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socratic · 09-12-2005, 11:07 PM

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.

09-12-2005, 11:07 PM	#7
socratic Registered User Join Date: Feb 2005 Location: Canada Posts: 183	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.