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<div class='quotetop'>QUOTE</div><div class='quotemain'>The difference in tyre pressures provides stability.[/b][/quote]
never heard that before. I had a blow out a few years back on the Putty road (Colo Heights end) on the right front @110km/h in a right hander.
Luckily it was the more lightly loaded side, and the 'fender tracked dead straight even under braking until the stop. It was an instant deflation, and I was driving on the rim. I was also running almost the same pressures front and rear. (IIRC @ 36 psi Bridgestone 235/85 MT's)
Generally, the tyre placard is a very good starting point, although the tyre placard for the 130 recommends 75psi in the rear in ALL conditions. (laden and unladen) If I ran that I'd have no tread in the centre of the tyres in about 5000km, the back end would be skipping all over the road, and I'd probably slash them to bits before that distance on our lovely gravel roads. Even with the OE Michelin XZL's, I started @ 50psi and dropped from there. Different tyres from OE also have differering constructions, with some being quite a bit stiffer or softer in the carcass, and so need a slightly different pressure to suit a given situation. Of course if the tyre size is different to OE, everything changes. Generally, a larger tyre (taller/wider) doesn't need as high pressures as a smaller one due to its greater internal volume, although this doesn't always hold true if the tyre uses a realtively soft sidewall/pliable case. More pressure may be needed here for support.
In road cars, I always ran more pressure in the front than the rear as there was much more weight over the front axle, as I do in our Patrol. (unless carrying a load) Any front engined race car I had anything to do with always ran more presure in the front than the rear for this reason. This was also borne out with lap times. Mid engined cars, even with their greater width/larger volume rear tyres always ran the opposite. e.g. Formula Fords run 17psi front, 19 psi rear hot pressures. Race tyre pressures are always checked hot, and adjusted accordingly. e.g. Cold days/weather, starting pressures may be higher, or a non abrasive track surface would use a higher starting pressure to achieve your hot optimimum running pressure. Differing suspension setups also require differing starting pressures, as they can work the tyre carcass differently, generating lesser or greater temperatures and consequently pressures.
Interesting fact on wet weather pressures. Any race car I ran that didn't use specialist wet weather tyres used much higher pressures in the wet than the dry
It didn't matter wether they were bias ply (Formula Ford, Historic Touring Cars, etc) or radials (production cars)
Any time I'd let the tyres down (at the request of a driver, 'cause everyone else was doing it) I'd be abused and we'd be as slow as everyone else. (I was accused by the driver at my first Historic meet with an XU1 of trying to kill him after setting the pressures where he insisted. He then proceeded to drive through the field in the second race and place, and he was a very cautious driver who hated the wet)
A couple of things are happening here. One is that the channels in the tyre are more defined with the higherpressure, the second and main one I think is that the reduced contact patch worked the centre portion of the tread much harder and actually was able to generate some sort of heat to generate grip. The tyres were always too cool after a race to actually check this with a pyro, but the fact remained that the cars were always much nicer to drive and much faster with silly high pressures.
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