Replace the chain

A chain is chosen first by the number of speeds on the cassette: an 11-speed chain is narrower than a 9-speed one, and a chain that is too wide rubs and skips. 12-speed chains are not always interchangeable between brands: follow the compatibility given by your drivetrain's maker.

Before buying, measure the wear on the old one. If it was past 0.75%, or 0.5% on 11-speed and up, the cassette may be worn as well: a new chain that skips under effort on the most-used cogs will confirm it. Single-speed bikes and hub gears use wider chains, often sold as single-speed chains.

Shift onto the small chainring and the smallest cog to slacken the chain. If it has a quick link, recognisable by its different side plates, squeeze it with the Quick-link pliers (or else Pliers), drawing the two neighbouring links together: it pops open.

With no quick link, set any link in the Chain tool (or else Multi-tool) and turn the handle until the pin is pushed right out of the link. Pull the chain off by one end. Keep it at hand: it will be your template for length.

The simplest method: lay the old chain and the new one side by side on a table and count links, not centimetres, since the old one has stretched. Cut the new one to the same number of links, counting the quick link, with the chain tool.

If the old chain was the wrong length, or on a new derailleur bike: wrap the new chain around the big chainring and the biggest cog without threading it through the derailleur, bring the ends together, then add two links, meaning one inner and one outer link. SRAM Eagle 12-speed drivetrains have their own method, described by the maker. On a single-speed or hub-gear bike, the chain only needs to let you pull the wheel back to tension it.

Feed the chain in from the back, over the smallest cog, then down through the derailleur cage: in front of the upper jockey wheel, behind the lower one, staying inside the small metal tabs of the cage. A chain that runs over a tab rubs and breaks the derailleur.

Then bring it forward to the chainring, passing through the front derailleur if there is one, between its two plates. Rest it on the small chainring without tensioning it. If the chain has lettering or an arrow, put it facing out from the bike, as most makers ask.

Bring the two ends together under the bike, where the chain is slack. A piece of wire bent into a hook between two links holds them while you work. Fit one half of the quick link into each end, then slot them together. Some links carry a direction arrow.

Remove the hook, bring the quick link to the top, between chainring and cog, hold the rear wheel with the brake and push hard on the pedal: the link snaps shut with a click. With a Shimano connecting pin, drive it in with the chain tool until it sticks out as far as its neighbours, then snap off its guide section with pliers.

Turn the cranks backwards while watching the chain pass through the derailleur: a stiff link makes the jockey wheel jump. Flex it gently side to side to free it. Check that the quick link is fully closed, with its pins at the ends of the slots.

Then go through all the gears, on the big and the small chainring. On the big ring and the biggest cog, the derailleur cage must not be stretched out horizontally; on the small ring and the smallest cog, the chain must not sag. Wipe off the factory grease on the surface if it is sticky, and ride carefully for the first few kilometres, building up the effort gradually.