Inrev

I experience the same thing even with proper alignment and pressures, worst happens on higher speed braking zones where it's a bit bumpy. I've ordered some suspension bits in order to minimize the problem.

malmasri

This is a faulty design of the rear suspension,I am not an engineer but I will try to explain what is the issue. Porsche decided to use the one piece rear suspension from the Cayman/ Boxster cars " straight up and down"...unlike the multilink that exist in a GT3 at high speed cornering and or heavy breaking the cars rear end wants to wander forward...very scarey when you are not expecting it and have not experienced it...I felt for the first time while breaking for turn 17 at sebring ...the reason is that the stock suspension actually causes the toe to decrease " toe in" and reduces the rear tire contact surface creating this issue. There is a solution that has been in the works for the past year and is available, it was developed by a race suspension engineer in cooperation with a South Florida race shop...it is based on a multilink 997 cup rear suspension, and it does work....I have it installed on mine and have never experienced this issue since.
Anyone who wants more information can contact Gavin at Autoquest.

http://www.autoquestcars.com/home

Yargk

Well, even on GT3 cups, the rear suspension goes toe-out under braking. It must do this because it goes toe-in under acceleration, which is something you want. The difference is just that the degree of toe-out increases more in the GT4 than the GT3, after an inch of suspension extension. Up to an inch of suspension droop the toe change is about the same in the two platforms. I think this is why just stiffening the car with the DSC module or springs helps because then the suspension doesn't move as much and stays where it is predictable. I could be wrong, I just drew these conclusions from looking at toe-curves posted here and elsewhere, along with personal experience on track.

Sven76

I quite like the lively back of the car, nothing wrong with that, just need to hold the steering wheel.

BryanCO

Great info fellas.

GT4Stu

I enjoy the lively back end too... thought it gave the car some charm. This time it just seemed a bit too active.

coldfire

I am still very much a novice on track, but I did play around a bit with ESC off during a few sessions and surprisingly found the car more stable and predictable under hard braking. Has anyone else experienced this? I'm curious if ESC intervenes under braking.

malmasri

You have the toe change backwards,and you are wrong about the characteristics of the GT4 rear suspension. Stiffer suspension in the CS is the reason many have crashed. I am going to let the expert explain this to the forum. Please be patient.

Yargk

I don't think I have the toe change backwards. The rear goes toe-out under braking and toe-in under acceleration. If it was the opposite, then the car wouldn't be lively under braking.

Also, there is no comparing the CS and street car spring rates.

street:
front 256 pounds/inch
rear 456 pounds/inch

CS original
front 1142 pounds/inch
rear 1256 pounds/inch

CS revised
front 800 pounds/inch
rear ? (I don't have this data, I guess 913 pounds/inch)

So the CS rates were too high, but also the shocks sucked. They went with softer springs. However, that doesn't mean the street GT4 wouldn't do better and be more predictable on track with
front: 336 pounds/inch to 448 pounds/inch
rear: 504 pounds/inch to 560 pounds/inch
These rates are stiffer than stock, but still very soft compared to the CS.

MarcD147

No need for DSC

Tire pressure is to high ( should be 34 hot) but that doesn't cause your braking issue

Get TPC toe links ( not a different brand) and adjust your toe settings. You need a little more than stock. Check Joe's thread or the track setup track

Ubermensch

Lots of good suggestions are car setup but I'd also consider the drivers influence on this behavior. Any information on brake pressure your applying? I had an interesting session with a coach a few weeks ago in which he pointed out how hard and short i was braking. This was making it difficult to transition to steering input and unsettling the rear. When I checked his brake pressure I saw i was applying roughly 2x the pressure and ending early. Braking a little softer, earlier and longer smoothed things out and make me quicker. You can also modify the weight transfer during braking with s little throttle (assuming you left foot brake).

GT4Stu

I'm sure driver influence was a factor too. I am far too inexperienced to know if my brake pressure was adequate or not. I didn't think I was being too heavy footed but anything is possible in the "heat of battle"
What I do know is that I was at the same track 2 weeks prior and didn't experience similar behavior. However, speeds were a bit lower (high 130's vs. high 140's) because we were running on the infield courses and using the NASCAR straights. Additionally, I had been on the NASCAR oval a year ago and the car was rock solid.
Really appreciate all the feedback and info guys!! Headed to the dealer Tuesday to have my toe settings looked at to start checking boxes.

Chris Hebert

Something else to check regarding allignment issues too - especially if you are using tarret end links..

I got an alignment with some tarett parts:
-Tarett Front Tie Rod
-Tarett Rear Toe Link
-Tarett Thrust Arm Bushing

allignment was where i wanted and 1st day was great. middle of 2nd day I had a Super uncomfortable feeling under braking at Summit Point. I finished the weekend running a few tenths slower but definitely felt like driving a boat braking into turn 1.

So, I came home. Visually I could see my right rear toe was out by 1 inch out compared to other. Allignment shop confirmed "outa-wack".

Turned out the Rear Toe can adjust under high stress (at least with Tarett). So, I got this:
-Locking Plate Kit, For Adjustable Rear Toe Link

If you felt more extreme tail wiggle under braking - you may be dealing with a rear toe out issue. And you may want to see if the stock rear toe links can accidentally adjust under load too..

Good luck and let us know if you determined a cause...

malmasri

Had a brain lapse...toe vs camber....my bad....Regardless, the GT4 that I have with a modified multilink suspension does indeed handle the way a track car should, stable under heavy breaking, stable under acceleration at high speed corners. IMO the poor handling problem have been solved, a well known pro driver described it as " the best handling GT4 he has ever driven ".

GT4Stu

Ok, so here's the prognosis...
Alignment was totally out of whack. Both front and rear, toe and camber! Not sure how this happened other than stresses on the suspension on track (as mentioned above) since I haven't hit anything on the road that would cause such an "adjustment".
I'll post the printout of the settings pre and post new alignment when I get the car back tomorrow or Thursday. Stay tuned...