WM, 465202 Checking Front PSCB Brake Discs: Information: Cracks

Normally, brake disc replacement is necessary if the thickness is less than the minimum thickness. Brake discs may have to be replaced because of cracks in rare, isolated cases. Up to a certain point, these cracks may be rated as not important. The following describe when to replace the 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 center of the friction surface to the inner and outer edges of the brake disc. Since different vehicle types have different brake disc sizes, there are also different maximum crack length limits.

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