Which brake system?

“A to B 41 was interesting, as always, but I’m a bit confused by Martin Fillan’s comment (Letters, A to B 40) about grease leaking onto brake drum linings and his suggestion about using a roller brake.What exactly is a roller brake? Is it better than a drum? And is it easier to replace? I can’t find one in the Sturmey catalogue, so whose is it and how much extra will it weigh?”

John Burnett
Swindon

Brakes are a fascinating subject, long neglected by this august publication. Broadly speaking, the problem faced by engineers since the invention of the wheel has been to produce a simple, tough device capable of transposing a large but weak hand or foot movement into a small but powerful force to push against a rotating body and slow its progress.The brake then needs to dissipate the considerable amount of heat generated – something that few bicycle brakes are very good at.The following devices are all available today to retard the progress of bicycles… Some more successfully than others:

1. Caliper brake

Long outmoded on motor vehicles, a caliper brake (sometimes called a side-pull) consists of a pair of curved arms or calipers pivoting somewhere beneath the headset bearings, with ‘blocks’ of friction material at their lower extremities. By the action of a pull rod, push bar, or more usually a flexible cable these days, the friction blocks are moved towards each other, squeezing the two outer faces of the wheel rim in the process.

The caliper is light and cheap, because the rotating element is already in place, but being completely exposed to the elements, it is badly effected by rain, grease, oil and grit. Different calipers and brake blocks are affected in different ways, but the most important element is the frictional co-efficient of the wheel rim material. Chromed steel lasts for ever, and works very well when dry, but loses most of its stopping ability in the wet. Aluminium is less effective in the dry, but relatively good in the wet, making it a safer material overall. Unfortunately, aluminium rims can wear away quite fast, especially on small-wheeled bikes.

The quality of the brake ‘feel’ depends largely on the friction material and the construction of the caliper. Poor calipers bend and distort when the brake is applied, giving a rubbery feel at the lever and/or judder or squeal.

Calipers are notoriously difficult to centre correctly, which can leave one brake block rubbing against the rim, and a wobbly rim will cause one or both blocks to rub intermittently. Generally, the rim disposes of heat quite successfully, but heat build-up can be a problem on long descents, particularly for heavily-laden or small-wheeled bikes. Excessive heat in the rim can cause tube failure and a catastrophic blow-out.

2. Band brake

A long-outmoded Edwardian technology, the band brake is nevertheless worth mentioning because these devices do still turn up in the rear wheels of Chinese bicycles once in a while. A band brake utilises a flexible band of friction material, firmly fastened at one end and wrapped loosely round a rotating steel drum.When the band is pulled tight by a lever, it wraps tightly around the drum, slowing its progress.This tendency for the brake to apply itself without undue effort from the rider is known as ‘self servo’.The bad news is that the effect usually disappears in reverse, so a band brake will not stop you running backwards down a hill…

Band brakes are cheap, low-tech devices, but the negative aspects go on forever.With the drum inside the band, there’s nowhere for heat to go, and being difficult to protect from the elements, water can slosh straight in, resulting in a near total loss of braking effort. Brake bind, shriek and squeal can be a problem too, especially after a good soaking.

3. Drum or hub brakes

More correctly an ‘internal expanding shoe’ brake.This was the standard motorcar and motorcycle brake for most of this century until superseded by cheap reliable discs, and remains a favourite on hard-used and/or heavy bicycles. The general layout is two curved aluminium blocks or ‘shoes’ faced with friction material, both pivoting at the same point, and pushed outward at the other end by a cam of some kind, to make contact with a metal (usually steel) drum. Like the band brake, drums exhibit a self servo action – the leading shoe tending to be drawn harder into contact with the drum, whilst the trailing shoe tends to be pushed away, and visa versa in reverse. A variant common on older motorcycles and cars was known as ‘twin leading shoe’ – much more effective going forwards, but virtually useless in reverse (see band brake).

Drums can be heavy, although much of the weight penalty is negated where the brake is combined with a hub gear, and modern manufacturing techniques can reduce the weight significantly.The shoes are largely immune from contamination, but internal and external sealing arrangements can be a bit crude. Sealing problems between the gear and brake components of a hub can result in grease contamination, which can ruin the shoes, and poor external sealing can allow water in, although this generally requires total immersion.Wet friction shoes lose most of their effect, and as they start to dry, a violent self-servo action can result in brake snatch and squeal.

Hub brakes are not good at dissipating heat, but they make do by transferring it into the substantial mass of the hub in the short term, where it can safely radiate away when the descent is over. If a good quality drum does overheat, it should gently ‘fade’, or become less effective until cooled. Adjustment is rarely required once the shoes have bedded in, and the progressive action and ‘feel’ of a hub brake beats most other types.

Drum brake exploded diagram. Left to right: back plate (fixed pivot above, moving pivot and lever below), brake shoes faced with friction material and steel drum.This is the Sturmey Archer ‘BR’ of 1932 – it survived, effectively unchanged, for 70 years

4. Back-pedal or coaster

Rare in Britain, but common elsewhere, the coaster is usually combined with a hub gear. Pedal backwards and a metal cone slides through the hub, pushing metal brake segments against the rotating hub shell. Operation can be a bit insensitive, with a lack of feel, although hubs vary. Being grease filled, a coaster brake is more or less immune from contamination. It also has no exposed levers and cables to go wrong, requires no adjustment, and lasts more or less for ever. Like the drum brake, heat dissipation relies on warming the hub on a descent, then allowing the heat to escape. In extremis, localised heating from the metal-on-metal contact can boil away the grease or even weld parts together, although I have never heard of this on a bicycle.

5. Cantilever and V-brake

Similar rim-squeezing action to the caliper, but the force is provided by two vertical arms, pivoting at the bottom and brought together by a cable pulling the top of the arms together.The only real difference between the cantilever and V- is in the way the cable pulls the arms. In both types, the brake blocks are mounted some way down the arms in order to gain a degree of mechanical advantage.

V-brakes have become the braking system of choice because they’re light and crudely effective in operation. Problems are as for the caliper brake – water and oil contamination, rim wear, difficulty with centring and heat build-up on long descents. Cheap V-brakes can be unpredictably fierce in operation, so many feature pressure limiting devices of various kinds (usually fitted in the cable) to prevent the front wheel from locking up. Other problems include squeal on cheaper units, judder, and frame or fork distortion when the brake is applied. On the positive side,V-brakes are simple to maintain.

6. Roller brake

Like so much else in the bicycle world, the roller brake is a Shimano invention, or re-invention. Combining elements of the drum, coaster and disc brake, the friction effect comes from steel rollers which are forced outward by a cam, pushing metal shoes against a rotating steel drum. Heat build-up can be a problem, but most designs incorporate a cooling disc, just like a ‘proper’ disc brake. Brake feel can be unpleasantly ‘woolly’ and vague compared to other types, and the shoes can make nasty metal-on-metal noises unless well greased.That’s also the good news, because water and oil won’t make much headway into a grease-packed unit.

7. Disc brake

This brake generally utilises a pair of friction pads, which are forced against opposite sides of a steel disc. Heat does build up in the disc (they can glow cherry red on a hard-driven racing car, or after stopping a high speed train), but the disc is well exposed, so heat is rapidly dissipated and fade is rare. Disc brakes have become the preferred means of de-acceleration on just about every wheeled vehicle, from aircraft to trains, cars and motorcycles. Progress in the bicycle world has been limited, mainly because the disc brake lends itself to hydraulic operation, which can add weight and complication. Pads may also bind slightly in the ‘off’ position, which can be frustrating on a fractional horsepower vehicle. Early bicycle discs were heavy, ineffective in the wet and noisy, but these problems have been largely eliminated by reducing the disc to a delicate tracery of struts.

In answer to the question, it isn’t easy to convert a bicycle to roller brakes because the Shimano Nexus system will fit only a Shimano hub. But if you have a Sturmey Archer or SRAM-equipped bike, there’s really no need – an upgrade to hub brakes is generally quite easy (not on smaller bikes like the Brompton, unfortunately). In practice, few roller, drum, coaster or disc brakes work badly enough to be worth changing.