On a bicycle, the cogset is the set of multiple rear sprockets that attaches to the hub on the rear wheel. A cogset works with a rear derailleur to give multiple gear ratios to the rider. Cogsets come in two varieties, cassettes or freewheels, of which cassettes are a newer development. Although cassettes and freewheels perform the same function and look almost the same when installed, they have important mechanical differences and are not interchangeable. The terms are sometimes incorrectly used interchangeably.
FreewheelsEditA freewheel (also known as a block or cluster) consists of the rear set of sprockets and a ratcheting mechanism in a single assembly that mounts on a threaded hub. Traditional rear hubs have a standardized right-hand thread (1.375 x 24 TPI) onto which a standard freewheel is screwed. This allows different brands of freewheels to be mounted on different brands of hubs.
The major disadvantage of the freewheel design is that the drive-side bearing is located inboard of the freewheel, and thus potentially far from the drive-side axle support. The farther from the support, the more flexing stress is placed on the axle which can bend or even break. Freewheels were common until the late 1980s. As the number of sprockets in cogsets increased beyond five and clusters became physically wider, the freehub design began to replace the freewheel design. Today it is rare to find a freewheel on a bicycle with more than seven speeds. Some new single-speed — especially BMX and utility bikes — and lower-end multi-geared bicycles continue to be manufactured and sold with freewheels.<p> Pedaling forces tighten a freewheel onto the hub, so no tool is required to install one. However, the ratcheting mechanism prevents loosening a freewheel simply by turning the cogs counter-clockwise. Instead, a freewheel can be removed from the hub with one of the many specific freewheel removal tools that engages a spline or set of notches on the outboard end of the freewheel. Removal often requires considerable effort due to the large torque involved in tightening up the freewheel, i.e. pedaling. Some freewheels cannot be removed intact. <p>
Cassettes are distinguished from freewheels in that a cassette has a series of straight splines that form the mechanical connection between the sprockets 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 threaded 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 wheel due to the lack of a contained freewheel mechanism.
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 keep the sprockets and spacers in the correct order and position when they are removed from the freehub body. When the sprockets need to be replaced due to wear or the user wishes to change gear ratios available, 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 1970s 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 higher quality derailleured bicycles use this newer design.
Number and width of sprocketsEdit
Over time, the number of sprockets in a cogset has increased, from three or four before World War II, to five or six used from the 1950s to the 1970s, to the eight, nine, ten or even eleven now found on modern bikes. As more rear cogs were added the cogset became wider, and the gears more narrow, and closer together. To accommodate the wider cogset the axle (measured O.L.D.) was lengthened, and the distance between the hub flanges was lessened.
Prior to the introduction of indexed shifting the spacing on the gears did not need to be standardized, and the width and sprocket spacing of the cogset was flexible.
With the introduction of indexed shifting and cassettes standardization became necessary in this regard. Shimano and Campagnolo both came up with independent standards, and SRAM later followed Shimano's lead in respect to sprocket spacing and cassette width.
For Shimano and SRAM the cassette spacing developed as follows: An eight speed cassette has (very nearly) the same spacing of cogs as a seven, but is wider. This results in functionally compatible shifters, but specific freehub bodies, or necessitating the use of a spacer with a seven speed cassette on an eight speed hub. Eight, and nine speed cassettes and freehub bodies have the same width, but with the gears closer together; as a result the shifters are not compatible, but they use the same freehub bodies. 10 speed cassettes are slightly more narrow than eight/nine so fit on their freehubs but need a spacer, conversely eight/nine speed cassettes do not fit on ten speed freehubs.
This progression has provided more fine adjustment of gear ratio, however the use of thinner metal parts has had the effect of shortening the life-span of the chain and cogs due to so called "stretching" of the chain. This is caused by frictional abrasion of the load-bearing surfaces of the chain causing elongation. As such, the chain and gears of a nine speed system require more frequent replacement than an eight. The narrowing of the hub flanges has created more "dish" in the wheel, something that weakens the wheel if all other factors are the same, however the improvements in the strength and reliability of spokes and rims has more than balanced this out and wheel strength is generally higher despite the increased dish.
Improvements in shiftingEdit
Shift ramps are complex tooth profiles, in the rear cogs and front chain rings, designed to pick up and drop the chain during shifting. They allow for shifting under greater load than was previously possible, and for smoother and cleaner shifting. The different systems are branded Hyperglide by Shimano, UltraDrive by Campagnolo, and OpenGlide by SRAM. The chain itself is specifically manufactured for ease of shifting, and to interface with a particular manufacturer's shift ramps; using a different type of chain will result in sub-optimal shifting.
- Park Tools webpage containing basic freewheel & freehub repair info
- How to replace a cassette cluster (video)cy:Casét (darn beic)