On a bicycle, the cassette, cogset or cluster is the set of multiple rear sprockets that attaches to the hub on the rear wheel. A cogset works with a rear derailleur as part of the drive train to provide multiple gear ratios to the rider. Cogsets come in two varieties, cassettes or freewheels, of which cassettes are a newer development. Although cassettes and freewheels perform the same function and look almost the same when installed, they have important mechanical differences and are not interchangeable. A freewheel (also known as a block or cluster) consists of either a single sprocket or a set of sprockets mounted on a body which contains an internal ratcheting mechanism and mounts on a threaded hub. Cassettes are distinguished from freewheels in that a cassette has a series of straight splines that form the mechanical connection between the sprockets and the cassette compatible hub, called a freehub, which contains the ratcheting mechanism. The entire cassette is held on the hub by means of a threaded lockring.
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