Wear Assesment
Checking Brake Discs (Wear Assessment)
Includes:
1. Visual inspection for cracks and crack assessment.
2. Checking brake discs for minimum thickness.
General
Two criteria may dictate replacement of drilled (perforated) brake discs:
1. Advanced stage of cracking in drilled (perforated) friction disc.
2. Disc thickness is below minimum due to wear (material abrasion caused by friction).
Both types of disc wear usually occur in service. Normally, brake discs will have to be replaced if the brake disc thickness is below minimum.
Only in rare cases (if brakes are subjected to racing-like loads for longer periods or if the friction surface is exposed to heavy temperature fluctuations) will perforation cracks progress far enough to require premature disc replacement.
Both condition criteria are described separately in the following sections.
1. Visual Inspection for Cracks and Crack Assessment
Note
Perforation cracks are caused by material fatigue due to severe, repeatedly fluctuating heat expansion. Disc temperature fluctuations of this nature that occur especially under racing conditions produce radial cracks in the perforation holes of the friction disc due to material fatigue (alternating thermal expansion). These cracks, on the other hand, will reduce tension in the friction disc to a certain extent, i.e. crack growth continues only very slowly.
The maximum admissible perforation hole crack length is 5 mm.
Further growth of perforation hole cracks or cracks at the edges of the friction disc impair braking comfort and reduce disc strength. For this reason, the components affected should be replaced as a precautionary measure.
Checking brake discs for cracks
- The brake discs have to be replaced (as a precaution) in the following cases: Length of perforation hole cracks is more than 5 mm (this means that no service life reserves remain if brakes continue to be subjected to severe loads) and/or if cracks appear at the edge of the brake disc (reduction of braking comfort and of breaking strength).
Examples of cracks assessment
Note
The crack thickness has been highlighted (to make identification easier) and therefore the cracks better visible than they actually are.
2. Checking Brake Disc Minimum Thickness
Notes
Along the innermost and outermost friction disc tracks that have no holes, wear of the corresponding brake pad friction area is lowest - compared to the center hole area - if the brakes are subjected to high contact pressures. As a result, less severe braking will lead to a corresponding increase of surface pressure in these areas and, hence, to increased friction disc wear.
The natural ratio of heavy to light braking will produce the typical wear profile of a perforated friction disc in virtually every case (inner and outer, smooth friction edge zone shows greatest wear. refer to illustration.
As opposed to smooth brake discs where the smallest thickness is measured in the disc center (effective frictional radius), the minimum thickness of perforated brake discs must always be measured at the inner or outer track of the friction disc that is worn to the greatest extent.
Checking minimum brake disc thickness
Use a suitable micrometer to measure the smallest brake disc thickness at one of the two smooth friction edge zones (at the friction edge zone that is worn to the greatest extent).
For the wear limit (minimum thickness), refer to the "Technical Data".
Example for area of greatest wear
The illustration shows the typical wear groove pattern (arrows) of a perforated 993 brake disc that was driven up to the wear limit under severe long-term operation (test operation).
Important note on new brake pads
Due to the relative high abrasive action of perforated friction discs, the wear pattern of newly fitted brake pads will adapt itself relatively rapidly to the wear pattern of the used brake disc. After introduction of optimized perforation patterns, the service life of perforated Porsche discs as well as of brake pad materials is now almost equal to the service life of smooth discs.