All you need to know about your next chain device.
Chain devices are important little pieces of metal and plastic that make sure your chain doesn't fall off right where you really don't need it to. Here's how to pick the right one for you;
Primary amongst all of a chain devices functions is to retain the chain. They do this via either singular top or bottom guides or a combination of the pair. Lower guides tend to feature jockey wheel-style rollers or sliders whilst the top guides are usually cages or shark fins which prevent the chain dropping towards the frame.
Bottom bracket heights are currently dropping faster than a microwave in a lift shaft so often, chain devices are utilised to offer up some protection to the chainring(s) too. They typically do this courtesy of a taco-style plastic guard which runs behind and along the chainring. The taco hits logs or rocks so that your frame and chainring don't have to.
Chain devices have shifted through the years from being complex collections of folded metal into smooth and simple components. The current fashion is to use tough plastics for the guides and guard whilst deploying steel or alloy to take care of the base plates.
First and foremost, you need to establish what standard your frame's BB tabs conform to. It's the bike industry so naturally there are a range of confusing standards. The most readily used are ISCG and ISCG 05 - be warned, a guide for one will not fit the other.
Remove crank, fit chain device, reinstall crank - that's as hard as it needs to be. Except it's often necessary to fiddle with a device to ensure that it sits perfectly centred on your chainring. If you're replacing a device, take a note of how many spacers you remove from behind the existing one and what angle it sits at in the tabs.