SUSPENSION COMPLIANCE 1

[bkndesign]

i am designing a rear suspension on a race car, but if i get the anti squat i want and other parameters i think will be helpful, i get a couple of milimiters of link error (compliance). i would like to know if anybody has some experience in using rubber or anyother elastic bushing instead of rod end bearings, and what was the result

 

[GregLocock]

Every (I think) production car uses elastic bushings in the suspension.

However, I really don't advise using them to circumvent kinematic errors, in fact the compliance effect will alter the antidive and rch that you have so carefully calculated.

If there is a really good reason for not being able to sort out the kinematics, then put one rubber bush in the arm furthest from the tie rod link.

If you want to take it further please post a sketch of your suspension.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[bkndesign]

thank you greg
following your advise, i kept working and i changed from 4 links and a mumford to 3 links and a mumford. now i have 0 compliance error, and i have 50% static anti squat, and 1.5 mm of toe-in with 2.5 degrees of roll because we look for roll understeer. does that sound OK?

 

[GregLocock]

Sorry I thought you were talking about a multilink IRS.

That sounds like far too much bump steer to me, it'll make the back end very darty on rough surfaces. That does depend on wheelrates admittedly.





Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

I think Nissan once promoted a similar feature as passive 4 wheel steer (they did it in the early 90s on the Bluebird). It was supposed to induce under steer on roll. In fact it induced instability on bump, especially on bump mid corner, which was particularly unnerving if you hit a bump mid corner where you wanted to tighten up by inducing a bit of over steer and the bump induced more under steer and sent you heading for the wrong side of the road or for the shoulder. Greg will understand when I say it was a rent a car that I picked up in Adelaide and returned in Melbourne and that I gave a workout along the Great Ocean Road. Bump steer was NOT a nice feature in that environment.

Regards
Pat
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[BrianPetersen]

A lot of rear suspensions have designed-in roll understeer, but 1.5mm toe-in at only 2.5 degrees of roll sounds excessive. Back-of-notepad calculation suggests that this is going to happen with only about 30mm of suspension movement (note; I don't know your track width, I took a guess). How much effect this is going to have, will also depend on the sidewalls.

Maybe design the chassis attachment points of the trailing arms with multiple holes at different heights, so you can experiment with how much roll understeer a.k.a. bump-steer you have.

I've only designed and built one suspension system, and that was on a home-built trailer years ago. In view of foreseeable bad side effects of both toe-in or toe-out on roll, I designed it with zero bump steer at nominal ride height (with leaf springs, this is determined by the heights of the attachment points). Works fine to this day.

 

[GregLocock]

Oh, a little bit of rear bump steer is ok, but of the order of 2 degrees/m of wheeltravel, ie in reality just enough to make sure it never goes the other way.

You do have the natural advantage with a mumford/3link setup that you will have compliance understeer.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[NormPeterson]

Stated another way, 1.5 mm of axle steer at 2.5° roll is about 5% (I think my quickie math is OK on this).

For a couple of reference points, the current Mustang (3-link with PHB) is around 2% with the driver on board, and the late 70's/80's US domestic GM metric-chassis intermediates are somewhere up around 8% or more. These cars are approximately the same size and weight, but on similar width tires and same-width wheels (though different diameters) the Mustang is by far the more nimble and easy to drive hard.

Getting rid of the overconstraint was a good move. Some people claim benefits from adding a separate lateral location device when all of the other links use compliant bushings, but even then the behavior can get "iffy" up around the limit.

Is this a Fox-body?


Norm