Flip-flop hub

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Flip-flop hubs are rear bicycle hubs that are threaded on both sides to take a fixed cog on one side and a freewheel on the other. This lets you switch between two gear options on one wheel without a derailleur or internal hub. You simply remove the rear wheel and flip it around to switch gears. They’re common on track bikes but can be used on other single-speed setups too.

Usually one side uses fixed-gear threading with a lockring, and the other side uses standard freewheel threading. The lockring has a left-hand thread to resist loosening when you apply braking torque. The other side accepts a freewheel with standard threads. There are a few rare variations where both sides are fixed-threaded, but those are uncommon.

Threading can vary. The most common standard is ISO 1.375" x 24 TPI, but older or different systems (British, French, Italian) exist. Because lockrings come in different sizes, it’s important to check compatibility with your hub manufacturer. Using the wrong lockring can damage the threads over time. Common lockring sizes include English/ISO, Campagnolo/Phil Wood, and Old French.

You can pair a fixed cog on one side with a freewheel on the other. The freewheel is usually the same size or a bit larger than the fixed cog to give either a coast option or easier pedaling for hills. The chain and frame must fit the gear sizes, and sometimes you’ll need to adjust chain length or brake setup when changing gears.

BMX-style flip-flop hubs add another twist. One side may have standard ISO freewheel threads while the other uses smaller BMX threads (about 30 mm x 1 mm) for 14T–15T freewheels. Some newer BMX hubs even support 13T freewheels on the smaller side.

Another BMX development is left-side drive, which moves the drivetrain to the left side to keep the chain and sprocket out of the way for grinding tricks. This requires a right-hand/left-hand flip-flop design and special hubs (examples include the ACS South Paw Claw and DK Dual Drive). Some hubs use mixed left/right threads with adapters to fit different sizes.

A third, rarer type has both sides threaded the same (ISO) but was not designed for two freewheels. Instead, one side might be used for a drum or disc brake, with a freewheel on the other side.

In short, flip-flop hubs give two gearing options in one rear wheel, enabling fixed vs. freewheeling or two different gear ratios, with several variations to fit different bikes and riding styles.