Maintenance
Rim Brakes and Disc Brakes Behave Differently When It Rains
Both stop a bike well when dry. In wet conditions the two designs face different problems, and only one of them has to clear water off the surface it grips.
Everything below about wet weather braking comes from what actually happens rather than from what is supposed to.
What holds up in practice
- Rim brakes suffer a delay while pads clear water from the rim.
- Discs sit higher and shed water faster, but rotors still glaze and contaminate.
- Wet braking distances are longer regardless of brake type.
What happens to a wet rim
A rim brake grips the sides of the wheel rim, which is the part of the bike closest to the road spray. In wet conditions a film of water sits on that surface, and the pads must sweep it away before real friction begins.
The result is a noticeable delay between pulling the lever and the bike slowing, which can be a second or more with some pad and rim combinations. Riders learn to apply the brakes briefly in advance to clear the rim, which works and requires anticipation. Grit suspended in that water is also dragged between pad and rim, which is why rim braking surfaces wear out in winter.
Why discs cope better with water
A disc rotor sits near the hub, further from the road surface, so it receives less direct spray than a rim does. Rotors are smaller and spin faster than a rim, which throws water off more effectively by centrifugal action. Most disc pads also have a coefficient of friction that is less affected by moisture than typical rim pad compounds.
The practical effect is a much shorter delay in the wet, which is the main reason discs became standard on transport bikes. This is a difference in degree rather than kind, since wet braking distances are longer on any bicycle.
What discs are not immune to
Contamination is the most common disc brake problem, and even small amounts of oil, chain lubricant or degreaser on a rotor ruin braking. Contaminated pads squeal loudly and grip poorly, and cleaning them is often unsuccessful because the material absorbs oil.
Never spray lubricant near the rotors, and handle rotors by the edges rather than the braking surface. Glazing occurs when pads overheat and their surface hardens, and it can sometimes be corrected by light abrasion and rebedding. Bedding in new pads by making repeated firm stops from moderate speed transfers material to the rotor and is necessary for full power.
Rim wear is a safety issue
Every wet kilometre with rim brakes removes a small amount of aluminium from the rim's braking surface. Rims become progressively thinner until the pressure of the tyre can deform or split the sidewall, which fails suddenly and completely.
Most modern rims have a wear indicator, either a groove or a dot, which disappears when the rim needs replacing. Check that indicator regularly on a bike ridden through winter, and feel for a concave profile across the braking surface.
This is why heavily used rim-braked commuting bikes need wheels rather than merely pads after a few winters.
Setup differences that matter
Rim brake performance depends heavily on pad alignment, cable condition and rim trueness, and a wobbling wheel makes braking uneven. Toeing the pads slightly, so the front edge contacts first, reduces squeal on many setups. Hydraulic disc brakes need occasional bleeding to remove air and replace fluid that has absorbed moisture over time.
Mid-traffic, mechanical disc brakes use a cable and need the same cable care as a rim brake, plus pad adjustment as they wear. Rotor rub is usually a caliper alignment or a bent rotor rather than a serious fault, and both are straightforward to correct.
Range and charging figures are quoted under conditions nobody commutes in.
Riding technique in the wet
Everything takes longer in the wet, including tyre grip, so leave more distance regardless of which brakes you have. Brake earlier and more gently, and do most of your slowing before a corner rather than in it. Painted road markings, metal covers and tram rails offer dramatically less grip when wet, and should be crossed upright.
By the second week of riding, wet leaves in autumn behave like a lubricant and are the most underestimated seasonal hazard on a bicycle. The best wet-weather braking improvement available to any rider is looking further ahead.
The takeaway
Discs shed water faster, rims wear thinner, and both need you to brake earlier when the road is wet.
Fix the small rattle. Every large repair began as one.
Questions readers ask
Are disc brakes necessary for commuting?
Not necessary, but they are noticeably more consistent in the wet and they do not wear out your rims. Well-maintained rim brakes remain perfectly serviceable if you anticipate the wet delay.
My disc brake squeals and barely works. Why?
Most often contamination from oil, lubricant or degreaser on the rotor or pads. Clean the rotor with a suitable cleaner and replace contaminated pads, as they usually cannot be recovered.
