How low before worrying about Roll Centre Correction.

[Burnsy]

I've been collecting various bits to set the suspension up better on my 242GT.

So far it is stock and worn appart from low king springs. I have an IPD Adjustable Panhard Rod, Torque Rods, Poly Bushes for the front and Koni Dampers to go on and was wondering at what ride hight I should be thinking about geometry issues and roll centre correction on the front?

Also is there enough clearance between the lower control arm mounts and tie rod ends to actually fit spaces when running 15" wheels?

[blkaplan]

You can benefit from roll correction even with lowering the car a minimal amount of 1" from stock.

[NM240]

I run negative roll center on my autocross car with no problems. In fact F1 cars run a little negative roll center. If your springs and shocks are dialed in and you do not bottom out your suspension then you should be ok. Bump steer and alignment changes at compressed suspesion may be an issue, but then again they always are.

[John2x240]

Ideally (I say that a lot don't I? ... Oh well) whatever adjustments you make on a Mac. strut, the lower control arm should be as close as possible to level with the ground with the car on level ground. To get this you may have to raise the upper strut mount, depending on your ride height.

[klr142]

Originally Posted by John2x240

To get this you may have to raise the upper strut mount, depending on your ride height.

That will change the ride height of the car(and as such, the roll center), but will not improve roll center once a car is lowered. It will only raise the car back up.

[Karl Buchka]

Originally Posted by NM240

In fact F1 cars run a little negative roll center.

What's 'negative roll center'? And LOL at comparing anything on an F1 car to a Volvo 240.

[Homer]

I can only assume he means below ground

[John2x240]

Originally Posted by John2x240

Ideally (I say that a lot don't I? ... Oh well) whatever adjustments you make on a Mac. strut, the lower control arm should be as close as possible to level with the ground with the car on level ground. To get this you may have to raise the upper strut mount, depending on your ride height.

Originally Posted by klr142

That will change the ride height of the car(and as such, the roll center), but will not improve roll center once a car is lowered. It will only raise the car back up.

No, but to be a bit more specific, with the LCA parallel to the ground and the body lowered, the upper strut mount must be raised the same distance as the car was lowered, or the strut rod has to be shortened. On my FB RX7, when I added a 1 1/2" bump steer corrector I had to raise the upper strut mount 1 1/2" to keep the LCA parallel to the ground with the same ride height. I'll see if I can find a photo.

[klr142]

Sorry, I meant to say that it will lower the car(raising the strut mounting point). If you install roll correction spacers, of COURSE you have to either lower the car by raising the mounting point, or more normally, by using lowering springs or coil-overs. The whole point of geometry correctors is to CORRECT the geometry that you screwed up by lowering the car.

I don't understand what you are trying to say.

[John2x240]

Before I added the bump steer spacer on my RX7 I did a bit of research in one of the Carroll Smith books and the G-Force Engineering set up manual for first gen RX7s. In brief, their info was that, for Mac struts, if the LCA is not parallel to the ground (level) the static front roll centre will be either too high or too low depending on whether the LCA is angled up or down. Furthermore, if the LCA isn't parallel to the ground, the dynamic roll centre will move around up to foot or more depending on the degree of roll, which can induce snap oversteer, even on a wet track. I learned this when I was coming out of turn one at Mosport just after I had bought the RX7, and wound up into the wall half way down to turn two, and that's when I started looking for a fix (while we were rebuilding the front end of the car ).

[mikep]

Originally Posted by blkaplan

You can benefit from roll correction even with lowering the car a minimal amount of 1" from stock.

Yes.
On a 240, a 1" drop puts the RC at ground level, but more importantly, at that height it migrates all over the place, inboard, outboard, up, down in turns and jounce. Bad juju. Wonky handling.

Originally Posted by NM240

In fact F1 cars run a little negative roll center.

Originally Posted by Karl Buchka

What's 'negative roll center'? And LOL at comparing anything on an F1 car to a Volvo 240.

True.
BTW, in F1, aero is always moreimportant than roll center.
http://www.ten-tenths.com/forum/showthread.php?t=114518
Do a search for Formula 1 Roll Center to read more.

Originally Posted by John2x240

bump steer spacer

Different animal entirely. Undesired bump steer is the steering angles changing too much as ride height changes, like when you put spacers on a 240 without changing the tie rod height as well. On a Corolla or other car with integral steering arm on ball joint adapter, it keeps the height of the tie rod close as you add spacers, as long as the spacers are parallel top to bottom.
A good example of drastic bump steer is an older ford with unequal length twin I-beams and unequal tie rod lengths.
BTW, all cars have toe change with ride height change, usually toe in on compression to make them more stable under braking. My autocross car (that has not run in 6 years) has been changed for toe out in compression.

[mikep]

In this thread, they all seem to think that a low roll center makes it resist roll more.
http://www.motoiq.com/forum/afv/topi.../aft/1190.aspx

[mikep]

http://www.physicsforums.com/showthread.php?t=326355

[John2x240]

Originally Posted by mikep

Yes.
On a 240, a 1" drop puts the RC at ground level, but more importantly, at that height it migrates all over the place, inboard, outboard, up, down in turns and jounce. Bad juju. Wonky handling. ...

... Different animal entirely. Undesired bump steer is the steering angles changing too much as ride height changes, like when you put spacers on a 240 without changing the tie rod height as well. On a Corolla or other car with integral steering arm on ball joint adapter, it keeps the height of the tie rod close as you add spacers, as long as the spacers are parallel top to bottom.
A good example of drastic bump steer is an older ford with unequal length twin I-beams and unequal tie rod lengths.
BTW, all cars have toe change with ride height change, usually toe in on compression to make them more stable under braking. My autocross car (that has not run in 6 years) has been changed for toe out in compression.

Well, being more the driver than set-up guru, which is why I turn to the experts for advice, I dug out my G-Force RX7 set-up manual and they call the spacer I have a Turn-in Spacer not bump steer. When my set-up guy set all the settings after the re-build, with the LCA parallel to the ground the ride height to the ball joint was 5 1/4" and the tie rod was also level to the ground, so maybe we're actually on the same track.

I guess I should mention here that the spacer in my car goes between the bottom of the strut and the steering arm which allows the tie rod to remain level to the ground.

[mikep]

Originally Posted by John2x240

Well, being more the driver than set-up guru, which is why I turn to the experts for advice, I dug out my G-Force RX7 set-up manual and they call the spacer I have a Turn-in Spacer not bump steer. When my set-up guy set all the settings after the re-build, with the LCA parallel to the ground the ride height to the ball joint was 5 1/4" and the tie rod was also level to the ground, so maybe we're actually on the same track.

I guess I should mention here that the spacer in my car goes between the bottom of the strut and the steering arm which allows the tie rod to remain level to the ground.

Right, like the Corolla I described. I only owned a 1983 RX7, so I didn't know if yours was the same.

[John2x240]

Mine's an '83 too, and as soon as I can restore my racing budget I hope to get it back on the track.

[quaylude]

Originally Posted by mikep

In this thread, they all seem to think that a low roll center makes it resist roll more.
http://www.motoiq.com/forum/afv/topi.../aft/1190.aspx

Roll center is the pivot point of the lateral g force when cornering that makes your car lean to the opposite side a lower roll center will make your car lean more, a higher roll center, there is less leverage therefore less body roll. A high roll center makes the car handle as if it had huge swaybars on it.

[mikep]

Which is why I said "seem to think". I guess I should have said "(incorrectly)" to clarify.
Also, there are valid reasons for keeping it low in some situations, none relating to Volvos and other cars with a live axle in the rear.