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Thermal judder occurs when differential thermal expansion is experienced around the rotor during a brake application. A temporary circumferential disc thickness variation (DTV) is created which can lead to brake torque variation. The cause of the uneven heating of the rotor can be an imperfection; often a permanent DTV caused by periods of off-brakes contact between pad and disc. Using a dynamometer test combining dynamic disc thickness measurement and pyrometric techniques the evolution of thermal DTV may be studied. It can be demonstrated that increasing thermal DTV is accompanied by increasing brake torque variation. Hot spot formation is also evident but this is shown not be the primary cause of brake torque variation.

The paper covers the development, validation and use of a technique to predict hot spot formation. The technique involves the use of a specially developed finite element able to deal with heat generation and wear. It is applicable to real braking geometries and duty cycles and an example of its use will be presented. Following the description of the technique its use to carry out a sensitivity study into the effect of pad and disc parameters upon hot spot formation is described. The parameters include pad and wear properties and the duty cycle.