Brake disc

How it works

The brake disc is the steel surface fixed to the wheel that the pads clamp against. It must be perfectly flat and thick enough to get rid of braking heat. Many bikes use floating discs: the braking track is linked to the hub by small rivets or 'buttons' that let it expand without warping. Over time the track wears thinner; the minimum thickness is stamped on the disc or given in the manual, and below it the disc must be replaced. A warped disc (bent by overheating or an impact) makes the lever pulse or the bike shudder under braking. A deeply scored disc, one blued by heat or contaminated with oil also brakes less well. Front discs are usually replaced in pairs, and always with new pads, because worn pads on a new disc brake poorly and damage it.

Maintenance tips

• ✓Measure disc thickness with a caliper or micrometer and compare it with the minimum stamped on the disc.
• ✓If the lever pulses under braking, have the disc runout checked with a dial gauge before replacing anything.
• ✓Never touch the braking track with fingers or a greasy rag; clean it only with brake cleaner.
• ✓Always fit new pads with a new disc, and torque the disc bolts to the manual's value, with threadlocker if specified.
• ✓A little play in floating-disc buttons is normal; a clear knock or worn buttons should be checked by a professional.