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.
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