How to Diagnose Excessive Disc Thickness Variation (DTV)

What is DTV?

When a rotors thickness varies at different locations, it is called disc thickness variation, or DTV for short. Typically, the average specification for DTV is .0005". On the other hand, the maximum allowable variation in thickness on a rotor is 5 ten thousandths of an inch. Because this measurement is so small, it is not measurable at the shop level.

Measuring the DTV

Most publications give instructions for measuring the DTV. Normally, the steps involve making 6-8 measurements at a specified distance from the edge of the rotor. Next, it'll typically have a chart for you to compare the measurements with each other. In some cases, the measurements will show more than .0005". Because of this, we know that the rotor has excessive DTV. Because the measurement is so small, it can only be observed in a laboratory environment.

Determining DTV

Since measuring the rotor in shop isn't an option, it's best to identify it via symptoms. For example, excessive DTV is the leading cause of high speed pulsation complaints. Likewise, DTV is typically more pronounced when stopping from speeds above 35mph. This is caused by the changing thickness of the rotor thus causing the caliper piston to move in and out. Similarly, the movement of the caliper piston causes fluid movement in the hydraulic system that is felt in the brake pedal.

Symptoms

When a vehicle is exhibiting high speed pulsation, then one or more of the rotors has excessive DTV. For instance, if the vehicle is disc/drum, then both front rotors should be serviced according to their thickness condition. In another example, if the vehicle is equipped with 4 wheel disc brakes, use the location of the vibration to help pinpoint whether it comes from the front or rear. Typically, if the steering wheel, dashboard, and/or brake pedal shake, the problem will be related to the front rotors. Otherwise, when the vibration comes from the seat of floorboard, it is likely to be a rear rotor issue. As a rule, DTV is more common on front disc brakes than rear.

Bonus Tip: You CANNOT measure thickness variation at the shop level. Because of this, you must rely on the symptom it produces to in order to know whether you have it or not.

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