Checking Rear Brake Discs (Wear Assessment)
Checking rear brake discs (wear assessment)Tools
Information
Brake disc wear assessment - General
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. A description is given in the following sections of when to replace 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).
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.
Bedding in the brake pads and brake discs
New brake pads require a bedding-in period of approximately 200 km! Not until then do they achieve their best friction and wear coefficient. During this period, the brakes should be subjected to full stress when traveling 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.
Checking rear brake discs (evaluating cracks)
Different vehicle types have different brake disc sizes. This produces differences 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).
The length of a crack can be measured with a steel rule, for example. -steel rule-
Note:
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.
As a precaution, brake discs must be replaced when:
^ the Maximum crack length: 0.5 x a mm (a = friction surface width in mm) is exceeded!
^ the Maximum crack width: 1 mm is exceeded!
^ the Maximum crack depth: 1.5 mm is exceeded!
Checking rear brake discs (minimum thickness)
Note:
Normally, brake discs are used until the maximum wear dimension is reached. 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 Workshop Equipment Manual, Chapter 3.4).
^ 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:
-A- Outer and inner wear areas.
-B- Central wear area.
The central wear area -B- is the area with the highest degree of wear.
- Measure the brake discs with a suitable micrometer at a minimum of three different places (-A, B, C-), offset by 120°.
Checking lateral runout (brake disc/wheel hub) and thickness tolerance
Note: Fasten dial gauge set with ball joint arm to wheel bearing housing.
For checking lateral runout two adapters are used. The brake disc and wheel hub can be measured with these.
Brake disc lateral runout
1. Remove the wheel and fit the adapter plate P 9510/1 with tightening torque 160 Nm (118 ft lb.).
2. Fit the adapter dial gauge set with ball joint arm NR.147-1 at a suitable position on the wheel bearing housing (schematic representation of front axle).
3. Fit the dial gauge with some preload and measure the lateral runout at the greatest external diameter of the brake disc.
Wheel hub lateral runout
Note: The front face of the wheel hub must not be corroded. If necessary, clean with fine emery cloth.
- Measure the wheel hub lateral runout at the front face of the wheel hub. You should always measure the external and internal diameters of the wheel bolt area.
1. Remove the brake disc.
2. Fit the adapter dial gauge set with ball joint arm NR.147-1 at a suitable position on the wheel bearing housing (schematic representation of front axle). Fit the dial gauge with some preload.
Brake disc thickness tolerance
- Brake disc thickness tolerance max. 0.01 mm
- Measure the thickness tolerance with a suitable micrometer screw within the braking surface at approximately 8 points.