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Disc brakes : powerful and reliable braking on demand

Along with suspension systems, disc brakes have been the major source of innovation in bicycles over the last decade. They are now fitted as standard on mountain bikes, some hybrid bikes and, increasingly, cyclocross bikes.

There are two parts to a disc brake: the calliper, which bolts onto specific mounts on your frame or fork and the rotor (or disc), which fits directly onto the hub of your wheel. Disc brakes have two distinct advantages over rim brakes. Firstly, they don’t wear out your wheel by abrading the rim every time you brake. Secondly, the harder, cleaner surface of the rotor makes for powerful braking. Mechanical disc brakes are easier to work on because they use standard V-brake levers and cables. Hydraulic brakes are more powerful and more resilient to wet, muddy conditions. Disc brakes are available to suit every purpose and budget, from super lightweight XC bling to workhorse heavyweights.

Braking power

Braking power is dependent on two things: the rotor size you choose to use and the piston arrangement within your brake’s calliper. Large rotors with diameters of 180–200mm (7–8 inches) give powerful braking with more modulation but are heavier than smaller cross-country rotors, which are typically 160mm (6 inches) or even 140mm (5.5 inches) across.

Callipers may have one, two or three pairs of slave cylinders (pistons); turin piston systems are typically found on cross-country bikes whilst four- and six- or three piston systems appear on downhill or all mountain bikes, where more speed needs to be scrubbed off quickly. The current crop of disc brakes are now significantly more powerful than their predecessors, though, with one-finger braking now possible across the spectrum.

Disc brake callipers are relatively simple to fit and need little maintenance as long as they, their pads and the rotors are all kept clean and free from lubricating substances. This includes road grime, which contains a high proportion of oily fuel residue and the grease from your fingertips, as well as the ubiquitous overspray from over enthusiastic chain lubing and system leaks from a damaged hose or piston seal.

The callipers themselves aren’t subject to the same barrage of dirt flung up by your tyres as rim brakes, so they will work better for longer in most conditions. Maintenance is usually limited to regular checks of the pads and rotors, plus an occasional bleed or fluid change. Bleeding is straightforward, if a little on the fiddly side, but do handle brake fluid, particularly DOT, with care as it’s corrosive to both paint and skin.

Calliper adjustment

Calliper adjustment is the same whether the brake is mechanical or hydraulic. It’s worth investing the time to get this right, as whilst it’s a fiddly task there is a marked performance difference between brakes with pads which aren’t hitting the rotor square and brakes where both pads are working exactly as they should. Most hydraulic brakes work by pushing both pads onto the rim at the same time, and function best if there is an equal gap between each pad and the rotor. Most mechanical brakes and some basic hydraulic brakes work by pushing the outer pad onto the rotor, which then flexes until it hits the opposite pad. This is more effective than it sounds but works best if the gap between the stationary pad and the rotor is as small as possible to minimize the distance the rotor has to bend.

Calliper mounting

The concept of standardized component manufacture was invented for gun making by Guillaume Deschamps for the French army. It encouraged the interchangeability of individual parts rather than the making of each gun as an individual mechanism. However, artisan gun makers were so resistant to a process they believed would damage their trade that they prevented the idea from being realized for more than fifty years. Bicycles are the same. Each manufacturer has its own preferred way of doing things and it takes time for any one way to be accepted by a majority. This turned out to be exactly the situation when disc brakes were introduced to the bicycle market – a few different standards fought for dominance, with International Standard and Post Mount as front runners.

Post Mount fixings use two threaded lugs in the frame or fork, with bolts that point roughly forward. International Standard mounts have a pair of unthreaded holes in the frame. Bolts pass through the holes, and thread directly into the calliper. The Post Mounts seem to have become the prevailing standard, as it’s generally considered easier to adjust the position of the brake callipers relative to the brake rotor. In either case, there are a range of converters available to fit one kind of calliper to the other kind of frame, and also to remount brake callipers to accommodate different rotor sizes, so don’t panic if your callipers don’t appear to match the fittings on your bike – your bike shop should be able to identify a suitable converter.