Brake Rotor/Disc: Testing and Inspection

PURPOSE
In the manufacturing of the brake rotor all the tolerances regarding surface finish, parallelism, and lateral runout are held very closely. The maintenance of these tolerances provide the surface necessary to prevent brake roughness.

Light scoring of the rotor surface not in excess of 0.38 mm (0.015-inch) in depth is normal. This condition does not affect the brake operation.

Some discoloration or wear of the disc surface is normal and does not require re-surfacing when linings are replaced.

SPECIFICATIONS - Specifications

LATERAL RUNOUT
Lateral runout is the movement of the rotor from side to side as it rotates on the spindle. This could also be referred to as "rotor wobble".

This movement causes the brake pad and piston to be knocked back into it's bore. This results in additional pedal travel and a vibration during braking.

Checking Lateral Runout
1. Tighten the wheel bearings to eliminate all freeplay.
2. Attach a dial indicator to a solid non-rotating portion of the hub assembly or suspension.
- The point of the styles must contact the rotor face about 25 mm (1-inch) from the rotor edge.
3. Move the rotor one complete rotation and observe the dial indicator.

4. Rotate the bezel on the dial indicator such that "0" is at the low deflection point.
5. Again rotate the rotor at least one complete turn and observe the needle deflection. Total needle deflection will equal lateral runout.
6. Readjust the wheel bearings.

MINIMUM THICKNESS
The thickness of a rotor is important for two reasons:
1. Rotors which are too thin are not able to properly absorb and release heat during heavy braking. This results in reduced braking capacity and brake fade.
2. Rotors worn below minimum thickness in combination with worn pads/linings can result in the caliper piston extending too far becoming cocked or jammed.

Rotor thickness should be measured at the thinnest point on the rotor. Any rotor which is worn below its minimum thickness should be replaced.

For accurate measurements, it is best to remove the caliper to allow for complete access to the inboard side of the rotor.
For more information on how to use a disc brake micrometer see Fundamentals and Basics. Fundamentals and Basics

PARALLELISM
Parallelism is the measurement of the thickness of the rotor at 12 or more points around the circumference of the rotor. All measurements must be made at the same distance in from the edge of the rotor.

Lack Of Parallelism:

MACHINING
Since accurate control of the rotor tolerances is necessary for proper performance of the disc brakes, machining of the rotor should be done only with precision equipment.

All brake rotors have a minimum thickness dimension cast into them. This dimension is the minimum wear dimension and not a refinish dimension. Do not use a brake rotor that will not meet the specifications, after refinishing. Replace with a new brake rotor.

NOTE: 1 inch = 25.4mm, so if your micrometer measures in inches and the specified thickness on the disc is in millimeters, convert millimeters to inches by dividing the specified number of millimeters by 25.4.
Example: Specification on disc is 17.8 mm.
Dividing 17.8mm by 25.4mm/in = 17.8/25.4 = 0.70 inches.