What specific tool is required to accurately measure rotor runout and why is this measurement critical?

A dial indicator is the specific tool required to accurately measure rotor runout. A dial indicator is a precision instrument that measures the amount of deviation or variation in a rotating surface. To measure rotor runout, the dial indicator is mounted to a stationary point, such as the vehicle's suspension or a magnetic base attached to a stable part of the vehicle. The indicator's tip is then placed against the rotor's surface. As the rotor is rotated, the dial indicator displays the amount of lateral movement, or runout, in thousandths of an inch or millimeters. Measuring rotor runout is critical because excessive runout can cause brake pedal pulsation, vibration, and uneven brake pad wear. Rotor runout refers to the amount the rotor deviates from a perfectly flat plane as it rotates. Even a small amount of runout can cause the brake pads to repeatedly contact and release from the rotor surface as it spins, creating vibrations that are felt in the brake pedal or steering wheel. High runout can also lead to accelerated wear of the brake pads and rotor, reducing braking efficiency and potentially creating a safety hazard. Exceeding the manufacturer's specified runout limit indicates that the rotor needs to be resurfaced or replaced.