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Ventilated disc                             Solid disc 

with Min. Th 28.10mm stamped on hub               with Min. Th. 8mm stamped on edge

The Minimum Thickness of a Brake Disc is stamped on the Disc during manufacture as shown above.

Definition

The Minimum Thickness of a Brake Disc is determined by a Motor Vehicle Manufacturer during the design of each particular vehicle and it defines the thickness at which a Brake disc must be replaced because it can no longer deliver the braking performance required of it.

The Risks

Observing the Brake Discs Minimum thickness is vitally important to road safety, as indeed  is observing the minimum tyre tread depth, exceeding either of these limits can negate the benefit of good brakes, a braking system which is not operating at its optimum can render useless a vehicle's other active safety systems. Using Brake Discs which have reached Minimum Thickness will lead to lowered braking performance and can even result in brake system failure. 

The Specification

When a Motor Vehicle Manufacturer specifys a Brake Discs Minimum Thickness a number of considerations are taken into account. Firstly regarding the other components in the system, the size and the weight of the vehicle and then the impact of this information has on the other dynamics within the brake system such as the system's capacity for:-

  • Heat absorption and dissipation
  • Caliper piston retention
  • Brake pad retention
  • Brake fluid retention

To explain each of these in more detail -

Heat Absorption and Dissipation

The function of a vehicles brake system is to convert a vehicle's kinetic energy into heat energy. In a Disc brake system this is done when a driver pushes a brake pedal which activates a power-boosted hydraulic system. Hydraulic pressure then forces the pistons to move inside  the callipers. The piston movement in turn forces the brake pads into the rotating Brake discs.  Friction then occurs between the Brake pads and the Brake discs generating heat which is then dissipated, mainly into the atmosphere by convection. As a Brake disc reduces in thickness, so does its ability to absorb and dissipate heat. Once Minimum Thickness has been reached, a significantly reduced braking capacity can result. Symptoms include premature brake fade and increased stopping distances.

Caliper Piston Retention

In the course of their life, Brake Discs and Brake pads wear down.  As the Brake discs and pads become increasingly thinner, the caliper piston moves further out of the caliper body to force contact between Brake pad and Disc. When Minimum Thickness is reached with fully worn pads, the extended piston can have insufficient support within the caliper bore. This can cause it to jam in the bore, causing brake drag or lock. Brake drag will lead to excessive heat build-up and possible brake fluid vaporisation. The result: increased stopping distances and, potentially, half-system hydraulic failure.

Brake Pad Retention

The combination of fully worn Brake Pads and Discs which are at Minimum Thickness can, in some caliper designs, allow the pad backing plate to jam between the caliper anchor bracket and Brake Disc. This will cause either brake drag or wheel lock-up. Both can result in a loss of vehicle stability. Alternatively it is possible that the pads will be torn completely free, severely impeding braking capacity and damaging other components.

Brake Fluid Retention

Under certain conditions when brake pads are fully worn and Minimum Thickness has been reached, the caliper piston may no longer form a hydraulic seal. This will result in leakage, causing increased stopping distances and, possibly, half-system hydraulic failure.