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Author: Subject: Roll center vodoo
v8kid

posted on 30/9/08 at 02:48 PM Reply With Quote
Roll center vodoo

Came across this in Engtips when searching for info on speed of load change. Wish I was clever enough to have thought of it myself.

Think Syd has a convert!!

I don't really understand the " This helps reduce the phasing between yaw and lateral acceleration " bit although I have a feeling I should. Can anyone help out?

"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."

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Paul TigerB6

posted on 30/9/08 at 03:43 PM Reply With Quote
Until i come to design my own race car, i've gotta be honest i'll never even attempt to understand all that lot!! I'll still sleep tonight anyway.
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mr henderson

posted on 30/9/08 at 04:22 PM Reply With Quote
Using designs that have worked well in the past in similar applications seems like a better place to start than reading a bunch of books which seem to offer a wide range of different opinions.

John






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Ians

posted on 30/9/08 at 06:06 PM Reply With Quote
v8kid.....Just as i was saying during the July debate if you have an uderstanding of pitch moments (viewed from the side). you gain a better idea of RCs when looking from the front..ians
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mikeb

posted on 30/9/08 at 06:15 PM Reply With Quote
a long time ago I studied vehicle dynamics, one of the most annoying things was the wildly different terminology and different ways of expressing things, often leading to confusion. This guy has uses a term roll force centre. In essense he is right the roll centre is the point that if you applied the lateral force you would get zero roll.

What he is differientiating is load transfer from the sprung component and in essense the rigid body load transfer, one goes through springs and dampers etc the other is just reacted through the suspension (like anti dive).

As for phasing, I think lateral acceleration rises slightly before yaw rate so there is a phase difference. e.g the car generates lateral force and lateral acceleration before the car starts to turn around its centre.

In the end I gave up trying to be an expert. Most race engineers setup cars through trial and error and dont know all the theory they have learnt what works.

For my build I'm hoping I have enough accuracy in my chassis so I dont have bump steer or different geometry left to right!

[Edited on 30/9/08 by mikeb]

[Edited on 30/9/08 by mikeb]

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Chippy

posted on 30/9/08 at 10:36 PM Reply With Quote
I have designed and built several cars, (sports), in my time and have always used roll centres of 3" front and 4" rear. I actually think that is a good height that works. The main thing on handling, I have found, is that the S and L arms are designed in such a way that the outer wheel when cornering stays flat on the road, in any angle of body roll, giving maximum road contact, and grip, and that I find is harder to achieve than getting roll centres smack on. Cheers Ray





To make a car go faster, just add lightness. Colin Chapman - OR - fit a bigger engine. Chippy

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