I went to Roebling Road this weekend for an HPDE. On Sunday I put my Pirelli scrubs on, I have 305/660r18's on the front, 325/680R18's on the rear. Outside temps was 70ish degrees, and cold air pressures were as follows, L/F 27psi, R/F 28psi, both rears were 30psi. I ran in track, sport 1 mode. after about 9 laps, tires were gripping well, but the psi was crazy, L/F 33 psi, R/F 34 psi, L/R was 41 psi, and R/R was 39 psi. Why would the rears pressures get to high and not the front. I watched the PDR, didnt look like I was spinning the tires anywhere? Any thoughts ? Has this happened to anyone else? This is my first experience with the Pirelli scrubs.

 

Anybody have any advice ?

 

Why are your cold rear pressures so high? Hell all of your pressures are way too high. Try 23-24 all around next time. You probably cooked the tires though. Target hot psi for pirellis is 29-32 depending on fast-but-high-wear or a little more longevity.

I bet the rears were sliding. Check pdr tire air temps on cosworth and see what shows. Rears should be way cooler than fronts.

for time trial type of event here is how mine turned out for a few hot laps. This is about a 10 minute long run . Cold psi this day was 24. But it was a cool day and I needed to get up to speed in 1 lap so I didn’t want too low of pressure. For HPDE I’ll often start at 22-23

 

OP, I agree with Bruno's post that all of your cold pressures were too high and you should be shooting for 29-32 hot pressures. However, there is something about the huge difference between cold and hot pressures with the rear tires. They may have been sliding like Bruno suspects or the moisture content in those two tires is higher. I ran into the moisture problem at the Glen with a set of Kumho's about 15 years ago. One tire would keep building high pressures and I would bleed it down to match the others but it would just go back up after the next session. Finally, somebody suggested letting all of the air out and refilling the tire. After that, the tire pressure behaved similarly to the other three tires. If the two rear tires were filled on a high humidity day the moisture content could be causing the higher increases in pressure as the tire goes from cold to hot.

Bill

 

Never thought about moisture in the tire... Ironically, Poor-sha suggested moisture in the rear tires as well. I was told by another guy at the event with same tires on his mustang, that the "sweet spot" on these were 32-33 "hot", and that they would only increase about 4 or 5 lbs in a session from cold. Plus running in "Sport 1", spinning the rears is usually not an issue. It was pretty cold this weekend in Ga., so I left the Michelins on until the last session, so really didnt get a chance to adjust anything. I definitely will drain all the air out and refill, start at a lower cold pressure. I also was thinking maybe the rear diff is getting hot. I just bought this car, it has 18K miles on, I changed all the fluids except the rear diff. I will do that this weekend.

 

Check and monitor along the way with a known good gauge. Forget the rest and DRIVE!