Checking Perforated Rear Brake Discs (Wear Assessment)
Checking Perforated Rear Brake Discs (Wear Assessment)
Checking perforated brake discs involves the following:
1. Visual inspection for cracks and evaluating cracks.
2. Checking brake discs for minimum thickness.
General
The condition of the parts can make replacement necessary under two criteria, depending on the wear of perforated brake discs:
1. Cracking in the holed (perforated) friction plate is in an advanced state.
2. Brake disc minimum thickness is not attained due to wear (material erosion due to friction).
In practice, both forms of disc erosion occur.
Usually, it is necessary to replace the brake discs when they fall short of the brake disc minimum thickness. Only in rare cases (for long-term race driving brake loads/very high temperature fluctuations in the friction surfaces), the cracking and perforating can proceed so quickly that a premature disc change has to be performed. Both criteria governing acceptable disc condition will be described separately in the following sections.
1. Visual Inspection For Cracks And Evaluating Cracks
NOTE:
- Perforation starter cracks arise through material fatigue due to frequent and extreme heat elongation fluctuations. These fluctuations in the disc temperature, which generally arise in racing conditions, tear the friction plate perforations in a radial direction due to material fatigue (changing heat elongation conditions). For their part, these starter cracks allow a partial relaxation of the friction plate, so that the speed at which the cracks develop is very slow.
- The maximum permissible perforation crack length is 5 mm.
- The growth of the perforation starter cracks or a tearing of the rim of the friction plate worsens the smoothness of braking, decreases the disc strength and therefore makes a precautionary parts replacement necessary.
Evaluating The Cracks On The Grey Cast Iron Brake Discs
- The brake discs must be replaced (as a precautionary measure), if the perforation crack length is over 5 mm (as there is no mileage reserve in the event of further extreme load) and/or the friction plate rim is torn (less smooth braking and an increase in the likelihood that a disc will break).
- In the evaluation of damage, the following drawings must be used.
A. Corresponds to a disc exposed to above-average load.
- Need not be taken out of the vehicle.
B. In parts, perforation starter cracks are over 7 mm long. Conditions after shock braking 1,200 times (minimum desired: shock braking 200 times).
- Brake disc can no longer be used.
C. Brake disc with torn friction plate rim -arrow-.
- Brake disc can no longer be used.
2. Checking Brake Discs For Minimum Thickness
- In the presence of high pressure forces, at the innermost and outermost hole-free friction plate track -arrows-, the brake pad friction surface is the least worn, compared to the middle holed area. As a result, in the case of weaker braking, a relatively higher amount of pressing on the surface arises in these areas and therefore greater wear of the friction plates. The natural balance between stronger and weaker braking ensures that in almost all cases the typical wear profile of a perforated friction plate arises (inner and outer smooth friction rim area) is the most worn -arrows-.
Arrows - Zone where most wear occurs in brake disc friction surface
A - Perforation friction zone (holed friction zone)
B - Smooth friction rim zone
NOTE:
- As opposed to smooth brake discs, by which the minimum thickness is measured in the middle of the disc (effective friction radius), in the case of perforated brake discs, the minimum thickness must be measured on the inner or outer friction surface track, -arrows- whichever is most worn.
- Brake disc thickness/wear limit: Max. 2.0 mm total wear permissible. Refer to the technical data for the new dimension and other dimensions.
- Measure the brake disc thinnest point with a suitable micrometer screw, at one of the two smooth friction rim zones -arrows- (at the most worn friction rim area).
Example of wear
The drawing shows the typical grooved wear profile -3 arrows- of a perforated grey cast iron brake disc, which has been used to the wear limit, under tough conditions i.e. constant operation mode (test operation mode).
Notes On Bedding In New Brake Pads
In the case of the perforated (holed) friction plate, the contact pattern of newly installed brake pads adapts more quickly to the wear profile of the previously operated brake disc than is the case with smooth brake discs.
In general, the following applies: New brake pads must be worn in (bedding-in period), their optimum braking effect is therefore not reached until after a few hundred kilometers. The slightly reduced braking effect must be compensated for by increased pressure on the brake pedal. The same also applies after a brake disc change.