The chain is a series of links which when turned around the chainset and rear sprocket propels the bike forward. By moving the chain between sprockets on the front and back, via the front and rear mechs, a series of gears can be selected. It is an often overlooked component but its condition and use has a massive effect on your cycling performance. If it’s working fine you don’t notice it; if it’s old and worn it will skip between gears and when it breaks you aren’t going anywhere.
Although chains all look the same care must be taken when choosing the correct one. Make sure the chain is compatible with the maker of your current transmission system as some chains are not compatible with other manufacturer’s products. Then make sure you have the correct gear amount, or speeds, i.e. 9,10 or 11 speed etc as chains become thinner the more gears you have. Lastly, please bear in mind that chains are supplied a certain length and will need to be shortened, with the correct tool, to fit each system which can differ due to the size of chainring and cassette sprockets.