lockup
A condition in which one or more of a vehicle's wheels suddenly cease to rotate due to the application of excessive brake torque, causing the affected wheel(s) to skid.
lockup: when brakes freeze a wheel solid
Lockup occurs when brake torque exceeds the maximum friction between tire and road surface, causing one or more wheels to stop rotating while the vehicle is still moving. The wheel ceases to roll and instead skids across the pavement, unable to translate the vehicle's kinetic energy into deceleration. This happens most commonly during hard braking, especially on low-friction surfaces like wet asphalt, gravel, or ice.
The physics behind lockup is straightforward: friction has an upper limit. The static friction coefficient between a tire and dry concrete might reach 0.9, but once a wheel stops rotating, kinetic friction takes over, often dropping to 0.7 or lower. More critically, a locked wheel cannot steer. A locked front wheel means complete loss of directional control; a locked rear wheel causes the vehicle's rear to slide outward, initiating a skid or spin.
Why lockup matters in practice
On modern vehicles with antilock brake systems (ABS), lockup is prevented electronically by modulating brake pressure many times per second, keeping wheels at the threshold of slip without crossing it. On older vehicles without ABS, lockup during emergency braking leads to longer stopping distances and loss of steering authority. Heavy trucks and trailers are particularly vulnerable because weight transfer during braking can unload rear axles, causing rear wheels to lock before front wheels reach their grip limit.
Different surfaces create different lockup thresholds. Wet pavement drops the coefficient of friction by 30 to 40 percent compared to dry conditions. Gravel, snow, and ice lower it further. Tire condition also matters: worn tread cannot evacuate water, reducing friction and promoting lockup at lower brake pressures. Brake fade, caused by heat buildup in sustained braking, reduces stopping power and can initiate lockup on the trailing axle even at moderate pedal pressure.
The term lockup is sometimes applied outside of braking: in drivetrains, a limited-slip differential or locking differential is said to "lockup" when it forces both wheels to rotate at the same speed, restricting slip. In compressors and pumps, "lockup" describes mechanical seizure or binding. In the braking context, lockup remains the standard term across automotive, heavy vehicle, and motorsport engineering.