A hub is the center part of a bicycle wheel. It consists of an axle, bearings and a hub shell. In the case of the front wheel hub the axle is fastened to the forkends with the use of a quick release clamp or bolts. The rear wheel hub is fastened to the dropouts or rear chain stays by the same means as the front. The hub shell has a flange which is where the spokes are threaded and then screwed into the rim. There are 3 general types of rear wheel hubs. Most commonly used is a freehub. Freehubs are designed to allow the cassette to slide over the hub which then works with the derailleur in changing of gears. In older bikes and track bikes a threaded hub is used which gear clusters and single cogs are directly screwed onto the hub. Lastly there are internal hubs. Internal hubs have the gears contained within the rear hub itself. Internal hub gears work by internal planetary, or epicyclic, gearing, in which the hub outer turns at a different, but adjustable, speed to the sprocket. Rear hub gears commonly come in 3 to 8 speeds but with variations up to 14 speeds. Internal hub gears are generally more reliable than external derailleurs because they stay cleaner, are almost weather-proof and require little maintenance simply due to the fact that they are not exposed to natural elements while riding.
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