GMPP Z/28 Suspension Too Stiff for Autocross?

[Retro69]

I installed the GMPP Z/28 suspension package on my 2011 SS around the first part of the year. I do a lot of autocross and am having a problem with the inside rear wheel lifting when powering out of hard turns. I never had this happen with my prior suspension (Pfadt 1.25 lowering springs, ZL-spec sways, Spohn delsphere toe links, trailing arms, poly bushings everywhere). On big open courses it seems to do quite well but on tighter, slower courses it is a constant problem.

I'm beginning to think the Z/28 springs are just too stiff for serious autocross duty. Has anyone had any experience with this? Anyone gone with a shorter, softer spring?

[Nick S]

I think this issue is more a testament to the rigidity of the body structure and maybe the stabilizer bar. I've seen lots of other cars (never another 5th gen though) pull an inside wheel up while going around corners in autocross.

As the weight transfers toward the outside of the corner and compresses the suspension on the outside corner, the inside suspension is then being "compressed" by the torque being applied to the stabilizer bar from the outside corner. If the body structure if very rigid and the stabilizer bar is fairly rigid, then the spring on the inside rear corner may not be able to overcome the force that the stabilizer bar is exerting on it.

Or I suppose the opposite could be true that the body rolls/lifts enough at that corner that the inside suspension reaches full decompression travel and as the body continues to roll it has to pick up the inside corner with it. Maybe the Z/28 shocks have less extension travel when and the suspension just could not travel any further downward to maintain contact with the road.

A video of this happening would likely be very helpful in determining what could be causing it. Maybe someone took a video or even a picture of you during your run and maybe there are some photos/videos you could study.

[shibbs]

Try going back to your stock SS rear sway bar, or a smaller rear bar. Maybe even try no rear bar at all with springs that stiff.

[VR Baron]

Coilovers. Friend running 1le 28mm rear bar when he put on kw clubsport coilovers with softer rear but a bit stiffer front springs, on his z28. Car wouldn't turn well in autocross, and he is a top driver. Now the car is unstoppable!

[FunkedOut]

Originally Posted by VR Baron

Coilovers. Friend running 1le 28mm rear bar when he put on kw clubsport coilovers with softer rear but a bit stiffer front springs, on his z28. Car wouldn't turn well in autocross, and he is a top driver. Now the car is unstoppable!

Let me try and translate this into my English and you tell me if I got this right...

You have a friend with a (5th gen?) Z/28 that is an excellent driver and couldn't get the car to turn well at autocross events.
He then swaps the rear sway bar for a 1LE bar, and installs coilover shocks that have softer rear springs and stiffer front springs. (Softer and stiffer than what?)
Now, he can't be beat at autocross events.

Did I get that right?

Sincerely asking, not trying to be an ass.

[FunkedOut]

Originally Posted by shibbs

Try going back to your stock SS rear sway bar, or a smaller rear bar. Maybe even try no rear bar at all with springs that stiff.

Sound like good ideas.
I'd think softening the rear springs/bar and/or stiffening the front springs/bar would move you in the right direction. The front to rear balance is what needs adjustment. You make the call on which way you want to go; softer or stiffer.

[Retro69]

Thanks for all the ideas so far...

Originally Posted by Nick S

I think this issue is .....ting on it.

Or I suppose the opposite could be true that the body rolls/lifts enough at that corner that the inside suspension reaches full decompression travel and as the body continues to roll it has to pick up the inside corner with it. Maybe the Z/28 shocks have less extension travel when and the suspension just could not travel any further downward to maintain contact with the road.

A video of this happening would likely be very helpful in determining what could be causing it. Maybe someone took a video or even a picture of you during your run and maybe there are some photos/videos you could study.

I know for a fact that the Z/28 struts have shorter travel than the OEM SS pieces. Plus the ride height is pretty close to what it was stock so there is a lot of travel already taken up so it wouldn't surprise me if that's what's happening.

No videos that I know of and the few stills I found are around course elements that don't exploit this ill behavior.

Originally Posted by shibbs

Try going back to your stock SS rear sway bar, or a smaller rear bar. Maybe even try no rear bar at all with springs that stiff.

Someone in the club also suggested disabling the rear bar. I'm a little nervous about doing this but I guess I should at least give it a try.

Originally Posted by FunkedOut

Sound like good ideas.
I'd think softening the rear springs/bar and/or stiffening the front springs/bar would move you in the right direction. The front to rear balance is what needs adjustment. You make the call on which way you want to go; softer or stiffer.

Given how stiff the Z/28 springs are I would probably tend to go softer in the rear.... just got to find someone who makes springs to fit.

[Nick S]

Just to clarify you installed the entire Z/28 suspension upgrade that Chevy performance sells correct? Just trying to make sure that the issue isn't being caused by having a piece mealed set up that isn't the entire kit that GM designed to work as a system.

[clarkkent]

Same question as Nick, the Z28 DSSV struts/springs are much stiffer than the SS. However the Z28 swaybars are much softer. They are made to be used together

If you're still using the stock SS swaybars that's probably the issue

[Retro69]

Originally Posted by Nick S

Just to clarify you installed the entire Z/28 suspension upgrade that Chevy performance sells correct? Just trying to make sure that the issue isn't being caused by having a piece mealed set up that isn't the entire kit that GM designed to work as a system.

All parts from the package were installed. The only other suspension mods are Prothane rear sub-frame bushings.

[FunkedOut]

Originally Posted by clarkkent

Same question as Nick, the Z28 DSSV struts/springs are much stiffer than the SS. However the Z28 swaybars are much softer. They are made to be used together

If you're still using the stock SS swaybars that's probably the issue

SS sway bars are 23/24mm diameter (front/rear)
Z/28 bars are 25/26mm
ZL1 bars are 25/28mm
1LE bars are 27/28mm
then there are aftermarket bars

[SomeGeoffGuy]

Originally Posted by Retro69

I installed the GMPP Z/28 suspension package on my 2011 SS around the first part of the year. I do a lot of autocross and am having a problem with the inside rear wheel lifting when powering out of hard turns.

Is it actually air gapping or is it just spinning the inside tire which you are equating to unloading? I was just thinking that if it was spinning, it could be why the Z28 has the helical diff.

-Geoff

[Retro69]

Originally Posted by SomeGeoffGuy

Is it actually air gapping or is it just spinning the inside tire which you are equating to unloading? I was just thinking that if it was spinning, it could be why the Z28 has the helical diff.

-Geoff

It's spinning... really easily. I'm sure my posi is not all that great but it just seems to spin way too easily... especially considering I never experienced this behavior with my prior set up.

I had actually thought about going with the Z/28 diff to try to deal with this that's a chunk of change that may not help with this specific problem if the wheel is lifting too much.

[SPCBA]

Originally Posted by FunkedOut

Let me try and translate this into my English and you tell me if I got this right...

You have a friend with a (5th gen?) Z/28 that is an excellent driver and couldn't get the car to turn well at autocross events.
He then swaps the rear sway bar for a 1LE bar, and installs coilover shocks that have softer rear springs and stiffer front springs. (Softer and stiffer than what?)
Now, he can't be beat at autocross events.

Did I get that right?

Sincerely asking, not trying to be an ass.

His friend did 1le rear bar with his KW club coil overs front stiffer rear softer.

My questions are is it lifting? Spinning? Or not rotating?
What is happening when it happens? What are you doing when it happens?

What's your alignment what's your tires. If you swapped to all that z28 shit should you be stock ride height?