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Old 09-10-2005, 08:05 PM   #1
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Tire Pressure?

My car is fitted with Good Year Eagle F1 tires (oddly enough made in Germany) 225/40ZR18 92Y Front and 275/35ZR18 95Y rear. Can anyone recommend the proper tire pressures for these tires?

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Old 09-11-2005, 08:40 PM   #2
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Reading Rainbow

I've read a lot of books in my day and none of them will change your life like the Holy Bible. But brutha, next on the list is Boxster/Boxster S Manual. It's definately worth the investment (both timewise & financially).

From system checks to warning lights, tire pressure rates to bulb sizes -this book has it all! Detailed illustrations make this read a piece of cake! Oh, and there's one special twist at the end when the Boxster... well, I'm not gonna give it away, you'll just have to read the book.

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Old 09-11-2005, 09:14 PM   #3
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Ah, Ya, I actually received all the manuals with my car and somewhere very near the front are listed the recommended tire pressures. Your right, these manuals, although not quite as valuable as The Holy Bible, are quite informative. Trouble is the tires on my car that I listed are not the tires Porsche put on the car. I think that matters. Right? I'll check Good Years website as well.
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Old 09-12-2005, 06:02 AM   #4
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the manual recommends pressures that are way too conservative for abundant understeer.
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Old 09-12-2005, 08:37 AM   #5
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29psi front, 36psi rear (YM 2000, YMMV).

It doesn't matter what tire you use, the pressure is determined by the vehicle and its' performance.
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Old 09-12-2005, 11:34 AM   #6
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Trick question: How do you reduce/eliminate the factory understeer, and get closer to neutral:

a. Set fronts 2 psi higher than rears
b. Set rears 2 psi higher than fronts
c. Increase psi uniformly in all four tires

Reason I ask, I have seen both answers a and b in other forums. Would seem only one could be correct.
June track day: I ran 36/36 cold based on advice that the fronts need more psi for turn in. Wound up about 40/40 hot. You could feel the understeer.
Friday track day: started 34 rear/32 front (cold) and wound up around 40 rear/38 front (hot). Car felt better balanced, but still with some mild understeer. I could get the tail loose if I worked on it. 90 degree day.

Comments:
- Seems to support "b". Wonder where "a" came from?
- Not sure I want to go to oversteer (would get a 911 for that). Neutral is nice.
- R/F 36/32 (cold) works fine on the street.

Open for comments/discussion -
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Old 09-12-2005, 11:07 PM   #7
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Limoncello,

I believe the generic answer to reduce understeer is to:

1) start with the factory prescribed pressures
2) increase the fronts relative to the rears until you feel an improvement

Increasing the front pressure usually gives better grip in the front under extreme cornering, thus reducing understeer. This makes sense if you extrapolate the OTHER way: i.e., REDUCE front wheel pressure. As pressure moves towards zero, you can imagine the wheel giving you less and less grip at becomes soft and deforms under cornering.

Note: this is for the general case, if such a thing exists. Our Boxsters may be different because of the fact that it's mid-engined and the tire sizes are different front to rear. But I think it the concept should apply. The concept may not apply under extreme circumstances however (such as extremely high tire pressures where maybe some extra air in the fronts could cause the tire to be massively overinflated and reduce the contact patch (I'm not sure about this though).

I have tried this out in low-speed autocross and on the street in front-wheel drive cars. My cars went from the factory understeer to neutral after adding maybe 4 pounds to the fronts. However, in one case, I saw significantly increased center wear on the front tires in street driving because the center of the tire is now higher than the sides due to the higher pressure.

I believe the factory tire pressures in "normal" passenger cars are set to provide understeer to be safe and to provide the best ride and longest tire life. Tires pressures set to maximize these things are unlikely to produce a neutral handling car.

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