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cogset
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.
==Freewheels==
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 CEI (Cycle Engineers Institute) thread was adopted as the international standard and is now known as B.S.C. - British Standard Cycle and is a standardized right-hand thread (1.375 x 24 TPI) onto which a standard freewheel is screwed.〔(【引用サイトリンク】 Sheldon Brown: Traditional Thread-on Freewheels )〕 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.〔(【引用サイトリンク】 Sheldon Brown: Freewheel or Cassette? )〕 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.
Pedaling forces tighten a freewheel onto the hub, so no tool is required to install one. The ratcheting mechanism prevents the freewheel being loosened when the sprockets are turned counter-clockwise. A freewheel can be removed from the hub with one of the many specific freewheel removal tools that engages a spline or set of notches on the outboard end of the freewheel. Removal often requires considerable effort due to the large torque that tightens the freewheel during pedaling, and some freewheels cannot be removed intact. Future removal of a freewheel is facilitated by grease on the freewheel threads before installation.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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