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I was at the track, 3rd session of the day (Streets of Willow), 2019 GS Z07 manual, and in the bowl heard a loud, sharp "whoosh", after which RR tire pressure went to zero. You can see from the picture one of the cords apparently split open.
It's always a little tricky knowing when to replace your tires and I was hoping to get at least another track day or two out of my OEM Michelin Pilot Sport Cup 2 ZP (335/25R20 rear). To me it looks like the RR tire had enough tread on it to run. I was surprised it was the RR, as the LR, also shown, gets I think more action on the mostly CW tracks I drive.
Right Rear
Left Rear
I normally run ~35psi at the track, but I was experimenting with a little higher temps (to try to avoid rapid edge wear), up to maximum 38 psi in the front, although the rears weren't heating up so much and the RR from the stats were always less than that.
Michelin web site, regarding tire pressure for the R tires it says:
HOT TIRES: The best operating pressure for MICHELIN® Pilot® Sport Cup 2 R tires is between 2.3 bar (33 psi) and 2.7 bar (39 psi) when hot, depending on the vehicle model and track. However, some vehicle models will need a higher inflation pressure than 2.7 bar (39 psi when hot).
If these numbers are roughly true for the non-R tires, then technically the pressures weren't out of range.
Anyway, the question is should this RR tire have blown considering the tread wear?
I should check the alignment, only outside car side is worn, so must be to much positive camberange, or toe in or out. For racing negative camber angle is expected to be needed.
Such a wide tire is more sensitive for that.
I am able to calculate a safe pressure for normal street use, but also for on track can be usefull.
Goal is to not overheat any part of tire, when driving the speed for wich pressure is calculated.
So for that deflection is important, made by load on tire/ pressure relation. Higher speed needs lesser deflection, so higher pressure for the load on tire.
Determining the real loads on seperate tires is the most tricky part in it all.
39 psi is way too high for a c7. 32-34 is where id shoot for. Higher psi means higher wear on the middle of the tires. Also these tires tend to overheat and delaminate. Your rear tires have delaminated. The oem cup tires really aren’t good track tires. They are hero lap times but don’t last at all. you should change over to another tire. Supercar 3r would be a good replacement.
also how many miles and how old are those tires? If it’s a c7 tire they gotta be getting pretty old.
You shouldn't run any more than 32 psi hot all the way around. However, that wear pattern is typical of the Cup2 tires. The tread block just outside of the inside tread block wears out before the two tread blocks on either side of it. All 4 tires will wear the same way when running on track. The wear pattern is somewhat similar to the mid 90s BFG R1 Groove of Doom.
In the last session when the tire blew, 3rd lap, the RR had a maximum pressure of 30.2 psi (maximum temp 86F). The previous session for all laps (15) the RR max was 34.8 psi (135F max temp).
The last track alignment was done ~1K miles, 9 months earlier by a reputable dealer. To me the tread depth in the center of the tires seems ok.
This was the 6th track day on the rears.
I guess what alarms me is that the tire had a blowout when the pressure, tread depths, and tread uniformity were not unreasonable. If it had been a front tire in a tight, fast turn, it might not have turned out so well. Of course there are other tire options (which I'll certainly look into now), but I've generally only read good things about these tires.
In the last session when the tire blew, 3rd lap, the RR had a maximum pressure of 30.2 psi (maximum temp 86F). The previous session for all laps (15) the RR max was 34.8 psi (135F max temp).
The last track alignment was done ~1K miles, 9 months earlier by a reputable dealer. To me the tread depth in the center of the tires seems ok.
This was the 6th track day on the rears.
I guess what alarms me is that the tire had a blowout when the pressure, tread depths, and tread uniformity were not unreasonable. If it had been a front tire in a tight, fast turn, it might not have turned out so well. Of course there are other tire options (which I'll certainly look into now), but I've generally only read good things about these tires.
jesus 6th track day? Mine delaminated after 2 days. 6 days is like 2-3 more than anyone else is seeing. I’m guessing they were delaminated and corded several sessions ago and you didn’t see it.
I see a not so great wear spot in about the same place on the other tire. I would probably run one more autocross on that tire but would be real hesitant to run a track session.
jesus 6th track day? Mine delaminated after 2 days. 6 days is like 2-3 more than anyone else is seeing. I’m guessing they were delaminated and corded several sessions ago and you didn’t see it.
OP, notice we even have a term for the wear situation you experienced: Delamination. Six-plus years ago when my first set of Cup2s did this I noticed that section of tread looked like somebody took a razor blade and scraped the rubber off that section of the tire. The blowout might not have been too bad if it happened on the front. The Cup2 ZP tire can handle a sudden loss of pressure when cornering hard although all nannies will automatically reengage and the car will slow to 50 mph. That's why we run ZP tires.
Yes, that's why I'm reluctant to use a non-ZP tire. After the blowout, I was able to drive comfortably 30 miles to the nearest tire dealer to see what my options were. I've been stung several times with tire issues at the track way out in the middle of nowhere, which is one of the reasons I bought the corvette, as zp tires are oem. (For peace of mind we even use them on our daily drivers.)
Hey JGN, I am sure you are counting your blessings you were not at 140 on T8 at Big Willow when this occurred! Every track has different demands on cars, and typically the higher speed tracks will wear out tires quicker. With all my sets of Cup 2s with a C6 Z06, I would only run Big Willow and CA Speedway, and the max track days I got out of them was 3, and the last few sessions were not pretty. No way would I ever run them a 4th day due to risk vs reward ratio inverting. Happy to hear you are healthy and you learned where the weak spot is on the Cup 2s!
Yes, live and learn. What's clear is that aggressive driving really hits the tires. I waited 5 track days in my brand new C7 before changing the fronts. The last time I changed fronts, after learning to push the car harder, was after only 2 track days. I had thought the rears should last a good deal longer than the fronts, so probably wasn't watching them close enough.
10-29-2022, 06:12 AM
Michelin Sport Cup 2 tire blowout on the track
