Information On Wear Assessment

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Normally, brake disc replacement is necessary after the thickness falls below the minimum. In rare, isolated cases (of very high loads for long periods) replacement may be necessary due to crack formation. To a certain extent these cracks may be rated as not important. A description is given in the following sections of when to replace brake discs.

Cracks occur as a result of material fatigue due to frequent and extreme thermal expansion fluctuations. Particularly in cases of very high loads for long periods, these thermal expansion fluctuations produce internal stresses. The result may be cracks in the friction surface. These cracks allow relaxation of the friction disc, so that the speed at which the cracks develop is very slow.

Cracks run from the centre of the friction surface to the (inner and outer) edges of the brake disc. As different vehicle types have different brake disc sizes, differences arise for the maximum crack length limits.

Maximum crack length limit is calculated as follows:

0.5 x a (-a- = width of friction surface in mm).

Fig 1: Identifying Brake Disc Friction Surface Width

Courtesy of PORSCHE CARS NORTH AMERICA, INC.

Larger cracks impair braking comfort and reduce brake disc strength, therefore making brake disc replacement necessary as a precaution.

Irrespective of the crack length, the brake disc must be replaced if the crack has extended as far as the outer and/or inner edge of the disc.