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Revealed! Secrets of brake discs

Published: 12 August 2001

You probably don’t spend much time thinking about your brake discs. They look pretty shiny… and that’s about it. But they’re one of the most important parts of your bike when you reach that panic moment.

If the discs aren’t doing their job properly when you yank on the lever, it doesn’t matter if you’ve got the latest six-pot calipers and race pads – if your discs aren’t up to spec you’re not going to stop.

In the past, most motorcycles were fitted with drum brakes both front and rear. Today, drums can still be found on the rear of some bikes and even on the front of some lower-powered machines.

The main problem with drums is that when continually operated for a long period, such as when descending a steep hill or stopping repeatedly from high speed, they will fade. When this happens, a greater effort is required for effective braking, and in extreme cases it becomes impossible to stop the bike.

Harder linings can be fitted, but they are less effective at low speeds. The answer came with the development of disc brakes for motorcycles.

A disc brake consists of an exposed disc which is attached to the hub of the wheel, and two friction pads. The pads are operated by hydraulic pistons mounted in a caliper attached to the fork, or in the case of a rear disc brake usually mounted to the rear swingarm. When the brake is operated, hydraulic pressure is applied to the pistons and the pads are moved into contact with the rotating disc. This way the bike is slowed down and the heat generated from the motion is conducted to the disc. As the disc is exposed to the air the heat is radiated quickly and so the brake can be used for extended periods without inducing fade.

The discs themselves are bolted to the wheel. Conventionally, they are bolted rigidly to the hub. But many high-performance bikes now have thin, light " floating " discs, where there is some room for movement built into the way the discs are attached to the hub. One of the many advantages of floating discs is that they are free to expand and contract when they heat up and cool down, which means they are less likely to distort or crack in hard use.

Ventilating discs also helps avoid problems associated with overheating. Drilling and slotting of the discs is often used to lighten the components, clear away debris at the pad surface and increase the surface area to aid cooling.

The diameter of the disc needs to be a compromise. A large disc will help with braking performance, but the extra size can bring two problems with it. One is weight, the second is that an increase in diameter also increases the radius of gyration, which will have an adverse effect on the handling and performance of the machine. The use of four and six-piston calipers permits a high effective radius with a narrow – and therefore light – working area.

Disc brakes are made from different materials for different uses. Road bike discs need to cope with all conditions and need to have a slow wear rate so that they last a long time.

They have to cope with the high temperatures reached when stopping from speed whatever the weather. They also have to be capable of withstanding sudden cooling, perhaps in heavy rain or if the bike is ridden through a puddle. Although some of these factors are also important on race bikes, the emphasis is different. Weight becomes more important on a race bike, the wear rate less so.

The vast majority of street bikes have stainless steel discs. Stainless steel performs well, but the main reason for using it is that it does not rust, and therefore retains a good appearance throughout the life of the disc. Pads are normally made of a sintered material, as this gives good wet and dry-weather performance. Stainless discs are relatively cheap, due mainly to the large quantities they are produced in.

Race bikes generally have steel or cast-iron discs, which are around 25 per cent more expensive than stainless steel. Both of these materials provide a 10 per cent improvement in friction over stainless steel and they also dissipate heat better. Both corrode, but this is less important on a competition machine which is not continually out in bad weather, and which will have the discs replaced sometimes more than once during the racing season. These discs require semi-metallic pads, of which there are around six different types, each giving a different " feel " . The actual type used is down to the preference of the rider.

The ultimate competition discs today are made of carbon-fibre. They are extremely light, which helps reduce unsprung weight, and are very powerful in the right set-up. The pads are also made of carbon-fibre. Carbon is not, however, practical for road use because it wears rapidly. Pad and disc wear at the same rate and that rate is increased in wet conditions, so on a race bike they need to be replaced several times during the season. The other disadvantage of carbon-fibre disc brakes is cost – they are around five times as much as cast-iron.

Other materials such as titanium and aluminium alloy have been used for brake discs, but are not currently popular – titanium because of cost and alloy because of distortion problems when the discs become hot.

Race technology has had a big influence on the design and in particular the appearance of discs fitted to road bikes – just take a look at the brakes on a bike dating from the ’70s and compare them with today’s machinery. Drilling and slotting became popular on the track before they were adopted for big-selling road bikes.

Looking to the future, there is a possibility of rim brakes becoming more popular. Here a large-diameter thin disc is fitted close to the rim of the wheel. It is attached to the rim rather than the hub. The caliper is inverted and works from the inside of the disc. First developed by AP Technology in 1983, rim brakes are currently appearing on some project bikes and are used on Buell’s new Firebolt and White Lightning. Otherwise it is likely that current brakes and the materials from which they are made will undergo refinement rather than any radical changes.

When should discs be replaced? Normally this will be when they have reached their minimum thickness quoted by the manufacturer – check your handbook. Replacement is straightforward. Remove the caliper and wheel, unbolt the old disc, then bolt the new one on, using new bolts if necessary or if recommended by the manufacturer. New pads must be fitted at the same time.

Replacement is not cheap - new discs for a Yamaha R1, for instance, cost around £150 each for the genuine components. But in normal use discs will last for 30,000-40,000 miles.