There is friction between everything

Friction and locomotion

The normal braking process:
Let us first look at the curve above from the perspective of a car that is supposed to brake.
When the brakes are applied, the brake shoes try to bring the tire to a standstill, the friction on the ground tries to keep it turning. The more the brakes are applied, the stronger the frictional force between the tire and the road, which is proportional to the coefficient of friction, and the slip slowly increases, the braking behavior is stable, which means that greater pressure on the brake results in greater braking force. If the frictional force exceeds its maximum (given by the curve above) when the brake shoes are tightened, the frictional force between the tire and the road decreases, whereas the force braking the tire remains, the tire stops (blocked) and slips completely uncontrollably without steering ability and the Braking force decreases; the braking behavior is unstable, which means that greater pressure on the brake results in less braking force.

On the right you can see the coefficient of friction (no distinction between static and dynamic friction) as a function of the so-called slip.

At the Brakes is the Slip the ratio of the speed of the rubber in relation to the road surface to the speed of the vehicle.

  • Slip 0: tire sticks;
  • Slip 100%: tire blocked;

At the Start up is the Slip the ratio of the speed of the rubber in relation to the road surface to the sum of the vehicle speed and the speed of the rubber in relation to the road surface.

  • Slip 0: tire sticks;
  • 100% slip: tire spins;
ABS and ASR
The graph above shows that for maximum power transmission to the road, both when braking and when accelerating, a slip of 5 to 10% would be necessary, depending on the road surface, and even up to 30% on gravel roads. Both ABS (A.ntiblockiersystem) and ASR (A.drivessliprcontrol) try to achieve this optimal slip through appropriate control. For this purpose, the wheel speed is measured on all wheels and compared with each other and possibly with the vehicle speed via a computer. This data is used to determine if the slip on a wheel becomes too great due to excessive braking or excessive acceleration, whereupon the braking force or the driving force of this wheel is immediately reduced. The area in which ABS and ASR should act is marked by the encirclement. It can be seen that the area has very different levels of slip depending on the road surface, so that the control must primarily be carried out by comparing all the wheels and not by using absolutely specified values.
The diagram opposite shows qualitatively how the brake pressure is regulated as a function of the wheel speed and the vehicle speed. The wheel speed is regulated in such a way that the slip always remains in the range just below the maximum coefficient of friction, but does not exceed it.