Brake Rotor, Checking, Steel Brakes

Special tools and workshop equipment required 

• Digital Dial Gauge:VAS 6668/1  or Brake Disc Vernier Gauge:VAS 6343  , not illustrated.
• Dial Indicator - 0-10mm:VAS 6079 
  Fig 1: Dial Gauge

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• Dial Indicator Bracket:VAS 6079/1  , not illustrated

• Respective wheel is removed. Refer to WHEELS AND TIRES .

Cracks, Evaluating

• Hairline cracks on the brake rotor friction surface are often noticed during brake repairs. Hairline cracks up to 10 mm long are not a technical flaw and do not justify a brake rotor replacement.
• Brake rotors and brake pads that have a worn through friction surface should be replaced.

Wear, Measuring

Due to varying contact pressures of the brake pistons to the brake pad there are different wear areas on the friction surface.

-- Brake rotor in the center wear area = measure the highest wear level

-- With Dial Gauge:VAS 6668/1  or Brake Disc Vernier Gauge:VAS 6343  at least three different locations -1-, -2- and -3- by -a- = 120° apart from each other.

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• If a measured value is below the wear value specified on the brake rotor: replace the brake rotors on the complete axle.

Brake Rotor Lateral Run-Out, Measuring

-- Only perform the measurement of the brake rotor lateral run-out, if the following conditions are met:

• There are vibrations when braking.
• Brake rotors are without scoring, hotspots, burn marks and blue discoloration.
• Wheels and tires are OK and balanced.

-- Tighten the brake rotor to the wheel hub using five wheel bolts placed underneath or five other bolts that are suitable. For the tightening specification. Refer to WHEELS AND TIRES .

• The completely installed bolts must not touch the wheel bearing housing behind!

-- Clean dirt and rust off the brake rotor.

-- Mount the Dial Indicator Bracket:VAS 6079/1  -2- at a suitable location on the wheel bearing housing.

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-- Mount the Dial Indicator - 0-10mm:VAS 6079  -1- with slight pretension and measure the lateral run-out at the dimension -a- from the outer edge of the brake rotor.

• Dimension -a- = 10 mm

-- When measuring, slowly turn the brake rotor via the threaded connection on the drive axle outer joint either by hand or using a hand ratchet.

• If the lateral run-out of the brake rotor is less than 0.06 mm: the brake rotor is OK.
• If the lateral run-out of the brake rotor is greater than 0.06 mm: measure the lateral run-out of the wheel hub.

Wheel Hub Lateral Run-Out, Measuring

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-- Remove either the front brake rotor (refer to BRAKE ROTOR, REMOVING AND INSTALLING, STEEL BRAKES ) or the rear brake rotor (refer to BRAKE ROTOR, REMOVING AND INSTALLING, STEEL BRAKES ).

-- Mount the Dial Indicator Bracket:VAS 6079/1  at a suitable location on the wheel bearing housing.

-- Mount the Dial Indicator - 0-10mm:VAS 6079  with slight pretension and measure the lateral run-out at the dimension -a- under the holes for the wheel hub.

• Dimension -a- = 5 mm

-- When measuring, slowly turn the wheel hub via the threaded connection on the drive axle outer joint either by hand or using a hand ratchet.

• If the lateral run-out of the wheel hub is greater than 0.02 mm: replace the wheel hub.
• If the lateral run-out of the wheel hub is less than 0.02 mm: replace the brake rotor.