front RC lower or higher than rear?

[spaxtracks]

Hello

I am designing a suspension for a light 240kg race car, I know that for most, the roll centre height is usually lower at the front than it is at the rear, but never really understood why. If someone could explain why can I not design them the other way round, that'd be great.

Thanks

 

[BillyShope]

Don't see why you couldn't. There's always the jacking problem with an independent suspension, of course, but I assume both roll centers are low and it just turns out that the front is higher than the rear.

As an aside, this brings to mind a story from a lecturer while I was in school. Seems Mercedes built a test vehicle with the roll axis above the CG. The car cornered like a speedboat! The translation of the test driver's comments came out something like this: "The car's handling gave me a feeling of extreme confidence...right up until the time I went off the road."

 

[GregLocock]

Yes, the active suspension guys commented that speedboat turns were the most confidence inspiring. That car spent a lot of time in the ditch!



Cheers

Greg Locock

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

 

[Forspeedrs]

I believe, keeping the RRC higher than the FRC will lend to an over steer condition, where the high FRC/low RRC will cause under steer. A flat roll axis = neutral steer.

 

[GregLocock]

That is pretty dangerous advice in general. Can you explain your thought process? Are you talking about linear range understeer/oversteer, or what?

Cheers

Greg Locock

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

 

[Yawing]

theres a WHOLE lot more to the vehicle balance than roll axis.

I think the higher RRC trend comes from cars that are front heavy and RWD. this usually means front heave rate of is higher than the rear for heave natural frequencies to be similar, yet this means if everything else is the same (RC height, ARB rates) it could tend to give U/S. So a higher RRC is a means to achieve a balance, without going up on RARB rates.

just a theory.

 

[Knap]

The equations we use in the WTW (since 1999) are the same as used by Claude Rouelle.

Geometric Weight Transfer - due to the component of lateral force, applied directly at the Roll Centre (RC). Geometric WT is reacted directly through the suspension linkages, and does not induce body roll.

It is clear that low roll centre give little geometric wt and most of the weight transfer goes through the springs (elastic wt), and is therefore delayed by the time it takes for the vehicle to take a set. Conversely, with high roll centre most of the weight transfer precedes the body roll, leaving a smaller amount of weight transfer to go through the springs.

The location of roll centre heights and the affect on geometric weight transfer vs elastic weight transfer is of importance in the set up of the car. Geometric weight transfer is a major influencer for cars of high front weight percentage and/or for FWD. Also for RWD with live rear axle. Also for current open wheelers with high downforce and little suspension movement.

In current open wheeler racing, geometric w.t. can be used because of the reduction in jacking affect: small suspension travel, wide track, long suspension arms to stop the RC height moving around so much relative to the chassis ie you don't get "progressive" jacking as the car rolls more. In fact, you need the geometric w.t. to help reduce the roll angle and suspension travel, while using less rear anti-roll bar, sometimes none at all.
From

http://www.racing-car-technology.com.au/wgttransex.htm