Impulse24,
Just a couple of points, the change in breaking is due to the radius of the tire. The larger the diameter, the longer the moment arm of the friction force at the road to the brake. (Think of it like a longer wrench on a socket...your hand is the road friction and the torque at the nut is your braking force.) So for a given velocity, to decelerate the vehicle at a give rate, the greater the force the brakes must exert. Same works the other way for engine torque for acceleration...
Next, the braking friction between the tire and the road is a function of the coefficient of friction for the two materials (the rubber and pavement) and the force between them, which is essentially the weight on that tire. The contact area does not figure into the equation in its most basic sense. When you consider local deformation of the materials, then the frictional force decreases with more surface area. With small contact area, things "dig in" due to local deformation. That's why you put a big pad under a small leg to get it to slide easier or why big wide tires don't have much traction on snow.
Bottom line, larger tires will hurt braking.
Jeff
