Disc brake noise: what it means and what to do

Disc brakes are better than rim brakes in nearly every way that matters and worse in exactly one: when they are unhappy, they tell you about it loudly, continuously, and in front of other people.

Most disc noise is trivial to fix, some of it is not a fault at all, and one cause is genuinely serious. Here is how to tell which is which.

Diagnosing by sound

Contamination is the big one

Brake pads are porous. Get oil into them — chain lube oversprayed, degreaser drifting across, a drop of hydraulic fluid, or a well-meaning hand — and the pad surface stops gripping the rotor properly. The symptoms are a loud squeal and braking that feels vague and requires more lever force than it should.

The important part: you cannot reliably fix contaminated pads by riding harder or braking more. That is the folk remedy and it mostly just wears them down. Clean the rotor thoroughly with isopropyl alcohol and a clean cloth; if the noise and the weak braking persist, the pads need replacing.

Prevention is straightforward and worth building into your routine: never spray anything near the rotors, mask them or remove the wheel when degreasing the drivetrain, and handle new pads and rotors by the edges. Washing your bike properly covers doing the rest of the cleaning without creating this problem.

The five-minute alignment fix

1. Loosen the two caliper mounting bolts just enough that the caliper can move.
2. Squeeze the brake lever firmly and hold it — this centres the caliper over the rotor.
3. Still holding, tighten both bolts evenly to the correct torque.
4. Release, spin the wheel, look through the caliper at the gap either side.
5. If it still rubs on one side, nudge the caliper very slightly and repeat.

This resolves the majority of rubbing complaints, and it is worth learning because the noise comes back periodically — after transport, after a wheel change, after a hard descent. If your bike has just been in a car or a bike bag, this is the first thing to check. Travelling with a bike covers the related transit problem: a lever squeezed in a bike bag with no rotor between the pads pushes the pistons together, so a spacer goes in before the wheel comes out.

Routine upkeep

• Check pad thickness monthly in winter. Wet, gritty riding eats pads at a startling rate — a set that lasts a summer can be gone in six wet weeks.
• Clean rotors with isopropyl alcohol, not degreaser. Degreaser leaves residue; alcohol evaporates clean.
• Bed in new pads properly. Ten to twenty progressive stops from moderate speed, not dragging. Skipping this is why new pads sometimes feel worse than the old ones.
• Replace rotors when they are worn or badly scored. They are a wear item too, and a worn rotor will chew through new pads.
• Get a bleed when the lever feel changes, not when it gets bad. Hydraulic systems degrade slowly and then quickly.

When to stop riding

Grinding means metal on metal, and that is not a noise to investigate at the bottom of the descent. Stop, look at the pads, and if there is no friction material left, do not ride the bike home on that brake. The same goes for a lever that suddenly pulls to the bar.

Everything else on this page is an irritation. Those two are the reason the pre-event check in the race-day mechanical checklist puts brakes in the week-out column rather than the morning-of one.