When to replace brake discs

All modern cars have brake discs. Some can be expensive. When you're advised to replace a set of discs, it's worth asking whether replacement is really necessary, or whether they can be skimmed at a much lower cost, and get more miles out of them. Here are some things to think about.
Brake discs are considered a throw-away item but they only need replacing when they wear below minimum thickness. This could be anywhere from 15,000 to 100,000 km depending on driving style and the type of brake pads used.

  • If there is a small amount of hub run-out, discs can wear unevenly. Each time the disc rotates, even when you are not touching the brakes, the pads will wear away one part of the disc. This eventually leads to disc thickness variation, which results in 'a shudder' or vibrations when the brakes are applied. A skim using an on-car brake lathe, correctly adjusted to eliminate run-out, will resolve this.
  • Some brake pads are much more aggressive than others. An aggressive pad is one that can produce a lot of dust and often causes uneven wear of the disc surface. This in turn can cause brake vibrations. A brake disc skim will remove this unevenness and any excess dust, and prevent brake vibrations.
  • On cross-drilled discs, the holes do not cover the entire disc surface, resulting in varying surface areas across the disc. Sometimes, this can result in disc thickness variation. This problem can be easily and cheaply resolved by having the discs skimmed whilst on the car.
  • New brake pads used on a partially worn disc will require bedding in, which can take 500 to 1000 kilometers and brake performance will be poor and noisy during this time. A skim of the disc surface of 0.1mm will solve this problem immediately.

Before replacing brake discs look at the condition of the discs and check to see if they can be salvaged, especially by machining the discs directly on the car.

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