Determining if one or more rotors has excessive DTV
The average specification for disc thickness variation (DTV) is .0005” (See Figure 24.1). The maximum allowable variation in thickness on a rotor is 5 ten thousandths of an inch. This dimension is too small to measure accurately at the shop level.
Most publications give instructions for measuring DTV. The typical steps involve making 6 to 8 measurements at a certain distance from the edge of the rotor and then comparing these measurements to one another. If they vary by more than .0005” the rotor has excessive DTV. Measurements this small can only be accurately done in a laboratory environment.
The only effective method to determine if one or more rotors has excessive DTV is by the symptom it produces. Excessive DTV is the leading cause of high speed pulsation complaints.
Excessive DTV is usually more pronounced when stopping from speeds above 35mph. The changing thickness of the rotor causes the caliper piston to move in and out. The movement of the caliper piston causes fluid movement in the hydraulic system which is felt in the brake pedal.
If the vehicle is exhibiting high speed pulsation then one or more of the rotors has excessive DTV. If the vehicle is disc/drum then both front rotors should be serviced according to their thickness and condition.
If the vehicle is equipped with 4 wheel disc brakes use the location of the vibration to help pinpoint front or rear. If the steering wheel and/or dashboard shake in addition to the brake pedal the problem is related to one or both front rotors. If the vibration is in the seat or floorboard it is most likely a rear problem. DTV is more common on front disc brakes than rear.
You CANNOT measure thickness variation at the shop level. You must rely on the symptom it produces to know whether you have it.