Roll Centre measurement and adjustment

[Steve]

Hi all,

Just wondering if anybody knows how to determine the roll centre of a car?

Also what solutions are available if they need adjustment.

Specifically for an S13, I am having trouble getting under the car to check the geometry with any great accuracy, any help would be appreciated

cheers

Steve

[blind_elk]

Roll centre calculation depends on the type / design / geometry of the suspension, eg macpherson struts are calculated totally differently to semi-trailing arms.

I think it's covered in a book by Fred Puhl (maybe Puhn). Worth owning. Check out Pitstop Bookshop in Perth (pitstop.net.au)

[Sydneykid]

Hi all,

Just wondering if anybody knows how to determine the roll centre of a car?  

Also what solutions are available if they need adjustment.

Specifically for an S13, I am having trouble getting under the car to check the geometry with any great accuracy, any help would be appreciated

cheers

Steve

Hi Steve, very hard to describe using words. Best to look for diagrams. Simply put;

S13's have MacPherson struts at the front, so the adjustment for roll centre is the inner lower bush verticle locatiion. Noting that the angle of the lower control arm needs to be maintained, so any adjustments at the inner bush must be matched by adjustments at the upright.

The rear is bascially double wishbone so the roll centre is determined by the verticle location of the inner upper and inner lower bushes of trhe control arms. As with the front, the angle of both control arms need to be maintained, so any adjustments at the inner bush must be matched by adjustments at the upright.

I have found very little issue with the S13/14/15 chassis as far as roll centre is concerned, but you can get problems when they are excessively lowered. This reduces the roll couple (difference between roll centre and centre of gravity) and they tend to hop (pig root is the term most often used) rather than slide progressively. The Japanese solution (as always) is to stick huge spring rates in so that there is practically zero roll. Then the roll centre becomes irrelevant because the only roll is tyre compression.

[Steve]

Thanks for that, I do think my car may be a bit low and was hoping to be able to check if it needs attention. I have looked at a couple of diagrams that describe how roll centre is determined - but climbing under my car, I dont know how I am going to take the measurements as described

Any ideas?

Also, is there a recomended bias between front and rear roll centre?

My S13 is quite low, and does hop a little on hard corners, but not when sliding.

The japs acutally make roll centre adjustable arms for the front, Nams and JIC are two that come to mind.

They only seem to make them for the front on silvias.

Thanks for your help

[djr81]

The differential between the front & rear roll centres will go a long way to determining how the car handles, ie a higher roll couple will give more grip for that particular axle. Most people dial this out by using adjustable anti roll bars, rather than adjusting roll centres. Put simply a stiffer anti roll bar gives less grip.

[Sydneykid]

Thanks for that, I do think my car may be a bit low and was hoping to be able to check if it needs attention.  I have looked at a couple of diagrams that describe how roll centre is determined - but climbing under my car, I dont know how I am going to take the measurements as described  

Any ideas?

Also, is there a recomended bias between front and rear roll centre?

My S13 is quite low, and does hop a little on hard corners, but not when sliding.

The japs acutally make roll centre adjustable arms for the front, Nams and JIC are two that come to mind.

They only seem to make them for the front on silvias.

Thanks for your help

Hi Steve, suggestions follow;

Roll centre projection is best done via a drawing, you won't be able to do it "on the car". You will have to take the measurements on the car and then draw it to scale. Change the drawing until you achieve the roll centre you want, then transfer that to metal work on the car.

Usually, in a front engine rear drive car, you would have a higher rear roll centre than than front, because the CoG is higher in the rear. It is the roll couple that is important.

Yep, hopping is a sign of too low a rear roll centre, thus the roll couple is too big and you get the amplified pendulum effect. Once the wheels are spinning you have basically zero grip, the problem is when you are setting up for the drift, while you have some rear traction. You get the pig rooting and you can't pick or control the slip angles. It looks like poor technique, but it is really chassis set up.

The front roll centre adjustment is more necessary because of the shortcomings of the MacPherson strut design. It is very sensitive to excessive lowering and the Japs love to have them looooooow.

Somewhere at home I have the drawings for a MacPherson strut roll centre on a Datto, if I can find it I will post it up.

PS; If you do roll centre adjustments on the front, you will also need to do bump steer correction. Because of the steering rack location, moving the lower inner mounting point really stuffs up the bump steer.

[Steve]

Thanks for the replies, couple more questions:

What is the roll couple??

(edit:) how do you correct for bump steer? dampening and wheel alignment? suggestions?

Also, for cornering the front seems to be looser than the rear, ie, it jumps around more - is this because I have more of a problem up front?

Cheers again

[Sydneykid]

Thanks for the replies, couple more questions:

1. What is the roll couple??

2. how do you correct for bump steer? dampening and wheel alignment?  suggestions?

3. Also, for cornering the front seems to be looser than the rear, ie, it jumps around more - is this because I have more of a problem up front?

Cheers again

I think this previous post answers question #1.......

have found very little issue with the S13/14/15 chassis as far as roll centre is concerned, but you can get problems when they are excessively lowered. This reduces the roll couple (difference between roll centre and centre of gravity) and they tend to hop

#2. Bump steer is a change in toe settings as the suspension moves up and down, such as over bumps, hence the term "bump steer". To correct bump steer on a MacPherson Strut car you have to change the relationship between the lower control arms and the steering arms. Usually by inserting a spacer between the steering arm and the upright (ie; at the steering ball joint). You can also remove bump steer by lowering (or raising) the steering rack, but this causes problems with location for the steering shaft. So most people do it at the steering arm ball joint.

#3. This can be caused by too small a roll couple, bump steer or excessive dynamic camber change. If by raising the front you improve the front end grip, then it is going to be one or more of these three things. If raising it doesn't improve the front end grip then it is a spring, shock, front stabiliser bar or caster problem. Note that you should always reset the static camber on a MacPherson strut car when you change the height.

Simple stuff, as I do it everytime we build a new race car:cheers:

[dangerous_daveo]

Easy way to tell if they car is too low. Look at all the arms, if they are flat then its too bloody low.

Also quick question, if my front is higher than my rear, what effect either positive or negitive (pardon the semi pun) would this have?

[Steve]

I think this previous post answers question #1.......

:Oops: sorry.

Yep, hopping is a sign of too low a rear roll centre, thus the roll couple is too big

This can be caused by too small a roll couple

Sorry, but now I am confused, the second quote was a response to my asking why my front is loose and tends to hop. So to clarify, does this mean that too big a roll couple on the rear has the same effect as too small a roll couple on the front - or if a roll couple is too big or too small it has a similar effect?

Cheers

I am trying to get the car as low as possible (for lower CG) but I dont want to sacrifice handling - hopefully I can find a work around if my car is 'too low'. Might have to investigate these jap roll centre adjustors. R&D can be costly...

[Sydneykid]

:Oops: sorry.

Sorry, but now I am confused, the second quote was a response to my asking why my front is loose and tends to hop.  So to clarify, does this mean that too big a roll couple on the rear has the same effect as too small a roll couple on the front - or if a roll couple is too big or too small it has a similar effect?

Cheers

I am trying to get the car as low as possible (for lower CG) but I dont want to sacrifice handling - hopefully I can find a work around if my car is 'too low'.  Might have to investigate these jap roll centre adjustors.  R&D can be costly...

Hi Steve, yep too large a rear roll couple has the same efect on the front as too small a front roll couple. Same logic as stabiliser bars, too big a rear bar makes the car oversteer same as too small a front bar.

Lowering the car too much has 10 dissadvantages and only 1 advantage, the lower CoG has stuff all effect on handling compared to the other 10. The reality is lowering the car is done for looks, not handling. We did a test for a magazine about 3 years ago on 4 X GTST's and the highest one was the fastest one around the circuit. The lowest one, was the slowest one.

Keep the height reasonable and spend the saved money on better tyres or bigger wheels or more power, the car will be faster, more sideways and much easier to control.

[agk]

Hi guys,

I dropped in here to see what i could learn. I am in the process of building a lotus 7 type clubman from the ground up and plan on using a skyline rear end and possibly front uprights and brakes also.

I have been doing heaps of research into suspension design recently and can perhaps offer some comments here.

Different roll centre front to back is not good.

An analogy I was give was if you get a shoe box and a long rod. Insert the rod at one end down low and out the other towards the top of the box, this is effectively you roll centre axis, rotate the box from side to side to mimic body roll of the car and you will see what happens to each of the 4 corners (tyres). Depending on your suspension set up one can not always get these points at the same height but from what I have read it is desirable.

If you have say a live axle in the rear and independent in the front the rear will almost always be higher there is little you can do about it.

I have also tried to include a pic which should show you the way to identify rc for mac pherson strut. Hope it works, ok no idea any tips on how to post an image??

Cheers

Andrew

[Steve]

Thanks again SK, could you possibly post up some of the dissadvantages to lowering a car? Also, why do they lower touring cars, F1, etc as much as they can before it starts scrubbing?

Hypothetically, if you can remove all the associated downsides of lowering a car, wouldnt it then be an advantage to have it lower????

agk, cheers, makes sense. I think you can use panhard rods to lower roll centre on live axle, not a huge difference from the research I have done though.

To insert an image, you must host it on a site first.

[djr81]

Hi guys,

I dropped in here to see what i could learn.  I am in the process of building a lotus 7 type clubman from the ground up and plan on using a skyline rear end and possibly front uprights and brakes also.

I have been doing heaps of research into suspension design recently and can perhaps offer some comments here.

Different roll centre front to back is not good.

An analogy I was give was if you get a shoe box and a long rod.  Insert the rod at one end down low and out the other towards the top of the box, this is effectively you roll centre axis, rotate the box from side to side to mimic body roll of the car and you will see what happens to each of the 4 corners (tyres).  Depending on your suspension set up one can not always get these points at the same height but from what I have read it is desirable.

If you have say a live axle in the rear and independent in the front the rear will almost always be higher there is little you can do about it.

I have also tried to include a pic which should show you the way to identify rc for mac pherson strut. Hope it works, ok no idea any tips on how to post an image??

Cheers

Andrew

Andrew,

Please be careful. Not sure who told you that different roll centres front to back is not good, but they are wrong. An inclined roll axis (think of a line drawn along the length of the car between the front & rear roll centres) is one of the most powerful tools that a chassis tuner has. Almost all cars have these to help compensate for poor front to rear weight distribution.

By way of an example the rear roll centre on a Watts linkaged live axle car (eg my old Foulcan, or most Lotus 7 clones) is defined by where the linkage picks up on the diff housing. This is what the V8 cars get changed in the pits. Changing the roll centre inclines the roll axis either more or less - depending on what the driver needs.

Hope this makes sense.

[djr81]

Thanks again SK, could you possibly post up some of the dissadvantages to lowering a car?  Also, why do they lower touring cars, F1, etc as much as they can before it starts scrubbing?

Hypothetically, if you can remove all the associated downsides of lowering a car, wouldnt it then be an advantage to have it lower????

agk, cheers, makes sense.  I think you can use panhard rods to lower roll centre on live axle, not a huge difference from the research I have done though.

To insert an image, you must host it on a site first.

Steve,

You need to be a bit careful with motorsport analogies. They lower F1 & touring cars as much as they can for the aero benefits - mostly more downforce from the front wing/splitter.

A lower car has, by definition a lower centre of gravity. If you have free reign in suspension design eg an F1 car this is an excellent starting point. If you don't eg a road car then it is probably best to forget about the c of g & instead concentrate on the roll couple and thence the roll axis. These are the important parameters - the cog is only half the story.

Lowering MacPherson struts too far can result in a larger roll couple than prior to modification because of the way the geometry is worked out. I really need to dig out the diagram to explain it properly...

[Sydneykid]

Hi guys,

I dropped in here to see what i could learn.  I am in the process of building a lotus 7 type clubman from the ground up and plan on using a skyline rear end and possibly front uprights and brakes also.

I have been doing heaps of research into suspension design recently and can perhaps offer some comments here.

Different roll centre front to back is not good.

An analogy I was give was if you get a shoe box and a long rod.  Insert the rod at one end down low and out the other towards the top of the box, this is effectively you roll centre axis, rotate the box from side to side to mimic body roll of the car and you will see what happens to each of the 4 corners (tyres).  Depending on your suspension set up one can not always get these points at the same height but from what I have read it is desirable.

If you have say a live axle in the rear and independent in the front the rear will almost always be higher there is little you can do about it.

I have also tried to include a pic which should show you the way to identify rc for mac pherson strut. Hope it works, ok no idea any tips on how to post an image??

Cheers

Andrew

Hi Andrew, interesting analogy, but a shoe box has a CoG that is the same as its geometric centre. This not the case with a car, it has different mass locations along it axis. It would be technically more correct to say that is balanced if the roll couple is the same front and rear ie; the roll centre and CoG at the front is the same distance apart as the the roll centre and CoG at the rear.

[djr81]

There is a scan of a diagram of how to calculate roll centres for MacPherson struts now included in my photo album thing (On the left). It comes from a book by Jeffrey Daniels published by Motor Racing Publications. Unfortunately the book was published in 1988 & I haven't been able to find a copy anywhere.

Also added bit about an inclined roll axis.