How do anti-lock brakes keep checking the wheels?

A braking vehicle can still move forward while one of its wheels almost stops rotating. If the wheel locks, the tire slides rather than rolling. An anti-lock braking system, or ABS, must notice excessive slip and respond: measure wheel motion, adjust brake pressure, and measure again. This rapid feedback helps preserve directional control during hard braking as road grip and wheel loading change.

The tire-rotation equivalent is its rotation rate multiplied by its effective rolling radius. A useful simplified measure of braking slip is the vehicle speed minus the tire's circumferential speed due to rotation, divided by the vehicle speed. If a vehicle moves at 20 meters per second while the tire-rotation equivalent is 14 meters per second, the slip is (20−14)/20 = 0.30, or 30 percent. A locked wheel has zero rotation speed and, under this definition at nonzero vehicle speed, 100 percent slip.

The illustration uses invented decision thresholds to expose the feedback mechanism. Above 20 percent slip, its controller reduces pressure. Below 10 percent, it increases pressure. Between those values, it holds pressure. Changing pressure changes braking torque, which affects wheel rotation; the sensor then supplies another measurement. Actual ABS controllers use more information, including wheel acceleration and estimates of the vehicle's reference speed.

The repeated loop matters because a single brake pressure cannot suit every moment of a changing stop. ABS limits excessive slip so that a tire can retain useful directional control instead of simply sliding while locked. It cannot guarantee the shortest stopping distance on every surface. The invented thresholds show how feedback works, not a production vehicle algorithm: speed, tire condition and firm or loose road surfaces still affect the outcome.