roll center and suspension geometry 1

[bigblock69]

I'm designing a kit car and trying to determine the best way to achive the goal of a vehicle that is lively and responsive, but not treacherous. I've got a computer program that can track roll center through various combinations of roll, and changes in chassis height, but wonder if it's really that meaningful. Going back to basics, I know if the car were on a frictionless surface, there would be no roll. As the amount of friction available to the tires changes, there would be more or less roll. The change in roll would not be instantaneous and would depend on the shock settings. It seems to me that roll is caused by tires pushing or pulling back against the suspension at the hubs, reacting against the sideways force pushing or pulling them through the suspension. Is roll center position an indicator of anything?

Thanks,

Pete

 

[GregLocock]

Is roll center position an indicator of anything?
yes. If you use the search engine you'll find several threads on RCH, some of which will discuss RCH migration.


Remember that RCH is 'fast' roll stiffness, so you don't want to go mad with it.






Cheers

Greg Locock

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

 

[carnage1]

as for old threads I saved this post and repost it occasionally because it gives a nice intro to roll centers.

blackbirdblue (Automotive) Jun 10, 2002
Roll Centre (excuse my UK spelling) has to be one of the biggest areas of confusion in suspension design that there is.

First, it isn't any sort of centre of motion, it's really a force centre. Second, for an independent suspension you can't arbitrarily combine the characteristics of both sides of the vehicle at the centre line.

The reason it matters is that it determines what proportion of suspension forces are transmitted via a "fast" mechanical route and what proportion are transmitted by a "slow" suspension spring/anti-roll bar/damper route.

The only things that matter in vehicle dynamics are forces on the tyres. A high roll force centre gives more load via the "fast" route and less via the "slow" route. Thus for a typical vehicle with higher rear roll force centre than front, the rear tyres load up faster during turn-in. This helps reduce the phasing between yaw and lateral acceleration and is generally A Good Thing.

If the roll force centre is below ground it means the suspension is "pro-roll" - the roll moment carried by the
sprung elements is greater than the inertial moment one might calculate using CG height and lateral acceleration. Motorcycle front suspension forks are similar in pitch.

In fact, the whole subject is better approached using the "anti-dive" logic applied to pitch motions rather than all this roll centre voodoo. Track down any half decent vehicle dynamics book and look at the anti-dive definitions then imagine them applied to roll.

One thing that isn't so obvious is that limit behaviour is helped by a low roll force centre and so some race cars have a rear roll force centre that plunges from above to below the front one to aid both turn-in and limit behaviour.
So you might find that lifting the rear roll force centre on your friend's car makes it technically faster but much more scary to drive and hence he'll return slower lap times.

As for camber, it has a lot to do with tyre wear but is really quite a small modifier on fundamental vehicle dynamics. There are a lot of people who will dispute that statement but none of them use any coherent maths to do it, only some very flawed reasoning.

In summary, think of roll-centres like anti-dive, reject any attempts to turn it into voodoo and find a good vehicle dynamics book. And set your static camber to maximise tyre life with the camber change characteristics you have, use tyre temperatures to predict tyre life without actually wearing them out.



Blackbirdblue
"A Honda Blackbird - in Blue - is my company car."

 

[bigblock69]

Thanks carnege1. That was extremely helpful.

Best,

Pete