Roll Centre Height? 4

[cfinister]

A question from a friend of mine. 'Should the roll center of the 1969 Pontiac GTO
race car be above or below the ground ? currently 500mm below ground with
factory set-up, and under 75mm bump the front camber goes from -3deg to
+1deg'

In my opinion. Large offset between C of G and Roll Centre is bad.

Also High roll centre is bad but I'm not sure of reasons.

Is it weight transfer?. Also I've heard of 'jacking' but don't know what it is??

Can anyone help with general tips on setting rollcentres.

 

[blackbirdblue]

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."

 

[MarkZE]

Just wanted to say that is the best explaination of Roll Centre characteristics I have ever read. I have played with roll centres on a couple of my cars and have learned the results of the changes I made hence I had an intuitive understanding of what I had done and why it worked. Your explaination has given me a few more things to think about in a theoretical sense now.

With my cars I have always attempted to put the roll centre back where the manufacturer had it in the first place but lower to the ground by the amount that the car is lowered hence keeping the CofG and roll centre in tha same relative position. After lowering the suspension etc the roll centres tend to move lower by more than the car is lowered.

I have always been of the opinion that roll centres below the ground are bad. I have one car set up with the front roll centre at about 80mm and the rear at 120mm which seems to work okay with the macpherson strut suspension and rear wheel drive solid axle watts link rear end. The car is a toyota corolla though Australian model. When the front roll centre was below the ground I know that the car felt to me like it needed a bigger front swaybar as it was quite "divey" on corners. It tended to have the outer front corner of the car roll (more like dive) a lot giving a very uneasy feeling to the driver. Rasing the roll centre fixed this. Some of this uneasy feeling may also have to do with the roll centre moving of centre after the rolling process starts also but I am not sure. I just know it was better after I raised the static roll centre.

I have alway knowns the rear should be higher than the front but never known exactly why. The idea of allowing the rear roll centre to move to be below the front in the limit is something I had never thought of either. I will have to look better at where my roll centres go after the cars starts rolling ;-)

I have always wanted to build an adjustable height pivot for a watts link These are used on the Australian V8 Supercars along with adjustable swaybars. They used to (before it was banned) have a lever in the car which moved a cable to the watts link pivot. Now that have the lever in the trunk which can be adjusted at pitstops only.

 

[79BBC]

A quick note about Roll Centers from personal experience is to keep in mind that your roll center moves through the range of your suspension motion. There are quite a few dicussions, all with very good points, about whether it is better to be above or below ground. The one thing you DONT want is a roll center that is both. Make sure your dynamic roll center stays on the same side of the ground plane as your static. Crossing the ground plane will cause an unloading of the vehicle suspension making it very unstable in cornering. I recommend using either a CAD program or strings to determine your roll centers at both extremes of expected suspension travel. Hope this helps.