What Causes Front Wheel Lock-Up?
2 Answers
Front wheel lock-up occurs due to excessive braking force between the brake pads and the disc or drum brakes. When a vehicle has significant inertia, it continues moving forward, causing the tires to skid rather than roll. This prevents the vehicle from changing direction, potentially leading to accidents. Even with low inertia, on congested roads or when following other vehicles too closely, wheel lock-up can cause sudden stops, increasing the risk of rear-end collisions. Wheel lock-up and sudden braking, resulting in loss of steering control, typically occur in vehicles without ABS (Anti-lock Braking System). In non-ABS vehicles, forcefully pressing the brake pedal rapidly reduces wheel speed. When braking force exceeds the friction between the tires and the road, the wheels lock up completely. Fully locked wheels increase tire-road friction, which, under high inertia, can cause skidding and loss of directional control. If the front wheels lock, the driver cannot steer the vehicle; if the rear wheels lock, the vehicle is prone to skidding. In ABS-equipped vehicles, during emergency braking, speed sensors installed on each wheel or driveshaft continuously monitor wheel speed. A computer calculates the wheel slip ratio (indicating whether the wheels are locked) and compares it to the ideal slip ratio. Based on this comparison, it adjusts the braking pressure—increasing or decreasing it as needed. The controller instructs the modulator to reduce hydraulic pressure in the wheel's brake cylinder, decreasing braking torque. After a brief interval, the original pressure is restored. This cycle repeats rapidly (5–10 times per second), ensuring the wheels remain rotating while maintaining maximum braking torque. Thus, ABS prevents wheel lock-up by keeping the wheels in a slightly slipping rolling state during braking, enhancing braking efficiency.
Last time my car's front wheels suddenly locked up during emergency braking, nearly causing a rear-end collision. Inspection revealed the return spring of the left front brake caliper had failed, preventing the brake pads from properly separating. Additionally, sticking brake caliper pistons are a common issue, especially after prolonged driving on muddy roads where dirt seeps into the guide pins causing poor retraction. The mechanic mentioned that brake caliper corrosion from winter de-icing agents is more prevalent in northern regions. Now I've made it a habit to have the technician remove the tires during each maintenance to clean the caliper slides and check the brake fluid's moisture content. By the way, if the brake disc warps by more than 0.05mm, it can also cause lock-up, which requires a dial indicator to detect.
