Fixed gear bikes or a 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 lockring is usually fitted to prevent the sprocket from unscrewing. Whenever the rear wheel is turning, the pedals turn in the same direction. By resisting the rotation of the pedals, a rider can slow the bike to a stop without the aid of a brake. Stopping is sometimes known as "skidding" in the fixed gear world. Skidding occurs when the rider opposes the pedals' movement hard enough to halt the rear wheel's rotation. 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 chainring).

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.

Advantages and Disadvantages

Fixed gear bicycles are ridden by cyclists for many reasons, such as their light weight, simplicity, low maintenance.

Many people who ride fixed-gear bicycles simply find it more enjoyable than or as an alternative to riding bikes with freewheels. Although the bike has only one gear, the lighter weight of a fixed-gear bike over its multi-speed freewheel equivalent can provide increased performance in some conditions. In slippery conditions some riders prefer to ride fixed because the transmission gives feedback on back tire grip.

Descending is more difficult as the rider must spin the cranks at a very high speed (sometimes at 150rpm or more), or use the brake(s) to slow down. Nevertheless, the enforced fast spin when descending is said to increase "souplesse" (a French word meaning suppleness or flexibility, usually referring to the human body), which improves pedaling performance on any type of bicycle.

Riding fixed is generally considered to encourage a more effective pedaling style, which translates into greater efficiency and power when used on a bicycle fitted with a freewheel.

When first riding a fixed gear, a cyclist used to a freewheel has a tendency to try to coast now and again, particularly when approaching corners or obstacles. Since freewheeling is not possible, this can lead to anything from a 'kick' to the trailing leg, up to a loss of control of the bicycle.