Flip-flop hub
Flip-flop hubs, also called double-sided hubs, are rear bicycle hubs that are threaded to accept fixed cogs and/or freewheels on both sides. There are several different types of flip flop hubs available for different applications. Their main purpose is to allow changing between two (and only two) different gear ratios on one rear wheel without the added complications of a multi-gear derailleur or internal hub gear, or between fixed and freewheel options. By removing the rear wheel and turning it around, the rider can switch between the two options. They are traditionally found on track bicycles, but can also be found on other single speed bicycles. Types Fixed gear/track bicycles Usually, one side has a fixed type stepped threading, and the other side is threaded for a standard freewheel. Occasionally you may find double-sided fixed hubs with stepped threading on both sides, but these are rare and only a few companies offer them. The fixed side has a stepped-down diameter thread to accept a lockring. The larger diameter thread on the fixed side accepts a standard threaded cog and uses the same size threads as the freewheel side of the hub. The most common standard I.S.O. thread size is 1.375"