Seeking advice on suspension setup: IP/GTS3 E36 M3 w/ Advance Design setup

[vinnymac]

Here's some info on the car:

E36 M3
IP/GTS3 configuration
3058lbs w/driver + fuel
Ground Control Advance Design suspenion
700lb front 6" x 2.5" springs
800lb rear 5" x 2.25" springs
GC "big" front sway bar (a hair off full stiff)
Stock 99 E36 M3 rear bar
235/40/17 Toyo RA-1 on all corners on 8.5" wheels
2.9 front camber
2.2 rear camber
1/8" front toe out
1/32" rear toe in

The car is handling great with very neutral steering and overall feels pretty good under braking and initial turn-in. The only thing I don't like is the rear end feels like it's "bouncing" a bit from apex to track-out as I approach full throttle and start to unwind the wheel. It feels this way in left and right hand corners equally. I had someone else drive the car and received the same feedback.

For those of you familiar with ADs. I have the front bump (blue) knob set at "2" and I have front rebound (red) knob set at the mid-way point between top and bottom (I believe it was 1.5 turns from the bottom).

I have the rear bump (blue) know set at "1" and I have the rear rebound (red) knob set at the mid-way point between top and bottom (just like the front).

Can someone clarify the rebound adjustment and how it will affect the handling? Will turning the rebound (red) knob down, i.e. bring it closer to the shock body, introduce more rebound or less?

This is my first experience was adjustable rebound and I'm still trying to learn it's application.

I would greatly appreciate some informed advice and/or links to any information that may be helpful.

Thanks all!

[JamesM3M5]

Get the shocks dynoed, first,and make sure they run through the range of clicks to show you the changes in damping force both compression and rebound.

[B.Watts]

The only thing I don't like is the rear end feels like it's "bouncing" a bit from apex to track-out as I approach full throttle and start to unwind the wheel. It feels this way in left and right hand corners equally. I had someone else drive the car and received the same feedback.

Unfortunately, based on my own experience and the dynoes of a few other sets of AD's that I've seen, I don't know if you'll be able to get rid of your problem with just a turn of the knob. The adjustment range on AD's just doesn't seem to be large enough IMO.

That said, to get rid of the rear bouncing, you want to increase your rebound damping. IIRC, that would be done by turning the red knob clockwise, "screwing it in" to the damper...feel free to correct me though, as it's been a while. If that doesn't solve the problem, you might consider having GC revalve them with additional rebound damping.

[Hellabad]

"bouncing" almost always means the suspension is moving too fast. Of course, "too fast" is a relative term, so it may be moving too fast, too fast compared to the front, or even the front may be moving too slow compared to the rear.

Realistically however, the rear of e36s, (and e30, e46 not so much for some reason), are very "busy" when you start to get up into the spring rates where the car is going quick. Even a stock e36 is "busy" in the rear suspension, follow one down the highway some day and you will see that the ride frequency is very different front-rear from the factory. (this is the only time I will mention ride frequency because we are talking about race cars not street cars. Lap time > ride frequency calculations, no matter what anybody says)

Sooo.... You need to slow down the back of the car. First make sure everything is set up correctly. The main thing is to be sure that you are not bottoming the bump stop or having anything cause the spring rate to be higher than you think.

The rebound knob and bump knob on AD rotate clockwise for stiffer (because that makes sense ). "Bouncing" is a cycling that usually requires an increase in both bump AND rebound, so I would
1) double check everything especially bump stops and springs.
2) make sure you are not running high spring rate and low tire pressure. It is very common if not almost always desirable to run MORE rear tire pressure than the front. (a pound or 2 or 3) You need to be sure you are not actually bouncing off the sidewall.
3) Increase rebound.
4) Increase bump.
5) Dont forget to DECREASE rebound after trying increased bump, to be sure you are not causing the problem with TOO MUCH rebound (very common)


Jay

[dmwhite]

3058lbs w/driver + fuel

this won't help you with your issue, but can you get any more weight out of the car? or do you have to run that heavy for gts-3?

[Eric1H]

I had/have a similair issue with mine. Only started after I rebuilt them, you may want to check the nitrogen level of the shocks, or check to see if you have any oil leaks. Could be a rebounf issue too, I am still toying with my adjustments after the rebuild.

[vinnymac]

this won't help you with your issue, but can you get any more weight out of the car? or do you have to run that heavy for gts-3?

Actually, my GTS3 weight is nearly 100lbs lighter than the 2970# IP weight. I still have a ways to go to get the car lighter. For example, I still have glass in both doors and all the window gear since I only have an open trailer and outside storage.

[vinnymac]

"bouncing" almost always means the suspension is moving too fast. Of course, "too fast" is a relative term, so it may be moving too fast, too fast compared to the front, or even the front may be moving too slow compared to the rear.

Realistically however, the rear of e36s, (and e30, e46 not so much for some reason), are very "busy" when you start to get up into the spring rates where the car is going quick. Even a stock e36 is "busy" in the rear suspension, follow one down the highway some day and you will see that the ride frequency is very different front-rear from the factory. (this is the only time I will mention ride frequency because we are talking about race cars not street cars. Lap time > ride frequency calculations, no matter what anybody says)

Sooo.... You need to slow down the back of the car. First make sure everything is set up correctly. The main thing is to be sure that you are not bottoming the bump stop or having anything cause the spring rate to be higher than you think.

The rebound knob and bump knob on AD rotate clockwise for stiffer (because that makes sense ). "Bouncing" is a cycling that usually requires an increase in both bump AND rebound, so I would
1) double check everything especially bump stops and springs.
2) make sure you are not running high spring rate and low tire pressure. It is very common if not almost always desirable to run MORE rear tire pressure than the front. (a pound or 2 or 3)
3) Increase rebound.
4) Increase bump.
5) Dont forget to DECREASE rebound after trying increased bump, to be sure you are not causing the problem with TOO MUCH rebound (very common)


Jay

Jay - thanks for the reply. I'll make some adjustments and see how the car feels the next time I'm on the track...which will be in a few weeks.

I'll also likely send in the rears for a rebuild after the next race in July. You guys did a superb job rebuilding the fronts earlier this year and looking back now; I should have sent in the rears as well.

Thanks again for the feedback.

[vinnymac]

Unfortunately, based on my own experience and the dynoes of a few other sets of AD's that I've seen, I don't know if you'll be able to get rid of your problem with just a turn of the knob. The adjustment range on AD's just doesn't seem to be large enough IMO.

That said, to get rid of the rear bouncing, you want to increase your rebound damping. IIRC, that would be done by turning the red knob clockwise, "screwing it in" to the damper...feel free to correct me though, as it's been a while. If that doesn't solve the problem, you might consider having GC revalve them with additional rebound damping.

Bryan - Thanks for the feedback. It looks like your advice is right in line with Jays so I'll give it a try. Cheers.

[dmwhite]

Actually, my GTS3 weight is nearly 100lbs lighter than the 2970# IP weight. I still have a ways to go to get the car lighter. For example, I still have glass in both doors and all the window gear since I only have an open trailer and outside storage.

ah, gotcha...i bet you could still get the car close to IP weight with the side windows and related mechanisms...

good luck with the suspension issue!!

[vinnymac]

ah, gotcha...i bet you could still get the car close to IP weight with the side windows and related mechanisms...

good luck with the suspension issue!!

Definitely, I was looking through the car this weekend and there are a lot of little things I haven't done yet which will help shave some of the weight.

My goal is to get it 35-50 pounds under weight so I can add weight back where I want it.