Flip-flop Hubs
Flip-flop, or double-sided hubs are threaded on both sides. Usually one side has a track-type threading (with lock ring), and the other side is threaded for a single-speed standard freewheel. It is possible to find them with track threading on both sides, but not easily. The cyclist can then change gears manually by reversing the drive wheel.
The usual way to use a flip-flop hub is to have a fixed gear on one side, and a single speed freewheel on the other. Ideally, the freewheel sprocket would be one or two teeth larger than the fixed sprocket. This allows for extended hill climbs at a lower ratio than the fixed gear provides. The freewheel in this gear also allows the cyclist to coast on a steep descent.
Read more about this topic: Single-speed Bicycle
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