Disc Brake System: Service and Repair

AH42.10-N-9406-07B Notes on Evaluating Brake Disks




Model WD1, WD2, WD3, WD6, WD7, XD1, XD2, XD3, XD4, XD5, XD6, XD7, YD1, YD2, YD3, YD4, YD5, YD6, YD7





Overview of possible brake disk conditions

1 Brake disk

A Network-like hair crack formation (heat blemishes)
B Cracks running at right angles to the friction track
C Unevenness in the friction track
D Through-cracks

k Friction track width
l Breadth and depth of cracks
m Permissible crack length

Scoring in the friction track

Score marks in the friction tracks of the brake disk are permitted up to a depth of 1.5 mm. If the score marks are deep, the brake disk will have to be replaced.





Crack formation across the friction track

F Crack pattern of a brake disk with evaluation of the largest crack (n). The brake disk shown here is located in the critical limits as the crack (n) is Approx.. 72 % of the friction track width.
G Through-crack to the cooling duct(arrow), the brake disk is no longer permissible.

Cracks across the friction track with a maximum width and depth of 1.5 mm are permitted provided their length does not exceed 75% of the width of the friction track. Most cracks stabilize in the center of the track and their length is 50 % of the width of the friction track.

When the crack length exceeds 75 % of the friction track width there is the danger that a crack will tear to the cooling duct. In this case the brake disk must always be replaced.





Signs of increased thermal load at the brake disk

H Brake disk with increased thermal load (carmine red discoloration in the cooling duct)
J Brake disk with lower thermal load (rust in cooling duct)

An increased thermal load at the brake is indicated at the brake disk by
^ carmine red discoloration of cooling duct,
^ crack formation,
^ Heat spot,
^ Lining coatings displayed.

If an increase thermal load is only apparent on one brake, check the brake caliper and caliper guide and repair or replace them if necessary. If all wheel brakes show signs of increased thermal load then as a rule the cause is attributable to a high load on the vehicle.

Heat spots on the friction track

Heat spots, dark spots on the friction track, are caused by an increased thermal load on the brake.

Heat spots usually consists of a network of tiny hairline cracks, which may be the preliminary stages of crack formation.

Heat spots are permitted but they may cause fluctuations in braking power and brake juddering. Install a new brake disk if there are any signs of heat spots.

Pad deposits on the friction track

Pad deposits are caused by an increased thermal load on the brake. Pad deposits can cause major changes in the friction coefficient as well as fluctuations in braking power and brake juddering or limited brake force distribution and chatter. Install a new brake disk if there are any signs of heat spots.

Damage and corrosion on the brake disk

Surface eruptions on the friction tracks, cracks in the neck area or in the bolt-on flange of the brake disk are not permitted. In these cases the brake disk must always be replaced.

Stubborn rust formation, in the same way as pad deposits, can cause changes to the friction coefficient. The brake disk must be replaced in such cases.

The brake disk is made of an anticorrosive cast iron alloy. If a vehicle is not driven for longer periods of time, a thin layer of rust may from on the friction tracks of the brake disk. This layer of rust has no effect on the working life of the brake disk and is usually removed when the brakes are applied.