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On a bicycle, the cassette or the freewheel is the set of rear sprockets that attaches to a hub on the rear wheel. The cassette is a later development of a cogset than the freewheel. Cassettes and freewheels are not the same, but because many bicycle users do not understand the differences, the terms are often incorrectly used.
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A freewheel consists of the rear cogset and a ratcheting freewheel 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 onto which a standard freewheel was screwed onto. This allowed many different brands of freewheels to be mounted on different brands of hubs. Unlike many cassettes, the individual sprockets in a freewheel can be removed or replaced if necessary. Most bicycles used this system of freewheels and threaded hubs until the late 1980's.

Over the last few years the cassette type hubs, called freehubs, have largely replaced the conventional threaded rear hub. A freehub incorporates the ratchet mechanism into the hub body. The ratchet mechanism is still replaceable on most hubs. Many users lament the loss of the brand interchangeability that existed with most freewheel systems; however, 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 center of the axle behind the freewheel. This greatly reduces the stress on the rear axle, so it does not fail as often and it can be lighter. Cassettes work so well that they have become the new standard. Most quality bikes made since the late 1980's have used this greatly improved design.

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 hub. The entire cassette is retained on the hub by means of a screwdown lockring. Some cassette systems from the late 1980's and early 1990's use a small threaded cog to hold on the larger splined cogs. Cassettes resemble freewheels, but 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 ratchet 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. This is unlike freewheels.

Number of Sprockets
Over time, the number of sprockets in a cassette has increased, from seven to eight, then nine, and now ten for racing bicycle groupsets. Up to eight speed, the spacing between sprockets was decreased and the rear spacing of the frame slightly increased to accommodate the greater number. With nine speed, the sprockets were made thinner, and even thinner still for ten speed. Because of the thinner sprockets, a thinner chain should also be used.

Shimano Hyperglide
In Hyperglide cassettes, the individual teeth on a sprocket are differently shaped, so that the chain can momentarily engage two gears during shifting. The exact shape of each tooth is designed by a computer taking into account the position of nearby teeth on adjacent sprockets. Shifting with this system is faster and quieter.

All modern Shimano cassettes use the Hyperglide system.
In bicycle dérailleur systems there is a series of ramps, varying gear tooth profiles, and/or pins along the faces of freewheel or cassette sprockets, or between the chainrings in a crankset, meant to ease shifting between them.

The design, developed by Shimano, improves on their earlier Uniglide design (which used beveled gear teeth instead of ramps), and was introduced as part of a commercially viable index shifting system. The Hyperglide ramps, along with laterally floating dérailleur jockey wheels, allows for enough "slop" in the system to make indexed shifting reliable, despite variations in shift cable adjustment and manufacturing or assembly tolerances. A Hyperglide freewheel or cassette on a bike with friction shifters can improve shifting there as well.

The individual sprockets on a Hyperglide cassette or freewheel are designed specifically to work with their neighbors. For example, the 18-tooth sprocket on a wide-range cassette (such as one for a mountain bike) will have a different ramp pattern than the 18-tooth sprocket on a narrow-range cassette, because the number of teeth on the neighboring sprocket requires a different ramp pattern for an optimal shift. As a result, cassettes are sold as a cohesive unit (rather than as individual sprockets) with all the sprockets on a given cassette designed to work with each other. However, some mixing and matching is possible for a custom gear range, as long as all the sprockets' ramps are compatible.

In order to ensure alignment of each sprocket with its neighbor, the freehub has a narrow spline at one position, and each sprocket has a corresponding wide tooth on its inside face.
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