How do anti-lock braking and traction-control systems detect when a car is beginning to skid?
Both systems measure wheel speed relative to each other. Each wheel has a disc that is similar to a gear that produces a electric pulse through a magnetic pickup that tells the computer the wheel speed. If any wheel or combination varies in speed upon braking, out of the computers limits, it overrides the manual brake pressure through the use of a system of hydraulic valving that cycles the brake pressure at the maximum appropriate rate without locking any wheel.
