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Replacing the hub sprocket

While hub gears can get along fine with far less attention than derailleur geared versions, it’s a mistake to think that this means that they can be neglected.

Just as on a bike with derailleurs, hub gears mean that you’re constantly putting pressure on the chain as you pedal and that inevitably means that the chain, sprockets and, to a lesser extent, the chainrings will wear out.

If left for too long, you will reach a point where the chain will slip over the sprockets under pressure. This is irritating at the best of times and dangerous in busy traffic. Chains are relatively easy to replace, but sprockets on the most common hubs are trapped behind a handful of hub-gear specific mystery components. These need a little care to remove and it helps a lot when you come to reassembly if you concentrate on the order and orientation of the parts that come off.

Replacing the hub sprocket - Step 1

Sprockets are worn when the teeth start to get very sharp or when the two sides of the teeth don’t look symmetrical. You may also find that the pressure of the chain on one face of each tooth has caused the surface of the tooth to splay outwards, making sharp lips at either side. It’s not worth waiting until the chain links actually starts slipping over the sprocket before you replace them. By that time the worn sprocket will already have started to damage the rest of your transmission.

Replacing the hub sprocket - Step 2

You’ll need a tool that you don’t often come across in bicycle repair toolboxes: circlip pliers. These are only useful for one job, which makes them a bit irritating to invest in, but it’s difficult to do the job without them. People do improvise with screwdrivers, but it’s far too easy to slip and stab your own fingers with the end of the screwdriver. Invest in the proper tool. These come in two variations: external and internal. External circlip pliers prise a circlip off the outside of a tube whereas internal ones lift them out of a groove on the inside of a tube. The kind you need here are external circlip pliers. You can get cheap versions that are supposed to be able to do both, but they’re usually fairly wobbly at either end and so quite difficult to use and, if they slip in the middle of getting a circlip off, they’ll most likely take a vindictive bite out of your fingers at the same time.

Replacing the hub sprocket - Step 3

A completely different reason for changing a sprocket would be to alter your gear ratio. The steps between each gear can’t be changed as they’re determined by the design of the internal parts of the hub. But the entire range of gears can be shifted in either direction, since this is determined by the ratio of the chainring size to the sprocket size. You can change either of these – fitting a larger chainring has the same effect as fitting a smaller sprocket, both will make all your gears a little bit higher. You’ll be able to ride faster on downhill sections, but will struggle a little more getting up steep ones. All the pictures below right show Shimano hubs. They’re the most common type, but there are others. SRAM have been making internal hubs for many years, which are reliable and last for many thousands of miles. The same sprockets fit on both types of hub. SRAM ones are held on by a similar circlip arrangement, but it’s easier to get to. The SRAM hubs don’t use a cassette joint, so the circlip and sprocket are revealed when you remove the back wheel.

Replacing the hub sprocket - Step 4

“It’s far too easy to slip and stab your own fingers with the end of the screwdriver”

Dropouts – vertical and horizontal – the merits and differences

This is one of those terms that sounds more complicated than it is. The dropout is the slot in the frame that the wheel bolts onto – it’s the same word for the front and back wheels. Traditionally, the slots on the rear wheel dropout were almost horizontal, allowing you to slide the rear wheel back and forward in the frame. However, this is unnecessary for derailleur gears, since the derailleur takes up the slack in the chain and so vertical dropouts became more common. They provide a very secure fitting, since you cannot pull the rear wheel out of the dropout however hard you stamp on the pedals. This is a particular problem when the wheel nuts/quick-release levers aren’t done up tightly enough or break, pulling the back wheel out of its slots and jamming it on the chainstays.

See also bike maintenance tips, tricks and techniques “Refitting the rear wheel”