Cassette (bicycle part)

On a bicycle, the cogset is the set of rear sprockets that attaches to the hub on the rear wheel. Cogsets come in two varieties, cassettes or freewheels, of which cassettes are a newer development than freewheels. Although cassettes and freewheels perform the same function and superficially look the same, they have important mechanical differences and are not interchangeable. Because many bicycle users do not understand the differences, the terms are incorrectly used interchangeably.¹

Freewheels

A freewheel (previously also known as a block or cluster) consists of the rear cogset and a ratcheting mechanism in a single replaceable assembly. Freewheels must be used with threaded hubs. The freewheel is attached to the hub by means of a right-hand thread. Traditional rear hubs had a standardized set of threads (1.375 x 24 TPI) onto which a standard freewheel was screwed.² This allowed many different brands of freewheels to be mounted on different brands of hubs. Most bicycles used this system of freewheels and threaded hubs until the late 1980s,¹ and many new single-speed, especially BMX and utility bikes, bicycles and lower-end multi-geared bicycles continue to be manufactured and sold with freewheels.

The disadvantage of freewheels is that they put more stress on the axle because the drive-side bearing is located inboard of the freewheel. This is less significant with single-speed freewheels as they are narrower, and so minimize the bearing displacement and the resulting additional stress on the axle.

Pedaling forces tighten a freewheel onto the hub, so no tool is required to install one. However, the freewheel mechanism prevents loosening a freewheel simply by turning the cogs counter-clockwise. Instead, a freewheel can be removed from the hub with the help of a specific freewheel removal tool that engages a spline or set of notches or prongs inside the outboard end of the freewheel.

Cassettes

Cassettes are distinguished from freewheels in that a cassette typically has a series of straight splines that form the mechanical connection between the gears and the cassette compatible hub, called a freehub, which contains the ratcheting mechanism. The entire cassette is held on the hub by means of a screwdown lockring. Some cassette systems from the late 1980s and early 1990s use a threaded small cog to hold on the larger splined cogs, the entire set referred to as a cluster. Cassettes resemble freewheels when installed, but are clearly different off the bike due to the lack a contained freewheel mechanism.

The sprockets are commonly sold as a set, called a "cassette". The sprockets in a cassette are usually held together by three small bolts or rivets for ease of installation. These bolts or rivets are by no means necessary, they just make it easier to keep the sprockets and spacers in the correct order and position when they are removed from the freehub body. Individual sprockets are also available. When the sprockets need to be replaced or the user wishes to replace them to change gear ratios, only the sprockets are replaced, not the ratchet mechanism.

The ratchet mechanism, known as the freehub body, is still replaceable on most hubs, but forms a structural part of the hub. Cassette systems have a major advantage in that the drive-side axle bearing can be out near the frame, rather than being back towards the centre of the axle behind the freewheel. This greatly reduces the stress on the rear axle, making bent or broken axles extremely rare.

Since their introduction in the late 1970s³ cassettes have been used on increasing numbers of bicycles, starting at the high-end and over time becoming available on less expensive bikes. Today all high-end derailleured bicycles used this newer design.

Number and width of sprockets

Over time, the number of sprockets in a cogset has increased, from three and four before World War II, to five and six from the 1950s to the 1970s, to (with the introduction of cassettes) seven to eight in the 1980s, then nine, ten and now eleven for Campagnolo's new road bicycle groupset. From seven to eight speed, the spacing between sprockets was decreased and the rear spacing of the frame slightly widened to accommodate the greater number. With nine speed, the sprockets were made thinner, and thinner still for ten speed. Because of the thinner sprockets, a thinner chain should also be used. The use of thinner metal parts in both these cases has had the effect of shortening the life-span of the chain and cogs due to "stretching" of the chain (frictional wear of the load-bearing parts of the chain causing elongation).

Improvements in shifting

Beginning with Shimano's Hyperglide system, and continuing with Campagnolo's UltraDrive, and SRAM's OpenGlide, groupset manufacturers have engineered cassettes with complex tooth profiling designed to pick up and drop the chain, which itself is specifically manufactured for ease of shifting, and to interface with a particular manufacturer's shift ramps. This, coupled with the adoption of combined brake and shift levers, allows for shifting under greater load than was previously possible, such as when out of the saddle.

Contents

Freewheels

Cassettes

Number and width of sprockets

Improvements in shifting

See also

References