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A freehub is a type of bicycle hub that incorporates a ratcheting mechanism, and the name ''freehub'' is a registered trademark of Shimano. A set of sprockets (called a "cassette") are mounted onto a splined shaft of the freehub to engage the chain. The ratcheting mechanism is a part of the hub, in contrast to a freewheel, an older technology, which contains both the sprockets and a ratcheting mechanism in a single unit separate from the hub. In many high-end and midrange bicycles, freehubs have replaced freewheel systems. A freewheel mechanism allows a rider to stop pedalling whilst the cycle is still in forward motion. On a cycle without a freewheel mechanism, the rider has to keep pedalling whenever the cycle is moving. ==Comparison to freewheels== The freehub concept answers several drawbacks encountered with the freewheel design: * Freewheels are threaded onto an axle hub, using conventional right-hand threads. As the bicycle rider pedals, the freewheel is continuously kept tight, as chain torque is in the right-hand direction. This becomes a problem when the freewheel needs to be removed. Having undergone high torque from leg muscles, it is difficult to loosen and remove the freewheels. A freehub, on the other hand, has cogs that slide onto an axially-splined cylindrical outer shell. A lockring or the last cog(s) are threaded onto the freehub. It is fastened to the wheel hub itself with a hollow retaining screw (for example, using a hex key on some models) through which the axle is inserted during operation. * The chain gear sprockets wear faster than the ratcheting mechanism. Replacing individual sprockets on a freehub cassette is easy compared to that on some freewheels. * The ball bearings for the wheel's axle are in the hub, but a multi-speed freewheel requires a considerable distance between the drive-side bearings and the drive-side frame dropout. This distance acts as a leverage force on the axle. Since the freehub can have its bearings near the end of the cassette (and the dropout), axle bending and breaks are far less common. Not all manufactures/models use this design. Those designs often use an axle made from oversize aluminum to compensate for the additional bending moment on the axle. Beyond removal from the hub and of the cassette, there is limited, if any, access for cleaning and lubrication. The part can be fabricated relatively inexpensively and is not intended to be serviced or disassembled with hand tools. The latter is only possible by means of specialized or shop equipment. The outer cup covering the ratchet pawls and bearings is pressed into place at the factory, secured by interference fit, leveraging the same inner threads of the shell into which the cassette lockring normally screws. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Freehub」の詳細全文を読む スポンサード リンク
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