SECTION 12: BRAKE AND AIR SYSTEM
DOB 1300-1556 | X3-45 Commuter PA-1648 Maintenance Manual First release Oct 2020
16.2.2 Wear Check of Disc
Measure the thickness of the brake disc at the
thinnest point. Be aware of possible burring at
the edge of the disc.
Total thickness of the brake disc
New condition = 1.77 in (45 mm)
Worn condition = 1.457 in (37 mm) (the
disc must be replaced).
If the disc dimension M ≤ 1.535 in (39
mm) or N≤ 0.039 in (1 mm), it is
recommended that the disc should be
renewed when the brake pads are
changed.
FIGURE 34: BRAKE DISC THICKNESSES AND OPTICAL
WEAR INDICATOR
Check the discs per axle at each change of
pads for grooves and cracks. If necessary,
replace the disc.
The figure shows possible surface conditions of
the brake disc.
A1 = Small cracks spread over the surface are
allowed.
B1 = Cracks less than 0.059 in (1.5 mm) deep
or wide, running in a radial direction are allowed.
Cracks to a max. length of 0.75 x ’a’ are allowed
(a = width of the friction ring).
C1 = Unevenness of the disc surface less than
1.5 mm deep is allowed.
D1 = Cracks going through to the cooling duct
or onto the inner or to the outer edge of the
friction ring are not allowed and the disc MUST
BE REPLACED.
The photograph shows a real-life example of
such cracks and wear grooves.
Note:
In case of surface conditions A1, B1 or C1, the
disc can continue to be used until the minimum
thickness A = 37 mm is reached.
Knorr-Bremse discs are normally service-free
and grinding when changing pads is not
necessary. However, grinding could be useful,
e.g. to increase the load-bearing surface of the
pads after severe grooving on the entire friction
surface has occurred. To meet safety
requirements, the minimum thickness after
machining must be greater than 39 mm.
In addition, the recommendations of the vehicle
manufacturer about the machining of the brake
disc MUST be followed.
WARNING
Grinding of splined disc is not allowed.
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