GMPP Z/28 Suspension Too Stiff for Autocross?

[Retro69]

Originally Posted by SPCBA

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?

Typically I get this when I make a chicane or in a slalom and am on the throttle. I'll feel the tire start to spin and this will upset the car. I try to drive through it but its definitely an interruption.

Front: -1.4 degrees camber and 0 degrees toe
Rear: -1.0 degrees camber and -0.10 degree toe.

I'm running Hankook R-S3 285's on all four corners.

Yes, pretty much at stock ride height.

I found five pictures that were take at the same event. The were not taken when I'm seeing this behavior but maybe they can tell something about how the car rolls...

[Nick S]

Do you run Autocross in Competitive Mode (two presses of the TC button)? If not then I can see where inside wheel spin could set off traction control and create a more noticeable interuption. If you put it in competitive mode then the Traction control is disabled but Stabilitrak is still active (at a reduced interference level though).

if it's just the rear inside wheel spinning easily (and not actually lifting off the ground) then I have heard of others having that issue before. Dropspeed was having that issue on road courses with his 1LE suspension set up. I beleive he has a Z/28 style diff (helical) in his 1LE now as well as the Z/28 suspension set up. I don't know if I've seen a report back from him on whether or not this resolved his inside wheel spin or not. Maybe you want to send him a PM and ask him about it.

[Retro69]

Originally Posted by Nick S

Do you run Autocross in Competitive Mode (two presses of the TC button)? If not then I can see where inside wheel spin could set off traction control and create a more noticeable interuption. If you put it in competitive mode then the Traction control is disabled but Stabilitrak is still active (at a reduced interference level though).

if it's just the rear inside wheel spinning easily (and not actually lifting off the ground) then I have heard of others having that issue before. Dropspeed was having that issue on road courses with his 1LE suspension set up. I beleive he has a Z/28 style diff (helical) in his 1LE now as well as the Z/28 suspension set up. I don't know if I've seen a report back from him on whether or not this resolved his inside wheel spin or not. Maybe you want to send him a PM and ask him about it.

Before I started having this problem I had always run autocross in Competitive Mode. I felt like it was not intrusive and if Stabilitrak were to kick in then I must be over driving the car anyway. When the wheel spins it still feels like Stabilitrak or Traction Control is backing off the throttle causing the upset. The last two or three events I've been experimenting with turning off all the nannies. I was surprised to find how much Stabilitrak was still having an influence as I spun out on a dry track for the first time ever. Undaunted I have continued to drive with everything off and the spinning is still interruptive and has me perplexed.

Sounds like Dropspeed's problem is definitely familiar. I'll PM him for sure.

[SPCBA]

It is semi-Popular to get the eaton for that inside wheel. Also there is some improvement to be had in your alignment for sure to help with rotation. JusticePete has a bunch of posts about castor and camber and toe.

Could have sworn the z28 set up is supposed to be lower. Cool car by the way goodluck.

[clarkkent]

Originally Posted by FunkedOut

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

Ah, thanks for the clarification. I must've be thinking about the 1LE bars

Originally Posted by SPCBA

It is semi-Popular to get the eaton for that inside wheel. Also there is some improvement to be had in your alignment for sure to help with rotation. JusticePete has a bunch of posts about castor and camber and toe.

Could have sworn the z28 set up is supposed to be lower. Cool car by the way goodluck.

I believe x25 said his 1LE dropped almost an inch, and that was without taking into account the 19" Z28 wheel swap. The wheel swap also dropped him a bit (since they have an overall smaller diameter)

[Retro69]

Originally Posted by SPCBA

It is semi-Popular to get the eaton for that inside wheel. Also there is some improvement to be had in your alignment for sure to help with rotation. JusticePete has a bunch of posts about castor and camber and toe.

Could have sworn the z28 set up is supposed to be lower. Cool car by the way goodluck.

Thanks!

The car ended up being higher than I had expected. I even had my alignment guy redo the bushing clocking to make sure there wasn't an issue there affecting the ride height.

Sent from my SM-G920V using Tapatalk

[VR Baron]

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.

Yes. He has always been one of the top drivers, previous car was a 1le.Havent talked to him as to why, probably what many have posted here. Rear Tire not getting traction would do it. Z28 is a track car not autocross. Bump setting is high and suspension is very stiff, probably to stiff for auotcross. You need to get the car to lean a bit and transfer wieght to the front tires and get tne car to rotate around the turns. To stiff and no bueno. Z28 spring rates are 305/610 or so, coilovers are 350/450, and changing rebound and bound dials it in. With settings he was gi en to use car is spot on. First time out on the coilovers he placed second in scca cam c class.

[FunkedOut]

Originally Posted by VR Baron

Yes. He has always been one of the top drivers, previous car was a 1le.Havent talked to him as to why, probably what many have posted here. Rear Tire not getting traction would do it. Z28 is a track car not autocross. Bump setting is high and suspension is very stiff, probably to stiff for auotcross. You need to get the car to lean a bit and transfer wieght to the front tires and get tne car to rotate around the turns. To stiff and no bueno. Z28 spring rates are 305/610 or so, coilovers are 350/450, and changing rebound and bound dials it in. With settings he was gi en to use car is spot on. First time out on the coilovers he placed second in scca cam c class.

good info. thank you sir.

[Zombie killer]

Originally Posted by VR Baron

Yes. He has always been one of the top drivers, previous car was a 1le.Havent talked to him as to why, probably what many have posted here. Rear Tire not getting traction would do it. Z28 is a track car not autocross. Bump setting is high and suspension is very stiff, probably to stiff for auotcross. You need to get the car to lean a bit and transfer wieght to the front tires and get tne car to rotate around the turns. To stiff and no bueno. Z28 spring rates are 305/610 or so, coilovers are 350/450, and changing rebound and bound dials it in. With settings he was gi en to use car is spot on. First time out on the coilovers he placed second in scca cam c class.

+1 I had same trouble with what I thought was the rear wheel lifting but it was because I was upsetting the car too much. I softened the front and let the front tires "turn" and roll the course better and let the rear end catch up to power out with about 10-15 degrees rear slip to turn the back end. Smooth in and smooth out didn't seem faster but it did make my runs faster.

[h018871]

Originally Posted by Retro69

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.

since you are spinning the inside tire, a couple of things: 1) the diff clutches are worn more and are now slipping, 2) the Z28 suspension does put a bunch of weight on the outside tires during abrupt maneuvers (auto-x).

an alternative to the Z diff would be to install a trutrac, that's not cheap either though. just another option

[VR Baron]

Originally Posted by Zombie killer

+1 I had same trouble with what I thought was the rear wheel lifting but it was because I was upsetting the car too much. I softened the front and let the front tires "turn" and roll the course better and let the rear end catch up to power out with about 10-15 degrees rear slip to turn the back end. Smooth in and smooth out didn't seem faster but it did make my runs faster.

Yep. I have the big pedders sway bars and while the rear bar can lift the wheel, thats not on the track , only on the street when turning from one street on to amother that changes
Angle upwards or through a gutter where you van loose traction. You cant go to stiff on shock settings if you have coil overs, with my Pedders supercars I keep them in the middle or upper middle range, more is no affect, softer not enough, and thats with 450/560 spring rates. Cars have to be controlled but also be abpe to get wieght onto the front tires, then also be able to swing that back end around. I find close to neutral is best

[Retro69]

Sounds like it's time to save some pennies for a Z/28 diff.

[L99CAMA2011]

Originally Posted by Retro69

Thanks for all the ideas so far...



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.



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.



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.

I have Z28 springs on my rear with Bilstein Pss10s set to 10 full stiff. I would recommend you try reducing the rear tire pressure. I could make my car understeer or be neutral by changing the front and rear tire pressure. My rear is stiffer than the Z28s because of the Pedders rear ZL-1 bar and other BMR goodies.

[Zombie killer]

I have my front sway bars set as soft as they will go. This photo was from this past weekend and you can see how much my front tire tucks under nicely and I get good front bite , but that does cause me to get some rear oversteer but only if I am too aggressive with the right foot. I personally like the way the front leans over but some my prefer a stiffer setting up front. Just sharing a different view of the front attitude on my Canaro.