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 μm (> 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: 
   1. 1.1. Surface roughness (Rk) = 6 μm -see illustration- (view 1)Fig 1
      Fig 1: Locating PCCB Surface Roughness 6 Micrometers (View 1)

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      Surface roughness (Rk) = 6 μm -see illustration- (view 2)Fig 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.

      Fig 2: Locating PCCB Surface Roughness 6 Micrometers (View 2)

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   2. 1.2. Surface roughness (Rk) = 56 μm -see illustration- (view 1)Fig 3
      Fig 3: Locating PCCB Surface Roughness 56 Micrometers (View 1)

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      Surface roughness (Rk) = 56 μm -see illustration- (view 2)Fig 4

      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!

      Fig 4: Locating PCCB Surface Roughness 56 Micrometers (View 2)

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   3. 1.3. Surface roughness (Rk) = 72 μm -see illustration- (view 1)Fig 5
      Fig 5: Locating PCCB Surface Roughness 72 Micrometers (View 1)

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      Surface roughness (Rk) = 72 μm -see illustration- (view 2)Fig 6

      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!

      Fig 6: Locating PCCB Surface Roughness 72 Micrometers (View 2)

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2. Worn brake discs: 
   1. 2.1. Surface roughness (Rk) = 108 μm -see illustration- (view 1)Fig 7
      Fig 7: Locating PCCB Surface Roughness 108 Micrometers (View 1)

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      Surface roughness (Rk) = 108 μm -see illustration- (view 2)Fig 8

      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 → 465402 CHECKING REAR PCCB BRAKE DISC (WEAR ASSESSMENT)  - section on "Information".

      In general, the following applies: Brake discs must be replaced if the surface roughness (Rk) is > 80 μm!

      Fig 8: Locating PCCB Surface Roughness 108 Micrometers (View 2)

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3. Worn Brake Discs (Special Cases): 
   1. 3.1. Surface roughness Rk = 108 μm, as for previous disc, but with some of the original friction surface remaining -arrow-  .

      Replace brake discs! To do this, follow the instructions → 465402 CHECKING REAR PCCB BRAKE DISC (WEAR ASSESSMENT)  - section on "Information".

      In general, the following applies: Brake discs must be replaced if the surface roughness (Rk) is > 80 μm!

      Fig 9: Identifying Worn Brake Disc

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   2. 3.2. Surface roughness (Rk) = 72 μm. Borderline disc, identified by the roughness, but also by the indication of wear and connected surface fractures -arrows-  , > 1 cm² .

      Replace brake discs! To do this, follow the instructions → 465402 CHECKING REAR PCCB BRAKE DISC (WEAR ASSESSMENT)  - section on "Information".

      Fig 10: Identifying PCCB Brake Disc Surface Roughness

      Courtesy of PORSCHE CARS NORTH AMERICA, INC.