progressive or regressive wheel rate?

[romkasponka]

When is useful one and other?
Thanks.

[romkasponka]

When is useful one and other?
Thanks.

[jack]

how about...linear?

[romkasponka]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by jack:
how about...linear? </div></BLOCKQUOTE>
linear is simple http://fsae.com/groupee_common/emoticons/icon_smile.gif

I jusy have read what it could be used with high downforce pack in formula cars when you have wheel bump (when you have large downforce)and in rally cars when after jump you have big wheel drop, and you can increase wheel rate.
But how much and what characteristic it should have? Did you saw any system like this?

[adrial]

Are you asking in regard to FSAE application or what?

[romkasponka]

Yes and in formula racing too.

[Denny Trimble]

One of the problems with nonlinear wheel rates is what happens in roll and pitch. If you have rising rates, then on corner entry you will be in the "stiff" part of the front suspension and the "soft" part of the rear suspension (due to pitch), which means understeer. If you're on corner exit, you will be in the "soft" part of the front suspension, and "stiff" part of the rear suspension ( <STRIKE>under</STRIKE> oversteer) (edited - beer on the brain, thanks for the catch Rob!).

You can use a 3rd spring if you really want rising rates in bump, but usually cars with heavy aero are the only ones who use this.

[Rob86]

" If you're on corner exit, you will be in the "soft" part of the front suspension, and "stiff" part of the rear suspension (understeer)."


errr... Oversteer yeah?

[Schumi_Jr]

[QUOTE]Originally posted by Denny Trimble:
One of the problems with nonlinear wheel rates is what happens in roll and pitch. If you have rising rates, then on corner entry you will be in the "stiff" part of the front suspension and the "soft" part of the rear suspension (due to pitch), which means understeer. If you're on corner exit, you will be in the "soft" part of the front suspension, and "stiff" part of the rear suspension (understeer).
QUOTE]

Exactly, but what if that condition is desirable for your particular application? You can achieve different u/s and o/s conditions in different parts of the corner. For example take a neutral-steer car with a 40/60 weight distribution. On a high speed course you may want an understeer state on corner entry to promote driver confidence (the first 1/3 of the corner makes or brakes the last 2/3). In mid-corner the car should be be neutral (see above). The car may inherently under steer on corner exit(rearward weight transfer, not too much power, aero pitch sensitivity) but the rising-rate rear rocker will help alleviate this.

There are tons of factor that determine the dynamics of a racecar. If you have a good understanding of the vehicle and driver as a system then you might find a non-linear response is desirable... I know of a highly succesful IRL team that uses a falling rate MR on their outside front suspension. Every rule-of-thumb you read says that falling-rates are a big no-no, especially for aero cars, but this team has no problem winning. I'm basically saying that if you REALY understand the problem then you might find some unconventional solution works best...

[Denny Trimble]

Aaron,
You're right, if that's what you want you can tune with it. It would be a good development project for the summer, if a team really had its stuff together. Apparently you guys do!

Still smarting about that 0.422 point lead you had on us... http://fsae.com/groupee_common/emoticons/icon_wink.gif

[romkasponka]

Do you have pictures of working systems?

[Z]

Romkasponka,

This is a big subject. Denny and Aaron just touched on some of it.

Generally, a rising rate is good for absorbing single wheel bumps, and for controlling widely varying aero downloads. This is why many people use rockers, because it is then easy to provide an aggressive rising rate with the rocker geometry, as on the rear of MotoX bikes. But along with Denny and Aaron's comments regarding handling balance, there is another disadvantage of rising rate "corner" springs (ie. the "coilovers" at each wheel). During horizontal accelerations (acc/brk/cornering) the car body will pitch and/or roll. Rising rate springs will mean that the end/side of the body going downwards will only move a little, while the end/side going upwards will move a lot. This increases CG height which is generally bad, especially during cornering! Falling rate springs will pull the body down during pitch and roll. This can be a good thing because of lower CG height, but a bad thing for single wheel bumps or aero loads... So most corner springs end up being close to linear rate.

If you use interconnected springing, such as a "3rd spring" or "monoshock" that controls two wheels in their "axle bounce mode", then you can give it a rising rate without effecting cornering behaviour (which is controlled by the "axle roll mode" spring). However, you still get CG rise during accelerating/braking...

If you use four wheel interconnected springing then you have much more freedom with rising or falling rates. However, hardly anyone does this, mainly due to a lack of understanding of the benefits available. As I said, this is a big subject...

Z

[romkasponka]

I think, i will make on the rocker a dozen damper mounting holes to have wide adjusment of rising wheel rate. http://fsae.com/groupee_common/emoticons/icon_smile.gif

[clausen]

I think almost everyone in FSAE would be best off with a linear wheel rate. Chassis dynamics are so complicated, interconnected and full of compromises that I think the best place to start is to try to seperate as many variables as possible. If you've got options and one of them is to make a given variable stay constant then DO IT! until you're sure that every other variable on the car is tuned to perfection (which would mean 1000's of hours of fsae testing with an excellent driver). Especially when the variable is such a critical one as wheel rate.