Hi there

 

I have a base carrera with 18's so my experience is somewhat different. I run at 34-35 front and 36-38 rear HOT with Michelin Super Sports.

I do as you say come off the track and quickly check pressures and release as needed.

If I get too high in rear pressures, say over 42-43, the car feels like its on ice. Once this summer I went too low and that was not good either. Flet like I was driving in mud.

With 19's your pressures seem to be about right but other guys running 19's can confirm.

best

 

Thanks for that.


Mine is set 33/39 cold for the road and I often see 44psi in the rear hot on the road but have to say it feels fine, but I've heard several mention that on the track over 40psi in the rear and it gets very greasy.

Does anyone know why this is?

 

----

 

40/52? I dare say not on a track.

 

I haven't noticed much interest in what engineers suggest on this issue of faith, but for your consideration Michelin suggests sports car owners add about three pounds to the manufacturer's road recommendation. A 997 light load setting is 34 front, 40 rear, so the suggestion amounts to 37/43.

They do not recommend bleeding air except in race situations. (Which implies people who know what they're doing, not just trying to get rid of a greasy feel. Forgive me, but you asked.)

In a private note a senior engineer told me they set up the cars for the 997.2 launch at 38/44 (cold of course) for Hurley Haywood and David Donahue and then bled the rears back to 50 psi during the day. He also remarked that he doubted many drivers would put heat in the rears like those two did. He went on to suggest that experienced drivers might want a couple of pounds extra in the fronts of a modern Porsche if it suits their driving taste. I'd already reached that conclusion from my own DE's and I endorse that -- if you fit the experience profile.

So. If you want the recommendation of a couple of engineers with track experience, set the tires cold at 38/44 or if you prefer to 39/45, keeping that six-pound differential. After some track time, you might want to bump the fronts two pounds for the second excursion if your experience and your handling preferences agree. If you do manage to keep enough heat in the rears that they stay over 50 psi when checked ... let's say ten minutes after exiting the course, then bleed them back to 50 psi. Otherwise, don't bleed air. We spec these things at a standard 'cold' temperature for good reason. A tire working for its living is supposed to warm up and the increased pressure from heat keeps the tire from getting ever hotter. That's a big part of the way tire designs work.

Not according to professional drivers and Michelin engineers it would appear. Not in my experience either, but you don't know me.

You understand the idea, but I seriously doubt tires as low profile as ours suffer from this unless you intentionally drop the pressures below recommended street values. Michelin said they suggest that 3 psi bump to improve transient response and then they implied it also reassured people about effects like roll over. They implied it while also scoffing at the likelihood with low profile tires.

I would class it with tires rotating on the rims. It can happen. We see it in race cars, and particularly with standing starts in high-power cars, and in aircraft. Rim bolts are used when that's really a problem, but I'm skeptical of it happening with road cars at track days. Not impossible of course, but I'd want to try some measurements of my own if someone complained of it annoyingly.

I wouldn't worry about either one. Just set your pressures at cold temps before heading for the track and don't worry about it after that unless you do something that might have damaged a tire. See what your TPMS reads after setting the track pressures you want with a quality gauge. Then you can watch how much they warm up during the event. It should be less than five pounds unless you're Hurley Haywood hanging around this forum for kicks.

Gary, hanging around

 

I set my MPSCs to 32/36 hot.
Then, and this is the most important part, I check temperatures with a pyrometer as soon as I come in from a run and adjust accordingly.
The temperature of the tires in specific areas tells you if you're under or over inflated. This is the most reliable way to set your pressures.
Don't forget to go by feel. Just because the label on the door jamb says 44 - that doesn't mean that the manufacturer you have will handle well at those pressures.
My MPSC's are great when they're at the 32/36 range with even temperature across the face of the whole tire even though the specified pressure is much higher than that at which I have them set.
Bottom line is that you want as much contact patch as possible without roll over and temperature is the most reliable way to find that out.