Bendix Brakes Provides Technical Advice on Disc Thickness Variation (DTV)
Disc Thickness Variation (DTV) is where the rotor or disc varies in thickness as indicated in the diagram.
The symptoms can be recognised by a torque variation i.e grab and release during braking. As the disc rotates through the caliper the variation in the disc thickness results in a variation in the braking pressure applied by the disc pads to the rotor.
The torque variation in the breaking is translated in vibration through the steering wheel, floor plan, seat or pedal pulsing.
DTV is the uneven wear of the rotor during service and 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.
Measuring is done by checking the thickness of the rotor using a Vernier Caliper 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.
In the preventative area Disc Thickness Variation will not occur if there is zero rotor run-out. Vehicle design, tolerance variations and general wear and tear may however mean that zero run-out tolerance is unachievable.
On-vehicle measurement of rotor run-out with an accurate dial indicator coupled with a good quality torque wrench to tighten the wheel nuts will minimise DTV effects. Less run-out leads to slower DTV generation.
Installed run-out should be less that 100µm (0.1mm) and when this cannot be achieved checks on the bearings, flange, stub axle and rotor hat should be made. Wheel nuts should be evenly torqued with the use of a torque wrench to avoid distortion.
For more information free call the Bendix Brake Advice Centre on 1800 819 666 or +61 3 5327 0211 from overseas (8am-5pm Monday to Friday EST), e-mail us at: brakeadvicecentre@bendix.com.au or visit our website by clicking here!
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