Explains the main types of disc brakes for bicycles and the different fitting systems. Discusses whether you can fit disc brakes to your existing bike.
Disc brakes are generally recognised as the most effective and reliable brakes available for bikes. So far unknown on road bikes, where weight and aerodynamics are most significant, they have become virtually universal on high-end mountain bikes and increasingly common in the middle ground too. They are also becoming common on bikes intended for urban use - city streets are one place where your life might depend on being able to stop quickly!
Not all disc brakes are alike. There are two main types, mechanical and hydraulic.
Mechanical disc brakes (sometimes called cable disc brakes) work on the same principle as rim brakes, except that the pads bear on a disc or rotor rather than on the wheel rim. They are relatively cheap, though still more expensive than a simple rim brake. Because cables are prone to stretch they require relatively frequent adjustment to keep them working effectively.
Hydraulic disc brakes, on the other hand, transmit braking force from lever to brake using a hydraulic fluid, which resists compression. The system is (or should be) completely sealed, which means it requires adjustment much more rarely. However, maintenance does require more specialised tools, and if problems do arise out in the wilds they can be impossible to fix.This is a rare occurrence, usually due to crash damage, but it’s a factor to be considered by anyone planning long wilderness rides.
While hydraulic systems normally require infrequent maintenance, they should still be checked regularly. Brake pads have a long life but do wear out eventually, and it is possible for air bubbles to enter the system. If this happens it is necessary to bleed it and possibly top up the fluid.
Brake systems differ in various ways. The most obvious is rotor size. Larger rotors give greater stopping power and are better at dispersing heat - which builds up through friction. Large rotors are usual on downhill bikes, where the extra weight is unimportant, while smaller rotors are typical on cross-country bikes. It’s quite common to have a larger rotor on the front wheel than on the rear.
Single-piston systems are the most basic, with only one ‘active’ brake pad and one static. Dual-piston systems equalise the braking forces on both sides of the rotor and are generally much more effective. Some downhill brakes have four pistons.
There are various fitting systems too. If you buy a new bike with disc brakes, this won’t be an immediate issue, but it’s important when replacing anything, from pads to rotors, to maintain compatibility, which often means sticking with the original manufacturer.
With all their advantages, it’s an attractive idea to fit disc brakes to an existing bike, but this isn’t always possible. In particular, the wheels must be suitable; you can only fit disc rotors to disc-specific hubs. You also need mounts for the brake callipers on the front forks and rear stays. It may be possible to get bolt-on adaptors but these aren’t really recommended. However, most suspension forks already have disc mounts. If you’re lucky you’ll have a disc-ready frame, as many manufacturers use the same basic frame for several bikes in a range.
If you have disc-ready wheels, frame and forks the next step is to verify what mounting system they use. There is an International Standard system for mounting the callipers, but it’s not universal.
There is also an International Standard system for attaching the rotor to the hub: it uses a six-bolt fitting. However, recent Shimano hubs use a different system, called CenterLock, which uses a splined fitting and lockring.
Unless you’re very confident, the best plan is to take the bike into a good bike shop so they can sell you the right setup - and probably fit it for you too.
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