# What causes a car wheel to skid under heavy braking?

Say you are driving down the road and and jam on the brakes in a car with no ABS, at what point does the wheel stop spinning and stop causing a skid.

I suspect it is when the friction between the brake disk and brake pad become greater than the friction between the tyre and the road?

Thanks

. . . . at what point does the wheel stop spinning and stop causing a skid?

If the wheels are not rotating and the car is moving then the car will be in a skid.

The condition that there is no relative movement at the point of contact between the road and the tyre is that $$v=r \omega$$ where $$v$$ is the speed of the wheel axles, $$r$$ is the radius of the wheel and $$\omega$$ is angular speed of the wheel.

When the brakes are applied the effect is that frictional forces due to the brake pads reduce the speed of rotation of the wheels which in turn means that there are frictional forces acting on the tyres due to the road in the opposite direction to the motion of the car.

If the condition $$v=r \omega$$ is maintained, ie there is no relative movement at the point of contact, then the frictional forces between the tyre and the road are static.
Applying the brakes harder, ie slowing the car down faster, requires the static frictional forces to be larger.

However there is a limit to the magnitude of the static frictional forces related to the coefficient of static friction $$\mu_{\rm static}$$.

If the maximum value of the static frictional force is insufficient to produce the required rate of reduction of speed then there will be relative movement between the tyre and the road at the point of contact, ie $$v>r\omega$$, and a skid occurs.

The important parameter is now the coefficient of kinetic friction, $$\mu_{\rm kinetic}$$, which is smaller than $$\mu_{\rm static}$$ so at the onset of a skid there is an almost instantaneous decrease in the frictional force trying to slow the car down which can lead to a loss of control of the car.

• Awesome, I think I have it sorted in my head now. Thanks – 5 8 Nov 2 '19 at 13:36

It is the static friction between the bottom of the tire and the road surface that keeps the tire from slipping or skidding (kinetic friction). Even though the tire is rotating, the bottom of a non slipping tire is in static contact with the road. When applied force is greater than static friction the wheel will slip. This can be caused by hard braking (skidding), extreme applied drive train power (peeling out), extreme cornering (sliding sideways), or a combination of these forces. Rain, ice, oil, or other properties can reduce static friction co-efficient between the tire and the road, making slipping easier.

• Thanks for the reply, much appreciated – 5 8 Nov 2 '19 at 13:34