A freehub is a type of bicycle

Bicycle

A bicycle, also known as a bike, pushbike or cycle, is a human-powered, pedal-driven, single-track vehicle, having two wheels attached to a frame, one behind the other. A person who rides a bicycle is called a cyclist, or bicyclist....

 hub that incorporates a ratcheting

Ratchet (device)

A ratchet is a device that allows continuous linear or rotary motion in only one direction while preventing motion in the opposite direction. Because most socket wrenches today use ratcheting handles, the term "ratchet" alone is often used to refer to a ratcheting wrench, and the terms "ratchet"...

 mechanism, and the name freehub is a registered trademark of Shimano

Shimano

Shimano, Inc. is a Japanese multinational manufacturer of cycling components, fishing tackle, and rowing equipment.In 2005, the company had net sales of US $1.4 billion. Bicycle components provided 75% of its sales income...

. A set of sprocket

Sprocket

A sprocket or sprocket-wheel is a profiled wheel with teeth, cogs, or even sprockets that mesh with a chain, track or other perforated or indented material. The name 'sprocket' applies generally to any wheel upon which are radial projections that engage a chain passing over it...

s (called a "cassette

Cassette (bicycle part)

On a bicycle, the cogset or cluster is the set of multiple rear sprockets that attaches to the hub on the rear wheel. A cogset works with a rear derailleur as part of the drivetrain to provide multiple gear ratios to the rider. Cogsets come in two varieties, cassettes or freewheels, of which...

") are mounted onto a splined shaft of the freehub to engage the chain

Bicycle chain

A bicycle chain is a roller chain that transfers power from the pedals to the drive-wheel of a bicycle, thus propelling it. Most bicycle chains are made from plain carbon or alloy steel, but some are nickel-plated to prevent rust, or simply for aesthetics. Nickel also confers a measure of...

. 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.

Introduction

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.

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
  Screw thread
  A screw thread, often shortened to thread, is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered thread...
  
  . As the bicycle rider pedals
  Bicycle pedal
  A bicycle pedal is the part of a bicycle that the rider pushes with their foot to propel the bicycle. It provides the connection between the cyclist's foot or shoe and the crank allowing the leg to turn the bottom bracket spindle and propel the bicycle's wheels...
  
  , 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
  Torque
  Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
  
   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
  Hex key
  A hex key, Allen key, or Allen wrench is a tool of hexagonal cross-section used to drive bolts and screws that have a hexagonal socket in the head .- Nomenclature :...
  
   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 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
  Dropout (bicycle part)
  A bicycle dropout is a type of fork end that allows the rear wheel to be removed without first derailing the chain. The more inclusive term fork end refers to a slot in a frame or fork where the axle of the wheel is attached....
  
  . 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
  Bending Moment
  A bending moment exists in a structural element when a moment is applied to the element so that the element bends. Moments and torques are measured as a force multiplied by a distance so they have as unit newton-metres , or pound-foot or foot-pound...
  
   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

Interference fit

An interference fit, also known as a press fit or friction fit, is a fastening between two parts which is achieved by friction after the parts are pushed together, rather than by any other means of fastening...

, leveraging the same inner threads of the shell that the cassette lockring normally screws into.

History

The concept of a 'cassette' or unit hub was devised and manufactured by British company Bayliss-Wiley in 1938 and won the Cyclists Touring Club (CTC) award for that year. On the Bayliss-Wiley design the freewheel unit was threaded to accept the sprockets. A different four speed design was manufactured by BSA Cycles Ltd in 1949 to accompany their BSA 4 Star derailleur gear. The BSA design had a splined freewheel unit (BSA part No.8-1913) which attached to the hub shell (BSA part No.8-701) and carried four sprockets.

Shimano

Shimano

Shimano, Inc. is a Japanese multinational manufacturer of cycling components, fishing tackle, and rowing equipment.In 2005, the company had net sales of US $1.4 billion. Bicycle components provided 75% of its sales income...

made their first freehub in 1978 in both the Dura-Ace, and 600 (later known as Ultegra) models. It was a significant improvement. It proved to be the first widely commercially successful freehub.

Freehubs, manufactured by various companies, are now common on mid- to high-end bicycles today. Nevertheless, freewheels continue to be fitted on some new bikes, especially single speed, and cheaper models of derailleur bicycles.

External links

• Bicycle Freewheels from sheldonbrown.com
• Exploded view of Shimano FH-7800 rear freehub