Breaking (locking wheels)

Gold Member
Why is breaking with the wheels turning more effective than when the wheel locks up?
Is it because there is more friction between the breaks and the disks (or drums) than there is between the road and tyre, or am I missing something.

russ_watters
Mentor
No, the key is always the friction between the tire and the road: static friction is always greater than dynamic (sliding) friction.

The brakes on a car are usually stronger than the tire road friction as it is quite easy to lock the wheels in motion.

Anti-locking does not reduce the stopping power to any significant amount, but while the wheels are turning you can still steer the car instead of heading straight towards the object that made you brake in the first place. Usually locked wheels cannot steer even if you turn the steering wheel.

ABS will reduce braking power but in the range of instead of a 30m full stop, it might be 30.5 m. The added distance is small so the ability to steer comes first

Gold Member
No, the key is always the friction between the tire and the road: static friction is always greater than dynamic (sliding) friction.

I'll read the pf page on friction.

*Edit*
I read it but don't get why static friction > dynamic friction.

Friction is the normal force times the coefficient of friction.

The coefficient of static friction is always higher than the coefficient of kinetic friction (moving).

You can witness this by pushing a large block. It takes more force to get it moving than to keep it moving once it's going.

russ_watters
Mentor
I read it but don't get why static friction > dynamic friction.
Essentially, when something is stationary, the rough surfaces settle against each other, but when the surfaces are moving with respect to each other, they don't. They'll skip from one bump to the next.

Friction is the normal force times the coefficient of friction.

Yup, people are often suprised that it's independent of a surface area term.

So. Why do wider tyres give more grip than narrow tyres?

Why do wider tyres give more grip than narrow tyres?

Yeah that question was bugging me for quite a while.

In bikes it is worth it to have greater stability during turns...but for cars.............don't know.

Friction is not independent on area, in practice. Small surface area will deform some materials, while to big and it might stick. The formula for friction is a bit more complicated than F=ymg. It depends on surface area (but it's mainly a small dependence if the surfaces are not at least on the order of 20x different) and speed of the two surfaces, this might explain why car tires should be wider, while on a bicycle nobody cares.

russ_watters
Mentor
The friction equation doesn't include surface area because it is a already part of the coefficient of friction. So that doesn't imply that friction is independent of surface area.