Brake technique in depth: rim drum and disc

Regardless of whether you are dealing with a city bike, a racing bike or a motorbike, the brake is about the same thing: converting speed to heat quickly and predictably. But the way to get there is different. Rim, drum and disc brakes are based on three different mechanical principles, and as a mechanic you must understand both — grasp the principle and be able to troubleshoot it when something is not working as it should.

On a rim brake, a cable pulls a caliper together, so two pads are pressed directly against the rim's edge. The principle is simple and easy to adjust, but it has a built-in weakness: the rim itself is the brake surface, and it wears over time. A worn rim can in the worst case fail during braking, so the rim brake requires that you continuously inspect the rim's condition — not just the pads.

In a drum brake, a cable pull rotates a cam inside the hub, which spreads two brake shoes outward against the inside of a drum. Because all the mechanics are enclosed inside the hub, the drum brake is unaffected by rain, snow and dirt — an advantage on everyday bikes and cargo bikes that have to work all year round. On the other hand, it is heavier than the other types, and because the heat has difficulty escaping from the closed drum, intensive braking can cause 'brake fade': braking power temporarily decreases because the material becomes too hot.

The disc brake is widely used today on both bicycles and motorcycles because it brakes powerfully and is less sensitive to wet conditions than rim brakes. There are two main types. The mechanical disc brake pulls, like the rim brake, on a cable — but here typically only one pad is pulled toward the disc, while the other remains stationary. The hydraulic disc brake instead uses a lever piston that forces brake fluid through a closed system to caliper pistons, which clamp both pads evenly against the disc and self-adjust as the pads wear.

• 01Mechanical disc brake: simple to adjust and repair, but typically less precise feel in the grip
• 02Hydraulic disc brake: powerful, self-adjusting and precise but requires bleeding when air in the system
• 03Both types require the disc to be clean, straight and free of oil — even a fingerprint can cause squealing brakes

Air is compressible, brake fluid is not, as a rule. If air enters a hydraulic system — for example when a hose is cut, or when the pads are worn completely smooth — it is felt as a 'spongy' grip that goes too far in before the brake takes hold. Bleeding is essentially about forcing fresh fluid through the system from caliper to grip (or vice versa, depending on manufacturer) in short strokes, whilst air bubbles are collected and removed, until the grip feels firm and precise again.

Manufacturers specify a minimum thickness for friction material on their pads — when that limit is reached, the pad must be replaced, regardless of how old or new it otherwise looks. The Danish regulation on bicycle design and equipment, on the other hand, does not specify a particular millimeter measure but a functional requirement: the bicycle must have at least two mutually independent brakes that work effectively. Your task as a mechanic is to translate that functional requirement into a concrete assessment of the individual brake in front of you.

“A brake you don't understand the principle behind you can only adjust by luck. A brake you understand, you can troubleshoot.”