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Derailleurs are cunning bits of kit. The way they work is simple: they take advantage of your pedalling action to move the chain smoothly from one sprocket to another. The name comes from the French for ’derail’ (pronounced simply ’de-railer’ or ’de-rail-yer’).

The rear derailleur hangs underneath the cassette and feeds the loose chain that’s returning from the chainset back onto the cassette. This is the part of the chain that isn’t under pressure – it’s the top part that’s doing the work as you pedal. The important part for changing gear is the guide jockey wheel, the one that sits closest to the cassette. It’s also called the top jockey, even when the bicycle is upside down. The derailleur works by using the cable to move the guide jockey across the cassette. Because this part of the chain is not under pressure, the chain will follow the guide jockey and move onto a different size sprocket as it is fed onto the cassette.

The chain needs to be moving to mesh with a new sprocket, which is why you have to be pedalling to change gear. If you pedal too hard, the chain will not be able to engage properly on the new sprocket and will slip and crunch as you try to change gear. The lower jockey wheel, also called the tension jockey, has a different function. It sits on the derailleur arm and is sprung so that it’s always pushing backward. It is there because you need more chain to go around a combination of big chainring and big sprockets than for a combination of small chainring and small sprockets. The tension jockey is needed to take up the slack, otherwise the surplus chain would drag on the ground.

The next step is telling the derailleur what size sprocket you want to be in. In the pre-derailleur days of bicycle racing, roadracers had two different-sized sprockets, one on each side of the back wheel. When they came to a hill, they’d jump off their bikes, whip off the back wheel, turn it round in the frame, refit it with the larger-sized sprocket engaged, jump back on and ride up the hill. At the top, they’d reverse the process. Derailleurs were invented because anyone who could avoid this palaver saved enough time to win races.

In 1951, after experimenting with different styles, Tullio Campagnolo invented a derailleur called the Gran Sport, which looks pretty much like those we use now and works in a similar way. The derailleur is bolted on just below the rear axle. The top part stays still, but the knuckle, with the guide jockey attached, is hinged at an angle. This means that as the guide jockey moves across, it also moves down, tracking the shape of the cassette. There is a spring across the hinge, pulling the two halves of the derailleur together. Consequently, left to its own devices, the spring will pull the derailleur so that the guide jockey runs under the smallest sprocket.

Finally, here’s where you tell the derailleur what you want. The shifter on the handlebars connects to a cable, which pulls the two parts of the derailleur apart. This moves the guide jockey across and down, and so pulls the chain onto a larger sprocket. Moving the shifter the other way releases cable, allowing the spring to pull the guide jockey and chain onto a smaller sprocket. This combination of cable and spring is common – V-brakes work the same way, with the cable pulling something into place and a spring returning it when the cable tension is released.

The two most important demands we place on our derailleurs are that they don’t affect the transfer of power from the pedals to the back wheel and that they shift the chain from one sprocket to the next as quickly as possible so that the change from one gear to the next happens without breaking the rhythm of the pedal stroke. In order to be able to deliver these objectives, the movement of the derailleur as controlled by the shifter needs to be very precise.

Derailleurs are right down near the ground, in a prime position for picking up all kinds of grit, dirt and mud. All that gunk will wear the pivots around which the movement of your derailleur hinges. It’s not surprising that derailleurs, especially rear ones, have a relatively short life. Keeping them clean and well lubricated helps, but if you ride hard, expect to replace them every year.

Slave to the rhythm the derailleur

Slave to the rhythm: the derailleur

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