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Discs – How Low Should You Go?

24 Aug 2020


Although it’s quite clear when brake pads become too worn/thin to be safe, the same can’t be said about brake discs. Even at MOT and VOSA level, conflicting information is given about the safety of disc thickness. Therefore, it is best to check the thickness specified by the vehicle manufacturer.

Disc thickness is calculated using key criteria such as heat dissipation and caliper/pad retention. The thicker the disc, the more material there is to dissipate heat and once the disc is thinner than the manufacturer’s specification, it will fail to disperse heat as intended when braking. This causes the discs to anneal (soften), the pad material to break down and performance to be compromised.

When the discs are too thin, the pistons in the caliper must push the pads further out to make contact with the disc. If the pads are also too thin, the caliper can overextend and the pistons are left unsupported resulting in uneven pressure or failure to retract. This causes brake drag or lock, generating enough heat to evaporate the brake fluid, leafing to total brake failure. Worst case scenario, the piston will no longer seal with the caliper, leading to the loss of brake fluid and eventual brake failure.

Furthermore, if the braking is very aggressive or the disc is very thin, the friction ring can shear away from the hub, leading to brake failure and/or loss of control.

Driving with low brake discs can cause vibration/shaking at the steering wheel and long brake pedal travel. Brake fade occurs quicker and the risk of brake loss is increased.

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