Bike disc brakes: hydraulic or mechanical

What does a complete disc brake include?

A disc brake brings together four elements: the lever fixed to the handlebar, the transmission of force by cable or by a hose filled with fluid, the calliper bolted to the frame or fork, and the rotor bolted to the hub, which the pads clamp. The brakes on this page are sold per wheel, front or rear, or as a pair. The hose on a rear brake is longer, since it runs to the bottom of the frame. Depending on the model, the rotor and adapter are supplied or have to be ordered separately: the product name often says so, with the words "with rotor" or "without rotor or adapter".

This type of brake is now fitted to most mountain bikes, gravel bikes and e-bikes, and to a good share of road bikes. It keeps the same performance in the rain, in mud or on long descents, a decisive advantage on a heavy or loaded bike.

Cable or fluid: three ways of transmitting braking force

In a hydraulic disc brake, the lever compresses a fluid that drives the calliper pistons. The system multiplies the force of your hand: one or two fingers are enough to brake hard, and the pistons compensate for pad wear on their own. It is the reference solution for mountain biking and on electrically assisted bikes.

A mechanical disc brake is operated by a cable, like a rim brake. It works with existing cable levers, can be adjusted with a simple key and repaired without bleeding tools, which makes it a sensible choice for long-distance touring or an everyday bike. The pad clearance is adjusted by hand as the pads wear.

A semi-hydraulic disc brake keeps a cable as far as the calliper, which contains its own small hydraulic circuit. It lets you keep cable levers while enjoying some of the power and comfort of hydraulics.

Two or four pistons: how much power for which bike?

A hydraulic calliper has two pistons, one on each side of the rotor, or four, two on each side. With four pistons, the pad is longer, the clamping force increases and the heat is better spread, for a little extra weight. A few guidelines:

• Cross-country, road, gravel and city: two pistons and rotors from 140 to 180 mm.
• Trail and all-mountain: two or four pistons, with at least 180 mm at the front.
• Enduro, downhill and electric mountain bikes: four pistons and rotors from 200 to 220 mm, to handle long descents without losing performance.
• Urban e-bikes and cargo bikes: a hydraulic brake that its manufacturer designs for these heavy bikes, with large-diameter rotors.

Mount, fluid, clamp: the compatibility points to check

The calliper is fitted to the standard provided by the frame and fork: Post Mount, the most widespread on mountain bikes, Flat Mount, the flat calliper of road and gravel bikes, or IS, an older standard that can be converted with an adapter. An adapter is also used to move up to a larger rotor, within the limit allowed by the frame and fork manufacturer.

The fluid is set by the brake brand. Shimano, Magura, Tektro and the Formula Cura family use mineral oil, as does the Sram Maven, whereas most other Sram brakes run on DOT fluid. Never use one in place of the other: the seals in each system can only withstand their own fluid. At the handlebar, Shimano I-Spec and Sram MatchMaker clamps attach the gear shifter to the brake lever, as long as both come from the same family. Finally, a hose cut to the length of the bike often needs bleeding.

Rotors, pads, levers: replacing a single part

When only one part is worn, there is no need to change the whole brake. Brake rotors and disc brake pads are chosen according to the calliper reference. A broken lever is replaced from our brake levers, a damaged calliper from our brake callipers. Hoses, adapters and bleed kits are waiting in bike brake parts; the whole bike brakes and parts section also includes rim brakes.

Which disc brake brand for your bike?

Clarks Cycle Systems, an English maker of braking parts based in Derby, focuses on equipping everyday bikes: its M2 and Clout 1 hydraulic brakes, with two pistons and mineral oil, are offered here with their rotor, including a Post Mount version.

Sram covers mountain biking with four hydraulic families: Level, with two pistons, for cross-country and trail; G2, the successor to the Guide, for trail; Code for enduro; and Maven, its most powerful brake, with four pistons and mineral oil, designed for downhill, enduro and electric mountain bikes.

Shimano offers its MTB brakes from Deore to XTR, by way of SLX and XT, whose Servo Wave lever quickly brings the pads close to the rotor to leave more travel for modulation. Saint is its downhill range, and the I-Spec EV clamp takes the gear shifter directly.

Tektro, a manufacturer specialising in braking systems, offers its four-piston Orion here: the HD-M750 version is designed for electric mountain bikes with a mid-drive motor and for thicker 2.3 mm rotors.

Formula, an Italian brand, groups its mineral oil brakes in the Cura family: the two-piston Cura for lighter riding, and the Cura 4, with four 18 mm pistons, for enduro and downhill.

Magura, a family business from Bad Urach in Germany and a pioneer of the hydraulic rim brake, gives its MT brakes a lever in Carbotecture, a light composite: the four-piston MT5 for aggressive mountain biking, and the two-piston MT4, also available as the MT4 eSTOP for e-bikes.

Disc brakes in the outlet and new arrivals

When a range moves to a new generation or finish, its brakes go into the brakes outlet: the right time to equip a second bike. Newly listed models can be found in new arrivals, and the summer sales and Black Friday are good times to upgrade your braking.

Frequently asked questions

How do bike disc brakes work?

When you squeeze the lever, you pull a cable or push fluid to the calliper. Its pistons then press two pads against the rotor fixed to the hub, and the friction turns speed into heat. Placed at the centre of the wheel, away from the spray that dirties the rim, the rotor keeps working well in the rain.

Why has my disc brake stopped working?

The most common causes are pads worn down to the backing plate, pads contaminated by oil or chain lube, or air in a hydraulic system, which makes the lever spongy. First check the pad thickness, then the lever travel. Contaminated pads are replaced; a spongy lever calls for a bleed.

Which disc brake should you choose for an e-bike?

The weight of the bike and its sustained cruising speed put more strain on the brakes. A hydraulic brake, preferably with four pistons on an electric mountain bike, combined with 180 to 203 mm rotors, provides the necessary reserve of power. Some models are designed for these bikes and accept thicker rotors.

Can you fit disc brakes to a bike with rim brakes?

This requires calliper mounts on both the frame and the fork, and hubs designed to take a rotor. Without these mounts, fitting them is not possible safely. In that case, a better-quality rim brake or new brake pads remain the best option.

Is the rotor supplied with the brake?

It depends on the model. Some kits are complete, rotor and adapter included, while others are sold without a rotor so that you can choose the diameter and fitting, 6-bolt or Center Lock, that suit your wheels. The name and description of each product state this.