A fixed-gear bicycle or fixed wheel bicycle is a bicycle without the ability to coast. The sprocket is screwed directly on to the hub and there is no freewheel mechanism. A reverse-threaded lock-ring is usually fitted to prevent the sprocket from unscrewing. Whenever the rear wheel is turning, the pedals turn in the same direction, this allows a cyclist to stop without using a brake, but rather by resisting the rotation of the cranks. One method for stopping without brakes is skidding. Skidding involves leaning forward to un-weight the rear wheel, allowing the cyclist to stop the rear wheels' rotation (and therefore that of the cranks) thus bringing the bike to a halt. A fixed gear bicycle can also be ridden in reverse.
Most fixed gear bicycles only have one gear ratio. Some have a sprocket
on each side of the rear hub, giving the rider a choice of two different
gear ratios. Such a hub may have a fixed gear on each side
(double-fixed) or a fixed gear on one side and a freewheel on the other
(fixed-free) also known as a flip-flop hub. To change gear, it is
necessary to remove, reverse and refit the rear wheel. Typically, the number of teeth on the sprockets will differ by one or two, for example 19 teeth on one side and 17 on the other, making the latter gear some 11 or 12% higher than the former (for the same chain-ring).
In the past Sturmey Archer made a fixed multi speed hub gear, the model ASC, allowing the rider to change gear while riding. Its successor company, SunRace Sturmey-Archer,
plans to produce a modern equivalent, the S3X, in the near future.
Fixed gear bikes are alternatively known as fixie bikes, or simply fixies.