Bicycles/Maintenance and Repair/Freewheels and Cassettes
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[edit] Freewheels and Cassettes
The freewheel is the device that makes it possible to roll without the pedals moving. A ratchet allows the rear sprockets to turn the wheel when the pedals are turned forward but also allows the wheel to turn even when the sprockets are not turning.
Freewheels are usually screwed onto the hub, which has threads to accept it. By design, pedaling forwards tightens the freewheel. This prevents it coming loose and also means tools are not required to fit a freewheel.
Removing the freewheel needs a "puller". A different puller is required for each type of freewheel. The most common design is the Shimano freewheel but others exist. The puller has splines that match splines inside the freewheel body and allow the freewheel to be unscrewed.
Freewheels are generally only found on older bicycles as the design has been superseded by the freehub. The freehub has the ratchet mechanism inside the hub body. The sprockets are sold as a cassette - a set of six to ten sprockets of different sizes (although sets of less than eight are now uncommon). The cassette is held onto the freehub with a lock-ring that have splines similar to those used on freewheels. To remove the cassette requires a chain whip to hold the cassette still against the freehub while turning the lock-ring.
