GT4Stu

I am hoping I can get some insight into an issue I experienced yesterday on track...


I had my GT4 on the NASCAR tri oval at Pocono where I experienced what I thought was abnormal handling behavior. Under heavy braking from pretty high (145+) speeds the rear end felt very loose. Now I understand that this is a characteristic of the car that I have read about and experienced before but this did not feel normal at all. I really had to fight the car to stay straight. It responded to inputs but felt really unstable for some hair raising moments. Also it seemed to wander a bit under heavy acceleration and high speed on the track and at higher highway speeds on the road.


Here are some details:
Car has 8,500 mi with 5 track days (3 on road courses and 2 on the Pocono tri oval)
Car is completely stock
MPSC2's with 5,000 mi
Pressure set at 29F/33R cold and got up to approximately 35F/38R hot (after the 1st 15 min session)
Stock alignment set to factory specs 4 weeks ago prior to a PCA DE event (full weekend with 8 25min sessions and I run in the green/novice group)


I have some thoughts as to what the issue may be:
Tires have heat cycled out or are otherwise shot
Should have started out with lower cold pressure
Alignment somehow got tweaked between the DE and yesterday's event (car was flawless at the DE)
A combination of the above


I would really appreciate any insight you all can offer.
Thanks!
Stu

ajw45

Sounds like a good time! Is the braking zone on a banked part of the oval? Braking into a banked turn can feel weird and the car doesn't want to stay straight, the rear will want to move out/up the banking. On the plus side, then you don't have to turn the car as much

FWIW, tire pressures sound pretty high, I try to run about 31/33 f/r hot. After my first event I noticed a change in my alignment, seemed like rear toe adjustment got a little loose at some point giving me a little pull to one side at speed and maybe some extra instability under braking. Revisited the alignment rack and everything has been perfect since....

vantage

If you want the car to feel a bit more stable under high speed braking, you can put your car on the high side of factory specs for rear toe-in. Allegedly this may cause some slight compromise in turn in, but probably not much. What is your rear toe?

Your hot pressures are maybe 4 psi too high.

Yargk

Rear toe-in at 3/16 inch total helps (about 5 mm total toe-in, 4 mm probably also okay, about 5/32). Stiffer front springs also helps. Oh yeah, and don't let the rear tire pressure go higher than 34 psi, or the front higher than 33 psi.

sdillon

FWIW, I never run my rears anywhere near that hard. My car always feels "skatey" if they're that hard. I try to keep them around 34F, 31R hot. I think I'm in the minority with that opinion, but I don't experience excessive wear in the rears so I can't think of a reason why not to run them fairly low. I also run the sways at full soft in front and full hard in rear, so YMMV.

jmartpr

What you felt is a Cayman thing....when you make such a violent weight transfer (either braking or accelerating) toe changes a lot and if you add tire pressures and other factors, well you get what you felt at the track. Like others have mentioned a good alignment will help a lot on this department but for me the thing that really helped in this area were the TPC rear toe links.....and after the DSC (also from TPC) and their latest Map the wandering is all gone, zero, nothing nada......the DSC even helps in controlling the weight transfer under braking.

Bill Lehman

I've driven the Pocono Tri-Oval with my 987.1 Cayman S. If I remember correctly, my speeds were a little higher. There is no heavy braking anywhere. The GT4, being a parts bin car, has a compromised rear suspension. Under hard braking the nose drops and the rear rises which introduces unfavorable rear toe out. The solution is to add more rear toe. Lots of data about this if you do a search. I run 3 mm per side and its still a little scary but I've gotten used to it.

GT4Stu

Thanks for the feedback everyone! This is really helpful.

It would seem that I was running my pressure way to high and that I need to check/adjust my rear toe. Any recommendations for starting cold pressures? I thought I saw some where that 28 all around was a good starting point.

Will the adjustment to rear toe affect normal street driving? My car is primarily for the street and its my daily.

Thanks again!

Yargk

I have to add, although the rear suspension is not as good as on the GT3, actually looking at toe curves for the GT4, it doesn't really seem all too different from the GT3 until the travel is over an inch. Perhaps you're getting there under heavy braking because the front suspension is so soft and if that's the case that is also why stiffer front springs help so much, they reduce the dive just enough to keep the toe in check. Outside of that, I'd think the suspension geometry differences should really only be apparent over mid-corner bumps that produce >1 inch deflection. Could be wrong.

O5C4R

Starting cold pressures are a mystery, treat the first session as warm up and adjust from there, at the end of some track days my cold pressures end up being right front 24 and left front 28 lol

johnr265

Agree with others, lower rear pressure a bit, more rear toe, and if that doesn't help, consider DSC unit. The DSC definitely helps with brake dive though I am not entirely sure I like the feel at other times. Jury is still out.

GT4Stu

This is all very interesting. I never thought of dive under braking being enough to lift the rear end and upset it so much. You can really go down the rabbit hole with all this stuff... and I'm just getting started!


Bill,
A question about your times at Pocono... Do you suggest that even at the end of the front straight coming in hot to NASCAR turn 1 there is no heavy braking? I would come in at around 150 and slow to around 110 just before turn in, which I thought required some firm pedal effort. Have you come in hotter and taken turn 1 significantly quicker in your 987? I found coming out of turn 3 at 105ish would only get me to 150 on the front straight in my GT4 and taking turn 1 at a bit over 100 was exciting Would love to hear your thoughts.
Thanks again all,
Stu

PCarOMFS

I've been to Pocono and ran the North course. I had a similar experience to you. Hard braking was very unsettling. The issue was primarily grip related for me. As soon as the tires heated up, the problem disappeared. I was running Cup2's and they were towards the end of their life.

Alan C.

After I put the TPC DSC unit in my car the twitchy rear feeling under high speed heavy braking pretty much went away. I agree with others on your hot pressures being on the high side.

GT4Stu

Yes, unsettling to say the least! The issue was most prevalent during my 2nd session when everything was hot... apparently too hot. I've also run the IMSA course and double infield and the twitchiness wasn't as bad. Maybe the speeds weren't high enough (high 130's to low 140's)?


May have to investigate the DSC unit after I sort out my pressures and toe settings first.