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Double-sided Hub
From WikiPedalia
A hub which can be fitted with a sprocket on each side, so that a one-speed bicycle can have two different gears, which are selected by removing the wheel and turning it around so that the other sprocket may be used. Also known as a "flip-flop" hub.
The most common application for double-sided hubs is for fixed-gear bicycles. Many fixed-gear fans have a fixed sprocket on one side, and a freewheel on the other, using one or the other as conditions change. This is discussed at more length in my article on Fixed Gear Road Bicycles.
