Fixed gear

A fixed-gear bicycle (or fixed-wheel bicycle, commonly known in some places as a fixie) is a bicycle that has a drivetrain with no freewheel mechanism. The freewheel was developed early in the history of bicycle design but the fixed-gear bicycle remained the standard track racing design. More recently the "fixie" has become a popular alternative among mainly urban cyclists, offering the advantage of simplicity compared with the standard multi-geared bicycle.

Most bicycle hubs incorporate a freewheel to allow the pedals to remain stationary while the bicycle is in motion, so that the rider can coast, i.e., ride without pedalling using forward momentum. A fixed-gear drivetrain has the drive sprocket (or cog) threaded or bolted directly to the hub of the back wheel, so that the pedals are directly coupled to the wheel. During acceleration, the pedal crank drives the wheel, but in other situations, the rear wheel can drive the pedal cranks. This direct coupling allows a cyclist to apply a braking force with the legs and bodyweight, by resisting the rotation of the cranks. It also makes it possible to cycle backwards.

As a general rule, fixed-gear bicycles are single-speed. A derailleur for gear selection would introduce chain slack, which would interfere with braking. Gear selection can, however, be accomplished with the use of an internally geared hub. For example, a Sturmey-Archer fixed-gear 3-speed hub is a fixed-gear multi-speed arrangement. Most fixed-gear bicycles only have a front brake, and some have no brakes at all.

A track bicycle or track bike is a form of fixed-gear bicycle optimized for racing at a velodrome or at an outdoor track. Some road racing and club cyclists use a fixed-gear bicycle for training during the winter months, generally using a relatively low gear ratio, believed to help develop a good pedalling style. In the UK until the 1950s it was common for riders to use fixed-gear bicycles for time trials. The 1959 British 25 mile time trial championship was won by Alf Engers with a competition record of 55 minutes 11 seconds, riding an 84 inch fixed-gear bicycle. The fixed-gear was also commonly used, and continues to be used in the end of season hill climb races in the autumn. A typical club men's fixed-gear machine would have been a "road/path" or "road/track" cycle. In the era when most riders only had one cycle, the same bike when stripped down and fitted with racing wheels was used for road time trials and track racing, and when fitted with mudguards (fenders) and a bag, it was used for club runs, touring and winter training. By the 1960s, multi-gear derailleurs had become the norm and riding fixed-gear bicycles on the road declined over the next few decades. Recent years have seen renewed interest and increased popularity of fixed-gear cycling.

...
Wikipedia