The minimum thickness is stamped on the outer diameter of the brake disc. The minimum thickness is the measurement across the width of the worn braking faces. If the thickness is below the advised minimum size, the disc must be replaced. The disc should also be inspected to see if it is wearing equally on both faces. If there are any signs of uneven wear then the caliper should be inspected to ensure correct operation and re-set as per the man-ufacturers specifications. If there is excessive wear across an axle then it is recommended that the distribution of the braking load between truck / trailer and other axles are checked.

Heat crazing, as illustrated in the image opposite, is the result of normal heating and cooling of the brake disc during service. These fine heat cracks are not detrimental to the performance of the brake. However, heat crazing can de-velop into deeper cracks. Therefore, any discs with radial cracks deeper or wider than 1.5mm should be re-ground, if the minimum thickness allows, or replaced. The early development of heat crazing can be an indication of an opera-tional problem.

Deep surface cracks, as illustrated in the image opposite, are usually the result of excessive heat. Brake discs with cracks going through to the cooling duct or from the inner to outer edge of the braking face must be replaced. Once the disc has been replaced the braking system should be checked for correct caliper operation and balance between wheels, axles and trailer.

Hard spots are slightly raised coloured areas on the braking surface and are created by non-uniform contact cycles creating localised graphite crystallisation in the surface of the cast iron. It may be possible to remove these hard spots by skimming the braking surface if the disc is within the minimum allowable thickness . However, continuation of this condition can, under extreme conditions, result in a thermal transformation of the casting structure "martensite". If this does occur we would recommend immediate replacement.

A grooved disc is defined by circumferencial scoring around the braking surface. If the depth of the grooves exceed 1.5 mm then the brake disc must be either re-ground, if the minimum thickness allows, or replaced. We would also recommend that the brake pads are inspected and replaced if the wear pattern has transferred to the friction surface.

The blueing of the braking surfaces indicates a high operating temperature. This can be caused by continued hard stops, an imbalance in braking or a malfunction in the brake caliper. It is not necessary to re-grind or replace the brake disc but the braking system and caliper adjustment should be checked.

Friction transfer, indicated by a layer of friction material on the braking surface, occurs when the brake pads exceed their effective temperature limit. At this point friction material will transfer onto the disc face at random intervals. Once this occurs the probability of material transfer is increased with central lines of friction developing, as illustrated in the image opposite. To resolve the problem the caliper should be examined to ensure correct operation along with distribution of the braking load throughout the system. The brake disc can be re-surfaced subject to minimum thick-ness allowance.

Some new brake disc designs have the ABS teeth incorporated the to the casting at the back of the disc. Unfortunately, despite an anti-corrosive coating the teeth on can corrode and cause faults with the ABS system. If the corrosion is not too severe the teeth can be cleaned with a wire brush and re-coated with anti-corrosive or high temperature paint, otherwise the disc must be replaced.