Brake Rotor Runout
The lateral wobble or variation in a brake rotor's surface as it spins, which can cause pedal pulsation during braking.
Brake rotor runout (specifically lateral runout) is the measurement of how much a brake rotor's friction surface wobbles side-to-side as it rotates on the wheel hub assembly. Runout is measured using a dial indicator gauge mounted to the suspension knuckle, showing deviations in thousandths of an inch. A healthy rotor should have virtually zero runout (typically less than 0.002 inches).
Excessive runout is caused by uneven torque on the lug nuts, rust or dirt trapped between the rotor hat and the hub face, or thermal stress that warps the rotor. As the rotor spins with runout, it contacts the brake pads unevenly, creating high and low spots on the rotor surface over time (known as Disc Thickness Variation, or DTV). This results in a noticeable shudder or pulsation in the brake pedal and steering wheel when stopping.
To fix runout, a technician must first clean the hub mounting surface thoroughly and measure runout again. If the rotor itself is warped beyond specifications, it must be machined on a brake lathe ('turned') to restore a flat surface, or replaced with a new rotor. Always torquing lug nuts to specification with a torque wrench helps prevent rotor runout.