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Freewheel
The mechanism that makes coasting possible. A ratchet mechanism that allows the rear sprocket(s) to drive the wheel when pedaled forward, but allows the wheel to turn forward independently even when the sprockets are not turning. In other words, the freewheel is the part which makes coasting possible.
Freewheels are normally sold with the sprockets attached, so this term is frequently used as a synonym for a cluster.
A standard freewheel attaches to a hub by screwing on to external threads that are part of the hub. The action of pedaling tightens the freewheel down on the threads, so no tools are required to install a freewheel.
To remove a freewheel requires a special tool, commonly called a "freewheel puller" or "freewheel extractor"
This tool is a splined unit that may be mounted in a vise or turned with a wrench. The splines engage matching splines in the interior (non-rotating) part of the freewheel body. Different brands of freewheels have used different spline patterns, but there is a recent tendency to standardize on the Shimano pattern.
Older freewheels had simple notches and matching extractors with two or four "bosses" (prongs.) This obsolete system was prone to failure, and it is easy to ruin the tool and the freewheel while trying to remove the freewheel. When using a boss-type freewheel puller, the tool should be secured against the freewheel by tightening down the axle nut or quick release skewer.
The standard I.S.O. thread for freewheels is 1.375 x 24 TPI, the same as for standard I.S.O bottom brackets.
See also my article on freewheels.
