Why have wide tyres for grip? If gravity is µR?

Question

If the formula for friction is
µR, where µ is the coefficient of friction and R is the reaction due to gravity (actually it is the same value as gravity with the direction reversed).
Where does having more surface area help in a better value of friction and hence a better grip.

Answer 1

That formula only works up to the point where the shear strength of the rubber is exceeded, beyond which the fictional force per *unit area* of rubber cannot go any higher. See all that black stuff on the track? That's rubber shearing off. So, the wider the tires, the more *total* shear force (aka traction) you get.

Answer 2

The point is that µR = µ (mg) is just the MAXIMUM that the friction force can withstand, beyond which it will slip. For example, if there is no one pushing on the box, there is no push to resist, so there is no friction force either. When you first start to push, the friction force will push back against you - until you exceed the limit of µ(mg), at which point it will begin to slide.

The point is that the limit depends on the value of µ, and the value of µ depends on the area of the exposed rubber: how much of it is in contact with the ground. So the greater the area (the wider the tire), the more sliding forces can be resisted by friction.

Answer 3

this concept of having more surface area to get better grip and friction can be visualize and explained by using an example. An F1 racing machine as we all know uses wide tyres because it has better grip on the road. narrow tyres means smaller surface area and we know that smaller surface area means less grip. So we can relate it to friction. To have less friction the tyres must be narrow meaning smaller surface area and to have greater friction tyres must be wider thus having greater surface area.

Answer 4

I'm not 100% sure but I think the coefficient of friction is only independent of the surface area for hard surfaces