Lesson #1: Terms

Lateral Run-Out (LRO): The amount of "wobble" in a rotor. Also known as the side to side movement as a rotor completes one revolution. In order to be properly measured, the rotor must be properly torqued and the mating surfaces must be clean and free of debris. The lateral run-out specification for each vehicle varies depending on how much the caliper piston retracts when your foot is off the brake pedal. If the amount of run-out exceeds the amount the piston retracts, then the brake pad will make contact with the brake rotor once each side per revolution. Over time, this scuffing will either build up material on the high spot of the rotor, as in the case of many ceramic pads, or remove material from the high spot of the rotor, as in the case of more abrasive metallic pads. This results in what is called Disc Thickness Variation.

Disc Thickness Variation (DTV): The amount by which the brake rotor's thickness varies when measuring with a micrometer in 4-6 different locations around the rotor. DTV is usually a symptom of excess LRO causing the rotor to scuff the brake pads each rotation of the rotor. It can take anywhere from 3,000-7,000 miles until the DTV is great enough to be felt in the brake pedal and/or steering wheel. Remember your rotor is making one complete revolution for each revolution of the tire – that's a significant amount of DTV inducing contact with the pads if the LRO is higher than manufacturer's specification!

Roughness Average (Ra): This figure represents the average height of the bumps on the rotor. With sharp cutting tips and a lathe in good working order, the Pro-Cut should leave a Ra between 30-60 micro-inches on a rotor surface after the cut. Providing a good Ra will help to reduce brake noise and bed the brake pads in quicker.

Rotor Matching: The process of match-machining a rotor on a car's hub with a Pro-Cut lathe. You are matching that particular rotor to that particular hub in that position (or phase). The rotor is considered "matched" because the patented computerized compensation system of the Pro-Cut lathe will ensure that the LRO is at or below minimum spec of .001" and that the DTV is removed by the subsequent machining process. This will provide the customer with a longer lasting smoother brake job.


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