Brake Disc Wear Assessment - General

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Normally, brake disc replacement is necessary if the thickness is less than the minimum thickness. 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. The following sections describe when to replace the brake discs.

Cracks arise through 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.

The cracks run from the center 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.

The maximum crack length limit is calculated as follows:

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

Fig 1: Identifying Width Of Friction Surface A

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.