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Brake Disc and Pad Minimum Thickness
Brake discs should be marked with their minimum thicknesses. They should be replaced before they reach this dimension. Vehicle manufacturers will also have a recommendation for when brake pads need to be replaced (for example, when there is 2 mm of friction material remaining).
So how do we arrive at these figures and why is it important to adhere to them?
The most important consideration is “piston pop”. If a caliper piston is pushed too far out of the caliper housing, it will pass the pressure seal, causing leakage and failure.
The piston must, therefore, be long enough to compensate for:
- The clearance between pad and disc
- Full disc wear
- Full pad wear
Variations caused by manufacturing and assembly tolerances also must be taken into account.
On single-sided calipers, the piston length compensates for wear on both sides of the disc as well as for full wear on both pads, making these pistons much longer than those used on opposed-piston calipers.
Wear also reduces the disc’s thermal capacity. This means that a fully worn disc will become hotter during a deceleration event than it would have if it was new. The further you go below what is recommended, the hotter this disc gets and the more likely you are to see fluid vaporization or a disc structural failure.
More obvious are the issues that come from the reduction in strength of the disc as it becomes thinner and the loss of friction coefficient of the pads when the friction material has worn away.
Finally, going “metal to metal” with your pads and disc will cause damage to the surface of the disc that may lead to prematurely scrapping the discs.
(Also, discs and/or pads should only be replaced as axle sets.)
Technical article provided by Alcon Components Ltd. Published by AlconKits with permission.