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 different size gear ratios on one rear wheel without the added complications of a derailleur or internal hub gear, or between a fixed and freewheeling option. By removing the rear wheel and flipping 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.
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