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Flip-flop hub
From WikiPedalia
A double-sided hub, intended to take a sprocket or freewheel on each side. The gear of a one-speed bicycle could be changed by removing the wheel, and installing it backwards. Most flip-flop hubs are intended to accept a fixed sprocket on one or both sides.
There is a special BMX variant of flip-flop hub which is intended to take a single-speed freewheel on each side. This type has a standard thread on one side, and a smaller thread on the other side, which fits special undersized 15- and 14-tooth freewheels. (There is not room to fit a freewheel mechanism inside of a 15-tooth or smaller sprocket, with a normal sized hub.)
