Originally Posted by
dscheidt
The contact patch pressure is only equal to tire pressure for a small range of pressures, which for a typical tire, are lower than most people run them at. Limits of carcass flexibility limit the range that it's true for; outside of that range, there will be a region of higher than predicted pressure at the front and rear of the patch, surrounding an area of lower than expected pressure. this effect is one of the reasons that tires at higher pressures are less efficient at transmitting power to the ground in poor traction situation.
Let me try to explain my point, as I think you missed something. Take 2 tires, the same construction, one is 23mm wide, the other is 32mm wide, at the same pressure, with the same load, the contact patch will be the same size. However the narrower tire will also have much less height from rim to tread, as it spreads out to the larger contact patch size. It's this lack of height that causes pinch flats, so the pressure required may be much higher in order to prevent them. In fact the pressure needed to prevent pinch flatting may be so high as to exceed the design specifications of the tire. For example with our example load, if the tire is rated 140PSI, but needs 180PSI to prevent pinch flats, you have too narrow a tire for the load.
Your right too about higher pressures, given two tires that are a specific width, one at lower pressure and the other at higher pressure, the higher pressure tire, will have a smaller contact patch, reducing the amount of rubber on the road to produce traction.