How E-Bike Disc Brakes Work

Your bike slows when you squeeze the lever, and that's the whole job. The parts between your hand and the wheel are few, and once you can name them, most brake problems become easier to describe.

The parts, from handlebar to wheel

The lever. It sits on the handlebar. Your squeeze starts everything else.

The cable or the hose. On some bikes a steel cable carries the pull. On others a hose carries brake fluid. Hydraulic and mechanical brakes are covered on their own page.

The caliper. This is the part that straddles the rotor, bolted to the frame or fork. Wikipedia describes it this way: "Calipers are attached to the frame or fork along with pads that squeeze the rotors for braking."¹

The pads. Park Tool describes a pad as braking compound bonded to a backing plate.² The compound is the part that touches the rotor. The metal plate holds it in place. See pad materials for the types.

The rotor. A flat metal disc fixed to the wheel hub. It spins with the wheel, so it's what the pads grab. Rotor sizes and mounting has the details.

What happens when you squeeze

On a cable brake, the cable moves a lever arm on the caliper.² On a hydraulic brake, fluid does that job. Either way, the pads close on the rotor from both sides. The pads squeeze the rotor, and the friction slows the wheel.¹

Some calipers move both pads. Others move only the outer pad. In that design the rotor flexes a little to meet the inner pad.²

Why the gap between pad and rotor matters

There's a small gap between each pad and the rotor when the lever is released. As pads wear, that gap grows. Wikipedia says that many hydraulic disc brakes have a self-adjusting mechanism, so the pistons keep the distance from the pad to the disc consistent.¹ Mechanical brakes usually need you to adjust the gap by hand, so check your own manual for how.

Symptoms and the part behind them

Start with what you notice, then look at the likely part.

• A rubbing or scraping sound while the wheel spins freely. Park Tool calls pad and rotor rub "the main symptom of a misaligned caliper," but it can also come from a wheel that isn't seated properly.³ Check that the wheel is seated, then follow your manual.
• One pad that seems to stick. On a hydraulic brake, dust and dirt can collect on a piston and make it stick.³
• A squeal when you brake. Several causes are possible. Noise and cleaning works through them.
• A lever that feels different. The cause could be worn pads, a cable issue or fluid trouble. How to tell when pads are worn is the place to start.

If you can't tell which part is at fault, follow the manual for your own brake, or ask a bike shop to look.