465002 Checking front brake discs (wear assessment)
WM 465002 Checking front brake discs (wear assessment)
Tools
Technical values
Information
- The pictures shown in this document may differ from the original.
- However, this does not affect the validity and relevance of the pictures.
Information on wear assessment
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 centre 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).
Width of friction surface a
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 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.
Checking
Checking front 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).
Width of friction surface a
The length of a crack can be measured with a steel rule, for example. -rule-
Brake discs
Information
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 front brake disc (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:
-A- Outer and inner wear area.
-B- Central wear area.
Wear areas A and B
The central wear area -B- is the area with the highest degree of wear.
Wear area B
- Measure the brake discs with a suitable micrometer screw gauge at a minimum of three different places ( -A, B, C- ), offset by 120°.
Measurement points for brake disc thickness
Checking lateral runout (brake disc/wheel hub) and thickness tolerance
Information
Fasten dial gauge set with ball joint arm to wheel bearing housing.
Two adapters are used for checking lateral runout. The brake disc and wheel hub can be measured with these.
Brake disc lateral runout
1. Remove the wheel -> 440519 Removing and installing wheel Removal and Replacement and fit the adapter plate 9510/1 with tightening torque 160 Nm (118 ftlb.).
Adapter plate on wheel hub
2. Fit the adapter dial gauge set with ball joint arm Nr.147-1 at a suitable position on the wheel bearing housing.
Dial gauge on brake disc
3. Fit the dial gauge with some preload and measure the lateral runout at the greatest outer diameter of the brake disc.
Wheel hub lateral runout
Information
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 outer and inner diameters of the wheel bolt area.
1. Remove the brake disc. -> 465019 Removing and installing the front brake disc 465019 Removing and installing front 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. Fit the dial gauge with some preload.
Dial gauge on wheel hub
Brake disc thickness tolerance
- For details of brake disc thickness tolerance, see Technical data/brake wear limits. -> 4600TW00 Technical data/brake wear limits
- Measure the thickness tolerance with a suitable micrometer screw gauge within the braking surface at approx. 8 points.