From Wiki : "A fixed-gear bicycle or fixed wheel bicycle, is a bicycle without a freewheel and usually only one gear ratio. The sprocket is attached to the hub without a freewheel mechanism, usually secured by a reverse-thread lockring.
Because there is no freewheel mechanism, fixed gear bicycles cannot coast. Whenever the rear wheel is turning, the pedals turn in the same direction. By resisting the forward motion of the pedals, a rider is able to slow the bike to a stop, without the aid of a brake. They can also be ridden in reverse, although the forward-oriented geometry of any bicycle makes this more of a stunt than a practical technique.
Most fixed gear bicycles only have one gear ratio. Some fixed gear bicycles have a sprocket on each side of the hub giving the choice of using two different gear ratios. To change gear, the rider has to stop, remove, turn around and refit the rear wheel. If the same chain is used in both positions, the difference between these two gears will be small. In the past Sturmey Archer made multi speed fixed hub gears which can change gear whilst riding."
Also something I ran into Fixed Gear Offroad Bike?!
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