Possible Duplicate:
Why do people recommend wider tyres in car for better road grip?
Static/kinetic friction says that the friction force is proportional to the normal force, via the coefficient of static/kinetic friction. Great.
Imagine that you have a sports car. You'd like to improve your lap times at a nearby race track. So, you buy larger (width) tires. Bigger contact patch, so your car sticks to the road better. But wait, the normal force on the car didn't increase, and the coefficients of friction of the tires didn't change. Just the contact patch. What explains the increase in grip from larger tires?
