Freehub

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A Shimano Dura-Ace freehub

A freehub is a bicycle rear axle assembly that incorporates a ratcheting mechanism. Separately, a set of sprockets (called a "cassette") are mounted onto a splined shaft on the freehub. In contrast, a freewheel contains both the chain gear cogs and a ratcheting mechanism in a single unit.

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 10-mm hex key on Shimano models) through which the axle is inserted during operation.
The chain gear cogs wear faster than the ratcheting mechanism. Replacing individual cogs on a freehub cassette is relatively easy compared to that on a freewheel.
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 has its bearings near the end of the cassette (and the dropout), axle bending and breaks are far less common. Shimano have negated this advantage in the latest Dura-Ace design, which no longer uses the freehub as a bearing surface. Perhaps to compensate for the additional bending moment on the axle, it is made from oversize aluminum.

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 that the cassette lockring normally screws into.

Freehubs became common starting in the late 1980s and remain so on mid- to high-end bicycles. Nevertheless, freewheels continue to be manufactured on some new ones, especially cheaper models.