Front With PCCB
Checking front PCCB brake disc (wear assessment)
Checking information on PCCB brake discs
General information on PCCB brake discs
Wear indicators
The brake discs have wear indicator plates in the friction surfaces. The wear indicator plates (six on each brake disc) are arranged close to the outer diameter, offset by 120 degrees -arrows- at three points. The three wear indicator plates on each friction surface (3 on the inside and 3 on the outside) are positioned directly opposite each other. Each wear indicator is approx. 2 cm2. The wear indicators are barely visible on new brake discs.
The minimum brake disc thickness (wear dimension) is stamped on the brake-disc chamber (e.g. Min. Th. 37.7 mm).
Basic information
Checking involves the following:
1. Visual inspection for surface changes in the brake disc friction surfaces and wear indicators. Indication of wear: Connected surface fracture ] 1 cm2 -arrow-, without adding up smaller fractures.
Connected surface fracture
2. Check surface roughness Rk of brake discs (max. 80 microns permitted) since this affects braking comfort (noises), braking behavior/pedal pressure and brake-pad service life.
Surface roughness of PCCB brake disc
3. Check brake discs for minimum thickness -X-. The minimum brake disc thickness (e.g. Min. Th. 37.7 mm) is stamped on the brake-disc chamber.
Minimum thickness -X- of PCCB brake disc
4. Check brake discs for edge damage -see illustration- (damage due to improper use).
Edge damage on PCCB brake disc
General information on replacing worn PCCB brake discs
Three criteria governing the condition of the parts can make replacement necessary depending on the wear of PCCB brake discs:
1. Surface changes in the brake disc friction surfaces (material fatigue/wear) at an advanced stage (indication of wear: connected friction surface damage ] 1 cm2).
2. Surface roughness of the brake disc friction surfaces too great (roughness depends on mileage and load).
3. Brake disc minimum thickness is not attained due to wear (material erosion due to friction).
In practice, all three forms of brake disc erosion usually occur.
Only in rare cases (as a result of long-term motor-racing brake loads/very high temperatures on the friction surfaces), surface changes can progress so quickly that the brake discs may need to be changed earlier than planned.
Brake discs that have turned blue at the brake-disc chamber and/or brake callipers that have turned a strong brown colour are clear indications of high loads over a long period of time, e.g. on the race circuit.
Replacing brake discs (notes/decision-making aid)
When replacing brake discs as a result of wear (see wear assessment) both brake discs must be replaced on an axle!
Irrespective of wear, a brake disc change may be necessary due to edge damage on the brake disc friction surfaces! In this case, only one brake disc can be replaced if the friction surface of the other PCCB brake disc is still 'as new'.
Wear indicators
The wear indicator plates (six on each brake disc) are arranged close to the outer diameter, offset by 120 degrees -arrows- at three points. The three wear indicator plates on each friction surface (3 on the inside and 3 on the outside) are positioned directly opposite each other. Each wear indicator is approx. 2 cm2. The wear indicators are barely visible on new brake discs.
The minimum brake disc thickness (wear dimension) is stamped on the brake-disc chamber (e.g. Min. Th. 37.7 mm).
Cracks (stress-relief microstructure) in the brake disc friction surface
Stress-relief microstructure in PCCB brake disc friction surfaces
Cracks in the brake disc friction surfaces must be regarded differently to cracks in grey cast-iron brake discs. These cracks are not regarded as wear criteria for PCCB brake discs and may even be present on new brake discs, depending on the manufacturer.
Information
Due to the composition of different materials in the friction layer and support body, the friction surfaces (even when new) are coated with an uncritical crack pattern (stress-relief microstructure). Individual relaxation cracks sometimes run along the perforated bores.
The stress-relief microstructure results from the way in which the brake disc surface is manufactured and does not pose a risk to safety. There is also no danger of the friction layers taking off from the support body, since the friction layers are almost free of internal stresses as a result of the stress-relief microstructure.
The stress-relief microstructure is sometimes clearly visible in new brake discs and the brake disc chamber side and the side that is opposite to the chamber can differ significantly from one another.
Information concerning bores (perforations) in the brake disc friction surface
Bores (perforations) in brake disc friction surface
Observe the following warning when cleaning or exposing the bores in the brake disc friction surface!
Notice
Work cannot be carried out on the bores in the brake disc friction surface
Damage to brake discs
- Clean the bores on PCCB brake discs with high-pressure cleaning equipment if necessary. Observe the relevant environmental and safety regulations.
- Do not machine the bores. Drilling or counter-sinking is not permitted.
Coating on the PCCB brake discs (potential causes)
The coating on the PCCB brake discs arises as a result of a high load and high temperatures, for example on a racing circuit. Check the following in such cases:
Bores (perforations) in brake disc friction surface
Ventilation (brake cooling): all components are present, secured correctly and undamaged.
Brake pad quality: If the vehicle is used for race-track driving, use appropriate brake pads.
Perforation bores -see illustration- in the brake discs must not be blocked. Clear them if necessary.
In the event of brake pad wear, the brake discs can still be used provided the brake discs satisfy all wear criteria. The perforation bores -see illustration- must not be blocked.
Information on bedding in new brake pads
New brake pads require a slightly longer bedding-in period for PCCB brake discs than cast iron brake discs.
In general, the following applies: New brake pads must be broken 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.
Checking PCCB brake discs (wear assessment)
Visual inspection of the surfaces of the brake disc friction surfaces (indication of wear)
Information
The friction surface changes occur due to material fatigue as a result of extremely high thermal load. These disc temperatures, which occur above all in racing conditions, cause a progressive material fatigue.
Effects on driving: The surface change worsens the smoothness of braking, decreases the disc strength and increases the brake pad wear and therefore makes a precautionary parts replacement necessary.
The following illustrations (PCCB brake disc wear appearance) are to be used for damage assessment.
PCCB brake disc wear appearance
1. Friction surfaces after normal brake load while driving on roads. No surface fractures present
Brake discs do not need to be replaced!
PCCB brake discs without surface fractures
2. Friction surfaces after a long journey and occasional higher load. Isolated cases of connected surface fractures less than 1 cm2 -arrow-.
PCCB surface fractures
No replacement necessary as the connected surface fractures are 1 cm2!
3. End of service life of friction surface or after highest load on a racing circuit.
Replace brake discs if the connected friction surface damage is ] 1 cm2 -arrow-. To do this, follow the instructions.
PCCB friction surface damage
The best way to detect a surface fracture is by checking the wear indicators. Based on the wear indicators, this means that at least one wear indicator -arrow- must have a surface fracture of ] 50 % (damage ] 1 cm2/entire surface of a wear indicator with a surface area of 1 cm2).
PCCB surface burnout on wear indicator
Evaluation of surface roughness Rk (max. 80 micrometers permitted)
Information
An increase in surface roughness impairs smoothness of braking (rubbing, shaking in steering wheel, scraping noises) and increases brake pad wear.
With a roughness (Sr) value of ] 80 microns (] 80 micrometer), a precautionary parts replacement is required.
The assessment is carried out visually, based on the diagram. Two illustrations of the same peak-to valley surface roughness (views 1 and 2) help to make a better assessment.
1. Brake discs that are OK:
PCCB surface roughness (Sr) = 6 micrometers (view 1)
1.1. Surface roughness Rk = 6 microns -see illustration- (view 1)
PCCB surface roughness (Sr) = 6 micrometers (view 2)
Surface roughness Rk = 6 microns -see illustration- (view 2)
New brake disc. Illustration is used for better classification.
The early stages of roughness are indicated by burnt-out carbon fibres on the surface of an otherwise intact, semi-matt surface.
The surface feels smooth.
PCCB surface roughness (Sr) = 56 micrometers (view 1)
1.2. Surface roughness Rk = 56 microns -see illustration- (view 1)
PCCB surface roughness (Sr) = 56 micrometers (view 2)
Surface roughness Rk = 56 microns -see illustration- (view 2)
In the advanced stages of fibre burnout and the initial destruction of the Si and SiC 'matrix', the depressions have become more pronounced. Large sections of the surface are still intact, the depressions form an even, fine-grained pattern.
The surface still feels matt, however, and the roughness is not yet pronounced.
No replacement necessary!
PCCB surface roughness (Sr) = 72 micrometers (view 1)
1.3. Surface roughness Rk = 72 microns -see illustration- (view 1)
PCCB surface roughness (Sr) = 72 micrometers (view 2)
Surface roughness Rk = 72 microns -see illustration- (view 2)
Borderline disc, which can still just be evaluated as OK As a result of more advanced damage to the surface, the depressions are larger, and above all deeper, compared with the previous surface condition.
On the one hand, the more extensive roughness is noticeable (only when a comparison is made); on the other hand, however, like before, "only" depressions are observed.
Replacement not yet necessary!
2. Worn brake discs:
2.1. Surface roughness Rk = 108 microns -see illustration- (view 1)
PCCB surface roughness (Sr) = 108 micrometers (view 1)
PCCB surface roughness (Sr) = 108 micrometers (view 2)
Surface roughness Rk = 108 microns -see illustration- (view 2)
Worn surface. Advanced wear has led to a situation where only the remains of the original surface are left.
It is clear that the roughness is caused not only by depressions but also by bumps.
Replace brake discs! To do this, follow the instructions.
In general, the following applies: Brake discs must be replaced if the surface roughness (Sr) is ] 80 microns!
3. Worn brake discs (special cases):
PCCB surface roughness (Sr) = 108 micrometers
3.1. Surface roughness Rk = 108 microns, as for previous disc, but with some of the original friction surface remaining -arrow-.
Replace brake discs! To do this, follow the instructions.
In general, the following applies: Brake discs must be replaced if the surface roughness (Sr) is ] 80 microns!
PCCB brake disc
3.2. Surface roughness Rk = 72 microns. Borderline brake disc detected based on roughness, as well as wear indication and connected surface fractures -arrows-, ] 1 cm2.
Replace brake discs! To do this, follow the instructions.
Checking brake discs for minimum thickness
Information
Minimum thickness of PCCB brake disc
For perforated brake discs, the minimum thickness -X- must always be measured on the inner or outer friction surface track -arrows-.
Wear limit: The minimum brake disc thickness is stamped on the brake-disc chamber (e.g. Min. Th. 37.7 mm).
- Measure the brake disc thickness -X- with a suitable micrometer screw or a brake disc gauge at the inner and outer friction surface tracks -arrows-.
Checking brake discs for edge damage
Information
Irrespective of wear, a brake disc change can be necessary on the brake disc friction surfaces due to edge damage.
In such cases, only one brake disc may need to be replaced if the friction area surface of the other PCCB brake disc is still completely OK.
The following criteria apply to edge damage assessment:
Edge damage on PCCB brake disc
Max. permissible width/depth = 2 mm
Max. permitted length = 10 mm
Max. 3 damaged edge areas permissible