Measure pressure when tires hot for track use

COURTESY OF GENERAL MOTORS

Reader wants to upgrade his tires on a 1998 Camaro Z28, like this one above, to 17-inch wheels.

John Mahler

SPECIAL TO THE STAR

Q: I am getting conflicting advice on optimal tire pressure for the Michelin Pilot Sport Cups on my Maserati for track use and would like your thoughts as well.

When the tires were first put on, it was recommended that the pressure not exceed 32 p.s.i. cold. The fronts wore quickly, and I just had a fresh set put on where it was recommended that I go to 36 p.s.i. cold.

I do plan another camber adjustment to take the fronts to at least a -2 (they are -1.4 now) so that will help either way but the pressure question remains. Do you have some thoughts?

A: I cannot give you a precise answer, because I don’t have all the facts. However, both those pieces of advice are meaningless. For track use, pressures are always measured hot. What you start with cold is irrelevant. It is what the pressure is after a session that determines what to do.

I would start by trying to get hot pressures at the factory settings. I believe the factory settings are 32 front and 32 rear on your car. After a session, I would bleed them down to that. Run another session, bleed them down to that again, etc.

Since your car likes to chew on outer edges, I would consider making the tire a bit more round across the tread face. It provides less grip but uses the shoulder less. For that, I would increase the hot temperatures by 5 per cent over factory or 10 per cent. That will reduce the wear rate but also lessen traction.

At the end of the day you will have bled the tires down quite a bit, so remember to add some air or they will look pretty flat by morning. R-compound tires take many, many hours to cool to ambient, and they will lose pressure as that occurs.

Please note from the Michelin Technical Guide to the Pilot Cups: Hot, the tires should be in the 32 to 36 p.s.i. range. So obviously that is where your car wants to be. To achieve that, Michelin recommends average cold starting pressures of 25 p.s.i. front and 28 p.s.i. rear, then fine-tune.

You cannot run these tires below 20 psi.

Q: I have a 1998 Camaro Z28. The tire size is P245/50R16. If I were to go to a 17-inch rim, what size would I need?

A: The diameter of your Z-28 tire is 25.65 inches. The correct upgrade to a 17-inch tire is size P245/45R17. The diameter of that tire is an almost perfect 25.68 inches. Plus, you gain 20 mm in tire width, almost an inch.

Email tire questions to John Mahler at thetireguy_1@hotmail.com. Please include vehicle's make, model and year, tire brand and size, as well as your name, address and phone number. Due to volume, personal replies cannot be provided.

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