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      06-22-2020, 11:31 AM   #1
smchuck
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tire pressure

Question about track tire pressure.

I'm driving a 230i xDrive with MPSS. I'm a competent Novice.

The door sticker says 32 psi front, 36 psi rear. This is independent of tire or driving style. I assume these pressures are for best for handling? These pressures don't do me any good for hot temps at end of session.

I've heard the MPSS run good at 40 psi. I could check pressure after each session and shoot for 40 psi both front and back.

I also chalk my tires. I can adjust pressures based on chalk marks.

Any thoughts/suggestions?
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      06-22-2020, 12:12 PM   #2
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For tracking you set via hot pressures; cold start point is often way off from the doorsill numbers. I haven't run MPSS, but similar Conti's gained about 7# from true cold, then gradually climb as you continue to heat them. I was running about 38-41# hot to try to prop up the outer edge some, but others w/ more experience w/ the MPSS may have more specific suggestions. You may want to use a pyrometer to look at variation across the tread to adjust this some (I use a $20 laser gun, since I'm mostly interested in relative spread rather than true temps). Once you start seeing the insides cooler than middle you are at- or over the optimal pressure; outside will always be hotter on our cars b/c can't set camber. And FWIW, I use roughly 35f/37r on the street; factory specs are biased to give good ride and a little more understeer, this sharpens up the steering response for me.
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      06-22-2020, 12:56 PM   #3
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Quote:
Originally Posted by Maynard View Post
For tracking you set via hot pressures; cold start point is often way off from the doorsill numbers. I haven't run MPSS, but similar Conti's gained about 7# from true cold, then gradually climb as you continue to heat them. I was running about 38-41# hot to try to prop up the outer edge some, but others w/ more experience w/ the MPSS may have more specific suggestions. You may want to use a pyrometer to look at variation across the tread to adjust this some (I use a $20 laser gun, since I'm mostly interested in relative spread rather than true temps). Once you start seeing the insides cooler than middle you are at- or over the optimal pressure; outside will always be hotter on our cars b/c can't set camber. And FWIW, I use roughly 35f/37r on the street; factory specs are biased to give good ride and a little more understeer, this sharpens up the steering response for me.
Thanks. I was thinking of DM'ing you rather than posting since you always give the most relevant responses to my questions. Sure enough, though, you responded and seem right on target. thanks. I've got an IR thermometer for my pizza oven so perfect, and from your post, it does look like around 40 psi hot for MPSS is about right.
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      06-22-2020, 11:34 PM   #4
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I was running ~38 psi hot, trying to keep the range close front and rear. (1) That's what the PSS needs to keep the sidewall stiff enough to not roll over on the stock front wheels, which you can check yourself with your chalk, and (2) keeping the stock psi balance front to rear was mildly understeer-y at the limit; keeping them the same front to rear helped the car to rotate more easily. Sport+, traction off
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      06-23-2020, 07:21 AM   #5
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Originally Posted by 230iZTR View Post
I was running ~38 psi hot, trying to keep the range close front and rear. (1) That's what the PSS needs to keep the sidewall stiff enough to not roll over on the stock front wheels, which you can check yourself with your chalk, and (2) keeping the stock psi balance front to rear was mildly understeer-y at the limit; keeping them the same front to rear helped the car to rotate more easily. Sport+, traction off
Thanks
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