What is an anti-lock braking system

The anti-lock braking system - ABS (1)

landing gear

Basics of the anti-lock braking system (ABS)

The anti-lock braking system (ABS), first produced by Bosch in 1978, prevents the wheels from locking in the event of emergency braking. The vehicle remains steerable and unexpected obstacles can be avoided. The ABS thus significantly improves driving stability and usually also shortens the braking distance.

The advantages of ABS at a glance:

  • Cornering forces and steering ability are retained, evasion is possible!

Over two thirds of vehicle manufacturers worldwide equip their new vehicles with ABS.



Some braking physics

The braking distance of a vehicle depends on the speed, the weight of the vehicle and the sum of the braking forces.

The maximum braking force depends on the wheel load and the current coefficient of friction between the tire and the road.

In the event of an emergency stop, the ABS causes the maximum possible braking force on each of the 4 wheels

Brake physics

Steerability means that sufficient cornering force is available on the front wheels when cornering. But if a wheel transmits 100% braking force, nothing is left for the cornering force. A locking wheel cannot be steered.

Kamm's circle of friction shows this relationship: cornering force can only be transmitted in a certain ratio to braking force.

Driving stability means that a vehicle remains directionally stable when braking. If, for example, the rear wheels lock, no cornering forces can be transmitted, the vehicle breaks away on the rear axle and skids.

Kamm's circle of friction

Questions to understand about slip and cornering force:

Diagram of cornering force (VW)

During the braking process, the braking force initially increases very sharply (with increasing slip) and then slowly decreases again. The side forces that act when driving and braking behave in reverse.

The lateral force that can be transmitted is called the cornering force. It is a measure of driving stability.

Sufficient cornering force can still be transferred in the working area of ​​the ABS.

Cornering force diagram

Brake force coefficient diagram (VW)

The braking force coefficient depends on the condition of the road and the weather conditions.

The maximum possible braking force for wet concrete is lower than for dry concrete. This means that the braking distance will be longer.

At very low coefficients of friction (snow and ice), only little braking force can be transmitted.

The ABS regulates the brake pressure so that the maximum possible braking force is always used.

The snow slip curve increases again from 90%. Reason: A snow wedge builds up in front of the locking wheels (without ABS), which shortens the braking distance. However, the vehicle cannot be steered. It's the other way around with ABS in snow: it can be steered, the braking distance is a little longer.

Brake force coefficient diagram

ABS control circuit



Questions to understand the ABS control loop
1. Who is responsible for the pressure build-up?The driver using the brake pedal.
2. What influences the brake pressure as a rule?The hydraulic unit.
3. Who controls the hydraulic unit?The control unit.
4. Where does the control unit get information about the turning behavior of the wheels?from the speed sensors.




Video tutorial on ABS from Bosch

The film is a bit older, but the effect is clearly shown.


Sources: Bosch, VW, Teves, AUDI, own graphics