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Flip-flop Hubs
Flip-flop, or double-sided hubs are threaded on both sides. Usually one side has a track-type threading, (with lockring) and the opposite side is threaded for a single-speed standard freewheel.
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 opposite side. The freewheel sprocket would normally be larger than the fixed sprocket, providing a lower gear.
Cyclists would use the fixed-gear side for most pavement riding, and save the freewheel for off-road use, or for getting home when they tired. Having the freewheel larger than the fixed sprocket gives cyclists a lower gear when they are using the freewheel. This makes it easier to climb. Since riders can coast when they are using the freewheel, the lower gear is no disadvantage on the descents.
Single-speed freewheels are commonly used on BMX bicycles, most shops that deal in BMX parts will stock them. The common size used for BMX is 16 tooth, but 17, 18, 20 and 22 tooth freewheels are available.
Note, there are two types of hubs called "flip-flop":
Fixed/free, this is the type we speak of above, with a "track" side and a freewheel side.
BMX type...BMX flip-flop hubs have two different freewheel threads, a standard one on one side, and a special smaller-diameter freewheel thread on the other, designed to work with special small freewheels (14, 15 teeth.) These are now quite rare, but if one is calling around looking for "flip-flop" hubs, make sure to get the right kind.
Please refer to the spec list from the order page of the bicycle one is considering for more details
Please check/tighten your lockring prior to prior to every ride