Why do my disc brakes rub and squeal?

Disc brakes have very small clearances by design — a few tenths of a millimetre each side. That means a small error produces an obvious noise, and it also means most disc brake noise is an alignment problem rather than a fault.

Safety

What you'll need

• Allen keys, usually 5mm
• Isopropyl alcohol and a clean rag
• A rotor truing fork, or an adjustable spanner (optional)

Step by step

1. Check the wheel is properly seated first.Open and reseat the quick release or thru axle and make sure the wheel is fully home in the dropouts. A wheel sitting slightly out of place moves the rotor relative to the caliper, and this is the single commonest cause of sudden rubbing after a wheel has been out.
2. Spin the wheel and listen to the pattern.Continuous scraping means the caliper is not centred. A rub that returns at the same point on every revolution means the rotor is bent. Nothing at rest but a squeal under braking means contamination or bedding-in.
3. Sight through the caliper against a light background.Look down through the slot with a white wall or a piece of paper behind. You can see which side is touching and by how much.
4. Realign the caliper.Slacken both caliper mounting bolts about a quarter turn so it can float, squeeze the brake lever hard and hold it so the caliper self-centres on the rotor, then tighten both bolts evenly a little at a time while still holding the lever. Release and spin.
5. Fine-tune by hand if it still touches.With the bolts just nipped up, nudge the caliper by hand while watching the gap, then tighten. Getting the last tenth of a millimetre often takes two or three attempts.
6. Straighten a bent rotor carefully.Find the bent section by spinning slowly and watching where it moves. Use a proper rotor truing fork, or an adjustable spanner opened to fit, and apply gentle pressure in the opposite direction. Work in very small increments — a rotor bends more easily than expected and over-correcting is easy.
7. Clean the rotor if the brake squeals under load.Isopropyl alcohol on a clean rag, both sides, then let it dry. Chain lube overspray onto a rotor is extremely common and instantly ruins braking.
8. Deal with contaminated pads honestly.Oil soaked into a pad cannot be washed out. Light contamination sometimes responds to sanding the face on clean abrasive paper and rebedding. Anything that has had chain lube or degreaser on it needs new pads — and find the source before fitting them.
9. Bed new pads in properly.About twenty firm stops from moderate speed, without coming to a complete halt, on a quiet road. Unbedded pads squeal and feel weak, and people often diagnose a fault that is just a lack of bedding.
10. Check pad wear while you are there.Look through the caliper at the friction material. Most pads should be replaced at around 0.5mm of material left. Running them to the backing plate destroys the rotor.
11. Test at walking pace before riding.Both brakes, hard, on a flat surface. If anything feels spongy or the lever pulls a long way, stop and take it to a shop.

Tips

• A little rub that only appears when you pedal hard is frame or wheel flex, and is normal on a light bike. It is not an alignment fault.
• Never spray anything on a bike near the rotors without covering them.
• New discs and pads always squeal a little until bedded. Give them a proper bedding-in before diagnosing anything.

Common mistakes

• Squeezing the lever with the wheel removed — On a hydraulic brake the pistons advance to close the gap, and once closed the pads will not go back over the rotor. A plastic transport spacer between the pads prevents it, which is why one comes with every new bike.
• Spraying chain lube or degreaser near the rotor — Any oil that reaches the rotor transfers to the pads and soaks in. Contaminated pads squeal, lose most of their power, and cannot be cleaned back to full function — they have to be replaced.

If it doesn't work

Questions people ask