I finally took the M3 to the track today.

under high speed cornering while accelerating I notice the rear of the car will "twitch" from side to side.

to clarify the rear will rock from 1 side to the other.

I'm not sure if spring rates would change this at all. Maybe your rear bushings are worn?

What are the modifications done to your suspension? Which spring rates do you have in now? What bushings, mounts, etc did you replace (or not replace)?

What tires, etc... Need some info. Does not sound like its a prblem with optimizing your spring rates.

GC c/o 375/450lb

new RATB installed few thousand miles ago. GC shims.

kumho MX street tires.

the reason I suspect that I don't have enough spring rate in back is that I had the same problem on my mustang. Although they are apples and oranges in suspension layout.

jack your car up and look at the rear springs. If you are experiencing coil bind then it would be easy to tell by looking at the spring itself, where the coils would touch one another if it was compressed enough. I am not sure if that would cause your problem, but your spring is not stiff enough for your application if this was happening.

Originally posted by fast4d
GC c/o 375/450lb

new RATB installed few thousand miles ago. GC shims.

kumho MX street tires.

I really don't know, but I was under the impression that a symtom of bad RTABs is when the car seems to rock side-to-side from the rear end.

I would suggest you lift the car and verify that the RTABs were installed correctly, in addition to inspecting the rest of the rear components.

Originally posted by RacermeX
I really don't know, but I was under the impression that a symtom of bad RTABs is when the car seems to rock side-to-side from the rear end.

I would suggest you lift the car and verify that the RTABs were installed correctly, in addition to inspecting the rest of the rear components.

no. this is a rocking motion. like a baby's cradle.

bad RTABs will make the rear unstable under cornering. different feeling. I know because I experienced it before I replaced the bushings.

Sorry OT, did you install the RTABs yourself?

yes. with the rented tool.

I had some coil bind issues due to incorrect bumpstops. I think the rears are bouncing off the bumpstops now instead of coilbind. this leads to believe the spring rates are too low.

Originally posted by fast4d
no. this is a rocking motion. like a baby's cradle.
This sounds like you have a bad shock or not enough damping for your springs and/or the ARB is way too strong for the setup. What dampers and ARB are you using and what settings?

Bob

they are the koni SA (yellow) ground control sells with their coilovers.

stock roll bars/bushings.

Call the Factory. As in call Ground Control and ask for Dale, thats me. No street car has an optimum set up. The optimum set up is far more aggressive than you have. Most guys start around 500 to 550 lb springs. You have a relatively soft set up, so you should go to a 7" 500lb rear and a 6" 450lb front. A little more harsh on the street, but better at the track. You can e-mail me at ddizzy21@hotmail.com, because I will be out of the office today.
Springs and Dampers do not create rocking motions even when they are not sufficient for your car. Something else is wrong. I ran your set up for months on my M3 and I go to the track as much as anybody. The spring you have should be easily controlled by your rear shocks. What shocks do you have? What length spring?

Originally posted by fast4d
yes. with the rented tool.

I had some coil bind issues due to incorrect bumpstops. I think the rears are bouncing off the bumpstops now instead of coilbind. this leads to believe the spring rates are too low.

OT again, sorry man.
Where in LA did you rent the tool?

it was rented by someone else. I borrowed it.

I forget the name it's done by internet/mail so it's not local.

Add rebound in the rear.