465002 Checking front brake discs - wear assessment
WM 465002 Checking front brake discs - wear assessment
Brake disc wear assessment - General
The condition of the parts can make replacement necessary under two criteria, depending on the amount of wear on cross-drilled (perforated) brake discs:
1. Cracks in the cross-drilled friction plate at advanced stage.
2. Brake disc minimum thickness is not attained due to wear (material erosion due to friction).
In practice, both forms of disc erosion occur.
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.
The maximum permitted perforation crack length is 5 mm.
Perforation crack length is too big (greater than 5 mm)
Checking grey cast-iron brake discs - evaluating cracks
No need to remove from the vehicle as the perforation crack length is small (less than 5 mm).
Perforation crack length is small (less than 5 mm)
Some perforation cracks are over 7 mm long.
Brake disc cannot be used (maximum permitted perforation crack length is 5 mm)!
Perforation crack length is too big (greater than 5 mm)
Brake disc with cracked friction plate rim -arrow-.
Brake disc cannot be used!
Friction plate rim damaged
Checking grey cast-iron brake discs - minimum thickness
Information
Normally, brake discs are used until the maximum wear dimension is reached. -> 46 Technical data/brake wear limits Only in a few exceptional cases is it advisable to skim the brake discs (if there is deep scoring or corrosion of the friction surface). In this case only the disc brake dressing machines approved by Porsche AG are to be used (see Tools and Equipment).
- Only as new brake discs may be reworked!
- Only rework the brake discs symmetrically, evenly from both sides!
As the pressure force of a brake piston on the brake pad varies in magnitude, different areas of wear arise on the friction surface. The brake disc can basically be divided into two areas of wear: outer wear areas and the central wear area.
The natural balance between stronger and weaker braking ensures that in almost all cases the typical wear profile of a perforated (cross-drilled) brake disc (friction plate) arises.
This means that the inner and outer smooth friction edge area is most worn.
Outer wear areas
Central wear area
- Measuring brake disc thickness
Information
Unlike smooth brake discs, on which the minimum thickness is measured in the middle of the disc, the minimum thickness of perforated (cross-drilled) brake discs must always be measured on the inner and outer friction surface track that is most worn -arrows-.
Wear areas A and B
-A- Central wear area
-B- Outer and inner wear area
Measure the brake disc thickness -X- in the area of the outer or inner friction surface track -arrows- , at a minimum of 3 different points ( -A, B, C- ) offset by 120°.
To do this, use a suitable micrometer or a brake disc gauge.
Measurement points for brake disc thickness
Bedding in the brake pads and brake discs
New brake pads require a bedding-in period of approx. 200 km (124 miles)! Only then do they achieve their best friction and wear coefficient. During this period, the brakes should be subjected to full stress when travelling at high speed only in emergencies. The slightly reduced braking effect must be compensated for by increased pressure on the brake pedal. The same also applies after replacing or machining brake discs.