The Importance of Measuring Brake Rotors

The Importance of Measuring Brake Rotors

Learn about lateral runout (LRO) and disc thickness variation (DTV) and why new rotors and pads alone won't always solve issues.

Measuring brake rotors is critical to performing a pulsation free brake job. It doesn’t matter if the rotors are new or old, they always need to be verified if they’re within specification. Lateral runout or LRO is defined as an inaccuracy of the rotor’s plates That does not allow them to rotate exactly in line with the flange. The rotor might have the same thickness, but the plates around the flange move up and down. In layman’s terms, it’s a wobble in the rotor. The wobble can be changed by machining the rotor to match the same plane using an on the car brake life. Replacing the rotor will not solve the LRO problem. LRO will not cause piston movement, but rather caliper body movement. This movement causes the brake pad to make contact with the high spot of the rotor and wear. This wear can cause changes in thickness to that area of the rotor.

LRO can cause disc thickness variation, or D-T-V-D-T-V is the difference between the thickest and thinnest parts of the brake rotor. Some engineers call it parallelism. When the brakes were applied, the thickest part of the rotor pushes the piston of the caliper back into the bore of the caliper. The thinnest part of the rotor causes the piston to extend from the bore of the caliper. This has implications for the hydraulic brake components. When the driver applies to the brakes, the pads clamp onto the rotor. The pads and pistons follow the rotors high in low spots. This piston movement causes brake fluid to be displaced, and the caliper becomes a pump. Since brake fluid is incompressible, this fluid movement is transferred to the master cylinder and eventually the brake pedal where it creates pulsation. New brake pads and rotors do not solve pulsation, jutter, or premature wear problems.

They only mask the cause for a short period. In fact, the only problem new pads and rotors can solve is the elimination of parts that are below specifications. The discard or minimum thickness of rotor is the dimension at which the rotor should be replaced. These specifications are not related to the ability of the rotor to absorb heat. Instead, the dimension is related to the brake piston travel and the integrity of the hydraulic system. If the rotor and pads are worn to the minimum specification, the travel of the piston is near the point where the seal and system could leak and the brake pedal will go to the floor. Thanks for watching.

This video is sponsored by The Group Training Academy.

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