C of G height/Roll ctr

[Paul Bearman]

Not sure if this is the relevant section to post this question, but here it is anyway.

As a novice on the subject, I have been doing some research into suspension geometry to try to understand how all the seperate components work together as a whole. One element I am not certain about is the relationship between Centre of Gravity Height & Roll Centre Height. I understand that a low Roll Ctr is preferable but how does this affect the C of G height? Would a high C of G height, say 16", affect a lower Roll Ctr height of say 30mm (1.18")?

 

[Lee]

Paul
That's complicated!
The car will roll about the roll centre. If you lower the roll centre, ie move it further away from the C of G, then all other things being equal, the car will roll more, because the roll moment is increased. So then fit stiffer springs to compensate which will alter the F/R weight transfer balance and hence under/oversteer etc, etc. And don't forget that the static roll centre is only at that point while the car is not rolling. As the car rolls, the roll centre moves.
Nightmare.
All good fun eh?

 

[Paul Bearman]

Why can't anything be simple!
So does this mean that CofG Height is proportional to Roll Ctr Height?

 

[Lee]

No, the CofG doesn't move. You can only change that by moving mass up or down, (or backwards or forwards). By moving the CofG down in this way, you reduce roll without changing any geometry. Hence trying to get engine etc as low as possible in the chassis, and the chassis a close as possible to the road (ideally this second exercise whilst raising the suspension pickup points by the same amount). This reduces the distance between the CofG and the roll centre and therefore the turning moment which produces the roll. In theory I believe you can prevent roll by designing the suspension to have its roll center at the same point as the CofG, but for some reason it doesn't work (I can't remember why). As you can tell I'm also a novice.
Using Allan Staniforths, Race and Rally car source book, I once went through the exercise of making scale suspension out of card and paperclips, and trying to get to grips with the roll centre movement. Drove me nuts after a while. This was before I had the 40. I haven't tried the same exercise with the 40 suspension.

Interesting to note that simply lowering the car can lower the roll centre more than the CofG, and therefore lead to an increased distance between CofG and roll centre, which leads to MORE not less roll.

Best of luck.

 

[Julian West]

I was told you can calculate the C of G using corner weight scales, somehow jacking one end or side of the car & measuring the change in reading on the scales.

However I think you need to be pretty smart with Trig to be able to do the calcs. I don't know how to do it myself but perhaps there is a more knowledgeable forum member that knows??

As for Roll Center, as far as I can remeber from reading Carrol Smiths books - VERY ROUGHLY! - on paper when viewed from either end of the car, you extend the lines of the wishbones & the contact patch of the tyre on either side & calculate it from the way the lines intersect. IIRC there is a whole chapter on this & it'll take more than a few lines to explain it properly!

/ubbthreads/images/graemlins/smile.gif

 

[Paul Bearman]

I'm ok with determining the Roll Ctr Height & Instantaneous Roll Ctr etc. What I would like to know is, if you lower the Roll Ctr height would it be beneficial to lower the CofG by the same amount?
The Lateral force going into a corner will act on the CofG making the car rotate about the Roll Ctr. I assume that by decreasing the CofG - Roll Ctr Height will cause the car to roll less? Or am I barking up the wrong tree?

 

[Lee]

Paul, lowering CofG in all pratical effect is beneficial.full stop.

If you leave the roll centre alone, while lowering Cof G you will get less roll.

If you lower CofG by 1" and lower centre of roll by 1", my uderstanding is the car will initially roll exactly the same, because total mass has not changed and distance between CofG and roll centre has not changed. If you move the initial roll centre, it will move differently to before so while rolling could be more or less depending on the change while rolling.

Sticking with the static situation, if you lower the roll centre by more than you lower the CofG, you will get MORE roll.

I think.

 

[Bob Putnam]

Roll will be roughly proportional to the difference in height between the CG and roll center, and it's actually possible to have negative roll with a very high roll center. There's a big problem with that effort though. A high roll center causes the outside tire to go laterally on jounce and rebound. It makes for a very unstable straight-line ride, and on cornering the car will be extremely twitchy if the road is less than perfect. The "easiest" car to drive has the roll centers low and relatively constant.

 

[Paul Bearman]

Thanks for all your replies, some interesting thoughts.
The reason I ask is that I have been toying with the idea of lowering the Roll Centre to 30mm at the rear & 10mm at the front. However all this is for the future as this will also involve some suspension mount modifications.
Some more thought & messing around with 'paper' CofG heights relative to roll Ctr heights is needed first.

 

[Malcolm]

I was under the impression you need the roll centre just above ground level but with a similar angle of attack as the ride height ie a bit lower at the front than the rear. No authority to back that statement up but heard it somewhere. To stiffen up, I would also point towards stiffer anti roll bars and not just stiffer springs. I went for just stiffer springs and it is fine in some circumstances ie track work but not so hot in others ie on bumpy roads.

Malcolm