Freehub

A freehub is an axle assembly, used notably for the rear axle of bicycles. Freehubs incorporate 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. In many high-end and midrange bicycles, freehubs have replaced the traditional, heavier and more exposed freewheel systems.

Introduction

In simple pedaled vehicles, such as bicycles, a chain is connected through a gear system from the pedals to an axle (on a bicycle, the rear axle). Unfortunately, such a system requires that the rider move their feet as the wheels do, so that even when one wishes to add no extra power to the vehicle, such as while coasting or during a downhill ride, the rider must maintain a forward pedaling motion. This is tiring, inefficient, and in some cases, particularly steep downhill runs, dangerous. For this, and a variety of other reasons, it is desirable to be able to add mechanical power "only when one wants to", and let the pedals rest otherwise.

Freehubs and freewheels fill this need. However, unlike a freewheel, the freehub is essentially an integrated part of the axle, making it lighter, smaller, less exposed, and, from a certain perspective, simpler.

It should be understood that freehubs can be used for many things beyond pedaled vehicles. They might be best thought of as a compact ratcheting system.

As Compared 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 cogs wear faster than the ratcheting mechanism. Replacing individual cogs on a freehub cassette is relatively 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 axel 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 that the cassette lockring normally screws into.

History

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 bikes, especially cheaper models.

External links

* [http://www.sheldonbrown.com/freewheels.html Bicycle Freewheels] from sheldonbrown.com
* [http://techdocs.shimano.com/media/techdocs/content/cycle/EV/bikecomponents/FH/EV-FH-7800-2252_v1_m56577569830537885.pdf Exploded view of Shimano FH-7800 rear freehub]
* [http://bicycletutor.com/freehub-body/ Freehub removal and installation] from bicycletutor.com