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      03-06-2011, 01:55 PM   #12
Lotus7
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Quote:
Originally Posted by kmarei View Post
well that arguement relates to every discussion on this forum
but in regards to this question
on the X3 if you should put the stock air pressure or more
more pressure means less grip
since more pressure means smaller contact patch
Sorry - Don't want to be argumentative, but your logic is just too simplistic.

Grip depends on the uniformity of pressure across the contact patch (as measured by spot tire surface temperature measurements) and shape of the contact patch as much or more than the final size of the patch. At low pressures, which usually include the factory recommended pressure, which is picked for the best ride/ handling compromise the ultimate grip (or cornering power, or lateral acceleration-however you want to define it) is usually not achieved. Most passenger cars will increase cornering grip as pressure is increased above the factory rec. setting for a while and then loose grip as the contact patch deforms and becomes smaller.

It's not just the number of square inches of contact, but the amount of contact pressure the vehicle is able to apply to each of those square inches of contact patch. the idea is to get the most uniform pressure on the most square inches. Having a slightly larger contact patch does not help if the pressure on EACH part of it is not the maximum possible. At stock or lower pressures that's what happens during moderate to hard cornering.

Anyone who has ever raced on a track or participated in an autocross will usually wind up with pressures substantially above the factory rec. settings if they want any chance of being competitive.

This is well documented in race oriented magazines such as "Grassroots Motorsports" where they routinely do tire tests on various chassis, as well as every basic book on how to improve your cars handling for competitive purposes. "Grassroots Motorsports" last comprehensive test on a Miata wound up using pressures in the 41 to 46 psi range for a car that has a factory rec. pressure of 32 psi. This was for a autocross at the TireRack's test track in Indiana.

Personally, EVERY street car and race car I’ve driven, road raced or autocrossed from my full race SCCA A-Modified Corvette and Lotus S-1 to my wife’s Subaru wagon has had significantly higher grip as well as better transient response at a tire pressure higher than the factory recommended settings. Sometimes it’s only a few psi more and sometimes it’s as much as 10 psi. I’m speaking about ultimate dry road grip independent of ride quality.

Plus, I'm not even getting into what happens to the tire patch shape at stock or lower pressures because of sidewall deflection lifting the tread and reducing the uniformity of pressure distribution across the patch.

Last edited by Lotus7; 03-06-2011 at 02:10 PM..
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