at normal ride height.

Z

(Edit: 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.)
02-21-2012, 05:31 AM
Tickers

Z,

As much as I love the idea of a simple go-kart type FSAE car, I look at the sketches and ideas in this thread and wonder where the simplicity is!

Teams aren't just using wishbones and push/pullrods because that's what "real" single seaters use; they use them because they're easy to make. The parts are almost all built from simple materials (rods, tubes, plates etc) that are all small and manageable to fabricate.

Sure, there can be "interesting" handling characteristics if the geometry is poor, but the teams who only care about getting a car going can just weld up a few tubes (with the required rod ends in bending) and get out on track. Teams who know what they're doing can spend a while analysing the kinematics for decent geometry, with a large amount of existing resources to guide them.

The ideas here require large parts and assemblies that would need to be fabricated, either by machining (expensive for large parts) or casting (expensive for anything). Not only are the systems you're advocating likely to cost more to produce than a double-wishbone setup (which puts off the poor teams), they'll also put off the lazy teams as they'll have little to guide them, meaning they have to design the whole thing themselves!

A go-kart would be fantastic, but unfortunately we're required to have suspension. In trying to accommodate the requirements while avoiding double-wishbones in the name of "simplicity" I think you've done the opposite.
02-21-2012, 08:56 PM
Z

Quote:

Originally posted by Tickers:
... I look at the sketches and ideas in this thread and wonder where the simplicity is!

Tickers,

Groan!!! http://fsae.com/groupee_common/emoti...n_rolleyes.gif I have heard this line so many times before...

People do things in ridiculously complicated ways, and then say "Oh, it's not difficult at all!"

Why???

Because that's the way they have always done it! http://fsae.com/groupee_common/emoti...n_confused.gif
~~~o0o~~~

You say, "Teams [use wishbones] because they're easy to make."

And, The ideas here require large parts and assemblies that would need to be fabricated, either by machining (expensive for large parts) or casting (expensive for anything)....

I suspect that the above is just a leg-pull, but if not, then it is pure crap! http://fsae.com/groupee_common/emoticons/icon_smile.gif

I have been through the "easy to make" process of wishbones, and while I agree that it is a mind-numbingly boring and brainless job, that fact is that making all those little gubbins, fishmouths, gussets, etc., MULTIPLIED BY EIGHT, takes a long time.

The two beams I drew would be fabricated from a few lengths of tube and some sheet-steel gussets (no "machining large parts", or "casting" - where did that crap come from!!!). From past experience I reckon it would take about half the time of a full wishbone setup.

But the even bigger gain comes from the simpler chassis with its much smaller number of hardpoints. Only four of these hardpoints need to be accurately positioned, and they are all on the centreline of the chassis. So much easier than when hardpoints are scattered all over the place.
~~~o0o~~~

It never ceases to amaze me how fearful Homo Sapiens are of change. Even the young ones!

Any pissweak excuse to avoid change will do, "... have to design the whole thing themselves!".

Any FSAEers with testicles out there????? http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z
02-22-2012, 01:46 AM
mech5496

Z, you have to admit there are terribly complicated ways to build a beam axle... Like the concept our team evaluates, which you might all see as long as we finish the manufacturing of our current car. Tickers, simple beams can be built very very cheaply and easily. Polished-up concepts to shave off some unsprung are a bit more complicated, but even if you say that it has the same complexity with a double wishbone setup, I'm pretty convinced there ARE performance gains in beam setups. You just have to think it as the whole system, not just the suspension... Right now you are staring at the tree, while missing the woods behind it....
02-24-2012, 01:40 AM
whiltebeitel

Quote:

Originally posted by Z:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by Tickers:
... I look at the sketches and ideas in this thread and wonder where the simplicity is!

Tickers,

Groan!!! http://fsae.com/groupee_common/emoti...n_rolleyes.gif I have heard this line so many times before...

People do things in ridiculously complicated ways, and then say "Oh, it's not difficult at all!"

Why???

Because that's the way they have always done it! http://fsae.com/groupee_common/emoti...n_confused.gif
~~~o0o~~~

You say, "Teams [use wishbones] because they're easy to make."

And, The ideas here require large parts and assemblies that would need to be fabricated, either by machining (expensive for large parts) or casting (expensive for anything)....

I suspect that the above is just a leg-pull, but if not, then it is pure crap! http://fsae.com/groupee_common/emoticons/icon_smile.gif

I have been through the "easy to make" process of wishbones, and while I agree that it is a mind-numbingly boring and brainless job, that fact is that making all those little gubbins, fishmouths, gussets, etc., MULTIPLIED BY EIGHT, takes a long time.

The two beams I drew would be fabricated from a few lengths of tube and some sheet-steel gussets (no "machining large parts", or "casting" - where did that crap come from!!!). From past experience I reckon it would take about half the time of a full wishbone setup.

But the even bigger gain comes from the simpler chassis with its much smaller number of hardpoints. Only four of these hardpoints need to be accurately positioned, and they are all on the centreline of the chassis. So much easier than when hardpoints are scattered all over the place.
~~~o0o~~~

It never ceases to amaze me how fearful Homo Sapiens are of change. Even the young ones!

Any pissweak excuse to avoid change will do, "... have to design the whole thing themselves!".

Any FSAEers with testicles out there????? http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z </div></BLOCKQUOTE>

Z, there is a limit to the steps in thickness of availible material for these giant leaf springs with big center pivots. I can cheaply buy coil springs in 5 lbf/in increments to tune my suspension, and a few click on the damper or a spin on my 5" long ARB blades. How does one tune this great big deal? A truckbed of leaves of spring steel?

Also, twist modes aside, there's a lot to be said about camber thrust and adjusting your suspension to give you the camber you need under roll to improve grip. Sidewalls are compliant, and often not to our advantage.

That said, this IS an interesting concept. They said rear steer could not work on the Thrust SCC, but some guy made it happen. Are you willing to help make it happen for a team, or just for a demonstration?
02-24-2012, 03:38 AM
Z

(More groaning... "like trying to put brains into statues....", grooooaaan....) http://fsae.com/groupee_common/emoticons/icon_smile.gif

whiltebeitel,

You say, "there is a limit to the steps in thickness of availible material for these giant leaf springs..."

At the top-left of the sketch it says "Z-Bar Concept". I drew the "centre-pivot leafsprings" for those "imaginatively challenged" students who can't figure out how a torsional Z-bar works. The lateral CPLs shown at bottom-right are the sort of thing that an apprentice blacksmith could make without a second thought. But a bit too much for you??? (Mumble, mumble, "education system, grrrr...")
~~~o0o~~~

"How does one tune this great big deal?"

Top-right! ERMD (=LLTD) = B/A. http://fsae.com/groupee_common/emoti...n_rolleyes.gif

(Key point: By geometry NOT spring rate!)
~~~o0o~~~

Are you willing to help make it happen for a team, or just for a demonstration?"

Read the Appendix of the SAE paper referenced above (just above Z-bar Sketch this page). As explained there, twist-soft suspensions are by far the most common for real working vehicles. Motorsports is in a tiny minority that hasn't figured it out yet. (Braindead!!!!)

Z
02-24-2012, 12:52 PM
whiltebeitel

Quote:

Originally posted by Z:
Read the Appendix of the SAE paper referenced above (just above Z-bar Sketch this page). As explained there, twist-soft suspensions are by far the most common for real working vehicles. Motorsports is in a tiny minority that hasn't figured it out yet. (Braindead!!!!)

Z

Any relation the the author?

It's a wonder more of your ideas have not caught on, with such a flowery personality and charm.
02-24-2012, 05:05 PM
Z

Quote:

Originally posted by coleasterling:
... but there comes a point when you have to actually make it and PROVE superiority.

coleasterling,

And for the Nth**n time, it is the most thoroughly proven concept in the ground vehicle industry!!!

Read the Appendix in the above paper, and then YOU PROVE to me how modern racecars (the odd ones out) are superior. http://fsae.com/groupee_common/emoticons/icon_smile.gif
~~~o0o~~~

"... you're just that crazy old guy who yells about how dumb kids these days are..."

The crazy thing is that I feel like the teenager living in the Old Folks Home.

OFs "Have some tea and digestive biscuits Z."

Z "Aw, but I want to go out and play. You know, do something interesting..."

OFs "No, no, Z, don't put any sugar in your tea! You'll get too excited. Now you don't want to miss Bingo this afternoon. You know how much we all enjoy that..."
~~~~~~o0o~~~~~~

whiltebeitel,

Yes.

"flowery personality and charm"?

The OFs like it.

Z
02-24-2012, 11:10 PM
Gruntguru

Quote:

Originally posted by Z:
ENGINE. The engine in the Twin-Beam sketch was supposed to be a Royal Enfield, based on info I found on various websites. It won't work as shown. I got parts of the old and new models mixed up...

But, funnily enough, the day after I posted the sketch I was tyre-kicking in a quad bike shop and guess what's sitting in the corner? Three old British bikes - a Norton twin, Matchless 500 single, and a Royal Enfield 500! (Old Z had a Matchless 500 engine that went in every "fun" vehicle he dreamt up. I clearly remember it NOT being a great success in the canoe (I was about 10). The first time the throttle was cracked open the propellor spun maybe five times one way, and the canoe did a half turn the other!)

So I had a good look at the RE, and I like it! If lots of REs were available, at the right price, then I would certainly consider one. The long term goal would be a turbo or supercharger (with standard CR~7:1), crank driven clutch and dog-neutral, then direct chain final drive ~5:1 (ie. ditch the gearbox).

Royal Enfield motorcycles are available new - made in India. http://www.enfieldmotorcycles.com/

Indian Institute of Technology Roorkee competed at FSAE Australasia in 2011 using this engine.
02-25-2012, 02:39 AM
Z

Gruntguru,

Unfortunately RE have updated the engine so that it and the gearbox are now of unit construction. The advantage I saw in the old engine was the separate gearbox that could be relocated to the front of the crankcases, for better overall packaging.

Or better yet, the gearbox could just be deleted, then some variation of single or dual chain drive used. The RE turns slowly enough that a direct final drive is possible. Less is more!

The old style engine had air-cooling with big fins, 2 valve hemi-head, approximately square bore/stroke, roller main bearings but pressure fed slipper big-end, dry-sump with separate inernal scavenge and pressure pumps and internal oil tank, and a direct drive alternator. Not too far from what I would want in a bespoke FSAE engine.

Z
02-25-2012, 06:03 PM
Z

Cole,

You say, "... I do have the opinion that your overall package is way out there for an FSAE car."

By "way out there" I guess you mean that you don't approve of it, and you think that it won't work?

Well, around the same time I was pushing the "brown go-kart" back in 2005, RMIT (with Geoff Pearson) built a car that was a step in that direction, and it was very successful. Quite a few more teams are now going in that direction (witness all the recent debate about "singles vs fours" that wasn't there in 2005).

However, the simple, lightweight single (with aero!) is still in the minority, so most FSAE students think that this approach is a "radical" step with lots of associated risks (eg. many think that they have to go "carbonfibre + titanium" to have a chance).

I am just trying to reassure these students that the KISS approach is in fact the easiest way to outright victory. (And please note, KISS does not mean mindlessly copying the unnecessarily complicated "standard" FSAE car.)
~~~o0o~~~

"....you've "already explained why" the single or two gear concept is better, but again, saying it and proving it are two different things. You speak these things like they are gospel when there are no results to back them up in FSAE."

Quote from Kevin Hayward in "Any way to objectively choose engine?" thread.
"Many cars in FSAE have done very well running the endurance in one gear. Even with heavy cars."

And, incidentally, Kev's old school, UWA, has successfully run soft twist-mode suspension for quite a few (?) years.
~~~o0o~~~

"... but dang those insults are getting old."

If you do something really stupid, and you are told that it is stupid, then it is only an insult if you refuse to believe that it is stupid.

A kick up the bum can be good for you. Thinking back, I wish I got a few more at certain times in my life. http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z
02-25-2012, 07:17 PM
Gruntguru

Love this thread. I have been advocating the single with aero and beams for a while. Now the "soft twist" concept has my attention but there are so many questions. For example - in the top left Z-Bar drawing, how would you adjust RMD? Adjustable side pivot location to change A/B would be a design challenge. Would fiddling the RCH's be sufficient? Perhaps an ARB at one end? And how to select the longitudinal location of the side pivots - at the CG? Slightly further back if running a spool?

These are some of the design challenges that would make teams wary of such a radical departure from "typical" FSAE layouts.
02-26-2012, 04:41 AM
Z

Gruntguru,

The top-left Z-bar sketch was mainly intended just to explain the concept. In practice it could be done, but the RMD wouldn't be easily adjustable, so you would have to work that out first.

That's why I also did the bottom-right sketch with its longitudinal Z-bars. These are harder to understand, but they make it easier to adjust RMD, simply by adjusting relative A and B lengths.

One of the main advantages of beams is that they can run very soft springs, so the suspension is already reasonably twist-soft. However, to adjust ERMD you have to change springs, which can be time consuming, expensive (need lots of springs), hard to make fine adjustments, etc.

So if you have beams with longitudinal Z-bars (and perhaps soft central coils like in top-left sketch) you can more easily adjust RMD via the geometric change to A/B lengths, a relatively simple bit of mechanical detail design.

Also, I would suggest to start with a soft damper-at-each-corner. So if the Z-bars get too confusing you can scrap them and easily go back to a spring-at-each-corner (on the dampers).

Z
02-26-2012, 05:33 PM
Z

I should also note that "Z-Bars" can be done "2CV style" (see 2CV pic somewhere? on these pages).

So let's say you are already running push/pullrods-and-rockers, and the pivot-axes of the rockers lie in a vertical-lateral plane. Now you can interconnect the pair of rockers (F&R) on each side of the car with a pullrod that acts as a Z-bar. Ie. when one wheel goes up, it pulls the rod, which forces the other wheel down.

Practically, the Z-bar-pullrod could be a ~4mm diameter cable with its ends wrapped around pulley sections on the rockers (like those machines in gyms). Somewhere along the length of cable would be fitted a compression spring, in the fashion of the springs used on tent ropes.

Importantly, the SPRING-RATE does NOT have any influence on handling balance via ERMD. The springs are there for comfort. The ERMD (=ELLTD) is determined soley by the geometry of the rocker arms (etc...), and is thus independent of bounce/pitch/roll motions of the car, and (importantly!) of twist in the roadway.

A system like this is the only justification for "rockers" that I can think of (ie. at least here the rockers are doing something useful!). Its practical implementation would be quite easy, being similar to current layouts, and allowing flexibility in the positioning of the Z-bar-pullrod/cable runs.

(Note also: Only three Z-bars total are required for sufficient control of the suspension of all four wheels. I'll let you students figure that out...)

Z
02-27-2012, 01:36 PM
coleasterling

Quote:

Originally posted by Z:

"Many cars in FSAE have done very well running the endurance in one gear. Even with heavy cars."

And, incidentally, Kev's old school, UWA, has successfully run soft twist-mode suspension for quite a few (?) years.
Z

How are you defining "well" and "success" here? Doing well, by my definition, is putting down an endurance pace capable of giving your team an overall win. To my knowledge, no team running a single gear has been able do this. That certainly doesn't support your concept.

How successful is UWA? My definition of success is winning the competition. Not to mention, sure, UWA is soft-twist, but they are still nowhere close to your overall concept.
02-27-2012, 03:06 PM
Gruntguru

Quote:

Originally posted by coleasterling:
How successful is UWA? My definition of success is winning the competition. Not to mention, sure, UWA is soft-twist, but they are still nowhere close to your overall concept.
-Cole

A pretty old list but I don't think WA's wins have been reversed.

Team Name # of Wins
Cornell University 9
University of Texas - Arlington 8
RMIT 4
Georgia Tech 3
University of Toronto 3
University of Wollongong 3
Texas A&M University 3
Sophia University 3
Graz University of Technology 2
USP São Carlos 2
University of Western Australia 2
University of Stuttgart 2
California State University - Pomona 1
Centro Univ. da FEI 1
Kanazawa University 1
Steven's Institute of Technology 1
University of Akron 1
University of Applied Sciences Graz 1
University of Houston 1
University of Maryland 1
University of Michigan - Ann Arbor 1
University of Texas - Austin 1
University of Wisconsin - Madison 1
Virginia Tech 1
Faculdade de Engenharia de Sorocaba 1
02-27-2012, 04:55 PM
Pete Marsh

Quote:

I wasn't claiming they don't have any wins. It was a genuine question. The statement still stands, however. They build a car that is nothing like the beam-axle concept.

There has been one more win since then, and a few more if you are only looking at dynamic wins. Not the same concept sure, but many similar outcomes. Ours is more of an "add on" way of achieving soft twist.

We have run the system for a long time, even after the sponsorship went away in the GFC. So it must have something going for it. It has no advantage in the design event, and until a judge drives it, will remain extremely difficult to justify to them.

On the flip side, Stuttgart has a pretty good dynamics record too, with a pretty similar concept but with a stiff twist mode, and the FSG track is far from flat.

So, hard to tell if the soft twist is much of an advantage, but clearly UWA, and some stop watches, thinks so.

Pete
04-11-2012, 08:36 AM
Will M

Quote:

Originally posted by Z:
A few weeks ago on the "suspension for spool" thread I said I would post a sketch of a Twin Beam-Wing concept. I think this is a more suitable thread, so here it is (warts and all http://fsae.com/groupee_common/emoticons/icon_smile.gif).

https://lh5.googleusercontent.com/-8...inBeamWing.jpg

(Edit: Made image bigger.)

Main features:

* This is an "aero above all" car. Everything else is there just to support the aero. I figure "drive on the ceiling" at less than maximum FSAE speeds (ie. DF=W at <100kph, (maybe <70kph?)). And more is possible!

* Aero is direct acting on the wheels, and easily adjustable for F/R balance via the flaps. Since wings are close to ground, and with wheels as skirts/end-plates, drag is low (no induced drag). The streamlined fuselage and wheel pods also lower drag for good economy.

* The aero centres of pressure are unlikely to be exactly above wheel axle lines, so some force acts on chassis through BJs causing some pitch/heave. This can be fixed by either; a) not worrying about it, b) reshaping the wings, or c) interconnecting the wings with two torsionally flexible spars running alongside the chassis and attached flexibly to the beams on their axle lines. Extending this last solution sideways gives a full width "live" undertray.

* Finally, a first year or "limited resource" team can do this car without the aero. Use a RE, B&S, or similar engine and you have a simple lightweight car that is quicker to build than the usual wishbones-and-pull/pushrods&rockers-everywhere cars. And, all other things equal, it will have high grip and benign handling because of soft springs with no camber change (not possible with normal independent suspensions).

Comments and criticisms welcome! http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

Z,

With the softer springs you should see more travel and I estimate from your drawing that a 2cm travel will cause about 2.5 degrees for caster change in the wheels and angle of attack for the aero (assuming the front and rear aren't linked). You might only see that much travel under heavy braking or hitting a bump and it might not make a big difference but it is another design consideration I wanted to point out.
04-11-2012, 10:02 AM
mech5496

How the hell did you figure that "2.5 degrees of caster change"?
04-11-2012, 02:28 PM
Will M

mech5496,

Unless I am completely not understanding the idea (I am not a suspension guy really) this should be right. Also I got the 45cm from Z's drawing.

William

http://s13.postimage.org/mfmip1r1j/001.jpg
04-12-2012, 05:35 AM
Z

Quote:

Originally posted by Will M:
I got the 45cm from Z's drawing.

William,

Actually, it is 50cm (lower right of picture).

Aerofoil AoA will only change about half your figure under extreme braking (even with +/-2cm F&R), but regardless of AoA the downforce can be kept fairly constant with the right profiles (ie. maintain a constant gap to ground). If the two main BJs are closer to centre of wheelbase (in side-view), then less AoA and castor change during pitch. Also there is quite a lot of anti-dive/lift in the kinematics, which lessens pitch under braking.

But the main point here is that you do NOT want to do a lot of braking or accelerating. Fast cornering, and a "constant speed car", is best! http://fsae.com/groupee_common/emoticons/icon_smile.gif

Also, the wings as shown are mainly for ease of construction. Just a "plywood" add-on to a simple beam-axle suspension suitable for a first year team. More advanced and powerful aero is hinted at in my quoted text in your first post.

Lastly, lots of castor change (+/-10+deg!) over bumps is NOT a problem. And FSAE tracks don't have bumps! http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z
04-12-2012, 06:22 AM
mech5496

Actually you could have NO (or very limited) pitch under braking by designing a certain amount of anti-dive and using a dedicated pitch/heave "spring" and "damper" (a polymer puck could replace both), directly on the sliding mechanism, so it does not affect roll at all. What's more, what is so bad about a degree of caster change?!
04-12-2012, 07:47 PM
Will M

I am actually a big fan of this car architecture, I'm just trying it for possible weak areas. The main problem I see with dynamic caster change is if it went from positive to negative or the other way around that could be disconcerting for the driver. Caster change is not bad, like bump-steer it is a knob you can turn but you do have to think about it.

Z,

Also. If you were building one of these how much adjust-ability would you add? If the geometry was locked in the beam could be very stiff and light... Maybe two sets one adjustable and then a second fixed one...

William
04-14-2012, 04:06 AM
Z

William,

"How much adjustability?

Good question. I didn't want to put too much detail into the earlier posts, but I'll briefly cover this important issue now. What follows applies to teams that are fairly new to racing, which is pretty much all FSAE teams, and certainly any that might try beam-axles (because very few examples to "learn from" (ie. copy http://fsae.com/groupee_common/emoticons/icon_smile.gif)).

The following also applies to any suspension type on smooth track circuit racing. So this is mainly about going fast around tarmac corners. Top of the list is most important, and less important further down.

Important Suspension Adjustments for Fast Cornering.
==============================================

1. Tyres - Type (carcass/compound), Size, Pressure, Rim Width.

2. Static Toe - to +/- one tenth degree.

3. Static Camber - to +/- one degree.

4. Front/Rear Roll Rates - ie. LLTD or ERMD (same things). Note Pitch/Heave rates are important for sprung-aero cars, but not considered in detail here.

5. Dampers - just enough to suppress resonant bouncing on above springs, or on the tyres.

6. Kinematics - RC heights, anti-pitch, camber recovery, etc.
~~~o0o~~~

Note that things like Ackermann and compliance are also very important in FSAE (in fact, close to top of list, depending on details), but will leave that for another time. Also a spectacular cock-up on something like Kinematics (say, severe rear bump toe-out) will push it to the top of list. And also not considering Z-bars (can be added later).

So from the above, and IMHO, the biggest gains come from lots of testing of different tyres, their hot pressures (anything from 0.5 bar (7psi) to 2+bar (30+psi) can be "optimal", depending on...), and also rim widths, which can make a big difference.

On the other hand, any kinematic issues can be covered by other adjustments (again, unless you truly stuff it up). A brief example is Formula Vee, which mandates Beetle suspension with ground level front RC and ~axle height rear RC, but corners as fast as any other similar single seater on same tyres (hint: high rear RC compensated by zero rear roll rate springing).
~~~o0o~~~

So for a beam-axle FSAE car I suggest the kinematics be non-adjustable, as in the sketch. That is, roll and pitch axes (strictly speaking, the beam's "cylindroid") as low as practical, which is just above floor level (to allow room for the brackets, bolts, etc.). (See note re: De-dion below.)

The spring-dampers can be direct acting as in sketch. LLTD adjusted by changing springs or, for fine adjustments, small changes to the angle or axle mounting points of the S-Ds might be used. Preferably do NOT use any ARBs, but if you must, then use only one (say, a stiff one at rear to compensate for a "spool" differential). Do not overcomplicate this! There are much bigger gains elsewhere. http://fsae.com/groupee_common/emoticons/icon_smile.gif

That leaves the two big ones on the suspension front, namely static Toe and Camber angles.
~~~o0o~~~

FRONT AXLE - Static Toe is easily adjusted via the steering tie rods. I suggest building one structurally efficient beam, then adjusting Camber and Castor (less important) at the king-pin. This is an opportunity for some good detail design, but even a crude overweight design won't cause too much harm here (as long as it doesn't break!).

REAR AXLE - More options here.

Live Spool, as one tube wheel-to-wheel. Here Toe and Camber are fixed at zero, but it is important to make everything stiff enough.

Live Spool, as two half axles with spline/CV at centre. This is the reason for the four bearings (eg. "pillow blocks"?) in the sketch. Toe and Camber now easily adjusted by moving relative position of inner and outer bearings for each half-axle.

Live Differential. This is as above, but with any sort of beam mounted differential (eg. open/LSD). Again, Toe and Camber easily adjusted.

De-Dion, with chassis mounted differential of any type (open/LSD/spool). This option may make the final drive easier, since the diff location is fixed wrt engine. This requires CV'd half-shafts and stub axles at the beam ends of similar type to independent suspensions. I suggest the stub axle housings have some means of adjustment for Toe and Camber, if only to correct for manufacturing tolerances of the beam. These can then also be used for "tuning". Again, a good opportunity for quality detail design!

Important De-Dion note. The side-view kinematics, or anti-pitch, changes when the diff is chassis mounted. A layout as in the sketch, but with De-Dion, will have lots of pro-squat under power. This can be ignored with a low power engine, or a beam-centre bump rubber (aka "third-spring", or "lateral Z-bar", as mentioned by Harry above) can be used to limit squat (axle bounce) but allow roll. Alternatively, a different kinematic location can give anti-squat (higher pitch axis) while maintaining a low roll axis. This is a quite simple structural change, but will require a sketch to explain it properly...
~~~o0o~~~

I also discussed suspension adjustments on this old thread.

Z
04-14-2012, 12:15 PM
Charles Kaneb

Z,

A couple things I've run into while trying to convince my team of the merits of a car with suspension other than A-arms all round:

1. With the required cockpit volume, side impact intrusion prevention, front and rear roll hoops, and supports for said roll hoops, you end up with a very torsionally stiff center section of the car. I'll mail a dollar to anyone with results showing a currently legal car with end-to-end torsional stiffness of less than 800 lbs*ft/degree. Any significant torsional deflection would have to be concentrated in a very small area near the back of the car. At this point, you'd have committed to weld in a nonadjustable twistable element with unknown damping properties that, if karting experience shows anything, will gradually change over a couple dozen hours of racing. Even if you figure out how to get good properties from this, there's nothing that says a tech inspector won't require you to weld in some supporting bars at competition.

2. I haven't had the time to draw up and do the stress analysis for a preliminary design of a beam axle rear end. I do not know whether it will save weight, or how much. The heaviest components in an FSAE suspension right now are the axle cups, the axle bearings, and the spring-damper units. If a beam axle rear end is not set up to be extremely stiff in roll, your sprocket will tip about the X-axis, and motorcycle chains or synchronous belts do not tolerate this well at all. A De Dion rear end is at least as complex as any double wishbone setup.

3. Vehicles which are soft around the Y-axis (twist mode) are used in many applications. Outside of racing and certain military off-road vehicles, there are very few vehicles where the main differences between success and failure are peak tire grip and controllability near peak tire grip.

My main experience with operation of a vehicle with a soft twist mode near its dynamic limits was on a driver-training day for garbage-truck drivers. The garbage truck consisted of a torsionally flexible twin-rail chassis with a rigidly attached cab, above which was "floated" a garbage compactor/storage box on flexible mounts. The truck entered a fairly tight, constant radius 180 degree turn, at a speed such that the tires squealed slightly. The chassis deflected about the Y-axis, storing up strain energy and keeping the inside rear tires on the ground. So far so good. Then the corner ended. The driver straightened the wheel. The chassis released its stored energy. The compactor box was tossed several inches in the air and landed in a slightly skewed orientation on the chassis. Meanwhile the rest of the truck wobbled back and forth...

4. Almost every chassis rules change since the start of FSAE has been anti-kart; in addition, up to 1/5 of the team score is in a design event with judges who are unlikely to have experience with design and optimization of karts. Just to get this out of the way, a kart with a 450cc motocross engine and a working vacuum traction system would absolutely wipe the floor with every FSAE car ever made. First, we're required to have some form of "working" suspension. Second, we're required to have a reasonably torsionally-stiff chassis. Third, we are required to have wheels that are at least 8" diameter (and no remotely competitive tire smaller than 10" exists); the resulting 18"+ tire diameter means that the suspension system must handle a large Y-bending moment during cornering and a large X-bending moment under braking. It will be lighter and cheaper to handle these moments with a small amount of material far away from the centerline of the axle than wih a large amount of material near it. Finally, the aerodynamic downforce generating components we're allowed to have are not terribly sensitive to roll or twist - more to pitch and heave.

The advantages of a fewer-moving-parts suspension have been heavily cut back by practical and rules constraints, while the risks of moving to one include possible disqualification, rules changes which would ban the car for next year and make what we learned about how to make one of these cars handle irrelevant, and a very real possibilty of a design-event score that's so bad it would wipe out any dynamic event gains this thing could make. When we build a car with four-wheel independent suspension by A-arms, with constant-rate passive springs and passive dampers, we are not simply polishing a turd - we are selecting a design that has proven to be at least OK at its job and will not be banned or drop-kicked in design.
04-15-2012, 07:36 PM
Z

Charles,

Thanks for comments. Here are some brief responses.
~~~o0o~~~

1. The mandatory chassis safety parts (roll-hoops, side impact,...) form most of the chassis for the Twin-Beam concept. No more torsional stiffness is needed, because beams allow soft springs. I would not suggest adding any extra torsional "softness" to the chassis, again, because the soft corner springs already give enough softness. The advantages of longitudinal Z-bars is that they can give a completely soft twist-mode, even with a stiff chassis and stiff roll-mode, this latter being necessary with wishbones with small camber gain (long FVSA). Also the Z-bars make handling adjustments (ERMD) very easy.
~~~

2. I think the possibility for weight reduction by using beams over wishbones comes mainly from the much simpler chassis with its smaller number of hard points.

You say "A De Dion rear end is at least as complex as any double wishbone setup."

I have to disagree. I reckon a detailed study of a De Dion would have it roughly half as "complex" (time to make, etc.) as double wishbones. Likewise, there are many other simple suspensions that are almost never considered. The "proof" of this is that almost all FSAE teams use the completely unnecessary push/pullrods-&-rockers, so they are clearly not concerned by useless complexity, and make no attempt at "simplicity". Well, other than talking about "KISS". http://fsae.com/groupee_common/emoticons/icon_smile.gif
~~~

3. "Outside of racing and certain military off-road vehicles, there are very few vehicles where the main differences between success and failure are peak tire grip and controllability near peak tire grip."

Well, how about every farm tractor ever built.... and all the earthmoving machines....? http://fsae.com/groupee_common/emoticons/icon_smile.gif All of these rely on massive grip, so almost always have a completely soft twist mode. (BTW, to be successful in motorsport you just have to keep your sponsors happy, and back of the grid is no disadvantage.)

"The garbage truck consisted of a torsionally flexible twin-rail chassis ... The chassis released its stored energy ... the rest of the truck wobbled back and forth..."

And therein lies the problem with providing a soft twist mode with a spring capable of storing a lot of energy. Better is completely soft, like tractors, forklifts, ride-on lawnmowers, etc., etc..
~~~

4. "...while the risks of moving to [a fewer-moving parts suspension] include possible disqualification, rules changes which would ban the car..."

Any comments from someone on the Rules committee???

"When we build [the standard suspension] we are not simply polishing a turd - we are selecting a design that has proven to be at least OK at its job and will not be banned or drop-kicked in design."

The sad part is that a "NO suspension" car would also be "OK at its job" in FSAE, but a lot easier and cheaper to build, and probably faster because lighter, more testing time, etc.

The really sad part is that you really are all "Polishing A Turd". And in building the "PAT" car very few students learn how real suspensions work, because real suspensions have to cope with real roads which have real bumps on them, whereas FSAE tracks are unrealistically smooth.

Z
04-16-2012, 05:32 AM
Will M

The debate that a go-kart is better fitted for an FSAE track than an FSAE car is has been going on at least since 5 years ago when I started reading this forum and probably stated 15 years ago. So there have been about 50 to 200 competitions since the debate started. We are not politicians doomed to argue about which century old econ book is best, we can objectively prove this one way or the other. Surely at least one FSAE official owns a decent shifter car. The test is simple, before each dynamic event they take their kart out and put down a time. This would give an objective comparison and would be a good way of checking track conditions. Getting decent historical data for FSAE is a real pain (the event book has car data but no results, and the online results have no car data). This would give at least one bench mark, and could be used to compare events in different years and locations to each other.

William
04-16-2012, 11:29 AM
mdavis

Will,

We do a decent bit of our testing at a local kart track, and knowing a handful of guys that run the TAG classes (100cc 2 stroke, single gear karts), we know their laptimes around various configurations. On the big track (considerably larger and more wide open than a typical FSAE course), the fast TAG's run 38-39 second laps, and our 2006 car (4 cylinder NA, manual shifter, no aero) with our current hotshoe runs about 39-40 second laps. If we cone down the course for practice to something much closer to an FSAE course, our go kart (clone motor = 200cc $100 Harbor Freight generator motor that makes something like 10hp) is likely faster than any of our 4 cylinder cars (we haven't taken lap times, but multiple drivers have driven similar configurations).
04-16-2012, 10:18 PM
Charles Kaneb

Quote:

Originally posted by Z:
Charles,

Thanks for comments. Here are some brief responses.
~~~o0o~~~

1. The mandatory chassis safety parts (roll-hoops, side impact,...) form most of the chassis for the Twin-Beam concept. No more torsional stiffness is needed, because beams allow soft springs. I would not suggest adding any extra torsional "softness" to the chassis, again, because the soft corner springs already give enough softness. The advantages of longitudinal Z-bars is that they can give a completely soft twist-mode, even with a stiff chassis and stiff roll-mode, this latter being necessary with wishbones with small camber gain (long FVSA). Also the Z-bars make handling adjustments (ERMD) very easy.
~~~

yyyy
Wouldn't a car that's soft in twist be less responsive to changes in ERMD/LLTD? I remember hearing about some older cars that couldn't have o/s or u/s tuned out of them because the chassis was too soft in twist - would you have to make changes to the twist stiffness to get some changes out of this?
yyyy

2. I think the possibility for weight reduction by using beams over wishbones comes mainly from the much simpler chassis with its smaller number of hard points.

yyyy
If your front end is legal, you are one or two tubes short of having all the hard points you need for a double-wishbone suspension. A legal rear box will have almost all of them already due to the need to triangulate the rear roll hoop support loads back to the main hoop floor in all three dimensions. You can save a couple tubes at the cost of making it harder to get in and out by making the main hoop supports come forward, which will give you almost complete freedom to get rid of the rear structure.

Thinwall steel tube is light. There will be many roughly 70-lb frames at FSAE/FSAEW. Your main hoop, front hoop, and side impact bars make up about half of that by themselves.
yyyy

You say "A De Dion rear end is at least as complex as any double wishbone setup."

I have to disagree. I reckon a detailed study of a De Dion would have it roughly half as "complex" (time to make, etc.) as double wishbones. Likewise, there are many other simple suspensions that are almost never considered. The "proof" of this is that almost all FSAE teams use the completely unnecessary push/pullrods-&-rockers, so they are clearly not concerned by useless complexity, and make no attempt at "simplicity". Well, other than talking about "KISS". http://fsae.com/groupee_common/emoticons/icon_smile.gif
~~~

yyyy
Double wishbome rear end:
2x upper a-arms
2x lower a-arms
3 spherical bearings per a-arm
2x toe control links, with 2 spherical bearings each, for a total of 16 spherijoints.

Four Link/"full-complication" De-Dion rear end:
2x lower longitudinal links
2x upper longitudinal links
2x lateral locating wishbones with toe links (you can substitute 4x lateral locating arms if you please, and I'll accept that with a laterally stiff center slider for the De Dion beam you can eliminate two of them and still get good toe control)
1x De Dion beam with rigid attachment to hub carriers and some sort of in-out sliding mechanism in the middle.
Total: 2 spherijoints per longitudinal link, for a sum of 8. 4 spherijoints per lateral wishbone, sum of 8. 16 total spherijoints plus one to-be-designed in-out slider mechanism on the De Dion beam.

"Simpler" De-Dion rear end:
2x longitudinal links leading directly forward from the hub carriers, pivoted on broad roller or needle bearings at each end.
2x lateral links leading to the center of the car, pivoted on broad roller or needle bearings at each end.
1x De Dion beam that absolutely needs an in-out slider mechanism as your axle lengths will change a lot.
I will contend that you will not achieve adequate toe and camber stiffness with this design unless your bearing assemblies can be EXCEPTIONALLY stiff in bending across the rollers/needles. The first way I can think of to do this is to build them as two-bearing assemblies with a "kingpin" between them. These will have to be pressed in and kingpin deflection, wear, and fatigue are all well-known problems in many applications where they are used. Total of 8 joints, plus the slider.

3-link beam axle with Panhard bar:
2x longitudinal links, 2x joints each
1x Panhard bar, 2x joints
Total of 6 joints. Second-simplest decent rear suspension I can think of (the Texas World Axle is my secret for now)
yyyy

3. "Outside of racing and certain military off-road vehicles, there are very few vehicles where the main differences between success and failure are peak tire grip and controllability near peak tire grip."

Well, how about every farm tractor ever built.... and all the earthmoving machines....? http://fsae.com/groupee_common/emoticons/icon_smile.gif All of these rely on massive grip, so almost always have a completely soft twist mode. (BTW, to be successful in motorsport you just have to keep your sponsors happy, and back of the grid is no disadvantage.)

yyyy
Farm tractors are limited in lateral and longitudinal force capabilities by the possibility of rollover about either the X or Y axis. They have an interesting compromise to consider - a low center of gravity will improve stability, which will broaden the range of terrain they can operate on and increase their tractive effort, but a high ground clearance is useful to clear obstacles, and more suspension travel will improve control. A lot of the time additional grip available at the tires will not provide any useful benefit! I agree with you that tractors and earthmovers are applications where traction and controllability are important near the static and dynamic limits of the vehicle.
yyyy

"The garbage truck consisted of a torsionally flexible twin-rail chassis ... The chassis released its stored energy ... the rest of the truck wobbled back and forth..."

And therein lies the problem with providing a soft twist mode with a spring capable of storing a lot of energy. Better is completely soft, like tractors, forklifts, ride-on lawnmowers, etc., etc..
~~~

4. "...while the risks of moving to [a fewer-moving parts suspension] include possible disqualification, rules changes which would ban the car..."

Any comments from someone on the Rules committee???

yyyy
As drawn, you have moving front suspension arms inside the cockpit and a heavily loaded front suspension pivot - at the back of a leading arm no less - that is right under the driver's knees. With typical FSAE design that places the driver's feet a large distance in front of the front axle, it will be in very close proximity to a rather delicate part of the driver's anatomy.

A concept of mine, with a simple axle tube with its two bearings mounted in rubber blocks, was rejected by my team's Design Review Board as very likely to be ruled illegal on not having working kinematic suspension.
yyyy

"When we build [the standard suspension] we are not simply polishing a turd - we are selecting a design that has proven to be at least OK at its job and will not be banned or drop-kicked in design."

The sad part is that a "NO suspension" car would also be "OK at its job" in FSAE, but a lot easier and cheaper to build, and probably faster because lighter, more testing time, etc.

The really sad part is that you really are all "Polishing A Turd". And in building the "PAT" car very few students learn how real suspensions work, because real suspensions have to cope with real roads which have real bumps on them, whereas FSAE tracks are unrealistically smooth.

yyyy
A passenger car's suspension doesn't actually have to work over bumps! If ride, roadholding, and handling actually mattered in a passenger car, Citroen, Mazda, and Lotus would not always be on the edge of existence. Almost every consumer willingly accepts a car that rides and handles like crap.

If I'm designing a suspension that actually matters, I'll consider more scenarios than uniform roll, one-wheel-bump, and two-wheel-bump. The point here is to design a suspension optimized for a fairly narrow scenario - a generally smooth, generally level track with a decently uniform surface. A broader range of surfaces is encountered in SAE Mini Baja, and it would be a lot of fun to try to clean up there by considering the dynamics a lot more carefully than a typical MB team does.
yyyy

Z
04-16-2012, 10:22 PM
Charles Kaneb

Quote:

Originally posted by mdavis:
Will,

We do a decent bit of our testing at a local kart track, and knowing a handful of guys that run the TAG classes (100cc 2 stroke, single gear karts), we know their laptimes around various configurations. On the big track (considerably larger and more wide open than a typical FSAE course), the fast TAG's run 38-39 second laps, and our 2006 car (4 cylinder NA, manual shifter, no aero) with our current hotshoe runs about 39-40 second laps. If we cone down the course for practice to something much closer to an FSAE course, our go kart (clone motor = 200cc $100 Harbor Freight generator motor that makes something like 10hp) is likely faster than any of our 4 cylinder cars (we haven't taken lap times, but multiple drivers have driven similar configurations).

The PAX index for (125/2t/415 lb) shifter karts is .952. The PAX index for FSAE cars is .989.

On a 60 second course, the world's fastest A-Mod would theoretically run 60 seconds. Maryland's FSAE car would theoretically run 60.66, and the fastest autox kart would be at 63 seconds.

As a former karter, I can categorically state that the karts used in autocross are some distance from the state of the art in karting, and that the cream of the crop of karting drivers is not represented. I'll try to prove that I'm not bluffing when I drag a ready-to-rip built ICC to an autocross and co-drive with the fastest ace driver I can find.
04-16-2012, 11:32 PM
mech5496

As any other design, there are up and downsides on a beam axle setup. The hard task is to identify the possibilities and flaws of each design, and then choose and go with the one that suits you better ACCORDING TO YOUR TARGETS. There is no thing such as "optimum solution". I am pretty convinced that a beam axle car can have similar (if not better) dynamic performance than a "conventional" FSAE car, and there are many examples out there that favor a simple-as-hell reliable car. A recent example I can remember of...

http://fsae.com/eve/forums/a/t...20065151#11120065151

Keep in mind that we are talking about GFR's 2011 car in comparison, which is one of the fastest FSAE cars out there. Also take a look at Rob's old school (Uni at Buffalo) and teams like ADFA, CalPolySlo, Mini Baja cars etc... http://fsae.com/groupee_common/emoti...on_biggrin.gif At least I was surprised! We might surprise quite a few in the not-so-distant future too...
04-17-2012, 05:23 AM
rjwoods77

BTW I looked up the results in 1996 from the Akron Bajamula and they finished 11th overall even after their fuel pump quit in the enduro. They ran a pure rotation swingarm for a rear suspension and just swapped the 18hp Briggs V-Twin for the spec Briggs single. Their baja car that year if memory serves me correctly was 320lbs and there is a 30lbs difference between the motors so they were probably at 350lbs for FSAE comp.
04-17-2012, 05:35 AM
rjwoods77

Dr. Paasch's post also really got me thinking that it would make a hell of a novel idea to do a Bajamula again but with Z's interconnected suspension utilizing front leading arms and rear trailing arms (2CV style) and just replace the "black box" or trailing/leading arm linkage configuration along with engine (45 degree leaned single and 90 v twin) in between events. It would make a much cheaper vehicle that would would be very competitive and be able to run dual competitions.
04-18-2012, 12:08 AM
Z

Charles,

"Wouldn't a car that's soft in twist be less responsive to changes in ERMD/LLTD? I remember hearing about some older cars that couldn't have o/s or u/s tuned out of them because the chassis was too soft in twist - would you have to make changes to the twist stiffness to get some changes out of this?"

Big subject here, but briefly.

The issue of "not enough chassis torsional stiffness" started, in circuit racing, when wings caused the springs to become much stiffer to take the aero loads. So, for example, the race engineer might have wanted the front tyres to carry most of LLTD so he stiffened even more the front springs/ARB. But the engine, mounted at the back, kept "leaning" on the already stiff rear springs, transfering all its load (ie. most of the car's mass) to the rear wheels. The pop-rivetted aluminium chassis didn't have the stiffness to pass any of the engine's LLT to the front tyres. So oversteer always, even with completely rigid front springs.

However (!), way back in the early 1900s the chassis was deliberately made soft. The production car would typically have its large and rigid engine mounted at four points (2F-2R) to the twin-rail chassis to torsionally stiffen it, mainly to stop the doors rattling, windows cracking, etc. For racing the engine was mounted at three points, one at the nose and two either side of the rear bellhousing. This allowed the chassis to twist, which was useful for grip on the rough roads (which was not helped by the stiff leaf spring suspension).

This arrangement was quite similar to the top-left of the Z-Bar Sketch, with the sketch's longitudinal leafsprings being the chassis rails, and the centre structure being the engine. During cornering the engine "leant" on the centre of the chassis rails, thus distributing its LLT about equally front to rear. If the engine had its mounts with 2F and 1R, then all its LLT would go to the front wheels and there would be massive understeer (all else equal).

I believe a similar thing happens with go karts when the seat mounting points are changed? The driver is the largest mass, and if during cornering his weight is transfered, via the seat mounts, closer to the front of the chassis, then US. If more to the rear, then OS.
~~~o0o~~~

Regarding complexity of De Dion versus rear wishbones.

In both cases the engine/diff/halfshafts/stub-axles are the same.

I would NOT do a "sliding-tube" DD just to avoid plunging CVs. I would do a beam as in the sketch, but without the two inner bearings.

So,
1x largish, but simple, triangular tube structure for the beam,
1x heavy duty apex BJ,
1x P&S, as in sketch,
2x (optional) bolted connections at "beam-ends/uprights", for camber/toe adjustments.

Again, the chassis now only needs two accurately placed hard points on its centreline at floor level (using stringline to align these and front points), and two less accurate points for the spring-dampers. The problem with wishbones is that more points have to be located accurately in 3-D, and their preferred locations don't always match the chassis nodes (eg. upper side-impact tube has to be at Z = 350mm (memory?), which might not be the best kinematic location for the upper wishbone mounts).

(Edit: Oops! I forgot that a DD needs different side-view kinematics. The above list should be changed slightly, but overall the "complexity" (time to build) is about the same. I might do a sketch of this option one day. It can be done with only 3 chassis hard points for the rear beam (the P&S is dropped).)
~~~o0o~~~

"Farm tractors..."

Ahhh, I love my tractors... I better not bore you. http://fsae.com/groupee_common/emoticons/icon_smile.gif
~~~o0o~~~

"As drawn, you have moving front suspension arms [the front beam] inside the cockpit and a heavily loaded front suspension pivot - at the back of a leading arm no less - that is ...
... in very close proximity to a rather delicate part of the driver's anatomy."

I would have both front and rear beams mounted UNDER the chassis's floor. The BJs would be attached to chassis with vertical bolts in single shear, to act as "fuses". In a big impact the bolt fails, chassis remains undamaged, and the beam slides under the floor (or car slides over beam). This also makes mounting of underwings easier. (I have grown fond of the "family jewels", and would not risk losing them.)
~~~o0o~~~

"A passenger car's suspension doesn't actually have to work over bumps!
Almost every consumer willingly accepts a car that rides and handles like crap."

Agreed. Sad, but true.

In the past I have suggested getting rid of the mandatory "+/-1 inch suspension" rule, and just scattering a lot of bumps around the track (say, molded rubber "cowpats", 2-3m (10ft) diameter, 0.1m (4") high in the middle, and tapering to very little at the edges).

Teams can then build a cheap no-suspension go-kart and pay the penalty of driving around the cowpats (or launching over them!), or else build a car with good suspension that allows them to take the fastest racing line, regardless of bumps.

This could be a long term benefit to consumers of production cars.

Z
06-26-2012, 02:11 AM
Dunk Mckay

Quote:

Originally posted by Z:
Important De-Dion note. The side-view kinematics, or anti-pitch, changes when the diff is chassis mounted. A layout as in the sketch, but with De-Dion, will have lots of pro-squat under power. This can be ignored with a low power engine, or a beam-centre bump rubber (aka "third-spring", or "lateral Z-bar", as mentioned by Harry above) can be used to limit squat (axle bounce) but allow roll. Alternatively, a different kinematic location can give anti-squat (higher pitch axis) while maintaining a low roll axis. This is a quite simple structural change, but will require a sketch to explain it properly...

Z

Z,
I might have completely misunderstood, my kinetmatics knowledge isn't great, but the pro-squat under power you talk about, is that due to the reation moment transmitted into the diff causing a rotation about it's axis. In which case displacing the diff decreases the effect this has on the chassis? For a moment there I thought I'd got it visualised in my head, but then I lost it, I need more experience.

I would love it if you could enlighten me. (If it sounds like I'm gunning for that sketch you mentionned, that probably because I am.)
06-26-2012, 05:53 AM
Z

Quote:

Originally posted by Dunk Mckay:
... the pro-squat under power you talk about, is that due to ...
If it sounds like I'm gunning for that sketch you mentionned, that probably because I am.

Dunk,

That sketch might take some time, so here's some that might help until then.

Figure 7 below illustrates "side-view antis" (eg. anti-squat under power, or anti-lift when braking) in terms of a suspension's "n-lines". N-lines are simply the lines "normal" to the direction of travel of any point in a linkage. Alternatively, the n-lines can be thought of as the directions of "no" motion.

Importantly, a suspension's n-lines are all you need to know, in order to understand its "antis".

In the figure the point being considered is the wheelprint (simplified to a point). Since the suspensions shown have only one degree-of-freedom (this needs more explaining, but...) the wheelprint can only follow a single, roughly up-and-down path. So the wheelprint has a "planar pencil of n-lines" that is perpendicular to its path of motion. (A "PP" is a bunch of lines, lying in a flat plane, and all passing through the same point, like the spokes of a wagon-wheel). Here we are interested in the particular n-line that also lies in the side-view (vertical-longitudinal) plane.

Importantly, in any linkage, forces can ONLY travel along n-lines. So, in the figure, any component of force acting on the wheelprint that lies outside of the plane of n-lines must necessarily be reacted by some other structure, which here is the spring-damper (not shown). So if the same arbitrary force acts on the wheelprint in all of the examples in the figure, then in each case the "component of force oustide of the n-line plane" will be different, so the spring-damper will be squashed to a different length, and the "anti" behaviour will be different.

https://lh5.googleusercontent.com/-c...echAntiFg7.jpg

Regarding an FSAE rear beam-axle, consider Figures 7a (similar to a "live" axle), and 7b (similar to a "De Dion" axle). In both cases consider the swing-arm pivot (towards top-right of each little sketch) as being much closer to ground, so well below the axle height. This gives similar side-view kinematics to my earlier "Twin-Beam" sketch, with the "apex BJ" = "swing-arm pivot".

Figure 7a now has a side-view n-line that is close to horizontal with only a slight rise to the front (right of pic). This gives a small amount of anti-squat.

Figure 7b (= De Dion) now has its n-line sloping quite steeply down to front. This gives quite a large amount of pro-squat. The question is how to push this n-line up-at-front, for a little anti-squat, while keeping the lateral n-lines close to horizontal, for good cornering behaviour. (Answer later...)
~~~o0o~~~

Meanwhile here is another figure that might help understanding of the distribution of wheelprint forces between the control-arms (ie. along their n-lines) and the spring-damper. Shown is a car accelerating longitudinally (eg. forward to right), but same principles apply for lateral acceleration (ie. cornering). Black arrows are the forces acting on the wheelprint or car. White arrows are some of the possible components of the black arrows. Fca = Control Arm force. Fsd = Spring-Damper force. H = Horizontal. V = Vertical. G = Gravitational. I = Inertial. F1,2V = Vertical forces on wheels 1 & 2. Etc...

https://lh5.googleusercontent.com/-a...chAntiFg10.jpg

(BTW, I uploaded the above pics somewhere (Picassa?) sometime ago, and I can access them easily via my old posts on this forum, hence this quick reply. New pics mean that I have to go shopping for a new cable for the old (perfectly good) scanner, or buy new scanner, or???, so will take more time.)

Z
06-28-2012, 01:33 PM
Dunk Mckay

That's awesome, really helpful, I can visualize it properly now.

Ok, so I've been pondering this twin beam-wing concept of yours, Z, since a friend suggested it a while ago. Starting at the rear wheels (front can come later, much later) I've been trying to figure out the best way be able to adjust camber and toe.

The best ideas so far are these:

-big solid uprights that mount onto the beam via interchangeable plates/spacers which have the camber and toe incorporated. The issue with this is that for stiffness these will have to be quite bulky. And even though you'll generally only need one setup for running the car you'll need to fine tune it to find that first, so you'll have to machine a large variety of parts to that kind of angular tolerance, which is gonna be costly (time or money or both).

-and upright mounted with a single B.J. at it's base to the beam end, then two adjustable tie rods between the upright and either arm of the beam. These of course will impede air flow (if only a little and above the wing), and load the beams such as they will be bent in the middle unless they are made fairly bulky as well. One also ends up thinking that perhaps double wishbones might not be that much more complicated.

Much as I'd like to say "stuff it" and come up with a design with fixed camber and toe, both design judges and the academic staff at my school would rip it out of us if we made that. I also think being able to find the ideal setup is not really something worth sacrificing for the sake of oversimplifying things.

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