Anti-lock braking system (ABS)

A locked wheel has no grip left for steering and, on most surfaces, stops the car more slowly than a wheel that is still just turning. ABS exists to keep the wheels on the edge of lock-up during a panic stop. Every new car sold in India has had it as standard since April 2019.

The hardware is a toothed ring and a speed sensor at each wheel, and a hydraulic unit, usually bolted in the engine bay, that sits between the master cylinder and the brake pipes. The unit holds a pair of solenoid valves per wheel, a small pump and an electronic control unit.

• The sensors report each wheel's speed many times a second.
• If one wheel slows far faster than the others, the ECU decides it is about to lock.
• An inlet valve closes to hold the pressure at that wheel, then an outlet valve opens to release some of it.
• The wheel speeds up and regains grip; the valves close and the pump pushes the pressure back up.
• The cycle repeats several times a second, for each wheel separately, until you lift off or the car stops.

In normal driving ABS does nothing; the valves are open and the pedal feels like any other. In a hard stop you feel a rapid pulsing through the pedal and may hear a buzz from the unit. That is the system working: keep your foot planted and steer. ABS shortens stopping distances on wet tarmac but not necessarily on loose gravel or snow, where a locked wheel can plough up a wedge of material. The same sensors and valves are the foundation for electronic brake-force distribution and stability control. Two-wheelers use the same idea in single-channel and dual-channel form.