Roll Centre & Camber

[Rusty]

I've been doing some more reading on suspension geometry and the roll centre of a Mc Strut design.

By lowering the car, I've lowered the roll centre a little due to the change in strut and wishbone angle. This is fairly uniform.

However, by inclining the rear struts to get more camber, I've lowered the roll centre more at the rear. This would explain the improved manners of the car on track last year, in combination with the camber increase at the rear.

The problem I have noticed is that the rear is now very inert, so the old back it in and power out driving style no longer applies.

I think that I actually need to reverse the top mount positions at the rear, to make the strut more upright. I can then use camber bolts to sort the camber out.

This should raise the rear roll centre a little (the strut will move by about 1 degree total).

I can then set the front top mounts to a neutral position and again use camber bolts to increase camber.

Ideally, I'd have 3 deg front, 2 rear, with the struts more upright.

Finally, some lower balljoint extenders would be a good plan, perhaps longer at the rear than the front, too.

Thoughts?

[Rusty]

Just been looking at the last alignment specs that I did prior to going a bit mental with the new alignment.

Front camber was 1.3 deg, now 2 deg, rear was 0.6 deg, now 2.5 deg! Castor was 2.5 deg now 4.2 deg.

The car used to be very lively on corner entry and all that was gone last time out, so that ties in. I'd like a bit of it back TBH, as you need a bit of help getting the thing in to the corner, so I think 1.5 deg rear is about right, with 2.5+ up front.

I can either use camber bolts up front or bring the top of the struts inboard, although I'm not 100% on that one..

[BosnianS2]

Front castor was 1.3 deg, now 2 deg, rear was 0.6 deg, now 2.5 deg! Castor was 2.5 deg now 4.2 deg.

You confused me a bit with this sentence above? Should one of those words be Camber instead of Caster??

[Rusty]

Yes, changed the thread.

[rynaqui]

How much have you lowered the car and is it on coilovers? What rollbars are you running? Perhaps fitting a smaller rear rollbar would make it feel more loose if that is what you are looking for?

I found that when I put a whiteline (stiffest setting) rear rollbar the steering became more direct and a bit more "boring" if that is the word. Though you do have the rear strut bar which also effects this somewhat, that's probably the biggest difference I've felt compared to my own car when I was in it last October. Unbelievable stuff that was

[Error404]

I think the relationship between front & rear roll centre is quite sensitive on the old coupe quattro.
Having the front axle roll centre lower than the rear means there is less proportion of the total load transfer on the front axle. All else being equal you have more grip to the front axle = less understeer
With your H&Rs and camber mounts you've probably dropped the rear roll centre more than the front, so the relationship has changed and the rear end isn't so loose. You could increase the rear roll stiffness some more, or increase the rear ride height to raise the rear RC. McStruts are a pain in the ***!

[BorisShakhov]

I've been doing some more reading on suspension geometry and the roll centre of a Mc Strut design.

By lowering the car, I've lowered the roll centre a little due to the change in strut and wishbone angle. This is fairly uniform.

However, by inclining the rear struts to get more camber, I've lowered the roll centre more at the rear. This would explain the improved manners of the car on track last year, in combination with the camber increase at the rear.

The problem I have noticed is that the rear is now very inert, so the old back it in and power out driving style no longer applies.

I think that I actually need to reverse the top mount positions at the rear, to make the strut more upright. I can then use camber bolts to sort the camber out.

This should raise the rear roll centre a little (the strut will move by about 1 degree total).

I can then set the front top mounts to a neutral position and again use camber bolts to increase camber.

Ideally, I'd have 3 deg front, 2 rear, with the struts more upright.

Finally, some lower balljoint extenders would be a good plan, perhaps longer at the rear than the front, too.

Thoughts?

It`s all a fine tuning. Strut inclination makes negligible change in a static RC position.
You should do some precise calcs (or better - more real tests) to get any benefits from RC position, especially dynamic RC and Roll Axis. I was playing this game for about 3 years now

As I can see in your post, you have made it more stable on corner exit? Is it bad? Put more torque!
And what is your target? What Lat G we are talking about?Or You want just to play with RC?

[BorisShakhov]

You could increase the rear roll stiffness some more

Car will be more unstable on entry, and could have snap oversteer because a roll bar needs roll to work but car has built-in understeer.

[Error404]

I think Andy wants the back end loose on corner entry. I would use stiffer springs rather than ARB if possible

[Rusty]

Yes, springs are something that need sorting at some point. That part of things hasn't changed, so I would like to get a little less stability out of it, as I had before.

[Error404]

maybe a bit of toe out on the rear?

[BorisShakhov]

Yes, springs are something that need sorting at some point. That part of things hasn't changed, so I would like to get a little less stability out of it, as I had before.

Springs would be nice, but I cant understand why Fr grip with small steering angle on the balanced car less than Rr grip with weight shift, torque and less slip angle. My guess that your front is not right. You should balance the car first on steady state then move to transient.

[BorisShakhov]

maybe a bit of toe out on the rear?

Tire pressure is better just for a try to understand where is a grip

[Rusty]

The extra rear camber has given good grip but there's not enough up front to balance it up. It's very stable, just not as adjustable as it used to be. Used to be able to get lift-off oversteer but now, it's much more stable. This has got to be mainly due to the 2.5 deg camber

Toe out at the rear is a good idea, I think I'll get some added in when I do the next setup. On the front, it's now 0 toe with 0.05 toe in at the rear, so again, that'll make the rear more stable than the front. It goes toe-in when the suspension compresses, so under power, it'll remain stable to a certain point.

I think I'll drop the rear camber to 1.5 - 1.75 and add some toe out and go for double that on the front (camber) with zero toe, as before. Castor will be 4.25-4.5 deg but I'm now not sure about tilting the struts inwards, as that'll tweak the roll centre down more. I may even go the other way and over-adjust the strut to hub bolts to raise the roll centre a mm or two.

[GazzaG60]

interesting reading but to be honest i dont really understand whats being said. i understand the basics around rollbars, shocks n springs but the rest is beyond me.

is there an idiots guide or an ideal setup even for a neutral car.

i found the change i made in top mounts shocks springs bushes and the whiteline rear helped the car feel more planted and steer better no end beyond that is a mystery.

my saloon is basically a coupe underneath. not sure if the shocks are angled different or not or other "things" maybe. it does feel better than a coupe though i know that. that was on stock stuff. when its back out its on the s2 stuff.

what would you suggest andy?