Disc Thickness Variation (DTV) - Issue 14

Disc Thickness Variation (DTV) is where the rotor or disc varies in thickness as indicated in the diagram.

DTV causes torque variation with symptoms such as (i) Pedal pulsation as the rotor passes through the caliper/pads with lower pedal efforts very noticeable and higher pedal efforts less noticeable. This problem is magnified as the rotor gets hotter i.e. after travelling down an incline with the brakes on. (ii) Steering wheel oscillation, left to right which is not noticeable if the rotors are in phase. (iii) Vibration through the floor of the vehicle.

DTV is the uneven wear of the rotor during service. Often this does not become apparent until 10,000 to 15,000 kms after disc machining or replacement. Poorly serviced calipers can accelerate the occurrence of DTV as can high levels of rotor run-out at fitment (greater than 0.10mm). This is not generally visible to the human eye and must be measured with a micrometer. It is often caused by off-brake wear resulting in flat spots or thin sections of the disc.

There can be thick or thin parts of the rotor which can cause the feeling of a variation in the clamping pressure between the brake pads and the rotor

Run out needs to be measured with a dial indicator with the rotor mounted on the vehicle however, DTV can be measured using a precision micrometer off the vehicle. This is done by measuring the thickness of the rotor 25mm in front of the outer edge. Measurements should be taken at a minimum of 10 points around the circumference and there should be no greater variance than 0.03mm between any of these measurements.

To correct the problem the rotor should be replaced or machined with a light skim to clean up any DTV flat spots. If the vehicle is using ceramic pads, the rotor must be machined as part of every brake job. Calipers should be overhauled, side pins must move freely and the piston seals must be checked for good condition or replaced. On-vehicle machining eliminates variation in axles, bearings etc.

To minimise the impact of Disc Thickness Variation rotors should be machined to clean up any flat spots. Where ceramic pads are used the rotor must be machined with every brake job. DTV will not occur if there is zero rotor run-out. On-vehicle machining eliminates variation in axles, bearings etc. Vehicle design, tolerance variations and general wear and tear may however mean that in some instances zero run-out tolerance cannot be achieved. Rotor run-out should be measured with an accurate dial gauge as less run-out leads to slower DTV generation. A quality torque wrench should be used to fasten “hat”-type rotors to the hub with wheel nuts and index rotors on the hub to minimise run-out. Installed run-out should be less that 100μm (0.1mm) however when this cannot be achieved checks should be made to the bearings, flange, stub axle and rotor hat. When tightening wheel nuts an air powered “rattle” gun should not be used but instead it is recommended that hand tools be used finishing with a quality torque wrench to avoid distortion.