Tire Pressures - A Cautionary Tale

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Tire Pressures - A Cautionary Tale

Postby Tom Helvey on Sun Oct 23, 2022 8:06 am

I was looking forward to tracking my new Supra at CVR this weekend.
It came with a free track day from NASA. It was a blast driving it at Buttonwillow but I didn't mess with the tire pressures, I just left them at the factory recommended setting for street and track (38 psi cold), I even left the traction control on. At the end of the day, you could tell the tires had been on a track but they didn't show much wear, good to go.

On my Boxster, I normally start the day at 28 to 30 psi square and drop the hot pressure to between 34 and 36 psi hot after each session. The Boxster is fully track prepped with lots of negative camber and I seem to get good grip and wear using that strategy. I carry a new set of tires to every event but if the old tires are still sticky and have some tread left on the edges I don't change them. (200 UTQG cheater tires)

I discovered that the Supra is different. Factory tires are Pilot Super Sports, 300 UTQG and, as previously mentioned, 38 psi.
This weekend I wanted to get a feel for the real cars handling (traction control off) and slowly build up to the limits to see where they are.
I started the first session by dropping 2 psi because I knew the tires would get hot and considered it in the 'safe' initial range and I took the corners at around 75% of what I would normally do in my Boxster. The tires were squealing nicely around 16 but no surprises. After the first session, the tires were hot and at around 45 psi, wear was OK but based on prior experience 45 psi seemed pretty extreme so I dropped them down to 38 (still in the 'safe' range?). Big mistake. :banghead:

I pushed the car a little harder on the corners during the second session. The car got a little loose on the track out for 10 but I attributed that to being less than smooth on the throttle and it seemed the tires were squealing a bit less on 16 (better grip or just slower?). When I returned from the second session I noticed there was a lot of wear on the drivers side front and I wondered if my tires were going to last the weekend, but the tires were still a few psi above the 'safe' range so I dropped them all down to 38 psi again.
After the third session, the tires were finished and I was done for the weekend. The outside tread on the drivers side front was gone and the tire was chunking, I was worried that I wouldn't make it home without a blowout.

The Vollig crew kindly refilled my tires to factory for the drive home (thanks), they were at 30 front and 32 rear after lunch and the first three run groups in session 4. Gary took a look at my tires and told me that they normally only chunk when they're under-inflated, DOH!

I want to track my Supra again, it was still fun even though I only got 3 sessions, but I learned an important lesson: Ignorance can be expensive.
Maybe next year after I get some new gomme from Bears. :surr:
Tom Helvey
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Re: Tire Pressures - A Cautionary Tale

Postby afilsinger on Mon Oct 24, 2022 10:14 am

It was cool to see your supra out there this weekend! There are a few different factors to consider when comparing the supra to your Boxster...

1: Weight - the supra is significantly heavier than even a lightly prepped Boxster - something like 600 pounds.
2: tire compound - The PS4S is a high-performance street tire - not an R compound competition tire. It is not designed to operate in the temperature window we often see on track - especially on a heavy car like the supra.
3: Pressures - 38 PSI is well outside the window for your PS4S tires - too much pressure makes them susceptible to overheating faster. Overheated PS4S tires delaminate and start to lose chunks of tread, and if you drive them like this, ultimately, they will fail. The correct hot pressure range for a PS4S is 32 to 36 PSI, with a strong preference to be at the bottom end of that window for optimal grip, as well as wear. That being said, 300TW tires are never going to wear well on track, especially compared to a 200TW R compound "track" tire that was designed to operate in these conditions. Your starting pressures will need to be in the 20s to achieve these hot pressures. Personally, I need to start my tires 10-12 PSI below my target HOT pressure to get in the window without going over - but your mileage may vary depending on your driving style.

RE your Boxster pressures, I suggest you lower them as well - try closer to 32PSI hot. I think you will find the car more confidence-inspiring and, by extension, far easier to find time on track.
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Re: Tire Pressures - A Cautionary Tale

Postby Jad on Thu Oct 27, 2022 4:13 pm

I might add that the Supra is likely set up by the Toyota/Bmw lawyers to push significantly from the factory especially with the nannies off. If you were putting too much steering input in to counter the understeer, you would overheat and chunk the front outside edges regardless of tire pressure.

So, there are numerous factors at work as part of your cautionary tale, but the key takeaway is always check your tires between session and catch the issue early like you did, not wait for a failure!
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