[Flieger]

It would be nice if it was only the Ff=mu*Fn principle from basic engineering classes. Unfortunately mu is not constant. Temperature is one variable affecting it and load is another. The coefficient of friction drops off with increased pressure on the rubber, and the dropoff is all dependent on the tire compound and construction. Furthermore, the tire construction has inherent stiffness that means air pressure is not the only thing affecting the size of the contact patch. Then you get into things affecting the shape of the contact patch...

Its not so simple, which is why the best engineers get paid the big bucks.