Cassette hub

On a bicycle, the cogset or cluster is the set of multiple sprockets that attaches to the hub on the rear wheel. A cogset works with a rear derailleur to provide multiple gear ratios to the rider. Cogsets come in two varieties, freewheels or cassettes, 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) 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. Threaded rear hubs were available in different thread patterns depending on the country of manufacture, French and British threads being the most common. British C.E.I. (Cycle Engineers Institute) thread was adopted as the international standard and is now known as B.S.C. (British Standard Cycle). It is a standardized right-hand thread (1.375 x 24 TPI) onto which a standard freewheel is screwed. This allows different brands of freewheels to be mounted on different brands of hubs.

The major disadvantage of the multiple sprocket freewheel design is that the drive-side bearing is located inboard of the freewheel, and as sprockets were added over time, moved the bearing farther from the drive-side axle support. This resulted in more flexing stress being placed on the axle, which can bend or even break. Multiple speed freewheels were common on quality high end bikes until the late 1980s. Suntour — Maeda Industries (Japan) introduced a compact 6 speed freewheel which reduced the spacing between the sprockets and was the same width as a standard 5 speed freewheel but required a narrower chain than the standard. The concept was copied by European freewheel manufacturers Regina (Italy) and Maillard (France). As the number of sprockets on a multiple freewheel increased clusters became physically wider, the freehub design overcame the axle/bearing problem with threaded hubs and began to supersede the freewheel design. Today it is rare to find a freewheel on a new bicycle with more than seven speeds. Some new single-speed — especially BMX and utility bikes — and lower-end multi-geared bicycles continue to be manufactured and sold with freewheels.

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Wikipedia