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Originally Posted by scoobanader
dont you want more pressure in the back tires then the front. so that the rear kicks out and follows the front tires?
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Tuning for under/oversteer via air pressure depends on the tire.
On normal street tires, let's use the RE-92s as an example, the biggest problem with grip at the limit comes from side-wall roll over. Basically, the tire deforms and you end up driving on the sides of the tires where there's not much grip. That's the sound you hear when the front tires are sqeeling and then they start to vibrate really bad. To increase overall grip on a "street" tire, you up the air pressure, because it helps to prevent roll-over.
On performance or race tires, let's take some Azeniz Sports as an example, the side wall of the tire is already very stiff. Adding air pressure to these tires actually reduces traction because it reduces the footprint of the tire on the ground.
So if you're running softer sidewalled street tires, you'd actually lower the rear pressures to get the car to rotate. Less traction in the rear = more oversteer. When I was racing on my RE-92's I'd run 44psi front 40psi rear, or there abouts. The car still had a ton of understeer, but this was about the best I could get it and still keep from destroying my sidewalls.
Now that I've got coilovers, swaybars and pseudo-race tires, My air pressures are primarily determined by tire temps. Idealy right after coming off course, you should have Inner, Middle and Outter tire temps (measured across the treadblock) that are equal. This mean that the entire treadblock is gripping as much as possilble. Using a combination of camber adjustment and air pressure, you can get pretty close. Then you use the adjustability of the suspension components to set under/oversteer. (I will also fine tune by changing rear air pressures a pound or two between runs.... more pressure = looser, less = tighter.)