Slip ratio is a means of calculating and expressing the locking status of a wheel and is vital to the effectiveness of any anti-lock braking system

Anti-lock braking system

An anti-lock braking system is a safety system that allows the wheels on a motor vehicle to continue interacting tractively with the road surface as directed by driver steering inputs while braking, preventing the wheels from locking up and therefore avoiding skidding.An ABS generally offers...

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When a vehicle

Vehicle

A vehicle is a device that is designed or used to transport people or cargo. Most often vehicles are manufactured, such as bicycles, cars, motorcycles, trains, ships, boats, and aircraft....

 is being driven along a road in a straight line its wheels rotate at virtually identical speeds. The vehicle’s body also travels along the road at this same speed. When the driver applies the brakes in order to slow the vehicle, the speed of the wheels becomes slightly slower than the speed of the body, which is travelling along under its own inertia

Inertia

Inertia is the resistance of any physical object to a change in its state of motion or rest, or the tendency of an object to resist any change in its motion. It is proportional to an object's mass. The principle of inertia is one of the fundamental principles of classical physics which are used to...

. This difference in speed is expressed as a percentage, and is called ‘slip ratio’.
The ideal slip ratio for maximum deceleration is 10 to 30%.

Slip ratio is calculated as follows: -
Slip Ratio % = [(Vehicle Speed – Wheel Speed)/Vehicle Speed] x 100