Why is compliance so important?

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Caito
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Joined: 16 Jun 2009, 05:30
Location: Switzerland

Why is compliance so important?

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I'm constantly hearing/reading about the importance of compliance. Mostly suspension wise, the camber/toe variations due to compliance are demons that will kill your handling.

Why do such tiny variations generate (as said) such a huge loss in handling performance? If the variations of the suspension are bigger than those of compliance, how can it be that 0.05° of camber change affects so much?

Let me add a non-related question, since I don't want to open another thread just for that. Why is it that track-width change with suspension movement is so important and must be kept to a minimum? How does it affect the wheel (or the car) a track change of 0.01''?


bye bye!

Caito.-
Come back 747, we miss you!!

Jersey Tom
166
Joined: 29 May 2006, 20:49
Location: Huntersville, NC

Re: Why is compliance so important?

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Who says its only 0.05°?

Hint: Can be a hell of a lot more, even on pro-series cars. That, is why it's an issue (with regard to camber and toe compliance).

Likewise, who says track width (mechanical or pneumatic) changes are only 0.01"?
Grip is a four letter word. All opinions are my own and not those of current or previous employers.

Belatti
33
Joined: 10 Jul 2007, 21:48
Location: Argentina

Re: Why is compliance so important?

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Caito wrote: Why is it that track-width change with suspension movement is so important and must be kept to a minimum? How does it affect the wheel (or the car) a track change of 0.01''?
Mechanical track change is called scrub and its a thing that if I was a suspension designer I would minimize as much as I can. In steady state cornering, scrub messes with the tyre contact patch, in the way you will have lateral loads variations. Ask Canapino about the time he was in Germany designing an F3 suspension and it was rejected because mechanical scrub was too big :wink:

Regarding toe changes, have in mind bump steer can chage it much more than those 0.05° you mention. BTW, where did you take that value from?
"You need great passion, because everything you do with great pleasure, you do well." -Juan Manuel Fangio

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dynatune
13
Joined: 28 Aug 2013, 11:03

Re: Why is compliance so important?

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Just have a look at the Bundorf SAE paper and you will know. Compliance is fundamental for creating stability. You can also have a look at http://www.dynatune-xl.com which explains some of the magic.

Cheers.

Lycoming
106
Joined: 25 Aug 2011, 22:58

Re: Why is compliance so important?

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For a primer, read this:

https://www.formulastudent.de/academy/p ... ompliance/
Jersey Tom wrote:Who says its only 0.05°?
Not me. Actually, 0.05 degrees of camber compliance is not too bad if that's your maximum. But, as seen below, it isn't always so stiff.

Image
Caito wrote:Let me add a non-related question, since I don't want to open another thread just for that. Why is it that track-width change with suspension movement is so important and must be kept to a minimum? How does it affect the wheel (or the car) a track change of 0.01''?
Track change scrubs the tire laterally as it articulates. Not great for tire wear. Also not good for a bunch of other things as others have mentioned. I should add that I feel quite certain that a modern F1 front suspension will have much more than 0.01" of track change even across it's limited range of travel.

bill shoe
151
Joined: 19 Nov 2008, 08:18
Location: Dallas, Texas, USA

Re: Why is compliance so important?

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Think transients instead of steady-state. When you turn-in hard you want the car to respond and settle as quickly as possible. If you have large compliance with camber or toe, then wheel has to move (possibly oscillate?) and then settle into a stable camber/toe position for the new loads. This takes time. I'm guessing that the compliance is lightly damped so an initial oscillation is quite likely. The tire compliance probably feeds on the suspension compliance to further extend the response lag. Bad for objective grip, and very bad for subjective feeling/confidence in the car.

It would be interesting to think of compliance issues in terms of the natural frequency and/or transient response time of a given compliance, rather than thinking about the deflection as a quasi-static issue once it has reached a new steady-state deflection.

I would love to see a high-speed high-res video of one of the typical compliant cars in the pictures. Any motorsport person ever measured suspension compliance in a way that could reveal the response time of something like camber or toe compliance?

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Tim.Wright
330
Joined: 13 Feb 2009, 06:29

Re: Why is compliance so important?

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Its mainly toe compliances which are the killer. In my opinion they are normally causing driver feedback problems not necessarily overall grip level problems.

One reason to avoid them is that they are "lying" to the driver about what is happening at the tyre's contact patch. The driver feels the rear tyre slip angle from the slip angle of the car and if a significant portion of that is made up of compliance steer, then it is contaminating the "signal" that the driver uses to feel the limit of the rear axle.

Also, as Bill says, in a transient situation toe and camber compliances introduce a delayed component of the vehicle response (the chronology is very roughly steering > front axle slip angle response > front axle compliance response > roll response > rear axle slip angle response > rear axle compliance response) Therefore any compliances on the rear axle may happen a long time after the initial steering response. Again this messes with the driver's feedback.

Its important to think of the driver as a controller in a feedback loop. Such systems have a finite "phase margin" where they become unstable due to phase delays in the feedback loop. Compliances and body roll effects are basically phase delays in the driver's control feedback loop.
Not the engineer at Force India

Greg Locock
233
Joined: 30 Jun 2012, 00:48

Re: Why is compliance so important?

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Bill, would you be looking at the effect of toe compliance (rear axle presumably) on yaw delay time? It's easy to work out the effect of toe compliance on linear range understeer, and the link between understeer and yaw delay is calculable, if not by me on a saturday morning.

bill shoe
151
Joined: 19 Nov 2008, 08:18
Location: Dallas, Texas, USA

Re: Why is compliance so important?

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Greg Locock wrote:Bill, would you be looking at the effect of toe compliance (rear axle presumably) on yaw delay time? It's easy to work out the effect of toe compliance on linear range understeer, and the link between understeer and yaw delay is calculable, if not by me on a saturday morning.
I think yaw delay is a reasonable way to look at it. However, I was thinking beyond linear range. Does this change the principle, or is there still a similar relation between understeer and yaw delay?

Greg Locock
233
Joined: 30 Jun 2012, 00:48

Re: Why is compliance so important?

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hang on and let me sort linear range out first!

dynatune
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Joined: 28 Aug 2013, 11:03

Re: Why is compliance so important?

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We have asked us that question (step steer into non-linear range ?) about 15 ears ago too and were quite skeptic about it. So we did some testing on that issue and we found out that there will still be a similar relation in the non-linear range at least a good correlation for some parameters and less good for others. I'll try to keep this as brief as possible.

We have done non-linear step steer testing on a "high performance" car with slick tires (worst case scenario) and a report with some of the results can be found here: http://www.dynatune-xl.com/documents.html

As can be seen in the validation report the step steer (page 7/8) was performed in real life to a final lateral acceleration value of 1g (considering that the tire friction coefficient was around 1.25 - as can be seen in the constant radius test max g) and the test worked out quite well in real life for such a high final g-level.

Obviously we encountered problems during the testing from which the most important ones were:

1) The obviously known one to find the "stable" final lateral acceleration value and not to "destroy" the tire before finding that point. We approached that point iteratively.

2) Whereas Yaw Gain and Lateral Acceleration Gain did not have a huge dispersion throughout the whole range of final lateral accelerations, the side slip angle proved to become at the highest g-levels in the transient phase quite peaky and more or less inconsistent. We blamed inconsistent tire temperatures - which changes the performance of the tire quite a lot - and the fact that as we all know dynamic slip angle measurements by itself tend to be quite problematic sometimes by themselves ....

So, in essence, considering all the boundaries the test in real life worked out better than we expected.

Now, in order to simulate the event with the linear bicycle step steer equations (Laplace Transformation) most of the results matched quite well when changing the total axle cornering stiffness from the value of "straight line running" towards the approximate "final" total axle cornering stiffness at the steady state lateral g level. As can be seen on page 7 and 8, Yaw Gain and Lateral Acceleration behavior matched pretty good. Vehicle Side Slip Angle behavior was as already mentioned in the real life test particularly "dynamic" so not finding a perfect match with a simple bicycle model was not a great surprise. But also here, considering the "elementary" state of the model and the complexity of the real world car, the results were surprisingly good.

For those who are interested in learning more on the effect of compliances on transient response functions, one can download on http://www.dynatune-xl.com a free demo version that allows to do all these linear type analysis including the possibility to investigate the effect of suspension compliance on the transient behavior and delay times. By entering a specific SWA one can also activate the above described procedure of venturing into the "non-linear" behavior.

Cheers,
dynatune

Greg Locock
233
Joined: 30 Jun 2012, 00:48

Re: Why is compliance so important?

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Thanks for that I'll have a read.

here's what i did on linear range stuff

http://greglocock.webs.com/vehicledynamics.htm

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