Roll rates in RCVD [Archive]

jose_90

03-04-2014, 05:29 PM

120 views and no responses :(

Anyway, I've kept doing ride and roll calculations (using the sprung weight for the case above) and I've run into some problems with the wheel displacement/frequency. I started out using a front frequency of 2,3 Hz as a baseline, and from that I calculated a ride rate, with the following data:
Full weight car+driver= 300 kg (I'm hoping for less, but just as an approximation)
Front weight distribution: 45%
Front unsprung weight: 26 kg
Front wheel sprung weight: 54,5 kg

So, Ride rate=4*pi^2*w^2*54,5=11382 N/m

On the other hand, I calculated the total load transfer, simply using
Load transfer to the outside wheels = Weight*Lat Accel*CG height/track=300*1,5*0,3/1,2= 112,5 kg
Which means that the front outside wheel should see a load increase of approximately 56.25 kg=551,8 N

Now I try to calculate how much suspension travel I would be using in that situation.
Wheel displacement = Load change/ride rate=551,8/11382=0,048 m = 48 mm
Which is way too much, ideally I'd like to be using less than 25 mm.

Now, my problem is that to solve that, I'd need to increase my front frequency (up to 3,1 Hz), thereby raising my rear frequency, and there comes a point when my rear ARB would need to produce negative roll stiffness to get the total lateral load transfer distribution I want. Either that, or choose a roll gradient under 0,9 deg/g, which is a bit low.

I guess that there are ways to work around this, but before that, I'd like to be sure that I haven't made some fatal mistake in my calculations.

Thank you very much for reading, and any comments will be appreciated!

Jose Maria Martin
ARUS Andalucia Racing
University of Seville

03-04-2014, 07:45 PM

Jose,

Yes, I am one of those people who read your first post but didn't offer a reply.

This is because I consider the RCVD, OptimumG, and the general automotive-cottage-industry approach to this subject to be puerile codswallop. Along with this approach (ie. of "ride frequencies", etc.) being unnecessarily complicated, it lacks accuracy, and, worst of all, it is a great hindrance to good understanding of suspension design and Vehicle Dynamics generally. For more of my views you can Search posts with "Z", "front and rear ride frequencies", "interconnected suspension", etc., as keywords.

Perhaps a better approach for you is to continue studying the problem as a simple mechanical system that can be understood by the age-old methods of Classical Mechanics. Your above post suggests that you have a reasonable grasp of these methods, so proceed as you have above.

As a first approximation you might consider only a single "sprung mass", with the wheel-assemblies being massless. Later you can also consider the effects of the masses of the wheel-assemblies (so now five masses in total). Note that now with the same wheel-rates and same cornering Gs, different suspension kinematics result in different body-roll-angles. And there are also the gyroscopic effects to consider (as I posted somewhere, and which include the spinning engine parts...).

Very briefly, part of your second post above has the right approach (IMO). That is;
* You want high cornering Gs.
* You also want a narrow track (so you can squeeze more easily through the slaloms).
* So, it follows (from simple FBDs) that during cornering you want the car to almost (but NOT QUITE!) get up on its outside pair of wheels.
* So, almost (but not quite!) 100% of load is transfered from inside to outside wheels.
* So, if you only provide +/- 2.5 cm of vertical wheel travel (as is required by Rules), and you want a little bit of travel left for any bumps that might be in the corner, then your wheel-rates should be such that "static deflection" = ~ 2 cm (ie. the static wheel load (Fz) compresses the spring about 2 cm).
* So, for your front wheels, Wheel-rate = ~ 50 kg/2 cm = ~25 kg/cm (or ~25 kN/m, or ~25 N/mm).

And should you ever want to calculate any REAL resonant frequencies, then please use the methods of Classical Mechanics, rather than the cottage-industry codswallop in the automotive textbooks.

And, very importantly, FSAE CAN BE WON WITHOUT ANY SUSPENSION MOVEMENT AT ALL! So the above calculations are largely academic.

And ARBs are possibly the worst kinds of springs to put on a car. If you feel that you must stiffen the Roll-mode over the other suspension modes, then "longitudinal-Z-bars" are much better than "lateral-U-bars" (= ARBs).

Enough for now...

Z

jose_90

03-05-2014, 05:07 AM

Hi Z,
Thank you for your time and your comments. I've read some of the stuff you've referenced in the forums. I do agree with you that wheel ride displacement is a more useful way to choose ride rates. In my case, instead of choosing a natural frequency in a range that I read somewhere (2,5-3,5 Hz or whatever) it allows me to choose a ride rate that I actually want (as in, I know that I want my car, under high load cornering, to be using x mm of travel).

Your point about longitudinal-Z-bars, interconnected suspensions, and non-linear springs and bumpstops (I read that in another thread) are very interesting, and I will definitely look into that for next year's design. However right now it's too late to introduce new things in the design, I think it is more important to just build a car and test it.

On the other hand, I want to know as much as possible about vehicle dynamics, so I am gonna keep reading RCVD, OptimumG, and any other sources, to allow me to take my own decisions.

Thanks again,

Jose Maria Martin
ARUS Andalucia Racing
University of Seville

Tim.Wright

03-05-2014, 07:50 AM

In the interests of a balanced discussion, I will counter some of Z's points.

The methods outlined in RCVD are simplifications and yes simplifcations require some cutting of corners. In this case, following the "traditional" method, you are largely constrained to end up with a traditional suspension with 4 springs and 2 anti roll bars. For passenger cars, this arrangement is a very good compromise between performance and complexity. Packaging anti roll bars is very simple. Packaging a longitudinal Z bar is a nightmare, and for what exactly?

At the end of the day, ANY model or method (including Z's) is necessarily a simplification and the traditional methods are quite a reasonable way to arrive at an adequate (but not optimum) solution without a lot of time or large amounts of input data. As you have mentioned, you don't have the time to do something out of the box so following a well established (and sucessful) route is a good choice from a risk management point of view. Even Z would agree, its more important to get the car built 1 month earlier than to agonise for 1 month more in the design phase about suspension rates.

The main problem though, from an educational point of view, is that often the traditional methods are taught as though they are the first principles of a suspension system whereas they absolutely are not. They are a (quite heavily reduced) simplification of how a suspension operates in roll and pitch. Additionally, they only have a limited range where they are valid (and this unfortunately does not include limit behaviour) and some suspension geometries cause the calculations to crash.

Your intuition should be telling you this anyway since these methods simply a complex system of 2 wheels + elastic tyres, 12-20 links located in 3D space, connected by a compliant chassis by compliant joints down into an equation with only 9 or so parameters (hrcF, hrcR, KwhlF, KwhlR, KrollF, KrollR, mass, mass_dist, cgh).

Regarding the book, there is too much good solid theory in RCVD (particularly the chapter on steady state stability and control) to simply brush it off as codswallop. My advice would be to follow whats in RCVD or in the Optimum G tips by all means but recognise you are cutting some corners by doing so.

stever95

03-05-2014, 11:42 AM

Excellent response Tim! Good thread going on here..

theTTshark

03-05-2014, 11:45 AM

To add to the debate of model accuracy.

""essentially, all models are wrong, but some are useful" - George E. P. Box
In Formula SAE, real racing, and the real world it is vital for us to understand how complex our simulations/models/calculations need to be. Is being more accurate providing more usefulness? Not necessarily. Is a more accurate model needed for someone just trying to learn the basics? Hell no. We can stroke our intellectual ego all day on this forum showing new and old FSAE people how brilliant we are, but what's the point? Make it useful. Make it work. Make sure you understand it. The conventional calculations that RCVD provides are certainly good enough for FSAE and really probably good enough for everything else. Now I know Z hates Claude, but I'm going to quote Claude on this one, "As engineers we work in deltas." Honestly it's not needed to get the calculations perfect as long as you're consistent. Get the car built. Test what is happening on the car. See what the difference is between real and theoretical. Make a setup change, see if the difference stays about the same. Make adjustments to the way you design the car the next year. Repeat.

Claude Rouelle

03-05-2014, 12:09 PM

Jose,
This is because I consider the RCVD, OptimumG, and the general automotive-cottage-industry approach to this subject to be puerile codswallop.
Z

To Z,

Since you write your blames on the public place and name me, I have to ask you publicly: What did I do to you to deserve such a brutal language?

jose_90

03-05-2014, 01:42 PM

Thank you for all your responses, it's been really useful. I know about models being just that, models, and only representing reality up to a point. I was just worried that I had done some stupid mistake in my calculations, and my results were going to be way off haha
Anyway, I am still inmersed in my excel worksheets, I wanna make sure I get some consistent results and then go and build the car!

Jose Maria Martin
ARUS Andalucia Racing
University of Seville

03-05-2014, 08:39 PM

Jose,

"120 views and no responses."

Better now? :)

(More for you below...)
~~~~~o0o~~~~~

Tim,

"In the interests of a balanced discussion, I will counter some of Z's points."

Thank you. I like balanced discussions too.
~o0o~

"...following the "traditional" method, you are largely constrained to end up with a traditional suspension with 4 springs and 2 anti roll bars. For passenger cars, this arrangement is a very good compromise between performance and complexity. Packaging anti roll bars is very simple. Packaging a longitudinal Z bar is a nightmare, and for what exactly?"

The "for what" answer is that Longitudinal-Z-Bars give FAR SUPERIOR PERFORMANCE. Especially so on bumpy roads, but also on moderately undulating roads. So, realistically, for all real roads. Very briefly, the soft Twist-mode possible means that wheel Fz loads change much less with the (very common) twist in the road, hence giving much better grip, more consistent handling, more comfortable ride, etc+++.

Until suspension engineers physically experience this, they will never appreciate it. As you say, the traditional method guarantees a stiff Twist-mode (which comes mainly from the ARBs). The traditional response to the poor performance is simply to blame the road for being too bumpy.

As for the "nightmare" of packaging Long-ZBs, well, you must have funny dreams! [<- Said in humorous tone.] I have promised some students who have PM'd me that I will do some more sketches on this soon, and they will happen... (yes, Tax Return finally done, and only 9 months late!).

Again briefly, there are, and have been, a great many cars that already have half of the Long-ZBs packaged. These are the cars with longitudinal-torsion-bar front-suspensions (eg. many late 1900s Chryslers, Renaults++, current Toyota Hiluxes++). Adding similar size torsion-bars rotated 180 degrees and connected to the rear suspensions, with the two bars on each side joined in the middle of the car, gives the required Long-ZBs. Easy packaging! See the 1950s Packard for the general layout.
~o0o~

"At the end of the day, ANY model or method ... is necessarily a simplification ...
Even Z would agree, its more important to get the car built 1 month earlier than to agonise for 1 month more in the design phase about suspension rates."

This is exactly my point.

The first paragraph of the OptimumG tip (linked by Jose) tells you to FIRST "pick a ride frequency", and then "calculate spring rate needed for the chosen frequency". So after agonising for months over your chosen (and fictional!) frequency, you still have to go through several pages of equations to get the spring-rates. My suggested approach can be done in a moment, and all in your head. Namely, wheel-rate = corner weight/chosen-suspension-displacement. Next, off for some testing...
~o0o~

"My advice would be to follow whats in RCVD or in the Optimum G tips by all means but recognise you are cutting some corners by doing so."

NOT "cutting corners", but rather taking a long, and circuitous, and difficult route through the blackberry-bushes, when the same answer is just a step away.
~~~~~o0o~~~~~

Trent,

"The conventional calculations that RCVD provides are certainly good enough for FSAE and really probably good enough for everything else."

As above. The "conventional calculations" introduce a fictional detour through the Fairytale Land of Frequencies, when no such detour is required! Why? Why bother?? What word is more suitable to describe this detour than "codswallop"???

Anyway, all this is due to the commonly held H. Sapien view that "progress" is all about making things more complicated, and difficult, and useless. Like bureaucracies, and Tax Returns, and the FSAE Rulebook...
~o0o~

"Now I know Z hates Claude..."

No, no, no, no, noooo...
~~~~~o0o~~~~~

Claude,

I DO NOT hate you. Not at all!

However, in 2002 I tried to enlighten you regarding some issues of Classical Mechanics, issues that would have helped both your understanding of Vehicle Dynamics, as well as all of the students (and industry professionals?) that you have taught since then. More recently on this Forum I have tried to do the same, and also to make you aware of many other issues (eg. interconnected suspensions) that you appear to currently not be aware of. For reasons that you only know, you have ignored this help, and, in fact, not even shown any interest in discussing it openly, or learning it for yourself.

I find this rather dissappointing. Mainly because your efforts make the next generation of young Engineers LESS knowledgable. It is all of future society that then pays the big price (ie. of undereducated Engineers who believe in "magic numbers", voodoo, whatever else...). Not me or you.
~o0o~

"What did I do to you to deserve such a brutal language?"

"Puerile codswallop" translates as "childish silliness/nonsense". Not too brutal?
~~~~~o0o~~~~~

Jose (again),

For your Spring-Damper design I suggest you look at U of Cincinnati's 2013 car (see links and discussion by Matt (mdavis) on the "FSAE Lincoln 2013" thread, in the Competitions section). They had direct-acting SDs (like Monash, winner of 2013-Oz), no ARBs, rubber spacers for spring-rate adjustments, and they did quite well! At most you might need three sets of springs (soft, medium, and stiff), with my earlier "static displacement" calc getting you in the ball-park for these rates.

With spring-rates solved, I then much more strongly suggest that you concentrate on controlling your suspension's toe and camber compliances, and get the car built ASAP so that you can start optimising the "nut on the end of the wheel" (ie. driver development!).

Z

Tim.Wright

03-06-2014, 02:14 AM

For the Z bars... packaging has changed a lot since the 1900's just because there is so much "stuff" in a car now (especially sports cars with the engine behind the driver) that the only way to link the suspensions like you mentioned would be hydraulically.

Anyway whether hydraulic or mechanical, a diagonal linking is an extra complication. Yes it will improve ride + body control but so will a lot of other complications. The manufacturers have decided its not worth the cost/complexity. I don't understand why this makes them a pack of morons...

Regarding the spring rate calculations... you are really nit picking there. Both calculations (based on displacement or frequency) are very simple and quick. To me they are the same thing and I find it hard to justify wasting much breath to argue about which is better.

jose_90

03-06-2014, 05:11 AM

Jose,

"120 views and no responses."

Better now? :)

Z

:D

For your Spring-Damper design I suggest you look at U of Cincinnati's 2013 car (see links and discussion by Matt (mdavis) on the "FSAE Lincoln 2013" thread, in the Competitions section). They had direct-acting SDs (like Monash, winner of 2013-Oz), no ARBs, rubber spacers for spring-rate adjustments, and they did quite well! At most you might need three sets of springs (soft, medium, and stiff), with my earlier "static displacement" calc getting you in the ball-park for these rates.

Z

Regarding the push/pullrod vs direct acting dampers, I have a question. I've read that one of the advantages of the push/pullrod method, is the reduction in unsprung mass. Now, unsprung mass is defined as the mass of the wheel, (...) etc and half (or, an undefined portion) of the mass of the connecting elements (spring, damper, bellcrank, etc). My question is, what is that portion? How could I calculate it?
I don't actually think that it is that important, for most purposes I guess 50 % is fine. I am only asking because, by that definition, a direct acting shock abosrber would actually reduce the unsprung mass, because you don't have the rocker or the pushrod. Right?

Tim.Wright

03-06-2014, 06:06 AM

Interesting question...

Remember that the unsprung mass is (another) simplification to represent the total "inertia" or "resistance to movement" of the total suspension. It is not a real mass but an "effective" mass which gives the same resistance to vertial motion as the complete system (of pushrods pullrods Z bars, dampers - only the moving part! etc etc...).

The best way to measure it is to move the suspension and measure the forces required to do so. The effective mass (your unspring mass) is then the ratio of the acceleration from the movement to the force required to do so or as Newton said it: M=a/F.

So anything that moves will be adding to the effective unsprung BUT some of those things will be contributing to it via its POLAR inertia not its mass. For example a rocker. Its mass is largely irrelevant because it doesnt move. Its inertia is what is resisting acceleration of the system.

So you can see, its not such a trivial thing to calculate. But given that your calcs are already a simplification (i.e. they are representing the complete suspension as a single point mass at the wheel centre) then if you are 10-15% out in your unsprung mass determination, its not going to be a show stopper.

Enjoy

03-06-2014, 06:38 AM

Tim,

For the Z bars... packaging has changed a lot since the 1900's ...
... the only way to link the suspensions like you mentioned would be hydraulically.

NOoooo!!! They are just two long straight bars that fit under the car. Very similar to ARBs, but longitudinal rather than lateral (and Z rather than U ends).

... whether hydraulic or mechanical, a diagonal [actually "longitudinal"] linking is an extra complication.

Again, noooo!!! Just three springs, namely 2 x longitudinal + 1 x lateral Z-bars, can provide the entire suspension for a car. Conventional suspensions have six springs, namely 4 x corner + 2 x ARBs. Which is more complicated? Which is simpler?

Furthermore, as I have explained many times before, the difference in performance is like night and day. But until suspension engineers exerience this, THEY WILL NEVER KNOW.

The manufacturers have decided its not worth the cost/complexity.

And here is the key point. The manufacturers, for the most part, do not have a clue about these sorts of things. Where are the papers? The books? The seminars? The in-house design reports with extensive experimental results, cost-benefit analyses, etc., etc? Certainly, Claude is not teaching this stuff in his OEM seminars. Or are you, Claude?

For the record, I am sure there were many French papers in the 1930s-50s. Also the Packard paper of the 1950s is a cracker, especially some of the comments in the post-presentation discussion. (Edit: Found it. "The New Packard Torsion Level Suspension", F. R. McFarland, SAE meeting June 1955, 560026, vol 64, 1956 SAE Transactions. Thanks Forbes!)

So I am sure there are SOME people deep in the bowels of the big OEMs that have some idea of these systems. But I am equally sure that those people DO NOT make the big decisions.

Regarding the spring rate calculations... you are really nit picking there. Both calculations (based on displacement or frequency) are very simple and quick. To me they are the same thing and I find it hard to justify wasting much breath to argue about which is better.

Like I said, THE DIFFERENCE IS NIGHT AND DAY. The conventional calcs will forever keep the auto-industry in the dark. You can't even start to calculate a "front/rear ride frequency" using the conventional approach on an interconnected suspension. Too hard! (Ask Claude.) So NO progress. Daylight is much better, IMO. Which is why I am wasting my breath... :)
~~~~~o0o~~~~~

Jose,

Regarding the push/pullrod vs direct acting dampers, I have a question. I've read that one of the advantages of the push/pullrod method, is the reduction in unsprung mass.
...
I am only asking because, ... a direct acting shock abosrber would actually reduce the unsprung mass, because you don't have the rocker or the pushrod. Right?

Well, IMO, you are absolutely RIGHT.

But I am not an expert, and the experts have so far been very reluctant to discuss this matter. Or to "defend their positions" on the matter, as they so often tell the students they must do. (In fact, one expert, Pat Clarke, has just recently notified me that I have received "1 infraction point" for my comments on this thread, with veiled threats that I will be banned from the Forum!. Yes, the Thought Police have learnt a lot from their trip to India! :))

Anyway, I recently discussed the effect of Motion Ratios on unsprung mass on this "How to design a bell crank for push rod suspension" (http://www.fsae.com/forums/showthread.php?11396-how-to-design-a-bell-crank-for-push-rod-suspension&p=118734&viewfull=1#post118734) thread. In brief, not only do the pushrod and rocker ADD their mass to the total unsprung mass for that corner, but their mass is multiplied by the MR SQUARED. So PR&Rs, especially when designed for high MRs, certainly do increase unsprung mass.

Comments and criticisms from the experts that support push/pullrods&rockers with high MRs, yet claim they REDUCE unsprung mass, most welcome!

Z

(PS. Just read Tim's post above. Could add a lot more, but no time just now. Well, for rotational motions the inertia is ~ M*R*R, and note which term is squared! And kinetic energy is ~ M*V*V, and note which term is squared! Ie., parts which must move fast are much harder to get moving...)

Tim.Wright

03-06-2014, 07:09 AM

NOoooo!!! They are just two long straight bars that fit under the car. Very similar to ARBs, but longitudinal rather than lateral (and Z rather than U ends).

They aren't just 2 longitudinal bars...

They also have the lever arm at each side, which on one of the axles needs to swap sides of the car. So you need longitudinal space to package the torsion bars (already getting tricky for a mid-rear engine car with no transmission tunnel) and then lateral space for the bars to swap sides of the car and swing an arc through the wheel travel movement without hitting anything. Its quite a significant increase in packaging space compared to an anti roll bar. And given that it needs to span the wheelbase of the car, I can imagine the diameter isn't going to be as compact as a typical anti roll bar either.

As luck would have it, I have open now a CAD model of the back of mid-rear engined car. You've got buckleys chance of fitting anything in longitudinally. I dont think you would get far either asking the manufacturer to reduce the luggage space in the front in order to swing the lever arms of the Z bars either. For a front engined car maybe.

For FSAE, I think it is a very viable solution which should be considered. I'm already considering a soft twist mode suspension for my own project car.

I respect your theoretical talent Erik, but in my opinion, one of your weaknesses has always been the jump from theory to reality. A lot of what you propose simply isn't practical these days. Particularly for modern cars which are becoming increasingly tightly packaged.

Tim.Wright

03-06-2014, 07:14 AM

One other thing I would add. The use of interters in F1 and some other racing categories is aimed at INCREASING the effective unsprung mass in order to get better control of the contact patch load variations.

So what Z says is right (about MV^2 and MxR^2) BUT its not nessarily a bad thing. Its not such a straightforward thing to analyse though.

I realised I didn't really address the question on the push rods either...

If you compare 2 setups with the same spring/damper and same wheel rate, I suspect (though Im not sure) that the effective unsprung mass contribution from the damper will be the same (since the work = force x disp is the same). The push rod and rocker then will be in addition to it. So I agree with Z on this point...

So unless your push rod setup allows you to use smaller dampers, I suspect you will gain unsprung mass.

Anyway, in my opinion, the choice of motion ratio (and therefore whether to use direct or push rod) should depend on what dampers you use and what damping rate you want at the wheel. Springs are easily made to size, but damping cannot be easily adapted to all motion ratios.

Silente

03-06-2014, 09:04 AM

Interesting discussions going on here.

Z, your input is always very interesting, above all from theoretical perspective. I always got some very good food for though reading your comments, but here i have to say i am pretty much with Tim. At least when calculating Wheel Rates or Suspension Frequencies, i don't see practically a big difference between the results we could get with your (formally more correct) approach and the one from RCVD or Tim.

Moreover, above all when we talk about suspension movements in the order of 20 mm, my experience has always showed me that the results i could calculate with the "classical" vehicle dynamics methods were very close to the ones measured on track (at least with properly calibrated sensors). Of course, as shown in the discussion regarding Anti jacking effects, this method is formally not correct (or, anyway, a simplification) but to be honest i think it could be considered correct within reasonable approximation boundaries.

Regarding the discussion about the unsprung mass definition, if the car was already built i would suggest to simply disconnect the pushrod/pullrod (or damper) from the rocker (chassis) and measure with the scale under the wheel your unsprung mass, keeping the car away from ground with jacking supports, for example (this would of course ignore the mass of the rocker and of the damper, but according to any theory we should use only a portion of these). Unfortunately, when the car is still on paper (or in excel, or in CAD) it is a bit more difficult, but i guess that the 50% approach gets close.

03-06-2014, 06:54 PM

Tim,

I respect your theoretical talent Erik, but in my opinion, one of your weaknesses has always been the jump from theory to reality. A lot of what you propose simply isn't practical these days. Particularly for modern cars which are becoming increasingly tightly packaged.

As I explained on another thread (somewhere?) I modified a car to "3-Z-bar" suspension about 30 years ago. It took about a weekend, or maybe two. I used an angle grinder, stick welder, and a bunch of miscellaneous junk, working under a tree. Sure, it was only a "paddock basher", but, I repeat, the difference in performance was like night and day.

I will happily bet $1M with any OEM, or smaller "Supercar" manufacturer, or anyone else, that I can successfully modify their "modern" densely-packaged car to 3-Z-bar suspension. The end result will be simpler, lighter, cheaper to make, and have far superior performance, to what they originally had. (I would happily bet $100M, but I only have $1M available right now.)
~o0o~

And here is where my practical experience trumps your theory (to continue the gambling analogy :)).

They aren't just 2 longitudinal bars...

They also have the lever arm at each side, which on one of the axles needs to swap sides of the car...

Once again, NOooooo!!!

Longitudinal-Z-bars connect side-pairs of wheel (which is why these, and longitudinal-U-bars, are often referred to as "side-pair" springs).

What you are referring to are "diagonal-Z-bars". These resist (ie. control by their spring stiffness) Heave and Twist motions, but provide NO resistance to Roll or Pitch motions. Briefly, such springs would be terrible for a car's performance (though not quite as bad as ARBs/lateral-U-bars!).
~o0o~

Given that this subject is so poorly understood in the automotive community, I will post more on it in the next few days, but on another thread.

For now I note that throughout the Packard paper referenced above they talk a lot about the "bounce and pitch ride frequencies" of the car, and also about the "centres" for these motions. This was, no doubt, following on from the work of people like Rowell and Guest in the 1920s. And these people only had pencil and paper for their calculations, or maybe slide rules. Back then, no computers to do all the work! (Note, however, that the calculations are extremely simple.)

Sadly, due to the decline of educational standards over the last 50+ years, the majority of you seem to think that the much more puerile concepts of "front and rear ride frequencies" are just as good.

(Oh, and BTW Pat, in case you don't have a dictionary, "puerile" means "childlike silliness". Hardly "inappropriate language"!)
~~~~~o0o~~~~~

Silente,

My main point is that FSAE is supposed to an EDUCATIONAL exercise for student engineers. As such, I reckon the students should be made fully aware of the limitations of whatever "theories" they are being taught, and they should also be pointed in the direction of any more accurate analyses, should they want to go further. In fact, RCVD has a section that covers this idea of a "ladder of abstraction" quite well.

I repeat that the more accurate "bounce and pitch frequency" analysis is extremely simple (for those of you who might want to calculate actual "frequencies", though hardly important in FSAE). But, sadly, very few people are even aware of it these days.

Also, considering this from the opposite direction, Jose started this thread because he thought (because he was taught?) that he had to FIRST choose a "ride frequency", before he could THEN calculate his "roll-rate". This is nonsense (which in polite English is often referred to as "codswallop", Pat), because it overcomplicates a much simpler matter. As I explained, the roll-rate can be calculated in one very simple step (in your head), with no need for any fictional "frequencies".

I have often been puzzled by students asking me "how do we calculate the spring rates". I say "puzzled", because FSAE conditions are so simple that I couldn't see where the problem was. I only became aware of the source of the student's difficulties when they started mentioning "ride frequencies". That is BAD education!
~~~~~o0o~~~~~

To sum up, in sociological circles there is the expression "fear of the new" to describe H. Sapien's common and irrational aversion to anything unfamiliar. (Well, to a certain degree it makes sense in the "wild", but it is still irrational.) In the modern automotive world, "bounce and pitch frequencies" and "interconnected suspensions" are such things.

But this behaviour should more accurately be described as "fear of the old, but forgotten"!

More on the Packard paper soon...

Z

dynatune

03-07-2014, 05:21 PM

I do fully agree with Tim. Just as with modelling the very basic theory of vehicle dynamics can be grasped "rather" quickly but the more one learns or the more one refines a model the more questions come up. There is absolutely nothing wrong with learning traditionally all topics the hard way. It just takes time and shortcut's leave by definition gap's that sooner or later are going to catch you. There are many good books about vehicle dynamics and suspension design but there is only one bible: http://books.google.it/books/about/Road_vehicle_suspensions.html?id=BHRTAAAAMAAJ&redir_esc=y .The book is very expensive and extremely tough literature but once understood of a fascinating simplicity that takes away all the glory of "guru's". Anybody that claims to be an expert in Dynamics must have read this book and if he did will confirm this statement.

At the end I would like to add my 2 cent's to a longitudinal Z-bar .... the most famous (european) car for having a (hydraulic) Z-bar concept was the Citroen 2CV and drove like a giant pile of c...p. Brake and Acceleration Pitch were amplified and any roll center height jacking on either end of the car raised the other of the car.... so much for the inherent disadvantages of that system.

In the interests of a balanced discussion, I will counter some of Z's points.

The methods outlined in RCVD are simplifications and yes simplifcations require some cutting of corners. In this case, following the "traditional" method, you are largely constrained to end up with a traditional suspension with 4 springs and 2 anti roll bars. For passenger cars, this arrangement is a very good compromise between performance and complexity. Packaging anti roll bars is very simple. Packaging a longitudinal Z bar is a nightmare, and for what exactly?

At the end of the day, ANY model or method (including Z's) is necessarily a simplification and the traditional methods are quite a reasonable way to arrive at an adequate (but not optimum) solution without a lot of time or large amounts of input data. As you have mentioned, you don't have the time to do something out of the box so following a well established (and sucessful) route is a good choice from a risk management point of view. Even Z would agree, its more important to get the car built 1 month earlier than to agonise for 1 month more in the design phase about suspension rates.

The main problem though, from an educational point of view, is that often the traditional methods are taught as though they are the first principles of a suspension system whereas they absolutely are not. They are a (quite heavily reduced) simplification of how a suspension operates in roll and pitch. Additionally, they only have a limited range where they are valid (and this unfortunately does not include limit behaviour) and some suspension geometries cause the calculations to crash.

Your intuition should be telling you this anyway since these methods simply a complex system of 2 wheels + elastic tyres, 12-20 links located in 3D space, connected by a compliant chassis by compliant joints down into an equation with only 9 or so parameters (hrcF, hrcR, KwhlF, KwhlR, KrollF, KrollR, mass, mass_dist, cgh).

Regarding the book, there is too much good solid theory in RCVD (particularly the chapter on steady state stability and control) to simply brush it off as codswallop. My advice would be to follow whats in RCVD or in the Optimum G tips by all means but recognise you are cutting some corners by doing so.

nowhere fast

03-07-2014, 08:32 PM

At the end I would like to add my 2 cent's to a longitudinal Z-bar .... the most famous (european) car for having a (hydraulic) Z-bar concept was the Citroen 2CV and drove like a giant pile of c...p. Brake and Acceleration Pitch were amplified and any roll center height jacking on either end of the car raised the other of the car.... so much for the inherent disadvantages of that system.

It’s worth mentioning that the 2CV was designed to be a cheap car capable of carrying eggs across a rough field without breaking them, not a sports car.

The 2CV suspension springing arrangement is different to what Z is advocating. It does effectively have longitudinal Z-bars to resist heave and roll, but it then has what is effectively a pair of longitudinal U-bars which resist pitch and warp. Note that it is the longitudinal U-bars that control the pitch issues you mentioned.

This arrangement is quite different to the 2 longitudinal + 1 lateral Z-bar arrangement which would result in zero warp stiffness. Then again, the 2CV Z-bar implementation is not hydraulic so perhaps you are thinking of a later model Citroen?

dynatune

03-08-2014, 06:03 AM

You are right it was linkage but if I am not wrong the 2CV had in the middle of the car, attached via linkage a sealed central unit with in it springs and hydraulic dampers - similar to modern f1 cars 3rd spring/packer - which remained in position when both axles had the same travel and moved/shifted when axle travel was different. Again to avoid any confusion, a Z-bar concept (independent of it mechanical design) makes the other side go down when the driving side goes up and a U-bar concept does make the other side go up when the driving side goes up.In parallel movement of both axle Z-bar work as heave (torsion) spring and U-bar are inactive. Even so the 2CV was designed as a cheap car capable of carrying eggs AND allowing the farmer to leave his hat on :), I cannot see what would be the enormous advantage of such a system on a sportscar (with most of all the negative characteristics). The generic behavior is the same to the 2CV. Sure one can tune and add-on other bits and pieces to compensate for some problems/characteristics. Beyond that how would one compensate for the "frequency" effect of changing speeds on the "wheelbase" induced connection ? McLaren has put some of these thought in their road car but used a mechatronic / hydraulic system which of course is a completely different approach - using the good bit's of the idea but not having the disadvantages.

Cheers,
dynatune, www.dynatune-xl.com

03-08-2014, 11:02 PM

May as well say a bit more about the two main topics going here...
~~~o0o~~~

CALCULATING "SPRING-RATES" BY USING "RIDE-FREQUENCIES".
================================================== ===
Consider the following three approaches to calculating your spring-rates.

1. Most basically, you start with the spring-rate definition of Ks = Force-on-spring/Deflection-of-spring.

You now estimate (ie. calculate, or guess, or...) the force F that is going to act on the spring, and then choose (ie. guess, or copy others, or ...) a deflection D that you are happy with. Then,
Ks = F/D.

This is a very simple calculation, is easily understood, and it doesn't require any sort of specialist "automotive expertise", so any engineer can use it. (BTW, the "conventional" method for determining roll-rates follows this approach, except that part of the calculation also requires the next approach.)
~~~o0o~~~

2. You start by inventing an entirely FICTIONAL concept known as "ride-frequency" Freq. You then formulate a clever looking equation that relates the desired spring-rate Ks to Freq, together with some other parameters such as the gravitational force W acting on one end of the car, the local gravitational acceleration G, and Pi.
Ks = 4*Pi^2*Freq^2*W/G.

Most of the parameters on the RHS are known or can be measured, but Freq must be chosen, usually from a table listing typical Freq values of conventionally sprung cars. There is no engineering ability, or deep understanding, required for this selection process of Freq. It is, in fact, very much like throwing a dart at a dartboard.

Many auto engineers and FSAE students (ie. above posters) consider this process quite acceptable, possibly because they like throwing darts, and they own a calculator which makes the Pi-squared, etc., bits quite easy.
~~~o0o~~~

3. A new consultancy firm of educationalists, perhaps called "OptimalGeeWhizPlus+++", develops a "new and improved!" method for calculating spring-rates. As with the growth of red-tape, bureaucracies, and societies in general, anything new-and-improved must, of necessity, be much more complicated, difficult, and generally useless, than what came before.

So OGWP+++'s new equation now includes a full dozen (count them!), brand new, and totally fictional parameters, known as Freqing1 through to Freqing12. All of these must be carefully chosen from various "Magic Number Tables" before the spring-rate can be calculated. I won't list the equation here, but rest assured that it only takes a couple of pages of Matlab code to solve it (easy!), and the answer is always given to 12 decimal places.
~~~o0o~~~

My point is, WHERE DOES IT ALL END!!!

By starting off down the path of fictional nonsense, the possibility opens for an infinite amount of absurdity to be added. And all this absurdity, via the "Magic Number Tables", locks-in all future designs into those that are already covered in the tables. NO MORE PROGRESS IS POSSIBLE!

The young suspension designers might think that they are doing real engineering ("and it's damn complicated, sophisticated, high-end stuff, at that!"), but they are really doing nothing more than throwing darts at a dartboard. Or maybe selecting a meal at a Chinese Takeaway ("A number 42, the Mongolian-4-corner-springs-with-2-extra-stiff-ARBs-in-sweet-and-sour-sauce, thanks!").
~~~~~~~~~~o0o~~~~~~~~~~

THE CITROEN 2CV.
==================
I guess the proof that so few people in the auto-industry have ANY understanding of interconnected suspensions is in some of the above posts (mainly Tim and Dynatune, who I believe both work somewhere in the industry).

As Nathan (and Dynatune) noted, the 2CV was designed to be the cheapest possible car for poor French farmers. As well as being able to carry a basket of eggs across a ploughed paddock to the markets on Saturday, it also had to take Mr and Mrs FF to church on Sunday, hence the high roof that allowed them to wear their hats. And extremely successful it was in these design goals. Its phenomenally comfortable ride, grip, and handling over very rough roads is widely acknowledged.

Anecdote 1. After a recent Australian "Rally Raid" that 2CVers regularly go on, some of them visited the Kinetic Suspension's test track in West Oz. (KS's is another approach to interconnected suspensions, and they were once sponsors of UWA.) After viewing some of Kinetic's test vehicles covering some very rough ground, one of the 2CVers covered the same course at low speed (harder because no momentum to carry the car through the hard bits), and quite effortlessly. The Kinetic staff just stood there dumbfounded, quite speechless for several minutes.

Anecdote 2. The soft springing keeps vertical wheel forces very constant over rough ground, so very consistent grip, both for driving traction, and also for consistent handling through corners. Nevertheless, the softness means that body roll is extreme during high-G cornering. It is said that during such cornering the driver will fall out of the side window before the car loses its grip on the road.

Anyway, the 2CV has (very soft) longitudinal-Z-bars, albeit implemented as pullrods acting on coil-springs. (NOT hydraulics, although the British-Leyland cars of the 1960s+ (Austin-Morris "Hydrelastic" and "Hydragas") did use glycol-based hydraulic interconnection. These used rising-rate at each end of the 2 x "hydraulic-longitudinal-Z-bars", with this rising-rate controlling Pitch, so no other springs were required.) The 2CV's Z-bars have, in effect, a falling-rate for Heave, which pulls the car down during fast cornering (ie. lowers CG).

The rubber donuts at either end of the main spring-canisters, that Nathan described as longitudinal-U-bars, have a very soft zone in the middle of their travel, which gives very soft Pitch and Twist. After this they act a bit like rising-rate, conventional one-per-corner, bump rubbers that control the final upward movement of each wheel. And many more interesting details... (Eg. the main dampers are quite conventional telescopic units but mounted at MR = ~0.3, and considered very effective. Inertial "anti-patter" dampers were also sometimes added, for even better ride on corrugations...)

Once again I stress that I have never seen a good engineering explanation of the 2CV's suspension, certainly not in English. I have seen many very BAD explanations, often completely wrong in the details, and almost invariably with the conclusion that "if it was any good, then we would still be using it...". Here is one such example.

Originally posted be Dynatune:
I cannot see what would be the enormous advantage of such a system on a sportscar (with most of all the negative characteristics). The generic behavior is the same to the 2CV...

Paul, you are looking at (but not seeing very well) a car designed for very rough roads, and then saying it would not make a good sportscar. That is not a "well-reasoned" conclusion. You could equally reason that since the 2CV's air-cooled, horizontally-opposed engine of 602cc (the big-block!) was not very powerful, then, quite obviously, Porsche are wasting their time, and the 911 will never be a successful sportscar!

For the record, a sportscar with 2CV style suspension, but with much stiffer longitudinal-Z-bars, would have stiff Heave and Roll (allowing low ride height and flat cornering), while still having much softer Pitch and Twist. The Pitch could be controlled with either a third lateral-Z-bar, or corner bump-rubbers similar to the 2CV.

Importantly, LLTD would be determined by the geometry of the Z-bars (NOT by the spring-rates!) and together with the soft Twist-mode, this isolates the handling balance from any Twist in the road. All modern, stiffly-sprung, sportscars have their HANDLING BALANCE DICTATED BY THE TWIST IN THE ROAD. This is the big difference!

Interestingly, this very obvious, and easily quantified, fact is never mentioned in any VD textbook I have ever seen (ie. either "ride comfort" is studied for a car travelling in a straight line on a bumpy road, or "handling balance" is studied for a car cornering on a perfectly flat road)!

Z

dynatune

03-09-2014, 09:43 AM

So Z, you are stating that the big difference is the excellent handling behavior of the "Z-bar cocktail" is most of all on twisted roads. Ok, that leaves about 99,9% of all racing cars that run on pool billiard like tarmac surface out of the equation because they would not benefit of that..... And the only race series that I know, that run on significantly twisted roads are Rallye Cars or Hill Climb Cars which are basically the same (I have worked on a few WRC cars) and all those engineers have not seen the benefit either in the last let's say 30 years or so .... very strange indeed. Either there are a lot of people that do not see as clearly or they are all ignorant ....

Let me share a secret from the ignorant automotive world: I have been responsible for the release of one or two sports cars in my life and the "bumpy" ride was in 9 of 10 cases caused by the request of the marketing department .... a tough guy wants a tough car .... many sports cars could/can be a lot softer without noticeable performance losses.

And by all due respect, if I might suggest one final thing; try to not consider that all other people are idiots, try giving the young engineers food for thought instead of bringing them down on every possible occasion and most of all try to avoid pretending to know where it all goes to. "There is no fool like an old fool" is if I remember correctly a very ancient saying, remind yourself of it sometimes :)

Cheers
dynatune, www.dynatune-xl.com

BillCobb

03-09-2014, 10:46 AM

And a bumpy ride makes for unhappy tires. Unhappy tires make corners bad. Bad corners make drivers mad. Mad drivers get shot by other drivers who carry.

So get Direct TV, enjoy the ride !

MCoach

03-09-2014, 09:04 PM

The point of inertia in suspension systems has been demonized in here. Has anyone actually put some thought into how much inertia in their system is acceptable? Should you aim to minimize it?

Yes? No?

Usually Formula 1 is a poor series to look to for correlation to what you're trying to calculate and make decisions based on, but they have engineered a 'J-damper' to put more inertia, without the weight, into their front and rear suspension systems to help better damp out the tire forces and control the "fictional" wheel oscillations and frequencies that occur. The tires are relatively floppy as the engineers have expressed in interviews and publications. I've looked into the math on it and it makes sense for them...

Does it for you?

03-11-2014, 08:59 PM

Dynatune (P?),

So Z, you are stating that the big difference is the excellent handling behavior of the "Z-bar cocktail" is most of all on twisted roads. Ok, that leaves about 99,9% of all racing cars that run on pool billiard like tarmac surface out of the equation because they would not benefit of that...

I have posted some more notes on Z-bars on the Suspension Design thread (page 25+) (http://www.fsae.com/forums/showthread.php?8950-Suspension-Design&p=119021&viewfull=1#post119021). In a few days I will explain there why racecars running on real race tracks (= billiard table smooth-ish) can also benefit from soft Twist-modes.

Most certainly, real "sportscars", on real public roads, can benefit even more. But as you suggest, most of these are "male jewellery" designed by the marketing department, so real performance is not so important.

On this last matter I would suggest to any young engineers who happen to be working in the auto industry, and who would like to try introducing these Z-bar suspensions, the best type of vehicle to start with might be 2WD utes/pickups, or commercial vans. Firstly, the tradesmen who use these on difficult "off-road" building sites (and then go fishing on the weekend) will love the greatly improved traction and ride comfort that comes from having all wheels firmly on the ground. Secondly, the hairdressers in the marketing department won't bother you.
~o0o~

... Either there are a lot of people that do not see as clearly or they are all ignorant ...
... try giving the young engineers food for thought ...

Here is some "food for thought" for the young engineers (I have given this many times before, but once again...).

The first steam-traction engines built two hundred years ago had soft-Twist modes. This, no doubt, inspired by Adam's first wooden cart. All farm tractors have soft Twist-modes. Same with all trucks, and trains. And from the largest earthmovers down to ride-on lawn mowers, all soft Twist-modes. And conventional passenger cars with reasonably soft springing also have softish Twist-modes.

The exception comes with sportscars and racecars. They have stiff corner-springs, so stiff Twist-modes. As noted, sportscars are male jewellery. And the tiny number of racecars are just "boys playing with toys". Importantly, in racing it is the participation that counts, not winning!

So which of the designers of the above vehicles do you think "see more clearly"?
~~~~~o0o~~~~~

MCoach,

The "J-Damper/inerter" was introduced into F1 after the earlier "Inertial-Dampers" were banned. These IDs (= mass-on-a-spring-in-a-can) are conceptually the same as used on the 2CV, and were introduced into F1 by Renault. The excuse for banning them, by Bernie and Max, was, as always, "to improve safety, and reduce costs"!!!

IIRC (could be wrong?), Massa was hit in the head by a stray J-Damper. IDs are intrinsically much safer because they can be (and were) buried inside the chassis or gearbox. And there is no doubt that IDs are a much simpler, cheaper mechanism than JDs. Also, IMO, the IDs were more effective at "stabilizing the aero-platform" on the floppy tyres (= F1 "suspension") than the newer JDs (well-reasoned counter-arguments welcome :)).

So, once again, the Rulemakers forced development in the less safe, more expensive, and less effective, direction!

Ah, don't you just love progress! (Not!!! :()

Z

(PS. Regarding your question about the importance of inertia/unsprung-mass, earlier I incurred an "infraction point" for saying that FSAE "can be won" with no suspension. Apparently this was an encouragement to the students to break the Rules. Naughty me! From now on I must remember to say that FSAE "HAS BEEN WON" by cars with no suspension movement. :))

Charles Kaneb

03-12-2014, 07:22 PM

Dynatune, "Rally cars..." - except for Citroen, who used the Kinetic system to whomp everybody until it was banned!

Charles Kaneb

03-16-2014, 02:59 PM

If i squint hard enough, I could see the potholes at Nelson Ledges as the pockets, but otherwise I don't get the billiard-table reference for race tracks in America.

dynatune

03-16-2014, 03:37 PM

There is nothing negative about a soft twisting mode IF it comes with no other major disadvantages. If the system is based on a mechatronic/hydraulic system - like McLaren uses - that permits to fruit from only advantages it is to be considered a step forward, if it comes with all the inherent disadvantages as earlier mentioned it remains in my professional opinion to be considered as a step backwards - and I see myself confirmed in my opinion by the choice of 99% of the automotive industry against a system like that. The market is by definition always right , whether one likes it or not .... and if for the guys for who "racing" is about participating and not about winning .... please stay on the right side of the track out of the way of the fast runners :D ..... Nothing against a Formula SAE car ... but it comes nowhere near to an F1 car that needs to "carry" at around 300kph 3 times it own weight in aerodynamic load so again I fail to see how one could create a "soft springed corner car" with these kinds of loads unless with a mechatronic/hydraulic system ...

Cheers,
dynatune, www.dynatune-xl.com

03-16-2014, 08:00 PM

...if [a soft Twist-mode] comes with all the inherent disadvantages as earlier mentioned it remains in my professional opinion to be considered as a step backwards - and I see myself confirmed in my opinion by the choice of 99% of the automotive industry against a system like that. The market is by definition always right , whether one likes it or not ...

Dynatune,

The auto industry is, most certainly, NOT making a "choice".

To make a choice you must first be aware of the options. As I explained before, the passenger/sports/racing auto industry is, for the most part, IGNORANT of these things. Just one example is that you are a "professional" working in the industry, but you have shown a considerable lack of understanding of how these things work (Edit: eg. what are "all the inherent disadvantages"???). So too, many of the other "auto-industry professionals" on this forum (eg. Claude, do you teach any of these things?).

Anyway, I may be trying to put brains into statues, but once again...

Current high-aero-downforce cars have "third springs" fitted front and rear (and as noted before, these should be called "seventh and eighth springs"). As explained in this "Suspension Design" thread post (http://www.fsae.com/forums/showthread.php?8950-Suspension-Design&p=119021&viewfull=1#post119021), these "third springs" are lateral Z-bars, so they control (ie. stiffen) Heave and Pitch modes without affecting the Twist mode. A high-aero car with only 4 x stiff-corner springs would be far worse than the current cars. Put simply, the current aero-cars are saved, to some degree, by their lateral Z-bars.

Now, if "real racers" ever become really serious about winning, then they will follow the vast majority of ground vehicles and design their cars with a soft Twist-mode (see earlier post about steam-tractors, etc., and see that their "market" demanded that they get it "right"). "Mechatronics and hydraulics" are NOT necessary. Frankly, they are just the typically mindless addition of complexity. Longitudinal-Z-bars are simple, and will do the job, although there are also a lot of other simple ways to do it.

Z

(Edit: PS. When did it become reasonable to argue, "I don't have a clue about this subject, and there are millions of other people like me, therefore we are all right!" ?)

ben

03-17-2014, 12:35 AM

Sorry Dyantune race tracks are not billiard table smooth. Far from it. The reality is that aero dominates so heavily in most categories that what would be considered "good" suspension characteristics often don't work well. Low warp stiffness could be a concern in terms of aero platform, and my understanding is that this was an issue for teams that have run interconnected at Le Mans for example, where road crowning would make the system desirable. My reference for this is the Crueat system that was run by RfH on a Dome 10 years or so ago.

Markets are "right" on their own terms. But we as engineers should at least recognise that those terms are sometimes incredibly stupid. Read Brave New World and formulate some engineering based answers. Wisdom (stupidity?) of crowds is not a good enough answer.

Two relevant Richard Feynmann quotes:

"The first principle is that you must not fool yourself and you are the easiest person to fool."

"Reality must take precedence over public relations, for nature cannot be fooled."

Ben

dynatune

03-17-2014, 08:44 AM

There is a difference between a public road, a race track and a WRC rallye stage ... in that contest most race tracks are "rather smooth" and yes many markets can be incredibly stupid and yes we engineers should not be blinded by that fact. However, if a very competitive "market" like the racing world with very bright engineers in all kind of racing series has "failed" to see the overall advantage of such a system and if on top of that the whole "common" automotive engineering world (with a few bright engineers there too) has failed to see - for what reasons whatsoever - over the last give or take 100 years an advantage, something might be not so good in the system as one would actually think. On top of that, if an FSAE car can win without any suspensions - as stated here - where does that put the "warp" theory ? These are just observations. For the record I am NOT saying that a low WARP stiffness is not beneficial to gain better handling of the car, but that that advantage may not come at the cost of a whole range of disadvantages.

Cheers,
dynatune, www.dynatune-xl.com

dynatune

03-17-2014, 09:30 AM

Dynatune,

The auto industry is, most certainly, NOT making a "choice".

To make a choice you must first be aware of the options. As I explained before, the passenger/sports/racing auto industry is, for the most part, IGNORANT of these things. Just one example is that you are a "professional" working in the industry, but you have shown a considerable lack of understanding of how these things work (Edit: eg. what are "all the inherent disadvantages"???). So too, many of the other "auto-industry professionals" on this forum (eg. Claude, do you teach any of these things?).

Anyway, I may be trying to put brains into statues, but once again...

Current high-aero-downforce cars have "third springs" fitted front and rear (and as noted before, these should be called "seventh and eighth springs"). As explained in this "Suspension Design" thread post (http://www.fsae.com/forums/showthread.php?8950-Suspension-Design&p=119021&viewfull=1#post119021), these "third springs" are lateral Z-bars, so they control (ie. stiffen) Heave and Pitch modes without affecting the Twist mode. A high-aero car with only 4 x stiff-corner springs would be far worse than the current cars. Put simply, the current aero-cars are saved, to some degree, by their lateral Z-bars.

Now, if "real racers" ever become really serious about winning, then they will follow the vast majority of ground vehicles and design their cars with a soft Twist-mode (see earlier post about steam-tractors, etc., and see that their "market" demanded that they get it "right"). "Mechatronics and hydraulics" are NOT necessary. Frankly, they are just the typically mindless addition of complexity. Longitudinal-Z-bars are simple, and will do the job, although there are also a lot of other simple ways to do it.

Z

(Edit: PS. When did it become reasonable to argue, "I don't have a clue about this subject, and there are millions of other people like me, therefore we are all right!" ?)

Z,

you are of course right. ... I did design my first 3rd spring F1 suspension in 1994 and ever since I have not understood the concept. Bummer, I wonder why we started with the concept anyway, and then it worked so well that we put also rollbars on top of that, stiff ones too to compensate for the lacking side springs ..... stupid us .... and then we tested for several years all kind of linkage systems with all kind of coupling variants ....(after the "active" F1 cars were banned we tested all kind of mechanical systems in order to simulate the possibilities, but we did not come anywhere close to the old "mechatronic" systems with our "analog" technology, again stupid us ...). Then later, when I tested in the "normal" automotive world some "zero" warp - suspension concepts with various suppliers in the late 90s, the automotive industry was to such an amount "not capable of seeing the light", that some very expensive prototype cars were build.

Dynatune

Moop

03-20-2014, 12:25 AM

Low warp stiffness could be a concern in terms of aero platform, and my understanding is that this was an issue for teams that have run interconnected at Le Mans for example, where road crowning would make the system desirable. My reference for this is the Crueat system that was run by RfH on a Dome 10 years or so ago.
I'm not really seeing why low warp stiffness would have an impact on the position/orientation of the underbody with respect to the road. For a given warp input, all the low warp stiffness means is that the warp input is taken up by the suspension, instead of being taken up by squish in the tires and twist in the chassis. If you have a high warp stiffness and the chassis is rigid in torsion, or at least very stiff relative to the tires, then tires will take up most of the warp input.

In both cases, the chassis is moving very little with respect to the road and the road is twisting under it. The only difference is whether the suspension or the tires take up the warp input. In both cases though, the warp input results in the underbody being at a "roll angle" with respect to the road at each end of the car. Am I missing something?

I could possibly see an argument for a soft chassis being beneficial in this case though. If the chassis was soft, it'd twist to conform to the warp input and result in little angle between the road and the underbody at both ends of the car.

03-20-2014, 10:34 PM

Ben,

Two relevant Richard Feynmann quotes:

"The first principle is that you must not fool yourself and you are the easiest person to fool."

"Reality must take precedence over public relations, for nature cannot be fooled."

YES INDEED!
~~~o0o~~~

Dynatune,

... but [soft Twist-mode] may come at the cost of a whole range of disadvantages.

And it is equally true that "pigs might fly".

But until I have reliable evidence of such, as opposed to just rumours, I will not be packing an extra-heavy-duty umbrella whenever I venture outdoors (flying pig poop is messy!).

So what are the "whole range of disadvantages" of soft Twist-modes?????

(Please note here the distinction between Warp-mode, which has equal up-or-down movements of the wheels, and Twist-mode, which allows LLTD to be fully adjusted, while still being completely Twist-soft. UWA knows how this works. Do you?)
~~~o0o~~~

Moop,

I agree with your comments on soft or stiff Warp and aero-cars. No difference really, assuming a torsionally stiff chassis. I will note that the Crueat car had a Warp-soft suspension of a type that I don't particularly like. Too many hydraulics that prevent the system from being easily adjustable in the right ways.

Specifically, if LLTD is dependent on the ratio of diameters of two different hydraulic cylinders, then how do you fine tune it, other than having to swap a cylinder for one of (very slightly!) different diameter? And, IIRC, their Roll and Pitch-modes were somewhat inextricably linked (with Heave also thrown into the mix). So the standard approach of having the car at high rake (= Pitch angle) at low speed for max DF, and then less rake at higher speeds, is harder to achieve.
~~~o0o~~~

BTW-1. I have only seen sketchy details of the Crueat (splnig?) system, so I could be wrong in above comments.

BTW-2. UWA's soft Twist-mode "Aerobeam" car solves the aero problems by allowing the main aero surface (= undertray) to twist and thus conform to the average road surface (ie. constant tunnel gap).

BTW-3. More on soft Twist-modes just posted on the "Suspension Design" thread (http://www.fsae.com/forums/showthread.php?8950-Suspension-Design&p=119179&viewfull=1#post119179).

Z

dynatune

03-25-2014, 03:56 PM

Ben,

YES INDEED!
~~~o0o~~~

Dynatune,

And it is equally true that "pigs might fly".

But until I have reliable evidence of such, as opposed to just rumours, I will not be packing an extra-heavy-duty umbrella whenever I venture outdoors (flying pig poop is messy!).

So what are the "whole range of disadvantages" of soft Twist-modes?????

(Please note here the distinction between Warp-mode, which has equal up-or-down movements of the wheels, and Twist-mode, which allows LLTD to be fully adjusted, while still being completely Twist-soft. UWA knows how this works. Do you?)
~~~o0o~~~

Moop,

I agree with your comments on soft or stiff Warp and aero-cars. No difference really, assuming a torsionally stiff chassis. I will note that the Crueat car had a Warp-soft suspension of a type that I don't particularly like. Too many hydraulics that prevent the system from being easily adjustable in the right ways.

Specifically, if LLTD is dependent on the ratio of diameters of two different hydraulic cylinders, then how do you fine tune it, other than having to swap a cylinder for one of (very slightly!) different diameter? And, IIRC, their Roll and Pitch-modes were somewhat inextricably linked (with Heave also thrown into the mix). So the standard approach of having the car at high rake (= Pitch angle) at low speed for max DF, and then less rake at higher speeds, is harder to achieve.
~~~o0o~~~

BTW-1. I have only seen sketchy details of the Crueat (splnig?) system, so I could be wrong in above comments.

BTW-2. UWA's soft Twist-mode "Aerobeam" car solves the aero problems by allowing the main aero surface (= undertray) to twist and thus conform to the average road surface (ie. constant tunnel gap).

BTW-3. More on soft Twist-modes just posted on the "Suspension Design" thread (http://www.fsae.com/forums/showthread.php?8950-Suspension-Design&p=119179&viewfull=1#post119179).

Z

Z, I hate to destroy your "religion" but when we started out testing "low stifness" warp cars in F1 - some guys like you were in those days convinced that it was pandora's box - the car's were actually behaving like lazy pigs .. not flying pigs but very lazy pigs ... since there was a flagrant lack of load transfer to make the car react sufficiently on corner entry. As I tried to explain to you, we came from semi active cars with all kinds of fancy mechatronics and hydraulics in there and tried to copycat the complex world with "simple" analog mechanical systems. We failed miserably, but we were of course a bunch of cunts .... excuse me for the phrase. As I also told you we put on stiffer bars and we went faster, the driver liked it better and that is the simple truth. The even harder truth was that the laptime was lower ... more ARB ... faster ... thus the simple conclusion was - in those days - that the warp system sucked .... Then we did in main stream engineering also try to reduce roll and warp rates for 3 reasons ... reduce cost, improve comfort & keep handling at the level the client would expect .... we did build cars with z-bars, without rollbar's and so on ... and we always found the OVERALL compromise better for "traditional" systems for one reason or the other. So much for that .... I'd like to end with a quote of a famous unknown ... "if you think that you have seen the light at the end of the tunnel, be aware that it might be the oncoming train that is going to plaster you .....". By all means do invent again roads that have been investigated decades ago .....

Cheers,
dynatune, www.dynatune-xl.com

03-25-2014, 07:14 PM

Dynatune,

The essence of your argument is this.

We failed miserably...

I know why you failed miserably. Do you?

Do you acknowledge that it is A HISTORICAL FACT that the "semi active cars with all kinds of fancy mechatronics and hydraulics in there..." were hugely successful, and swept all before them? So successful, in fact, that they were banned from all forms of motorsport, including FSAE. (And for the usual motorsport reason of "we want to see races between the drivers, not between the backroom boffins!".)

Do you acknowledge that IF YOU WERE SUCCESSFUL when you "tried to copycat the complex world with "simple" analog mechanical systems", THEN THE CAR WOULD ALSO BE SUCCESSFUL? (The alternative is that you think that current dumb-suspension cars work better than full-blown active-suspension cars. Is this your view?).

Finally (and this is the important one), do you understand the difference between a soft Warp-mode that gives " a flagrant lack of load transfer to make the car react sufficiently on corner entry.", and a Twist-mode that, as well as being completely soft, can also allow an arbitray amount of LLTD, so as to suit the car?

I know for sure that many of the active-suspension experts DO NOT UNDERSTAND this difference. (And note here, the role of such "experts" is to prevent progress, by claiming that they are doing it.)

Anyway, if, as you are suggesting, all that is required to prove a concept wrong is to show one failed attempt at it, then, quite clearly, conventional double-wishbone-and-eight-spring suspensions must be complete and utter failures! The back half of EVERY grid is full of them!
~~~o0o~~~

If anyone has any technical/engineering thoughts on this subject, then please post them. Because, honestly, I am finding this typical H. Sapiens (the "wise one"!!!) attitude of "Awww... it's sooo haaaard, because it's soo new (... err, well, old-ish but out-of-fashion now) ... and now I'm going to have to learn all new stuff ... which is making my head hurt... awwww...", extremely boring.

Z

03-25-2014, 07:50 PM

Ooops! I just put a long post-with-pics here, but it was meant for the "Suspension Design" thread (see link a couple of posts up). Same subject, different room. I will try to confine future rants to that thread.

Z

(PS. Noah, I will answer your PM'd question in a few days on the "SD" thread. Apparently, my PM boxes are near full and I have to do a clean-out... grrrrr... :()

nowhere fast

03-25-2014, 09:54 PM

03-25-2014, 10:58 PM

Nathan,

You are right that road-graders have excellent ride and traction over very uneven ground. These, and some of the other vehicles you mention, are briefly covered at the end of the SAE paper 2000-01-3572 "Balanced Suspension".
~~~o0o~~~

But I have, perhaps incorrectly, been using the terms warp and twist interchangeably. I don’t recall a formal definition of these terms, and have been using the term ‘warp mode’ to refer to any mode where the front and rear axles roll relative to the chassis by different amounts (opposite directions implies a negative amount); bringing the wheel prints out of a flat plane. Others might also be using their own slightly different definitions of ‘warp’.

And this is, indeed, the source of the problems. Nobody bothers DEFINING anything these days. I guess because it is much easier to argue against something, when there is no clear definition of it!

Whenever I have discussed "All-Wheel-Modes" in detail, I have always tried to point out that these modes can be defined in many different ways (in fact, four infinities of different ways!). I have also stressed this by giving examples of some of the different ways in which this can be done. Your post that you link to above suggests that you also understand this difference.

I suspect that many of the problems that come from classic "Warp-mode" thinking (ie. from the active-suspension era, and possibly Dynatune's problem) is that it is IMPLICITLY ASSUMED (though rarely, if ever, explicitly stated!) that the Warp-mode (Edit: and also the "Roll-mode") MUST BE symetrical left-right and front-rear. This means that the only LLTD possible, EVER (!), is 50F:50R. Hence, (maybe?) Dynatune's insistence that they had to "add bar" to make the car handle.

Anyway, it seems this problem stems from the H/P/R/Warp thinking being initially used only as a means of DESCRIBING suspension movements, rather than as a means of CONTROLLING them. In the active-suspension days these H/P/R/W DESCRIPTIONS of wheelprint positions were sent to a computer, and the computer then determined what forces should be exerted by the suspension in these modes to get the car to handle appropriately. To do this the Warp-mode had to be "active", in that it had to be able to forcefully "warp" the four wheels one way or the other, to achieve the appropriate LLTD. Hence, the common phrase from that time, "Warp-Roll coupling".

The way I have always described H/P/R/Twist suspensions is that the definitions of these different modes are CHANGED to suit whatever handling characteristics are required. Specifically, to get the right LLTD while still allowing soft Twist-mode, you must change the "definition" of your Twist-mode (Edit: actually, you must subtlely change the definition of your Roll-mode, and the way it behaves during Twist motions. All explained at length elsewhere).

But the bottom-line is that none of this will ever be understood by the wider industry, while the experts keep insisting that;
"It will never work, because I tried it, and I am an expert! And I do NOT have to explain the details of why I couldn't get it to work, because I am an expert!"

Which, I think, is what you said above.

Z

dynatune

03-27-2014, 11:49 AM

Dynatune,

You are telling us that soft warp suspensions did not prove to be beneficial in your experience, but you aren’t providing an engineering interpretation of why that was the case. I’m not saying this as a criticism; but because I am genuinely interested to hear your reasoning on why it was not beneficial. People like me that have no experience with soft warp cars* are left to reason our way through the issue in our own minds, and this is why merely telling me that your experience suggests a conventional suspension is better than soft warp suspension is an unconvincing argument.

* I have driven several soft warp vehicles including a forklift, wheel loaders, and a grader. The grader in particular was very smooth over bumpy terrain; how it would handle around a racetrack is another matter though.

Z,

I understand how LLTD can be adjusted on a zero twist stiffness car and have posted about it before:
http://www.fsae.com/forums/showthread.php?6433-Fantasy-Car&p=21157#post21157

But I have, perhaps incorrectly, been using the terms warp and twist interchangeably. I don’t recall a formal definition of these terms, and have been using the term ‘warp mode’ to refer to any mode where the front and rear axles roll relative to the chassis by different amounts (opposite directions implies a negative amount); bringing the wheel prints out of a flat plane. Others might also be using their own slightly different definitions of ‘warp’.

Nathan,

The main problem as I said was the fact that the car due to it's low warp stiffness did not sufficiently quickly build up (diagonal) load transfer which lead to a rather slow build up of tire slip angles, which was generally interpreted by the drivers as a "lazy understeering pig" setup. Imagine a road car with soft roll bars and then putting some really stiff bars in it ... the car changes from day to night . ... Beyond that there were issues with roll center heights (that do also affect the LLTD) which made finding a good setup a pain. If one considers that for instance on flat surface circuits like Magny Course we put into the car the stiffest springs and rollbars in the car that we could find (to keep the tires as hot as possible ....) one can see that there are many other factors that can come into play than "just" having a good idea .....In those days I had the pleasure of working together with some of the brightest vehicle dynamics engineers and we did look at all the options known to man-kind in those days. And with us there were many other teams looking into the matter. The 90s were famous for all the developments on suspension technology. Apart from the fact that some people here in this Forum do tend to "overestimate" their seniority, I have always worked on the principle that "if an idea of mine has not won the hearts of the clients and especially if the clients are very capable of judging whether the idea works or not, something might not be as good about the idea as I initially thought". And especially having tried it myself, together with many other clever people, especially having seen what "alternative" technology can do, and especially since neither of us all ha been able to make a faster car than a "conventional" car made this particular idea no "winner". These are just some simple facts, experienced in a world where one was prepared to spend 1.000.000 £ for each 0.1sec of lap time, without any politics or "presumed" so called expert knowledge.

If anybody believes he can do better (since in his visionary wisdom obviously all the "other ones" in the past have been complete & utter idiots, or if active now, are at the least ignorant peers) this person is either the long awaited genius or one of those poor souls that will wander around the globe forever waisting his time in trying to baptize the non-believers ....

Cheers,
dynatune, www.dynatune-xl.com

Claude Rouelle

03-27-2014, 01:44 PM

Dynatune,

Can only agree 100 % with you on the vehicle dynamics experience and explanation of warp stiffness as well as the understanding of both usefulness and appreciation of different engineering methodologies. But I am only one of the codswallop idiot of this forum.

BillCobb

03-27-2014, 02:59 PM

Actually, there is an important difference between the technical aspects of twisting. Warping is the displacement of the elements of a beam along the twist axis. It is caused by the non-colinearity of the shear center axis and the centroidal axis of a beam.

So,,,, a solid tube does not warp but twists. A slit tube will warp as it is twisted. The edges of the tube extend in opposite directions.

For example, in a twist axle type of (rear) suspension, the U or V beam is usually oriented in such a manner that the edges welded to the control arms are steered when the vehicle rolls during cornering. The orientation of the shear (warp) center defines whether the steer by roll characteristics are understeering or oversteering (or neutral steering). Anyone care to speculate why a U or a V section shape is chosen ??

When it comes to body/frame/ chassis warping, the same observation is made. (The body/frame is not a tube, that's a mistaken assumption, called a tube steak). When the structure is twisted from induced roll or single bump inputs, the fore/aft displacement (warping) of the suspension attachments is considered and can be measured and quantized.

One difficulty in the understanding of all this is that the torsional stiffness (moment stiffness distribution) is certainly NOT distributed evenly down the length of the body/frame. The powertrain contributes vastly to the body stiffness because of the engine containment strength requirements. And, the powertrain is the most likely cause of twisting moment reactions from bumps and cornering. Its inertance and inertia change the tuning needs for roll moment and roll damping tuning. Something as simple as a pencil brace or an underslung cross cable can significantly alter the TLLTD percentages (front to rear). Modal analysis tools are often used to identify the mode-shapes active in each type of event and structural and other parts are chosen or altered to reinforce or accentuate 'modal participation' or modal 'isolation'.

That's warping (with a twist) from a structural mechanics perspective. (I wrote an SAE paper on this that's a lot older than most of you).

nowhere fast

03-27-2014, 10:55 PM

Dynatune,

I see your point that in some circumstances a setup that is ‘worse’ in terms of additional tyre scrub or tyre vertical load variation over bumps can improve lap times by bringing the tyre temperature up.

If the issue with soft warp suspensions was the rate of diagonal load transfer and not the amount; then I don’t understand why bumping up the stiffness of the longitudinal Z-bars (the springs which resist roll in this case) would not speed up the transfer in a similar way to increasing the roll stiffness of a conventional car by reducing the final roll angle. Of course other factors including damping and kinematic load transfer become more prominent when we are talking about transients.
A soft warp suspension introduces a new set of compromises; roll stiffness may be linked to heave or pitch stiffness depending on the implementation instead of being linked to warp stiffness as is the case with ARB's, maybe there are some issues here. It seems that stiff roll and heave modes would be desirable for a race car though, considering conventional race cars are often fitted with ARB's + 3rd springs.

I have no horse in this race, and am open to hearing both sides of the debate. I just find this topic interesting.

Anyone care to speculate why a U or a V section shape is chosen ??

I’ll bite. Is it because the shear centre lies a long way from the centroid in U and V sections compared to other shapes? (Shear centre some distance below the U, while the centroid is somewhere inside the U).

Loz

03-28-2014, 01:21 AM

For example, in a twist axle type of (rear) suspension, the U or V beam is usually oriented in such a manner that the edges welded to the control arms are steered when the vehicle rolls during cornering. The orientation of the shear (warp) center defines whether the steer by roll characteristics are understeering or oversteering (or neutral steering). Anyone care to speculate why a U or a V section shape is chosen ??

I’ll bite. Is it because the shear centre lies a long way from the centroid in U and V sections compared to other shapes? (Shear centre some distance below the U, while the centroid is somewhere inside the U).

The orientation about the axis of symmetry of the axle dictates the orientation planes of shear induced warp at the axle ends. The shear deformation (warping) either contributes to roll steer (gain or loss) for a beam orientated like up or down (looking like an n or U ) or even camber gain/loss if the axle is orientated like [ or ]. This is for a very basic prismatic axles and is dependent on how they are attached to the control arms. Feasibly the axle could be used to correct or enhance roll steer induced by the angle of the control arms relative to the body.

Another particular reason for having an open section axle though is that it significantly reduces the torsional stiffness of the axle hence preventing it from acting like a large and very stiff ARB (i.e. decoupling the roll and pitch/heave modes). As Nathan says the shear centre is significantly offset from the centroid and as the centroid is likely to lie very close to the axis of the couple exerted on the axle produced by the control arm movement, the torsional stiffness is reduced as much as possible as is the transverse shear stiffness of the axle section. This results in more warp at the axle ends and potentially a significant amount of slip angle loss/gain at the wheel with subsequent ramifications for changes in yaw contribution from the wheel pair (understeer/oversteer effects). The roll stiffness contribution from the axle then also changes the LLTD during roll hence undesired changes in handling can creep altering dynamic behaviour.

For relatively simple axles (mechanically), twist axles still have a lot going on in terms of how they behave structurally and kinematically and how they can alter the forces at the contact patch.

BillCobb

03-28-2014, 08:19 AM

Very Good replies to both of you. Comforting to know that some people still know the WHYS of car designing instead of the common alternative.

Now about that inclined roll axis stuff....

03-30-2014, 08:14 PM

I often wonder if this Forum isn't hosted by the "Society for the Appreciation of the Dramatic Arts, and Other Trivial but Delightful Indulgences...". You know the sort of place, where NO well-reasoned thoughts, or numbers, or facts, or any sort of rational thinking AT ALL is required to support your argument. Nope, you just call everyone "Daahlink...", and then try to scratch their eyes out! :)

Here is an example.

Originally posted by Dynatune:
... since there was a flagrant lack of load transfer to make the car react sufficiently on corner entry.

Question asked by Nathan:
You are telling us that soft warp suspensions did not prove to be beneficial in your experience, but you aren’t providing an engineering interpretation of why that was the case.

Reply to Nathan given by Dynatune:
...the car due to it's low warp stiffness did not sufficiently quickly build up (diagonal) load transfer which lead to a rather slow build up of tire slip angles, which was generally interpreted by the drivers as a "lazy understeering pig" setup. [* See Note 1.]
...
Beyond that there were issues with roll center heights (that do also affect the LLTD) which made finding a good setup a pain. [* See Note 2.]
...
In those days I had the pleasure of working together with some of the brightest vehicle dynamics engineers and we did look at all the options known to man-kind in those days. And with us there were many other teams looking into the matter. The 90s were famous for all the developments on suspension technology. [* See Note 3.]
...
[followed by lots of hissing, and snarling, and scratching of eyes...]

* Note 1. If we are allowed to get technical for a moment, then it is my interpretation that a "slow build up of tire slip angles" means that the tyres are rolling in the direction in which they are pointed (ie. they have a SMALL SLIP-ANGLE!). So if the driver has actually steered the front-wheels into the corner, then the front of the car will also being "turning in to the corner" just as quickly. So, quick turn-in, and NO UNDERSTEER.

So, Dynatune, can you please give a technical explanation for why a car with low front-wheel slip-angles can be described as a "lazy understeering pig"?
~o0o~

* Note 2. Roll Centre heights have no direct relationship with springing, in that they can be adjusted entirely independently of each other. Or put another way, if your RC heights are inappropriate for the car, then that is a problem related to RC heights, and not to the type of springing used.

So, Dynatune, can you please give us some technical insights (with as much detail as possible, maybe even some numbers) as to why you, and the other "brightest vehicle dynamic engineers", had difficulty separating the effects of RC-heights and modal spring-rates, and their F/R LLTDs?
~o0o~

* Note 3. I lived through the 1990s. I cannot remember a single significant development in suspension design from that time. At least not one that had not already been thoroughly investigated at least 30+ years earlier. And then, perhaps, forgotten by the "brightest vehicle dynamic engineers"...

Anyway, I might be asking too much here (geez, 3 x tech questions, all at once!!!), but...

Dynatune, do you understand how a "mechanical" suspension can be completely Twist-soft, and still be able to give any LLTD from 100%F:0%R to 0%F:100%R? (And, BTW, also any ratio outside that range, though not recommended.)
~o0o~

Well, I would love to talk more "engineering" than that, but perhaps too much here already (hiiiissss, snaarrrlll!!!). :)

Z

(PS. What the heck! Bill's "warping" of flexible structures (= twist-beams) is a bit different to a suspension's movement of its four wheelprints out of a flat plane, but interesting nonetheless. I think Nathan and Loz got it right...
* Shear-centre lower = roll->toe-out-of-corner = +OS.
* Shear-centre higher = roll->toe-in-to-corner = +US.
* Shear-centre forward = roll->less-camber-gain = +OS.
* Shear-centre backward = roll->more-camber-gain = +US.
* And, most importantly, twist-beam-near-chassis-pivots = easier-packaging-of-junk-between-the-wheels, hence inverted "U/C-shape".
Also, lots of interesting historical "twist-beams" from the 1930s, but best not flood the Forum with too much tech at once! :))

Tim.Wright

03-31-2014, 06:43 AM

* Note 1. If we are allowed to get technical for a moment, then it is my interpretation that a "slow build up of tire slip angles" means that the tyres are rolling in the direction in which they are pointed (ie. they have a SMALL SLIP-ANGLE!). So if the driver has actually steered the front-wheels into the corner, then the front of the car will also being "turning in to the corner" just as quickly. So, quick turn-in, and NO UNDERSTEER.

So, Dynatune, can you please give a technical explanation for why a car with low front-wheel slip-angles can be described as a "lazy understeering pig"?

I'd interpret "lazy" as being a fast build up of slip angle on the front tyres, but without the corresponding lateral force. So something is delaying the build up of lateral force. Possibly some load dependancy on the relaxation lengths maybe? I know they are supposed to be constant, but given that response times to steering are in the range of milliseconds, an increase of relaxation length of 100mm will add 3.6ms to the response time.

Also, I guess that with very different normal loads on the front tyres during turn in, a soft warp suspension might need a different steering setup w.r.t. KPI, caster and ackermann to get the same response as before.

By the way, in the 90's, in F1 there were a lot of attempts at hydraulics and other interlinked systems to try to mimic the behaviour of the active systems...

dynatune

04-03-2014, 10:55 AM

Tim is correct with the interpretation of "lazy" but it was not the front causing this behavior, it was actually the retarded delay of building up of slip angle on the rear tires. Compare it to a step steer maneuver where on measured data one can see the "braking" free of the back end at the "famous" initial negative slip angle turning into positive and the small pike followed by drop off in the lateral acceleration followed again by the increase towards peak values.

Yes indeed in the 90s there a lot of attempts to mimic the behavior of the active systems .... I was trying to explain that 3 pages ago, not with much success though - that all the interlinked systems were not all as good....

Cheers,
dynatune, www.dynatune-xl.com

04-03-2014, 09:06 PM

Dynatune, do you understand how a "mechanical" suspension can be completely Twist-soft, and still be able to give any LLTD from 100%F:0%R to 0%F:100%R?

Dynatune,

?????

Z

dynatune

04-04-2014, 05:29 AM

No Z, I obviously do not understand and with me all those engineers that worked in the 90s on the conceptual evaluation. Now try reading carefully these few words: "it did not work" and ask yourself which part of that phrase is not clear to you.

Cheers,
dynatune, www.dynatune-xl.com

Tim.Wright

04-05-2014, 04:15 AM

dynatune

04-05-2014, 06:34 AM

Interesting. So it seems the suspension may have been too front biased in terms of load transfer on turn-in. Were there any advantages in rear end stability on corner exit? Or were there problems there too?

Those were definitely interesting times. I would not dare to say that the front suspension was too much front biased, there were issues with the tires, a tire war (and later especially when in 98 the grooved tires came other issues), there were issues with weight distribution and so on, the usual stuff. Beyond the linkage ideas and concepts we also started in those days in order to make the car "more" agressive in it's reactions to work with extreme rising rate rocker ratio's and also rebound stops to introduce drastic changes in wheel rates in extensions on one corner of the car to provoke transient diagonal load transfer effects. It did however make the "setup" of the car incredible complex.

The good thing about the "active" cars was the engineers could give the driver a car that would exactly do what he wanted on a certain point on the track. Something like "if v= .. and SWA= .. and ay = ... then set left rear pushrod actuator = ... mm" and that "option" would only come into place once and not interfere with other parts of the track where a different setup was quicker.

As I said before and I would use this as my final comment in this thread here: There is nothing wrong with aiming for a low "twist" stiffness and if executed with an active mechatronic system there will be advantages as history in the racing and OEM world has shown. If executed however purely with a passive analog mechanical system the racing and OEM world has found no "overall" benefit. Everyone can do what he or she likes with my 2 cent of information.

Cheers
dynatune, www.dynatune-xl.com

Tim.Wright

04-05-2014, 02:01 PM

I have been playing about with the longitudinal Z bars today to try and see in more detail what effects they have on the wheel loads under different conditions. I'm interested in this mainly because I am developing a car design for myself and wanted a deep understanding of the suspension's function. For anyone interested I have been posting my ramblings on my design here: Open Source Racecar (http://www.f1technical.net/forum/viewtopic.php?f=14&t=13521&hilit=open+source+race+car)

While I still believe (at the moment) that for production cars a 2x ride springs and ARB is an acceptable compromise between packaging and ride quality - for a prototype or race car you could repackage or accept some complexity/weight penalty if there really is some performance to gain by running another system.

Enter the Z bar...

I have modelled a system with one ride spring per corner and 1 longitudinal Z bar per side linking the front and rear wheels. The Z bar has different lever arm lengths on the front and rear suspension connections to reach the desired roll stiffness distribution.

I have put this into an analysis spreadsheet which I developed in my spare time. It takes a stiffness martix of the suspension as an input and calculates modal stiffness and responses of the body due to lat/long load transfer, warp inputs at the contact patch and single/dual wheel bump events. I started this spreadsheet over a year ago for the purpose of investigating interlinked suspensions.

It models the chassis/suspension system basically like a 7 post rig. I.e. displacement inputs are given at the contact patches and the body is free to respond vertically and in roll and pitch. Additionally roll and pitch moments can be applied to represent lateral and longitudinal accelerations.

I have modelled the Z bar suspension in this spreadsheet and compared them to:

A traditional suspension system with springs/ARBs
A mechanical interlinked system (all four wheel linked)

All suspensions are matched for roll rate, roll distribution and where possible vertical stiffness and pitch stiffness.

So in terms of responses to vertical loads at the CG here is the comparison:

https://lh4.googleusercontent.com/-7NUqeYtAOkc/U0BOZWv-_zI/AAAAAAAAAe4/dPbb1e9A9eM/s0/linear_vertical_model_longZbars_01.jpg

I have seen that with the Z bars I cannot acheive my desired vertical stiffness and spring centre (point where vertical forces give no pitch angle) simultaneously. This is because I set the bar stiffness to match the roll rate and roll stiffness as a first priority. The body mode shape plot on the bottom shows the vertical displacements of the front and rear axles with the CG shown as a circular marker and the spring centre marked with an X marker. The traditional system and the interlinked system have identical responses, and the Z bars have a very different response.

After some algebra on the system equations, it seems that the vertical and roll responses are not independant like is the case in a traditional suspension. Worse still, the resulting vertical mode that I have got is almost twice as stiff as the traditional suspension and biased too far in front of the CG (23% of the wheelbase). This means it will pitch a lot due to vertical accelerations which is bad for ride (on a road car) and bad for aerodynamics (on a race car).

Another disadvantage, particularly for a race car, is changing either the Z bar stiffness OR the springs results in changes to the ride AND roll AND pitch rates together. Nothing is independant so tuning at the track is going to require a calculator. Any desired roll distribution changes will require disassembly of the main ride springs which on a GT car is a significantly annoying thing to do.

Its not all bad for the Z bars though. There advantages lie in the low warp stiffness:
https://lh6.googleusercontent.com/-tsirIKXbr7k/U0BOa3MqmbI/AAAAAAAAAfA/XCqp25rXe5o/s0/linear_vertical_model_longZbars_02.jpg

You can see that I have tuned all 3 suspensions to have the same roll stiffness distribution and same total roll stiffness (top half of the page).

On the bottom half we can see the response to a warp input at the contact patch. Here all three suspensions conform to the warp input identically in terms of the suspension travel, but the Z bar system and the interlinked system show significantly lower load transfers on the front and rear axles. Most of the load transfer on the Z bar suspension is due to the ride springs. This is an obvious advantage as the tyres will see less contact patch load variation due to road irregularities.

After a discussion here a few weeks ago, I have also added an analysis on the sensitivity of the elastic load transfer distribution to a warp input. This confirms Erik's calc which show that very small warp inputs have a very large effect on the elastic load transfer distribution:

https://lh4.googleusercontent.com/-fZIY57QyOYs/U0BOb0dQYnI/AAAAAAAAAfI/-WBmZDX155o/s0/linear_vertical_model_longZbars_03.jpg

Here we can see that the elastic load transfer distribution (ELLTD) changes by 4.6% PER MM of warp input at the contact patch. The Z bar suspension reduces this down to 1.0%. The interlinked suspension can basically eliminate it but this is at the cost of more parts and complexity.

A few other points I found which are not shown above:

I calculated energy dissipated by the body due to single wheel vertical inputs and saw no significant advantages of the interlinked system compared to the traditional system. In fact the interlinked suspension has the disadvantage that a vertical input at one wheel affects the loads on all of the wheels whereas on the traditional suspension only the other wheel on the same axle has a load imposed. When you then calculate the energy from the force and displacement required for the body to find its response equilibrium its the same for the traditional and interlinked system.

The Z bar suspension is about half a stiff in pitch as the trditional system. This coupled with the large distance between the CG and the spring centre will likely result in pitch oscillations which will need to be managed. Then there is the problem of managing pitch angles under braking (critical in terms of aerodynamics) and squat under acceleration.

So, after all that, I have to say my opinion remains unchanged regarding interlinked systems on road cars and race cars i.e. I think they are too complicated for serial production road cars and especially the Z bar solution which is reasonably simple but for me has too many critical disadvantages. For racecars, I think there are advantages to managing the warp mode better and in classes with open rules, I think you could have an advantage with an interlinked suspension, but I feel that Z bars are not the way to go.

However, given that I am a moron, I may have overlooked something so I'm up for a discussion.
https://lh3.googleusercontent.com/-MCr8FkVQ2uk/Uy22OP8tNXI/AAAAAAAAAeg/OULUi_PHOME/s288/homer.jpg

Just a short disclamer: The design work on my own car (which includes this analysis) has been done outside of my work (in the cottage industry), so my findings and designs done there are more or less "open source" and I'm happy to discuss them. However any aspects which cross over with what I'm doing at work obviously I can't be so open about. So far I've done very little at work on ride, so most things are fair game for the moment but don't take offense if hold back on answering some things.

Enjoy.

Tim.Wright

04-06-2014, 09:20 AM

Another reservation I had some time ago about soft warp suspensions is that without any stiffess or damping the warp mode is free to oscillate relatively uncontrolled on the tyres only which are famously underdamped. So I think there could be a good possibility of the wheel hop mode (approx 25Hz) being excited in a way which could kill grip or destroy ride quality depending on what your targets are.

Jay Lawrence

04-06-2014, 11:10 PM

Tim,

Thanks, that was excellent!
Your comments regarding pitch are what I was trying to get my head around earlier (i.e. surely Z-bars would be 'pro-pitch' and therefore perhaps not suitable on an aero dependent platform).

PS. I hope your car plays La Cucaracha on its many horns.

04-06-2014, 11:22 PM

Tim,

Thanks for sharing your numbers (yes, real numbers at last! :)).
~o0o~

I have only had time for a quick look at your linked blog of your project car. Just a few quick comments here (all "IMO"):

* For all-round, easy-fast-driving (race or road) I would suggest stretching the wheelbase to ~2.6 metres. Keep F/R CG position and the Yaw-inertia similar to what you have (ie. passengers and engine centralised), but move front-wheels forward and rears backward a bit. This increases the Yaw-control-forces, while leaving inertial resitance low, so giving faster reaction to the driver's steer-inputs. Yaw-damping also increases, so less chance of suddenly "losing it".

* Lower the CG by using a Subaru engine (easy 200+ hp, and dirt cheap). The shorter-wheelbase, higher-CG, rear-weight-biased cars, like the Lotus (and also most off-road buggies), suffer from lack of front-grip on corner exit. Essentially, a moderate amount of Accelerating-Gs can reduce your 39%F down to very little. This then requires a "point and squirt" driving style, where the car has to be pointing down the exit straight before power is applied. Lower CG means less load-transfer OFF the front-wheels for given Acc-Gs , = more predictable handling.

* Reduce KPI to as small as possible (eg. 0 degrees, wrt wheel-camber). This easiest done with "large offset" wheels, but if you have to accept "Steering Offset" (= "scrub radius") of 20+ mm, then better than large KPI (IMO).

* (Edit: And <700 kg total is a reasonable target (all steel, no CF!).)
~o0o~

Back to your Z-bars post above. More details please! :)

I suspect you are setting up a Z-bar "straw-man" so you can burn him down. Namely, you have added longitudinal-Z-bars to a four-corner-springs suspension (so 6 springs in total), when a 4 x Z-bar suspension would be a better comparison (2 x long, 2 x lat). Note that U-bars (eg. ARBs) can NOT carry Heave loads, so require other springs to do that. On the other hand, Z-bars can do that most important job of holding the car up, so no other springs required.

Anyway, I will comment more on the details of your above post in a few days (work to do...). Meanwhile, can you clarify the contents of the "stiffness matrix" entries? And can you give the spring-rate details for the three different suspensions you are comparing (I suspect they are in the matrix, but...)? Oh, and also the details of the "4-wheel-fully-interconnected-suspension"?

And, yes, that conventional suspension (4 x corner + 2 x ARB) sure does have a lot of load transfer PER MILLIMETRE of road induced warp-motion! :)

Z

(Edit: Jay, Longitudinal-Z-bars are NON-pitch. They don't control it at all (except through rising-rates, etc.). That is where lateral-Z-bars come in (ie. they control Heave and Pitch).)

Tim.Wright

04-07-2014, 07:44 AM

Thanks for the commets on the project car... especially the comment on the weight is interesting. I've also been thinking about revisiting the mass distribution but this is another discussion.

I suspect you are setting up a Z-bar "straw-man" so you can burn him down. Namely, you have added longitudinal-Z-bars to a four-corner-springs suspension (so 6 springs in total), when a 4 x Z-bar suspension would be a better comparison (2 x long, 2 x lat). Note that U-bars (eg. ARBs) can NOT carry Heave loads, so require other springs to do that. On the other hand, Z-bars can do that most important job of holding the car up, so no other springs required.

No straw man, like I said I've been researching this over the last year because I want to understand for ride/body control myself rather than relying on preaching from yourself, or anyone else about which system is the "best".

I added 2 longitudinal Z bars to a conventional system (replacing the anti roll bar) because the conversation seemed to go in that direction a couple weeks ago. I will update the spreadsheet with 4x Z bars later this week when I have time.

Anyway, I will comment more on the details of your above post in a few days (work to do...). Meanwhile, can you clarify the contents of the "stiffness matrix" entries? And can you give the spring-rate details for the three different suspensions you are comparing (I suspect they are in the matrix, but...)? Oh, and also the details of the "4-wheel-fully-interconnected-suspension"?
Very quickly the stiffness matrix is the stiffness coefficients (k01 - k16) for the system of equations;
FZfl = k01xZfl + k02xZfr + k03xZrl + k04xZrr
FZfr = k05xZfl + k06xZfr + k07xZrl + k08xZrr
FZrl = k09xZfl + k10xZfr + k11xZrl + k12xZrr
FZrr = k13xZfl + k14xZfr + k15xZrl + k16xZrr

Where the body is fixed and vertical displacements are applied to the wheels. The suspension displacements and body movements are then solved in another matrix equation based on a freebody diagram of the chassis. I will detail all of this also later this week, right now the only documentation on this is in my head.

The 4 wheel interconnected system I will also detail later but it started off as a purely mechanical system but I have changed parts of it to hydraulics because it became a packaging nightmare. It has 3 spring/damper units and can control pitch, roll and vertical stiffness' independantly. Which reminds me to ask you, how do you propose to implement damping on the 4xZ bar system?

(Edit: Jay, Longitudinal-Z-bars are NON-pitch. They don't control it at all (except through rising-rates, etc.). That is where lateral-Z-bars come in (ie. they control Heave and Pitch).)

What Jay says has some sense to it. In that if you give a vertical input to the front axle (i.e. driving over a speed bump), the rear axle is forced away from the chassis (against the ground) and the reaction on the body lifts the rear up. This has the desired levelling effect of the body BUT at the cost of imposing a load variation on another wheel.

Like I mentioned before, one disadvantage of all of these interconnected suspensions is that a force input on one wheel affects the loads on all the others. So I'm not so convinced that the control of contact patch load variation is going to be unconditionally better.

04-09-2014, 01:09 AM

Tim,

I look forward to more details. Meanwhile, here are some general comments on your first big post above.
~o0o~

... - for a prototype or race car you could repackage or accept some complexity/weight penalty if there really is some performance to gain by running another [suspension] system.

This issue of extra "complexity..." always amuses/annoys me (depending on my current mood).

For the sake of all you students who may not be aware of this yet, the fact is that the vast majority of H. Sapiens will swear on their grandmother's grave that ANY NEW APPROACH, to doing ANYTHING, is ALWAYS far more complicated and expensive then "the way we have always done it...". This is regardless of any objective assessment of the matter.

Numbers do not matter. Part-counts do not matter. Number-of minutes-to-make do not matter. Very often, NO ASSESSMENT AT ALL is made before the "new" approach is declared "far too complicated and expensive".

Example 1. A proposed suspension that only uses 3 springs (or less!) is considered "too complicated". In contrast, current race suspensions that have 8 springs (4 x corner + 2 x ARB + 2 x "third-springs") are considered simpler. Go figure...

Example 2. An unconventional Swing-Arm suspension, which requires one part, the "Arm", attached to chassis by 2 x BJs, is considered more complicated than the common Double-Wishbone+Toe-Link, which has four parts (upright + 2 x wishbones + toe-link), all attached to chassis by ~8 x BJs. Grooooaan...

You students are, I guess, entitled to fool yourself with these irrational opinions. But I don't think it fair that you should try to fool your fellow students.
~o0o~

I have seen that with the Z bars I cannot acheive my desired vertical stiffness and spring centre ...
it seems that the vertical and roll responses are not independant ...
Worse still, ...
which is bad for ...
Another disadvantage, ...
Nothing is independant ...
a significantly annoying thing ...

All the above claims of "bad things happening" stem from a lack of understanding of Z-bars. Specifically, the bold section above is pretty much the DEFINITION of longitudinal-Z-bar behaviour! So, why the "it seems"?

Longitudinal-Z-Bars couple Heave and Roll, but are independent of Pitch and Twist (with "non-linear" qualifications, as noted before). Lateral-Z-Bars couple Heave and Pitch, and are independent of Roll and Twist.

By contrast, corner-springs couple Heave, Pitch, Roll, and Twist. So any suspension with these has NO INDEPENDENCE, whatsoever.

And conversely again, a fully-interconnected-suspension has completely INDEPENDENT control of each of the all-wheel-modes (when appropriately defined). And it can be extremely simple.

A Z-Bar suspension is somewhere in the middle of the above two. It connects two-wheels at a time, so can provide twice the "independence" of conventional suspensions, but only half the "independence" of fully-interconnected systems.

Bottom line here, all of Tim's "bad things" can easily be resolved. Best way is to simplify the system (toss the corner-springs).
~o0o~

Its not all bad for the Z bars though. There advantages lie in the low warp stiffness:
...
Most of the load transfer [from warp input] on the Z bar suspension is due to the ride springs.
...
the [conventional suspension] elastic load transfer distribution (ELLTD) changes by 4.6% PER MM of warp input at the contact patch. The Z bar suspension reduces this down to 1.0%. The interlinked suspension can basically eliminate it but this is at the cost of more parts and complexity.

ALL of the "load transfer from warp input" comes from the ride (= corner) springs.

Once again, note the prejudiced assumption that the "interlinked suspension" comes at a "cost of more parts and complexity". I stress again that this is an UNQUANTIFIED opinion (ie. no numbers) of someone who has just dipped their toes in the waters of interconnected-suspension, so really knows very little about them.
~o0o~

In fact the interlinked suspension has the disadvantage that a vertical input at one wheel affects the loads on all of the wheels ...

In fact, this is the great ADVANTAGE of interconnected-suspensions.

A single-wheel-bump hitting a conventional suspension causes a large force to lift that corner of the car. As the car-body lifts, the wheel-loads on the other three wheels change. Typically, the two wheels closest to the one hitting the bump have their Fzs reduced, and the diagonally opposite wheel has its Fz increased.

This is clearly seen when racecars clip the apex curb with their inner-front-wheel:
If the car is front-heavy (say, tin-top), then inner-rear-wheel lifts completely off the ground, and outer-front has lesser Fz.
If the car is rear-heavy (say, rear-mid-engined), then outer-front-wheel lifts completely off the ground, and inner-rear has lesser Fz.
This is easily understood by considering the plan-view of the car, and CG position wrt the two lines that connect diagonally opposite wheelprints.

A single-wheel-bump hitting an interconnected-suspension car has ALL FOUR wheels sharing the increased Fz forces that readjust the body position of the car. The amount of "sharing" is determined by the details of the interconnections, and can be varied arbitrarily (including so that it acts like a conventional suspension).

Consider that a "fully-active" suspension is often considered the "ideal". Here, some sort of computer determines the share of the car's total Fz load that each wheel must carry, in any given situation. That is, all four wheels respond to any given event. An interconnected-suspension does effectively the same thing, but with the geometry of its interconnections acting as an analogue computer.

Note, here, that a "change of program" for a mechanically-interconnected-suspension requires a change in the geometry of the linkage. However, there is no reason that this cannot be done whilst driving, say, in the same way that a brake-balance bar can be adjusted.
~o0o~

... will likely result in pitch oscillations which will need to be managed....
... the problem of managing pitch angles under braking ... and squat under acceleration.
... my opinion ... interlinked suspension ... too complicated ...
... Z bar solution ... too many critical disadvantages... not the way to go...

Once again I stress that Tim has just dipped his toes in the waters. It seems that he is frightened that it might be too cold, or there might be all sorts of nasties swimming around down there, or something terrible is about to happen...
~o0o~

However, I may have overlooked something so I'm up for a discussion.

Well done, Tim! Very brave!

Now, as I have pointed out countless times before, the vast majority of ground-vehicles use a soft Twist-mode. Lots of other people are doing it.....

So, MAN-UP boy (!!!), and just bloody-well jump in!!!!!!!

(Z mumbles... "Geeeez, what is it with these woosy-kids these days...".
But, to be fair, Tim has been brave enough to dip his toes in... :))

Z

04-09-2014, 01:12 AM

More comments regarding posts subsequent to Tim's first big one:
~o0o~

Another reservation I had some time ago about soft warp suspensions is that without any stiffess or damping the warp mode is free to oscillate ...
... good possibility of the wheel hop mode (approx 25Hz) being ...

All suspension types, including single-wheel, two-wheel-interconnected, all-wheel-interconnected, +++, can be independently damped. The less the modal stiffness, the less damping required to suppress oscillation (from Critical-Damping ~ Sqrt(M.K)).
~o0o~

No straw man, like I said I've been researching this over the last year because I want to understand for ride/body control myself rather than relying on preaching from yourself, or anyone else about which system is the "best".

I agree 100% that you have the right approach. Just be careful that your own prejudices don't mislead you.
~o0o~

Which reminds me to ask you, how do you propose to implement damping on the 4xZ bar system?

Many ways are possible. Damper-in-parallel-with-spring-element is one way (damps only that mode). Someone recently posted an image from UQ showing such an implementation (though I think they are changing that...).

However, as I have said many times before, (IMO) "dampers are crutches" that help a car with bad suspension get around faster. The analogy is with someone who has broken legs, and they can "run" faster with a crutch under each arm. However, with two healthy legs, the crutches just get in the way...

Ultimately, the goal is to have springing that is clever enough that NO damping is needed. Damping, after all, is just friction that requires more fuel to be burnt, and thus slows the car down. A fully-active suspension does not, and should not, try to dissipate energy. Instead, it snubs out any oscillations by adjusting the Fz forces appropriately...

The intermediate, and adequate, solution is simply to put just enough damping in to compensate for the not-quite-good-enough springing. Four corner-dampers, each of which damp all H/P/R/T modes, is more than enough. Just three corner-dampers, or maybe even just two, can do it, depending on details.

(And please don't anyone say that "using two dampers is TOOOO complicated..."!)
~o0o~

... if you give a vertical input to the front axle (i.e. driving over a speed bump), the rear axle is forced away from the chassis (against the ground) and the reaction on the body lifts the rear up. This has the desired levelling effect of the body BUT at the cost of imposing a load variation on another wheel...

"...at the cost of..."???

Do the numbers! They are particularly easy in this side-view, "bicycle-model".

In brief, the conventional suspension has two large Fz force spikes at the front and rear wheels (of the bike) as each passes over the bump. A soft-Pitch suspension has two much lower Fz spikes, felt, and SHARED, at BOTH WHEELS, as each passes over the bump. On the football paddock it is called "team-work". It wins games!

This better bump-absorbency from soft-Pitch was the original motivation for Packard's adoption of Z-bars (the French also saw the soft-Twist advantages). Unfortunately, it is still not well explained in any of the VD textbooks that I have seen. And BTW, the issue of too much Pitch motion during Accel/Braking is best solved with highly non-linear Pitch-springing (falling-rate around centre of range, then rising-rate at ends of range).
~o0o~

Enough for now...

I look forward to more details.

Z

Jay Lawrence

04-09-2014, 11:24 PM

Z,

Your childish attack on Tim's post aside ("MAN UP boy (!!!) ..." really?), can you explain how you could have non-linear pitch springing whilst creating a stable aero platform and having soft twist mode? Given that torsion bars can't have variable rate (?) I'm assuming you'd set up your 'lever arms' at each end such that they are falling/rising rate. Doesn't this mean that you have to travel to the extremes of your lever arm to get sufficient stiffness for pitch (without bottoming out), which would involve significant chassis movement to achieve (and hence non-stable aero platform)?

04-10-2014, 08:28 PM

Jay,

One day when you are old, and tired of hearing the kiddies moaning,
"Aww, but it's just TOOO haaard... And something really bad might happen to me. And [whinge, whine...]..."
then you will learn that everyone is happiest shortly after you pick up the kiddies and throw them in the deep-end. Sure, there is a little spluttering to begin with, but after that lots of FUN! :)
~o0o~

... can you explain how you could have non-linear pitch springing whilst creating a stable aero platform and having soft twist mode?

So simple ... and all explained before. But once again...

Consider UWA's recent "diamond-springing" (reg. TM?) cars that have front and rear beam-axles, with a spring at the centre of each beam that together control Heave and Pitch. (Roll is controlled entirely independently with a central Lateral-U-Bar+2-x-Balance-Beams, with the BBs forming the aero-tunnels). The beam-axles-with-central-spring are effectively Lateral-Z-Bars. So, overall, the car is completely Twist-soft (except for some friction, etc.).

The beam-centre-springs, namely the "W-springs", serve also to horizontally locate the beams wrt body, but ignore this for now, and just consider a vertical spring at the centre of each beam. It should be obvious that this spring can be implemented in any of countless different ways. Steel-coils, multiple-pre-loaded-steel-coils-in-series-and-parallel, torsion-bars-with-crazy-linkages, rubber-bands-with-cables-and-funny-pulleys-and-cams-and..., etc.

A lot of these can be very simple to make, yet also have very non-linear rates. They are everywhere! (Ask your teachers, or ask for your money back!) The non-linear rates can make the Pitch behaviour of the car very stable, yet also very good at absorbing bumps. Furthermore, separating the Heave and Pitch spring-rates of an UWA-like car is quite straightforward. I have covered all this many times before.

Bottom line, all this is easily doable. Just because you can't think of a way to do it right now, does not mean it is impossible. Or even "TOOO haaard...".

Z

Jay Lawrence

04-10-2014, 11:10 PM

Thanks Z, when I wrote my question I had the Packard image in my head and couldn't reconcile the variable rate idea with it.

ben

04-11-2014, 07:41 AM

So just to clarify Z on the aero tunnels acting as balance beams. So if you move the longitudinal position of the U-bar attachment along the beam you change the roll stiffness distribution?

Ben

Pete Marsh

04-11-2014, 07:56 AM

So just to clarify Z on the aero tunnels acting as balance beams. So if you move the longitudinal position of the U-bar attachment along the beam you change the roll stiffness distribution?

Ben

That's correct Ben. ALL elastic RMD is distributed by the central U bar, and is distributed F/R according to the drop link location, which is of course adjustable. Future iterations will have driver adjustment, allowing balance trim on the fly. The kinematics of the link, and hence RMD, is very stable through all suspension motions.

ben

04-11-2014, 02:25 PM

So warp soft with tunable LLTD...

Much as Z rubs people up the wrong way I actually have as big a problem with people denying something can be done by just stating "we were really smart yet didn't manage it"

Ben

dynatune

04-24-2014, 06:13 PM

04-24-2014, 10:16 PM

So where is this discussion now going to .... ?

Dynatune,

Tim last said that he will be giving more details soon. I have the patience to wait...

He also said he will be comparing an all-Z-Bar suspension with the other more conventional suspensions, and with his fully-modally-independent suspension. And he has already confirmed that a stiff Twist/Warp-mode makes a thorough mess of LLTD when the car is on any slightly uneven surface (ie. per MILLIMETRE of Warp!).
~~~o0o~~~

... the already anticipated disadvantages are fully confirmed...

I hope this is NOT the reasoning you use in your professional capacity!

Tim gave ONE PARTICULAR EXAMPLE (4 corner-springs + 2 longitudinal-Z-Bars) that had some disadvantages. A slightly different choice of spring-rates would have eliminated those disadvantages. An all Z-Bar suspension would be even better. A fully-interconnected set-up even better again.

By your reasoning, all conventional suspensions can be deemed "fully confirmed" catastrophic disasters, simply by choosing a SINGLE BAD EXAMPLE of such a suspension.

Very sloppy thinking.
~~~o0o~~~

I asked you quite some time ago, several times, if you know how to build a suspension with zero-rate Twist-mode, but also with fully adjustable LLTD. As Pete confirmed to Ben above, the STUDENTS at UWA know how to do that. They have known how to do that for at least 10 years.

So, once again;
Do you know how to do it?
Did you know how to do it 20 years ago?
Will you ever give us the details of what you did 20 years ago, when you "failed miserably"?
Or, otherwise, how are we to draw any conclusions regarding the reasons for your failure?
~~~o0o~~~

"I have seen here "just" a lot of "this should work" but no hard facts .... at all, just ideas ......no hard facts ... no car ... no nothing ..... show us ... that it works .... show us that we were incompetent 20 years ago and make me eat my words."

Once again, all the wooden-carts, and farm-tractors, and earth-movers, and +++..., FOREVER, have soft Twist-modes. And the 2CV, and Packard, and all ride-on lawn-mowers, and fork-lift trucks, and +++..., FOREVER!!!

I built my first 3 x Z-Bar car ~35 years ago. It worked very well indeed.

Racecars from F1's "active-suspension" era had soft Warp-modes (in their definition). And they worked very well indeed.

UWA's cars that have won several FSAE events had soft Twist-modes (different to Warp). They worked very well indeed. (And so will their current cars, when they manage to train their students NOT to leave small pebbles INSIDE the brake M/C! :))

Dynatune, you are in a tiny minority. You have locked yourself in a small cupboard, and closed your eyes. You should get out more.
~~~o0o~~~

"... what is wrong with "we were really smart yet didn't manage it" ..... ? Some guys (like Isaac Newton..."

Newton, who, in the modern era made some of the greatest advances in our understanding of how things work, said,

"... to myself I seem to have been only like a boy playing on the sea-shore, and diverting myself in now and then finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean of truth lay all undiscovered before me.", and,

"If I have seen further it is by standing on the shoulders of Giants."
~~~

A century earlier, Michelangelo, who as a young man carved the statue of David, and in his middle age painted the ceiling of the Sistine Chapel, and in his final years designed the dome of St Peters (still the largest such dome in the world), said on his deathbed, at age 89,

"I regret ... that I die just as I am beginning to learn the alphabet of my profession."
~~~

Dynatune, who "failed miserably" to make a mechanical COPY of the pre-existing semi-active-suspension cars, said,

"I ... with some of the brightest vehicle dynamics engineers ... did look at all the options known to man-kind..."

That is the height of arrogance.
~~~o0o~~~

Students,

BEWARE THE EXPERT WHO THINKS HE KNOWS IT ALL!

Z (<- A nobody who knows nothing, but likes driving his soft-Twist-mode tractor... :))

BillCobb

04-24-2014, 10:44 PM

It's the sprung mass that rolls. There is additional roll inducement from tire overturning moments acting through the roll by camber kinematics of the suspension. (In many cases, this can be worth 10% extra induced roll). The tire ovtm is also a track modifier. (not race track but lateral axle dimensions) Also, there is an interesting roll inducement via the aligning moment camber characteristics of the tires. This is what makes a vehicle waddle in the pavement troughs created by heavy semi-trucks in warm asphalt. Yes there is extra roll in the entire vehicle coordinate system because of tire spring rates.

If you ever plan to study vehicle roll-over, all these factors plus the dynamic effects of roll velocity will allow you a complete enough understanding of roll gains . That means damper performance in both velocity directions and both ends of the vehicle. Keep in mind that dampers usually have different compression and extention rates, so this complicates the 'roll center' location(s) during a maneuver. Once you've been in the presence of a dynamic K&C machine, all this will be evident. You don't even need a 'dynamic' K&C machine. Running a static hydraulic powered machine in a square wave instability mode will jostle the vehicle enough to see all these things.

BTW: With 'slow' road speed and high steer angles, the influence of caster angles must be factored in, too. This alters the TLLTD estimate.

PS. I'm fascinated by the explanations of 'slow slip angles' and spikey things in the step response signals. You really ought to try a little simulation of various understeering and oversteering and neutral steering system models. All this will then be engineering science and not mystery.

dynatune

04-25-2014, 02:36 AM

Z,

So you now need to relay on Tim to prove that you are right :D, I would have expected to see from you some of his calculations way earlier to convince people, that would have given your statements some added credibility ......

Professionally I am used to look at hard facts, in order to make the correct decisions. This does by no means mean, that I am ignorant or too arrogant to see any pro's of such a system . My point of view -as stated weeks ago - is that, if the system brings more disadvantages than advantages, I would not go for it.

Let me repeat some key points (again for a passive analog mechanical system only, for using in a race environment) that are still standing :

Advantages:

Higher Comfort - Confirmed, yet not needed in race cars
Better "Diagonal" Grip - To be confirmed for a mechanical system (active F1 cars confirmed)

Disadvantages:

Increased Pitch sensitivity - Problematic for Braking/Acceleration especially on aerodynamic pitch sensitive cars. To be corrected (?)
Increased Components and Complexity of Installation. Added linkages (or Hydraulic lines --> if going to hydraulics one can take the next step and become full active)
Increased Complexity in Car-Setup and Tuning.

And I have not even looked at factors like costs, durability and so on that in for automotive OEM's will be important ... those items above are just the "functional" pro's and con's

Then, as I mentioned when "we" looked at it 20 years ago in F1 we found next to drivers complaining about a sluggish response of the car one other major disadvantage. The car was never faster than a "conventional" car ...... and this was not just "us" but all other teams who were trying to reproduce a mechanical system that would be as good as the active system. And ever since those days the racing environment (where still a lot of money is being spend on finding lap time has failed to incorporate this concept ... I wonder why ;) ...).

So my dear Students,

Be open minded, but do always ask the right (and hard questions). Be critical to others but mostly to yourself and your ideas. Listen and find guidance from experiences made in the past by your colleague's and reflect. In your future professional life your manager will ask you sometimes hard and provocative questions to get the facts out of you. You will need to be prepared and convince him by facts, not fairy tales. Reacting emotionally, telling him, that he is arrogant, ignorant and not professional is most likely not going to help your case but will probably be a great way to limit your career .....

I guess I will be back in 3 weeks or so :)

Cheers,
Dynatune, www.dynatune-xl.com

luxsosis

04-25-2014, 04:56 AM

Increased Components and Complexity of Installation.

Have you had a look at UWA's car and concept for soft-twist? I don't need all my fingers to count the suspension components! (excluding fasteners)

How many parts does a typical double A-arm wishbone, pushrod-rocker suspension with coilovers, third springs and ARBS have?

Pete Marsh

04-25-2014, 09:36 AM

So where is this discussion now going to .... ? 3 Weeks after having stepped out of this discussion and more than 2 weeks after the last comment I cannot see what was all the fuss about. Besides the excellent contributions from Tim and his conclusions that the already anticipated disadvantages are fully confirmed - much progress has not been made .... - and if I may say so .... what is wrong with "we were really smart yet didn't manage it" ..... ? Some guys (like Isaac Newton & Leonardo Da Vinci showed us what we can do - or cannot do) ... I have seen here "just" a lot of "this should work" but no hard facts .... at all, just ideas ......no hard facts ... no car ... no nothing ..... show us ... that it works .... show us that we were incompetent 20 years ago and make me eat my words :)

Cheers,
Dynatune, www.dynatune-xl.com

PS... this is a motivation ....

Thanks for that.

Drove one today actually. It goes great, seems to do all the things people would associate with a fully functional suspension system. Soaks up the bumps, keeps the wheels pointing the right way etc. It is super stable and consistent over bumps, which you simply don't notice exist. This is in contrast with a traditional double wishbone independent vehicle I also drove today, on the same track, that requires much steering and pedalling through dips and bumps while at the limit. From track side it is visually very smooth, to the point of seeming to be going slowly, yet it is actually 4 sec/km faster. (superior power and tyres though), The point being it goes fast while looking slow.

SO - What hard facts would you like? We have a car, it works, you can see it, (or at least pictures of it) , on the internet. You are welcome to come and visit us and see it for yourself, or something similar will be at FSAE-A this year.

I'm not suggesting this idea is superior to all before it, or "the new black", but I do take offence to these assertions from afar that it doesn't exist and can't work. Even from someone with F1 experience.

Pete
UWA Motorsport

dynatune

04-25-2014, 11:12 AM

Pete,

I am not trying to make assertions, nor wanting to offend anybody professionally. I have just been asking and will keep on asking critically whether the car is actually faster than a conventional sprung car. Nothing more, nothing less. I hope that that is still permitted here.

Cheers,
dynatune

Claude Rouelle

04-25-2014, 12:26 PM

Pete,

Just to add to Dynatune question" "fair apple to apple comparison" same or roughly same power, weight, tire and aero and of course track and track conditions.

Pete Marsh

04-25-2014, 09:24 PM

Pete,

Just to add to Dynatune question" "fair apple to apple comparison" same or roughly same power, weight, tire and aero and of course track and track conditions.

Hi Claude,
I would dearly love to be able to answer that for you, but alas things at UWA are not what they once were. I don't believe we will ever be able to show that within the team, as we have been soft twist for a very long time, and our last traditional car has much larger dimensions than any modern FSAE car. Even comprehensive data continues to escape us with various Gremlins always spoiling the efforts. Simply two sets of identical tyres is beyond our resources these days.

Perhaps if another big team with a strong VD background made the change we they would be in a position to really know the difference. I'm thinking a Jayhawks, Cincinnati or a Stuttgart type of team.

HOWEVER - The main goal of the concept is NOT a physically faster car, but rather a car than can be driven faster, on a track you have only walked, with only one chance to get it done, with (at best) an intermediate level driver. This is all about a car that is not intimidating at the limit, such that student drivers will have a go when the chips are down. (for non FSAE drivers, the pressure to not stuff up is pretty intense) A wild monster with (virtually) no suspension, the best tyre, and a professional driver, will almost certainly be faster. I don't think anyone would dispute that. But for this level of performance on a single unpractised lap we are talking about a VERY special driver. The type that is not at uni, but rather being PAID to drive as their career, and hence is never going to available to us.

So why build a car for them?

Have a look at the Lap one and two time difference in enduro and our Autocross record. Your 'faster' car needs to be a LOT faster to catch up once your drivers find their feet and start to get going. I think the record of our soft twist cars when we were a big team is pretty impressive. We won overall dynamics a lot. We attribute a lot of that to building a friendly car that doesn't want to kill you, that's why we continue to pursue those ideas, even though our team is positioned differently these days an we are no longer a player at the top level.

Pete

04-25-2014, 10:47 PM

Originally posted by Z:
Dynatune,
...
I asked you quite some time ago, several times, if you know how to build a suspension with zero-rate Twist-mode, but also with fully adjustable LLTD.
...
So, once again;
Do you know how to do it?
Did you know how to do it 20 years ago?
Will you ever give us the details of what you did 20 years ago, when you "failed miserably"?

Dynatune,

I must conclude, from your repeated refusal to answer the above questions, that YOU DO NOT KNOW how to design an effective interconnected suspension. Not now, and certainly not 20 years ago. This discussion would be much more helpful to the students here if you simply admitted this.

Your gross lack of understanding is further confirmed here.

Disadvantages:
Increased Pitch sensitivity - Problematic for [blah, blah...]
Increased Components and Complexity of Installation. [blah, blah...]
Increased Complexity in Car-Setup and Tuning...

When I first read this I thought it must be a leg-pull. But assuming you are serious...

* "Pitch sensitivity"??? - That was explained just a few posts before yours! The Pitch-rate can be set to whatever the designer wants (assuming, of course, that the designer knows how to do it!).

* "Increased Complexity..."!!! - Explained by Luxsosis. But, honestly, how can you claim that 8 springs, all with INTER-DEPENDENT VD behaviour, is simpler to design, build, or set-up, than 3 springs, each INDEPENDENTLY controlling Heave, Pitch, or Roll?
~~~~~o0o~~~~~

Professionally I am used to look at hard facts, in order to make the correct decisions.

Students,

You may be wondering why Dynatune is trying to discourage you from exploring interconnected suspensions, and why he is doing this by presenting only his opinions (and highly illogical and factually wrong ones at that, as above), with very little presentation of "hard facts" at all. (From a quick count, I have asked him for just some of these "hard facts" 5 times now, but still nothing...)

The reason, IMO, is perhaps best put down to those age-old vices of Sloth, Greed, and Pride. (Yes, I know that nowadays these are peddled as virtues (eg. "Greed is good!", "You're a very special person, so put your feet up and delight in our Tim Tams..." <-true advert, which includes Gluttony!), but that is just part of our society's current down-hill slide. :))

Anyway, you can understood Dynatune's opinions when you see that the role of "Experts" (like Dynatune, and Claude++) in our societies, is to PREVENT PROGRESS.

These Experts have put in the hard-yards in their early years, working long hours in difficult and poorly paid positions. But, now, having eventually learnt quite a lot of the "Orthodox RCVD", they expect to be rewarded for it. So they should now be able to put up their feet (Sloth), dispense morsels of advice in return for substantial fee$$$ (Greed), and be treated as highly respected, "the brightest VD engineers", of the time (Pride).

But what happens when PROGRESS sticks its ugly head in the way?

All of a sudden the Experts are back to being Nobodies. In fact, it is even worse since they are now the Dunces who have been getting it so wrong for so long (a real hit to their Pride!). And now they are also Know-Nothings, so cannot command their Expert Fee$ (not good for the Greedy). And, worst of all, they have to go back to school and start that whole learning thing, all over again. No more Slothfulness now! Aaaarrrghhh!!!

At the moment, neither Dynatune's software package, nor Claude's software and seminars, cover the sort of interconnected suspensions discussed in the last few pages (and in many other pages on this Forum). If either of these two Experts are to include these things in their work, then they will first have to admit that they have been wrong so far (or at least negligent in not covering these things), and then they will have to do quite a lot of work learning this stuff.

So quite a lot of Humility and Diligence will be required.

I won't be holding my breath waiting for them to change... :)
~o0o~

BTW, the above resistance to progress is especially strong in the academic world. Google "paradigm shift" to see just how irrational is their resistance to new ideas. Academics are ranked and rewarded according to their grasp of ideas, so any shifting of the ground here has earthquake-like consequences on their careers.

Yep, for many of them, PROGRESS = DEATH!

Z

(PS: A quote from Max Planck: "a new scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die, and a new generation grows up that is familiar with it.")

Jonny Rochester

04-25-2014, 11:27 PM

04-25-2014, 11:56 PM

Z,
I would be interested to see some more fully developed drawings of your suspension ideas...

Jonny,

Firstly, NOT my ideas. They have been around since the 1920s, at least. ("Z" and "Z-Bars" is a coincidence. :))

Secondly, you must by now have seen drawings of the 2CV, Packard, etc. And there are countless more if you search through patents. And you can look at UWA's cars, past and present.

But I will post some more sketches of simple, soft-Twist, interconnected-suspensions that are suitable for FSAE, err... soon-ish (when lots of rainy days, and no other hassles...).

Z

Mitchell

04-26-2014, 02:46 AM

Z,
I have nothing invested in this conversation and nothing to sell. But as a young (40) and impressionable engineering student that could possibly build a fresh FSAE car for 2015/16, I would be interested to see some more fully developed drawings of your suspension ideas. So far I have seen some very very basic concept drawings. If we see some developed designs, then others will no longer be able to argue about packaging constraints or how things couldn't work etc...

All you need is a concept sketch. Packaging is up to you.

There will be two soft twist cars a FSAE-A, assuming UWA are doing something similar this year. Feel free to come have a chat with the UQ guys.

dynatune

04-26-2014, 05:17 AM

Z,

you are too funny. Remind me to buy a pint of beer next time I am Down Under.

You still do not seem to understand how the real world works he ? I am the one that needs to be convinced, I am NOT trying to convince you or anyone else here. Convincing people is being done in a certain way and whilst I am not going to judge your engineering skills, your social skills can surely use some polishing ...

Now try reading this little role play, it might help you a bit better understand what I am talking about:

Imagine that I am your boss and that you are a bright engineer with a quest for low warp/twist suspensions. Let's assume for the moment that I am indeed as ignorant as you think I am. So you come to my office and start all exited about your new project. I do have my doubts and ask you to provide some facts. You do not have those, you have many stories and ideas which sound good but no facts. I have my sincere doubts and send you back to do your homework better to convince me. After a few weeks you come back and the only additional argument that you have is that a colleague (not even you..) made some calculations that show some pro and some con's. Interesting but still not convincing. You are getting all grumpy and emotional calling people ignorant & incompetent. So, being the good and understanding boss I am - do not forget I am still as ignorant as you think I am - I offer you to bring in another expert to evaluate your idea and strangely this expert does ask the same questions that I did. So, by this point in time I understand that I was not as ignorant as you thought I were and I look at you again asking the same questions. You hit the roof and claim that the "experts" must convince you that it does not work, that they are anyway overrated and in fear of losing their influence, that they are incapable of understanding new ideas and thus are blocking progress. So by asking a few critical questions - one by a presumed ignorant boss and one by a presumed expert - you hit the rev limiter. Now ask yourself: Which boss would actually defend such an employee with a possible good idea in front of his superiors, or which investor would put money in it ?

So again, as I asked you several times, do show us (and of course I mean you, not waiting for other people here to save your day) with hard facts that there is an advantage in LAPTIME for such a pure analog mechanical system on a racetrack.

Sincerely Yours,

dynatune

luxsosis

04-26-2014, 06:51 AM

You still do not seem to understand how the real world works he ? I am the one that needs to be convinced, I am NOT trying to convince you or anyone else here. Convincing people is being done in a certain way and whilst I am not going to judge your engineering skills, your social skills can surely use some polishing ...
dynatune

While we are waiting...

Could you tell us what sort of soft-twist layouts you tried in F1 to mimic the active suspension setups?

From the sounds of it there will be 2 low warp stiffness cars at Aus this year, I'm curious to see if either of them resemble what engineers in F1 conceptualised...

rwstevens59

04-26-2014, 03:36 PM

Uh...Gentlemen,

Being very new here I will tread lightly but I still feel the need to comment.

I would say that this thread has drifted very far off the path of the original question of calculating spring roll and ride rates. Static deflection or the somewhat fictitious ride frequency/roll gradient methods. Personally after doing the calculations on the race cars I play with both methods are valid and give results that correlate well with what is found to actually work on the car under race conditions. The frequency/roll gradient numbers while possibly not being 'real' in the sense of true classical vibration analysis do provide a quick indication of whether similar cars setups are stiff or soft relatively speaking. That said, move away from similar cars and the frequency/roll gradient doesn't mean much of anything and you have to start back at square one with static deflections under anticipated loads.

This, in my view, is where the treatment of this subject and many others in the accepted authoritative VD texts breaks down as a teaching tool. As others on this thread have pointed out a lot of simplifying assumptions have been made to create usable 'hand-calc' models as a first pass or a rough check of data obtained from more sophisticated models. It would have been very beneficial to have included the derivations of these simplified models in say Race Car Vehicle Dynamics (or any of the others) because the student young or old would understand how he got to where he is at and not just try to plug in numbers (never good). I do understand that this would have stretched RCVD into two volumes at least and I don't think the authors ever envisioned this book to be as widely used as an instructional text.

As far as the seemingly religious argument about conventional vs. inter-connected suspension systems is concerned, while I find some of the discourse very interesting and the alternate ways solving the race car or four wheeled automobile suspension design problem a healthy pursuit. The current and seemingly long running battle is accomplishing nothing productive. And it is certainly not in any way beneficial to the students trying to learn about automotive design. IT IS DRAMA. And maybe that is what everyone on this forum enjoys.

If I were a student seeking information on how to construct my first FSAE car there is a wealth of information here but too much is obscured by the seemingly large egos that in my view just muck up the works.

If you want to teach the next generation then teach them. But don't do it from a position that would make them begin to wonder if they want to become an engineer at all if this forum is any example.

My two cents. Agree to disagree. Present your particular brand of VD and believe you me, the kids will figure out who is right in the end. You can take that to the bank.

Off my soapbox.

Sorry for the rant.

Ralph

Scott Rinde

04-26-2014, 07:14 PM

This IS Drama.
This IS a high quality representation of real life.
This IS Good Stuff.

<Rant ON> "If you can't see how this is beneficial, then you must have the most dull-witted caricature of an engineer in mind."

Sorry for the rant, but it needed to be said.

Scott

exFSAE

04-26-2014, 07:50 PM

If I were a student seeking information on how to construct my first FSAE car there is a wealth of information here but too much is obscured by the seemingly large egos that in my view just muck up the works.

True of racing at all levels, amateur and professional :) Though mucking it up more is that there's also a wealth of BS floating around which is often sold as gold. What's that quote.. something to the effect of the biggest danger not being ignorance but the illusion of knowledge.

I only looked at the first post here, but ultimately I get the impression this comes down to the trap of looking for the equation, or the most "accurate" simulation tool or what have you. Paraphrasing several popular quotes by George Box... all methods / simulations / models are wrong. All of them. Chasing perfection isn't the way to go, and over-elaborate methods are often not the way to go. Best to go with the most simple and economical choice which still works well enough to get you a solution to a problem. Yes, even in racing where you're "chasing small gains." (Losing sight of the big picture while chasing minutiae is its own topic...)

mdavis

04-26-2014, 08:22 PM

Hi Claude,
I would dearly love to be able to answer that for you, but alas things at UWA are not what they once were. I don't believe we will ever be able to show that within the team, as we have been soft twist for a very long time, and our last traditional car has much larger dimensions than any modern FSAE car. Even comprehensive data continues to escape us with various Gremlins always spoiling the efforts. Simply two sets of identical tyres is beyond our resources these days.

Perhaps if another big team with a strong VD background made the change we they would be in a position to really know the difference. I'm thinking a Jayhawks, Cincinnati or a Stuttgart type of team.

HOWEVER - The main goal of the concept is NOT a physically faster car, but rather a car than can be driven faster, on a track you have only walked, with only one chance to get it done, with (at best) an intermediate level driver. This is all about a car that is not intimidating at the limit, such that student drivers will have a go when the chips are down. (for non FSAE drivers, the pressure to not stuff up is pretty intense) A wild monster with (virtually) no suspension, the best tyre, and a professional driver, will almost certainly be faster. I don't think anyone would dispute that. But for this level of performance on a single unpractised lap we are talking about a VERY special driver. The type that is not at uni, but rather being PAID to drive as their career, and hence is never going to available to us.

So why build a car for them?

Have a look at the Lap one and two time difference in enduro and our Autocross record. Your 'faster' car needs to be a LOT faster to catch up once your drivers find their feet and start to get going. I think the record of our soft twist cars when we were a big team is pretty impressive. We won overall dynamics a lot. We attribute a lot of that to building a friendly car that doesn't want to kill you, that's why we continue to pursue those ideas, even though our team is positioned differently these days an we are no longer a player at the top level.

Pete

Pete,

First off, thank you for considering my alma mater in the same sentence as Stuttgart and Kansas, both of which I consider to be very good teams. At Cincinnati, the years we did well, we got a lucky mix of people that were willing to put in the extra work. UC is primarily a one year team, so there is a ton of turnover and very little knowledge transfer (this was not the case from 06-08, and the results speak for themselves). We tried a soft twist mode car back in 2009 after seeing the UWA car at Michigan 2008. That car was unfortunately let down by the drivetrain and engine controller, and a lot of people severely discounted the idea based on the implementation, rather than the core issue at hand (spool diff, effectively non-useable ECU).

Second, do you have any pictures of the UWA car showing the attachment of the longitudinal Z bar to the suspension elements? I've been able to find pictures of the lateral Z bars (W springs), but the longitudinal Z bar seems to be very elusive. I'd be very interested in seeing the method you guys use to adjust LLTD as well.

And I agree 100% with you about designing the car for the people that are going to be driving it. Our 2013 car was incredibly softly sprung by FSAE standards, but it gave the drivers a lot of confidence and made the car incredibly easy to use very early on in the development cycle.

-Matt

Marshall Grice

04-27-2014, 11:52 AM

Second, do you have any pictures of the UWA car showing the attachment of the longitudinal Z bar to the suspension elements? I've been able to find pictures of the lateral Z bars (W springs), but the longitudinal Z bar seems to be very elusive. I'd be very interested in seeing the method you guys use to adjust LLTD as well.

-Matt
the undertray/tunnels are the longitudinal z-bars. you can see the central pivot and lltd adjustment method in this shot.

http://sphotos-f.ak.fbcdn.net/hphotos-ak-prn1/60945_256867894442833_214606088_n.jpg

mech5496

04-27-2014, 02:35 PM

Pete, appreciate the insight on UWA "twist-soft" over "traditional" systems. Matt, as Marshall pointed out above, the tunnels with their pivot points act as Z-Bars in the UWA car. My understanding of the systems is as such:

Front and rear beams (and theri pivots at the W springs) act as lateral z-Bars, and not the W-springs alone. These act as pitch/heave springs and double as beam axle locating devices as well, making for a very smart integrated design. Side tunnels and theri pivot points act as longitudinal z-bars (or as side leaf-springs in Z's concept sketch). LLTD can be adjusted via adjusting the pivot point lengthwise, and is equal to the distance ratio A/B. Interestingly, the "pivot point" should initially be at the same point as the car GC lengthwise and is really easy to determine. The U-bar in the above picture, besides locating the "pivot points" acts also as a roll spring or ARB but for all four corners.

Once more, this is purely my understanding of the UWA concept. Pete or Kev feel free to correct me if I am wrong!

04-27-2014, 09:32 PM

Wow, I have Sunday off, and a page full of posts...

Let's get the pissing contest over with first.
~~~~~o0o~~~~~

Imagine that I am your boss...

Dynatune,

Ha ha... Keep dreaming... :)

"You still do not seem to understand how the real world works he ? I am the one that needs to be convinced,..."

Your arrogance continues by assuming that I NEED to convince you. Why should I bother? Honestly, why?

But assuming I wanted to, how could I do it? Well, I would start by challenging you to a wager for a very substantial amount of money. How much can you afford to lose? I will happily match it! :)

We each then take very similar cars (same size, mass, engine, tyres, etc.) and fit our respective suspensions. For statistical significance these cars are raced at many different tracks, with many different drivers. To give you a leg-up I let you choose three-quarters of the tracks and drivers.

I know your chosen tracks will be very smooth, because your suspension can only work when it DOES NOT HAVE TO WORK (ie. it will have negligible movement). However, my chosen tracks will be real tests of the suspensions. It follows that my car will have similar speed to yours on your tracks, but your car will not even be able to complete a single lap of my tracks!

So, on averaged laptimes, you will be the LOSER. But even though I win the money, you will most likely complain that "It wasn't fair. Real racetracks don't have bumps. I want my money back...". And you will remain unconvinced.

So, once again, here is "how the real world works", according to Max Planck.
"A new ... truth does not triumph by convincing its opponents ..., but rather because its opponents eventually die, and a new generation grows up that is familiar with it."
~~~~~o0o~~~~~

Ralph,

Believe me, when I was at school I much prefered Maths and Science to the Drama classes. And I still do.

All the technical stuff on this thread could be boiled down to about one page. I have had several shots at it. And so has Tim. (And I look forward to more of Tim's work, as I still have some comments regarding his last posts.). And, like Luxsosis above, I wish Dynatune would just post some "hard facts" (eg. numbers, sketches, etc.) of his 1990s work.

But apparently we live in a world where "reputation" trumps "reason". Anyone who was ever an "F1 Engineer" is always absolutely right, and must be believed regardless of any "hard facts", or the lack thereof......

Anyway, I will stick with reasoning...
~~~~~o0o~~~~~

exFSAE,

...the trap of looking for the equation,...
... all methods / simulations / models are wrong...

Agreed (within reason!).

Recently Ralph (above) posted some links to educational resources on the "Beam-Axle..." thread. One of these is a series of videoed lectures of "Classical Mechanics" by MIT Professor Walter Lewin. I plan to comment on those on the BA thread (so far I'm less than half way through the ~40 x 50 minute lectures).

But worth pointing out here the HUGE emphasis Lewin puts on estimation of errors, "A measurement is USELESS without knowledge of the UNCERTAINTY!!!". (He SHOUTS a lot, too. :))

Students might like to go to the end of lecture 13 (http://ocw.mit.edu/courses/physics/8-01-physics-i-classical-mechanics-fall-1999/video-lectures/lecture-13/) and guess why the experiment there is a failure (measured time >> predicted time). Bad measurements? Bad equipment? Bad equations? Bad something else?

I reckon I know the answer (quite easy), but eternal fame and glory to the first student who gives the right answer here. :)

BTW, when Scientists get their error estimates wrong, they just have dud experiments/theories. When Engineers get their tolerances wrong, jumbo-jets fall out of the sky, and lots of people are killed.
~~~~~o0o~~~~~

Matt,

... do you have any pictures of the UWA car showing the attachment of the longitudinal Z bar to the suspension elements?

I touched on this in a post at the top of page 7, this thread. As you (and others) noted, Heave and Pitch are controlled by the TWO Lateral-Z-Bars (ie. the beam-axles) and their W-springs, with each LatZB only interconnecting TWO wheels.

BUT!!!, the Roll-Mode is ENTIRELY INDEPENDENTLY CONTROLLED by a Lateral-U-Bar + 2 x Longitudinal-Balance-Beams (with these last two LongBBs being similar to Longitudinal-Z-Bars).

IMPORTANT NOTE. If any of the all-wheel modes of H/P/R/T is to be ENTIRELY INDEPENDENT of all the other modes, then it MUST have a linkage that connects ALL the wheels.

Adjustment of LLTD is as explained by Marshall and Harry...

For more information, try:

* SAE paper #2000-01-3572, "Balanced Suspension" (see Figure 4 for above details).

* US Patent 6,702,265 B1, "Balanced Suspension", (lapsed, and with many, many words and figures, because the Lawyers charged by the page!).

* Also Racecar Engineering article (March 2000) with same name.

* Also some recent posts on the "Suspension Design" thread (bottom page 25+).

(I can email you the SAE paper if you want).

Z

MCoach

04-28-2014, 12:13 AM

To answer Z's question with what I believe is the correct answer:

The kid who yelled out friction was only partially correct, but not really. It's involved, but for this I will assume it is the same for both systems.

If the two circuits are creating pendulums, with respect to how accurate they keep time then it makes sense to compare their 'Q factor'. The Q factor is something to do with the bandwith and oscillations of the system.
The first system has a very low period of oscillation with 3 cycles over 22 seconds. The second system has 10 oscillations over 22 seconds, meaning that if we can assume the friction in the system is same for each, but the mass of each 'pendulum' is vastly different and the period of oscillation is vastly different the Q factor will noticebly different. The Q factor is directly related to the error of consitency over time in the system and the first system has a (relatively) high Q factor meaning that it may have a negligible effect on the outcome over the several seconds that it is being recorded for. The second system has a low Q factor where the error is able to outweigh the uncertainty of the curvature itself.

By measuring things in periods I believe Walter (my hero) is trying to point this out subversively to come back to it at a later date.

04-28-2014, 12:41 AM

MCoach,

[Insert appropriate quiz-show sound that indicates wrong answer.] :)

I see it as a lot simpler. It has some relevance to FSAE.

Hint 1: Consider the "right equation" for working out how fast an FSAE car accelerates (ie. straight-line, drag-race style)...

Hint 2: NI & NII (paraphrased) "The impressed force causes a change in the QUANTITY of motion"...

Z

(Edit: Gave Hint 3, but deleted because maybe too many hints already...)

MCoach

04-28-2014, 01:49 AM

Not sure where you're going with this,

F = ma (NI)

If you're talking about the air resistance on the object, then that's factored into the 'Q factor' in the all consuming 'friction' section in the equation.

A larger object with lower velocity and higher interia will slow less to the same air drag force compared to a smaller, lighter and higher velocity, (each without a driving force).
Gravity is a restoring force and without friction would allow the pendulum to oscillate indefinitely.

rwstevens59

04-28-2014, 07:58 AM

MCoach,

No, air resistance, while present as well as a small amount of friction, is not the reason for the much longer than estimated period. Lewin is sneaky and is actually making a lead in to what is coming up later in his lectures. Like Sherlock Holmes think hard about what was obviously, but not intuitively, different between the air track and the ball and track experiments.

Ralph

mdavis

04-28-2014, 07:01 PM

the undertray/tunnels are the longitudinal z-bars. you can see the central pivot and lltd adjustment method in this shot.

That is the picture I was looking for. For some reason, I was thinking that the LLTD was adjusted via some form of Z bar, which is why I was really confused. Seeing that it is a U bar makes a whole lot more sense.

Matt,

I touched on this in a post at the top of page 7, this thread. As you (and others) noted, Heave and Pitch are controlled by the TWO Lateral-Z-Bars (ie. the beam-axles) and their W-springs, with each LatZB only interconnecting TWO wheels.

BUT!!!, the Roll-Mode is ENTIRELY INDEPENDENTLY CONTROLLED by a Lateral-U-Bar + 2 x Longitudinal-Balance-Beams (with these last two LongBBs being similar to Longitudinal-Z-Bars).

IMPORTANT NOTE. If any of the all-wheel modes of H/P/R/T is to be ENTIRELY INDEPENDENT of all the other modes, then it MUST have a linkage that connects ALL the wheels.

Adjustment of LLTD is as explained by Marshall and Harry...

For more information, try:

* SAE paper #2000-01-3572, "Balanced Suspension" (see Figure 4 for above details).

* US Patent 6,702,265 B1, "Balanced Suspension", (lapsed, and with many, many words and figures, because the Lawyers charged by the page!).

* Also Racecar Engineering article (March 2000) with same name.

* Also some recent posts on the "Suspension Design" thread (bottom page 25+).

(I can email you the SAE paper if you want).

Z

Erik,

I've read your SAE paper before (it's actually posted a couple of places online, which is where I was able to find it), but it's been a long while, so I'll re-read it. I've learned a lot since then, so hopefully it'll have some more meaning.

As for the longitudinal U bar, yeah, I was misunderstanding things pretty significantly. Seeing the picture (along with the explanations), I have a much better understanding of the UWA system, at least on a very high conceptual level. The intricate details and various stiffness values, those I'm still working on (I've got the pen and paper going tonight to look at some calculations similar to Tim's spreadsheet, but I work slow and I'm not all that smart, so we'll see how it goes).

-Matt

Loz

04-28-2014, 10:32 PM

Well for starters it is not a closed system and there is not a conservation of momentum.
Secondly, the ball is rolling and the other sliding (with minimal rotation).
Assuming a rigid ball and track, the external impressed force (gravity) is opposing the torque vector of angular momentum of the ball rolling up the hill thus "reducing" its quantity of motion and slowing it down during each half of the period it is going up hill.
If the ball and the track were not infinity rigid, there would also be some component of rolling resistance imparted as a result of deformation of both the ball and track, somewhat similar to that of a car tyre.

mech5496

04-29-2014, 01:45 AM

That is the picture I was looking for. For some reason, I was thinking that the LLTD was adjusted via some form of Z bar, which is why I was really confused. Seeing that it is a U bar makes a whole lot more sense.

Matt, the U-bar does NOT adjust LLTD. LLTD is adjusted by the relative distances from the pivot point of the longitudinal z-bar (tunnels) to the axles. In a side view, if A is the distance from the pivot point to the front axle and B the distance from the pivot point to the rear axle, their ratio A/B determines the LLTD, regardless of the existance of the U-Bar (it could be replaced with another U-bar with different ratio or a solid chassis mound with (almost) no difference regarding LLTD)

EDIT: Let's assume a car with a WB of 1600mm. Then, for an LLTD of 40/60% front to rear, the pivot point should be 40%*WB=640mm from the rear axle, and 60%*WB=940mm from the front axle. Giving 30mm of adjustment on each side of the slot (60mm in total) can modify the LLTD from 42/58% to 38/62%

mdavis

04-29-2014, 09:24 AM

Matt, the U-bar does NOT adjust LLTD. LLTD is adjusted by the relative distances from the pivot point of the longitudinal z-bar (tunnels) to the axles. In a side view, if A is the distance from the pivot point to the front axle and B the distance from the pivot point to the rear axle, their ratio A/B determines the LLTD, regardless of the existance of the U-Bar (it could be replaced with another U-bar with different ratio or a solid chassis mound with (almost) no difference regarding LLTD)

EDIT: Let's assume a car with a WB of 1600mm. Then, for an LLTD of 40/60% front to rear, the pivot point should be 40%*WB=640mm from the rear axle, and 60%*WB=940mm from the front axle. Giving 30mm of adjustment on each side of the slot (60mm in total) can modify the LLTD from 42/58% to 38/62%

Harry,

When you say the pivot point, are you talking about the center of the U bar, or the point on the undertray where the drop link attaches? Or are you talking about the pivot point where the U bar attaches to the upper chassis? When I think of "pivot point" I think of the latter, but I don't think that is the point you are talking about.

My current thinking would say that adjusting the point on the U bar where the drop link attaches to the bar would change the roll stiffness of the setup, but would have little/no impact on the LLTD (maybe at high angles between the bar and drop link, but I would imagine this to be very small). The way (from my seeming lack of understanding) is that to adjust LLTD, you change where the drop link attaches to the undertray. By my understanding, this would change the motion ratio between the undertray movement and U bar movement, thus changing the LLTD. How far off am I?

-Matt

mech5496

04-29-2014, 10:05 AM

Matt, my THINKING (as I am not sure about it) is the same as yours, i.e. the drop link mounting point to the undertray is the one adjusted to adjust the LLTD. However, the U-Bar/undetray MR does not affect LLTD, only roll stiffness. LLTD is dependant only on the drop link/undertray point position. Again, this is only my understanding of the system and I might be wrong.

Loz

04-29-2014, 10:05 AM

When you say the pivot point, are you talking about the center of the U bar, or the point on the undertray where the drop link attaches? Or are you talking about the pivot point where the U bar attaches to the upper chassis? When I think of "pivot point" I think of the latter, but I don't think that is the point you are talking about.

The pivot point being referred to is the attachment point of the droplink on the undertray tunnels.

My current thinking would say that adjusting the point on the U bar where the drop link attaches to the bar would change the roll stiffness of the setup, but would have little/no impact on the LLTD (maybe at high angles between the bar and drop link, but I would imagine this to be very small).

Correct, it would have no impact on the LLTD.

The way (from my seeming lack of understanding) is that to adjust LLTD, you change where the drop link attaches to the undertray.

Correct

By my understanding, this would change the motion ratio between the undertray movement and U bar movement, thus changing the LLTD. How far off am I?

The ratio of movement between the undertray and bar does change, but this isn't what changes the LLTD. All a motion ratio change between those parts does is increase roll stiffness in the same way that reducing the lever arm length or changing the torsion bar dimensions would do.
Keep in mind the pure roll mode is equivalent to having the wheels fixed (to the ground) and the chassis rolling about some arbitrary axis equally at the front and the rear (or one side pair of wheels goes up, the other goes down by an equal amount).
As Harry suggested, the LLTD is set by varying the location of the attachment point of the droplink to the tunnel. It is the ratio of concern is ratio of distances from the drop link forwards to the front axle and the distance aft to the rear axle.

Aside from decoupling roll from pitch/heave, a nice effect of a setup like this is that load that is actually transferred from one side to the other is independent of front/rear mass distribution and presumably completely decoupled from load transfer during combined roll/pitch.

mdavis

04-29-2014, 10:12 AM

Loz/Harry,

It sounds like we're on the same page, I'm just off on my wording/naming of some things. I feel better about my level of understanding at this point.

-Matt

04-29-2014, 09:50 PM

Matt,

I have been a bit busy lately, but thanks to Harry and Loz for giving good explanations of the UWA Roll control system.

The gist of the UWA system is that during cornering the car-body Rolls because it has an "external" Roll-COUPLE acting on it, from the net horizontal tyre forces acting at W-springs level (ie. roughly at the "Roll-Centre" height), together with the equal and opposite horizontal inertial (centrifugal) force acting at CG height. This 2 x horizontal-forces Roll-Couple is counteracted (= brought to equilibrium) by an equal and opposite 2 x vertical-forces Suspension-Roll-Couple acting from-undertray->to-car-body. These 2 x vertical-forces act along the drop-links, pushing the outside end of U-bar up, and pulling inside end of U-bar down, thus "resisting the body Roll".

Of course, each vertical drop-link force has an equal and opposite force acting from-car-body->to-undertray. So the outside-drop-link pushes down on the outside-undertray-tunnel, and the inside-drop-link pulls up on its inside-undertray-tunnel. Next, the two undertray-tunnels act very much like brake-balance-bars, and longitudinally distribute their respective drop-link forces to their respective front and rear wheels, in the inverse ratio of distance from drop-link to wheel (as per Harry's example).

As a semantic note, I usually refer to elements like UWA's tunnels as Longitudinal-Balance-Beams because their function is primarily to distribute forces according to their "lever" geometry (ie. very much like Brake-Balance-Bars). By contrast, I refer to any structure that behaves similarly to a LongBB, BUT ALSO PROVIDES SPRINGING, as a Longitudinal-Z-Bar.

Note that UWA's tunnels (LongBBs) could be very rigid, and their Lateral-U-Bar could also be very rigid, but nevertheless they could still have a soft Roll-mode by replacing one of their drop-links with a single Spring-Damper. So UWA's complete Roll-mode control package consists of 1 x Lat-U-Bar + 2 x drop-links + 2 x Long-Balance-Beams, and any one (or more) of these elements can be made the "springy" one.
~o0o~

I guess I should do another sketch or two to cover all this, as there is some interesting stuff that has not yet been explained.

Briefly for now, U-Bars, Z-Bars, and single-wheel springs, can all be explained as detailed variations of conceptually the same thing. Namely, a simple, straight, lever!

Similarly, a fully-interconnected, and therefore fully-modally-independent 4-wheel suspension, is simply a number of different variations of 3 of these levers connected together. By varying the lever ratios this suspension can imitate any conventional suspension (not including non-linearities.)

Putting this in terms of "cogitatio caeca" (= "thinking blind", or algebra) we can say that the three forces acting on any of these simple levers are related as,

Ff = F1 + F2

Where;
Ff = force at fulcrum (assume to be acting on car-body),
F1 = a x Ff = force from drop-link 1,
F2 = b x Ff = force from drop-link 2,
a, b = fractions representing relative size of F1 and F2 (say, as % of Ff), and determined by the geometry of the lever.

Interestingly:
If a is SAME SIGN as b, then Z-Bar.
If a is OPPOSITE SIGN to b, then U-Bar.
If a = 0 OR b = 0, then single-wheel spring.

BUT!!!, when "thinking blind" we have no idea where Ff's LoA is. So we have very little idea of Ff's affect on the car-body. So I guess I should do some sketches...
~~~~~o0o~~~~~

Regarding Lewin's failed experiment at the end of Lecture 13, I am itching to give more hints, but there are already lots of good clues given above.

But I can't help myself, so here is a Red Herring.

The experiment does, indeed, fail because there is friction involved (in one of its senses!). But this friction does NOT dissipate any energy (in another sense of the word "friction"). So the experiment would still fail if this friction was replaced with an entirely FRICTIONLESS mechanism that had the same effect!

Bonus points for making sense of above paragraph. :)

Z

04-29-2014, 11:49 PM

MORE ABOUT UWA'S INDEPENDENT ROLL-MODE CONTROL.
==================================================

Some more things that need explaining...

I assume everyone has a picture of UWA's system in their mind. Namely, there is a Lateral-U-Bar that has its "backbone" pivotted to the car-body, just behind the MRH. The ends of the LatUB have drop-links that go down to the two undertray tunnels, and these tunnels act as Longitudinal-Balance-Beams. The 2 x ends of each of the 2 x LongBBs can be considered to act directly down on the 4 x wheelprints.

Assume (to keep the numbers simple) that wheelbase is 1500 mm, and the drop-links attach to the tunnels/LongBBs at 900 mm rearward from the front-wheels, so 600 mm forward of rear-wheels. Thus any vertical force on a drop-link is transferred, via its LongBB, in the ratios 600/1500 = 40% to the front-wheel, and 900/1500 = 60% to the rear-wheel. That is, the forces are as per Archimedes' "Law of Levers" (so nothing new here! :)).

Now consider the all-wheel suspension modes of:

HEAVE - With above picture in mind it should be obvious that the four wheels can all move together in Heave (ie. all equally up, or all equally down, wrt the car-body) with NO RESISTANCE from the above linkage (other than a bit of friction in joints, etc.). That is, the LatUB simply pivots about its "backbone", and NO PART OF THE LINKAGE IS STRESSED. Therefore, this linkage exerts NO FORCES on the wheelprints during Heave motions.

PITCH 1 - Assume that the "Pitching" occurs around the drop-link-to-LongBB ball-joints. Obviously these BJs don't offer any resistance (again, neglect any small friction), so NO FORCES on the wheelprints from this type of "Pitch".

PITCH 2 - BUT!!! What about "Pitching" about a lateral-axis at MID-WHEELBASE? Now there will be some rotation about the drop-link BJs, and also some rotation of the LatUB wrt car-body, as in the Heave case. But, again neglecting small friction, NO FORCES on the wheelprints from this type of "Pitch". (Please draw side-view sketch to convince yourselves of this).

TWIST - Assume that this "Twist" IS DEFINED similarly to Pitch 1 above, but with opposite rotations of the two LongBBs, as seen in side-view (eg. so left LongBB rotates nose-up about its Drop-Link-BJ, and right LongBB rotates nose-down about its DLBJ). So, again, the total linkage only has a small rotation at the drop-link BJs, and so again NO FORCES on the wheelprints from this type of Twist.

Thus, the above Roll-mode control linkage ONLY RESISTS Roll motions, and exerts NO RESISTANCE against any Heave, Pitch, or Twist motions, AS THEY ARE DEFINED ABOVE.
~~~o0o~~~

BUT!!!

WARP - Let us, somewhat perversely perhaps, DEFINE this mode to be the EQUAL and opposite vertical movements of the car's diagonal pairs of wheelprints. This means that if we consider the originally flat plane containing the four wheelprints to be Warped in this way, then the "Nodes" (= points of no displacement) of the resulting warped plane are along the longitudinal-centreline of the car, and along the lateral-centreline (= mid-wheelbase) of the car (see sketches somewhere...).

VERY IMPORTANTLY, this now also means that one drop-link must move upward, and the other drop-link must move downward, because they are rearward of the node points. The Lateral-U-Bar is thus twisted, and thus UWA's Roll control system RESISTS THIS TYPE OF WARP MOTION.

So, it is NOT WARP-SOFT!!!

BUT!!! It is still COMPLETELY TWIST-SOFT!!!!! :)
~~~o0o~~~

There is nothing paradoxical, at all, in the above explanation.

Here I should mention that I have very little idea of how the F1 "active-suspension era" engineers went about their work, or what DEFINITIONS they used. There is very little information about this in the public domain (and Dynatune ain't talking...).

But I do suspect that many of the workers at that time somehow "burned into their minds" the notion that the Warp-mode MUST BE DEFINED as above. And therefore any adjustment of LLTD away from 50:50 MUST NECESSARILY require an ACTIVE Warp-mode. I have certainly heard that view expressed several times, and used alongside phrases such as "Warp/Roll-mode coupling...".

This is why I have always used the name "Twist-mode" to distinguish it from the "Warp-mode" behaviour. I have also used "Bounce-mode" to distinguish it from the more symetrical "Heave-mode" (and this relates to Tim's earlier comments regarding "spring centres" and "aero Pitch sensitivity", which are (non-) problems that are easily solved with the appropriate definitions.) I would have chosen different terms for "Roll" and "Pitch" too, because these can also be defined differently, but I still can't think of good enough alternative words...
~~~o0o~~~

Bottom lines:
Sloppy, and unconsidered, use of DEFINITIONS = "we failed miserably".
Careful and considered choice of DEFINITIONS (right from the beginning!!!) = you can have a completely soft Twist-mode, AND any LLTD you want.

It really is much easier to solve problems, when you have a clear understanding of what you are talking about. :)

More to be said, but enough for now...

Z

Pete Marsh

04-30-2014, 02:34 AM

"WARP - Let us, somewhat perversely perhaps, DEFINE this mode to be the EQUAL and opposite vertical movements of the car's diagonal pairs of wheelprints. This means that if we consider the originally flat plane containing the four wheelprints to be Warped in this way, then the "Nodes" (= points of no displacement) of the resulting warped plane are along the longitudinal-centreline of the car, and along the lateral-centreline (= mid-wheelbase) of the car (see sketches somewhere...).

VERY IMPORTANTLY, this now also means that one drop-link must move upward, and the other drop-link must move downward, because they are rearward of the node points. The Lateral-U-Bar is thus twisted, and thus UWA's Roll control system RESISTS THIS TYPE OF WARP MOTION.

So, it is NOT WARP-SOFT!!!

BUT!!! It is still COMPLETELY TWIST-SOFT!!!!! "

As the body is completely free around the roll axis this is not quite true. The body simply assumes a roll displacement proportional to the deviation of the link position from 50:50, and no force or spring stress is required. SO still soft warp. This is much same as a Trike (or bulldozer) if taken to the extreme, with the body roll angle determined by the one axle, but still with no weight transfer between the wheels due to the road surface change. There will obviously be inertial effects there if the motion happens quickly, but with a typical setting for a slightly rear heavy car very close to 50:50, and the relatively low frequency of the warp mode, we ignore them and treat it as zero warp. In our implementation there is some warp stiffness within the lower floor with it being a giant flexure so as to achieve the relative motion between the wheels. This is however very low, with measured warp stiffness less than 10% that of a 4 spring and 2 U bar car with equivalent roll stiffness.

Here is a good image of the system. Adjustment and function has been covered by others here already. Major difficulties include managing (mode separated)damping ratios with high unsprung without resorting to separate ones everywhere, achieving required displacement and relative stiffness profiles of "M" springs (or W if you prefer) and the lower floor, and steering system and front upright packaging.

Pete

04-30-2014, 06:55 AM

As the body is completely free around the roll axis this is not quite true. The body simply assumes a roll displacement proportional to the deviation of the link position from 50:50, and no force or spring stress is required. SO still soft warp.

Pete,

The above is actually what I was getting at when I mentioned the phrase "Warp/Roll coupling". With the body fixed in space, and NO Heave, Pitch, or ROLL-mode motions of the wheelprints, and the wheelprints then moved only in a symetrical (50:50) "Warp" motion, then the Lateral-U-Bar is definitely twisted, and this particular definition of Warp is resisted.

However, your car is still most definitely Twist-soft (neglecting any stiffness of the undertray), in accordance with the particular definition of "Twist" that suits the drop-link positions.

Anyway, the above might sound like I am splitting semantic hairs, but once the wrong definitions get burned into the mindset of people designing these things, I think it can lead to long term misunderstandings. As I mentioned before, one of these misunderstandings is that "If you want to change the LLTD away from 50:50, then you MUST have forces acting in the Warp-mode". This is true of the Warp-mode, but not true for an appropriately defined Twist-mode.

Z

mech5496

04-30-2014, 07:18 AM

Z, in a symmetric (50:50) warp motion, the U-bar indeed twists, resisting the motion; the question is by how much? It is a question easily answered by doing some simple, mostly geometrical calculations. At your numerical example for instance (1500mm WB, 20mm movement in all wheels resulting in a 40mm 50:50 wrap motion) the pivot point moves upward ONLY by 4mm, and that's an extreme example (wrap movement too large, WB too small for FSAE). 4mm movement on pivot point, acting on a U-bar lever of let's say 200mm, gives an approximately 1.2 degrees twist in the U-bar. I think it is pretty straightforward to calculate the desired roll stiffness contributed by the U-bar in such a setup ant then calculate the U-bar contribution to the wrap stiffness, but still I think it would be insignificant.

EDIT: To compare with, a 2deg roll movement, assuming that the left-to-right pivot ponts are 1m apart, means a 17.5mm upwards/downwards movement on the U-bar links.

04-30-2014, 07:48 PM

Harry,

I will probably have to repeat this several more times, but the problem here is the high-level conceptual understanding of these things (or the lack thereof).

As I mentioned before, I have never been able to find any detailed papers, etc., about F1's adventures with this "modal" suspension thinking. But I have heard, quite often, mention of "THE Warp-Mode" as being ALWAYS, AND ONLY, of the "50:50" type (ie. as if the Longitudinal-Balance-Beams MUST pivot about their mid-points).

The problem with this thinking is;
If the GOAL is to have "The Warp-Mode" completely soft, then the Elastic-LLTD MUST BE 50:50!

As soon as you LOCK yourself into this type of thinking, then you can NEVER change handling balance by adjusting the Elastic-LLTD as UWA does, and as you and Loz correctly explained. You can NEVER have a "Twist-soft" suspension that gives 40:60 LLTD (as in my earlier post). And you certainly can NEVER have a "Tractor-soft" suspension (with 0:100 LLTD, as mentioned by Pete), even though these latter suspensions are just as "road-twist compliant" as the 50:50 Warp-soft ones.

Instead, when locked into the 50:50 Warp thinking, you might try adjusting Total-LLTD by changing the "Roll-Centre" heights (actually lateral n-line slopes!) at each end of the car, as Dynatune said they did in one of his few details. But, apparently, even the "brightest VD engineers" of the 1990s had great difficulty doing this. And here I must wonder why they even bothered with RC heights, when the "variable-LLTD-Twist-soft" approach is so much easier. :)

So, to repeat again, the problem is a conceptual one. Namely, locking yourself into a particular way of thinking because someone, somewhere, sometime, ARBITRARILY DEFINED something in a particular way (eg. "Warp MUST be 50:50"), even though that definition may not be useful to you.

Z