ajsrider

I took my 2001 911T with 18000 miles on the odometer and a set of brand new MPSC tires on the wheels to Road Atlanta this weekend. This was my first experience with almost-racing tires and the increase in grip was of course phenomenal. BUT, there was also some very peculiar behaviour: When accelerating out of fast right handers (such as turns 12 and 1) the car was extremely twitchy, almost like it was walking around the turn. To the instructor it felt like the rear end was twitching. To me it felt more like the front wheels gripped alternately causing me to make small alternating corrections. The frequency of these alternations was in the range once or twice per second I guess. I started out with 36 and 44 psi (cold), and dropped first by 4 psi and then by an additional 2 psi but didn't notice too much difference. However, when I drove solo, the phenomenon was much reduced.

Any comments, ideas suggestions?


Thanks,

ajsrider

jrgordonsenior

I drive them on a 993TT with 235/295's. The recommended pressure from Michellin is 26/28 cold, 32/34 hot. If that's correct, you were well over pressured. I ran them at those temps this past weekend at Cal Speedway reaching 160+ on the banked oval and they worked perfectly. If you had 44 psi in those rears I'm not surprised it felt twitchy. I imagine as your rears slipped your front wheels were engaging and that does feel different especially if you're not used to stepping it out....Next time out try lower pressures and see how it feels. Aslo ambiant temps affect these tires too. How warm/cold was Atlanta?

ajsrider

The temp varied from the 40's on Saturday morning to the 70's on Sunday pm. The problem was least noticeable on the last run on Sunday, which is also when I was alone in the car.

Where do you get the Michelin recommendation? Should I assume that it trumps the one from Porsche, which says 36 front 44 rear.

BTW, what do you think of using nitrogen. I had that and I still saw about 5-6 psi increase from cold to hot. I had been told it would not increase tnat much.

GreggT

I start with 25/28 F/R..............goal of hot 32 front, 33-36 rear......you need some temp to get max from these tires.....over 200' if you are workin them.

jrgordonsenior

http://www.cb-racing.com/pilotsport_technical.pdf
http://www.cb-racing.com/PilotCup_Presentation.PPT

From Tire Rack:
http://www.tirerack.com/tires/tirete...otsportcup.jsp

RXDOC

ajsrider:
I was at RA last weekend in HPDE3. In fact, your instructor approached me and talked with me about your problem. The only thing I could come up with was tire pressure! 36/44 is way too high a starting point. I would see 8-9lb increase after every run. Our TT's should not be twitching anywhere on this track! I have worn street Bridgestone PS0-2A's. I started with 30/38 lbs. Every morning.

ajsrider

jrgordonsr, GeggT, RXDOC:

Thanks, guys. I looked up the Michelin info I Got from jrgordonsr, which of indeed indicated 25/28 F/R cold. I will try that the next time. On the highway, though, I assume i should go with the 36/44 per the Porxche spec?

GreggT

Continuous highway they need to be up there. Around town they could be a bit more comfortable.....say 34/40

jrgordonsenior

I would try Greg's recommedation above first. But there's another very important requirement to run these tires: They need lot's of negative camber! Regardless of how much pressure you run in them, they need at least 1.5 degress of camber, perferably 2 or better. Just look at the tread pattern and you'll understand why. They will wear out the thinner outside pattern if they're not angeled back in at the top (negative camber). I had 1.7 degrees of front, 1.5 rear and wore the fronts out with 8 track days. The outsides were gone, the middle was worn, but the insides looked like new tires. I just added camber plates and increase to 2.5/2.3 and I'm sure I'll get better tire mileage, increase my turn-in speeds, and hopefully pick up a few seconds.....