Runout

📖 YOUCANIC Automotive Glossary

Runout is the measurable deviation from perfect circular rotation of a rotating component — how much it wobbles as it spins. In automotive applications, runout is measured on brake rotors, wheels, wheel hubs, and axle flanges. Lateral runout (side-to-side wobble) is the most commonly measured, using a dial indicator placed against the rotor face or wheel mounting surface while rotating the component by hand. A new brake rotor typically has less than 0.001 inch (0.025mm) of lateral runout. Excessive runout causes the rotor face to oscillate as it rotates, pushing the brake pads in and out with each revolution, creating a pulsation felt through the brake pedal and steering wheel during braking.

Brake pedal pulsation is one of the most common brake complaints, and excessive runout is the primary cause — not warped rotors as commonly believed. Even a perfectly machined rotor develops thickness variation (DTV) over time if installed on a hub with excessive runout, because the high spot contacts the pad more frequently, wearing unevenly. DIYers should measure hub flange runout before installing new rotors — if hub runout exceeds 0.002 inch, the hub assembly should be replaced or the rotor indexing position changed to find the minimum combined runout. Always clean the hub mounting surface of rust and debris before rotor installation, as even a small rust flake can create enough runout to cause pulsation.