Definition of Lockup

The point at which a tyre starts to skid during braking. A tyre’s maximum braking force is developed when it is on the verge of lockup, so a car’s shortest stopping distances are produced when its front and rear tyres approach lockup simultaneously. This is very hard to achieve under varying conditions of load and traction, so in reality one end typically locks up before the other.

Front-wheel lockup is inherently more stable than rear-wheel lockup.