A fixed-gear bicycle (or fixed-wheel bicycle, commonly known 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 advantages of simplicity compared with the standard multi-geared bicycle.

 

 

 

 

Most bicycles 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 the forward or downhill energy of the bike and rider. A fixed-gear drivetrain has the drive sprocket (or cog) threaded or bolted directly to the hub of the back wheel, so that the rider cannot stop pedalling. When the rear wheel turns, the pedals turn in the same direction.[1] This allows a cyclist to apply a weak braking force without using a brake, by resisting the rotation of the cranks. It also makes it possible to ride backwards although learning to do so is much more difficult than riding forward. 

As a general rule, fixed-gear bicycles are single-speed. A derailleur cannot be fitted because the chain cannot have any slack, but hub gearing can, for example a Sturmey-Archer fixed-gear 3-speed hub, in which case is a fixed-gear multi-speed arrangement. Most fixed-gear bicycles only have a front brake, and some have no brakes at all.

One of the perceived main attractions of a fixed gear bicycle is low weight. Without the added parts required for a fully geared drive train—derailleurs, shifters, cables, cable carriers, multiple chain rings, freewheel hub, brazed-on mounting lugs—a fixed gear bicycle weighs less than its geared equivalent. The chain itself is subject to less sideways force and will not wear out as fast as on a derailleur system.

  Also, a fixed gear drivetrain is more mechanically efficient than any other bicycle drivetrain, with the most direct power transfer from rider to the wheels. Thus, a fixed gear requires less energy in any given gear to move than a geared bike in the same gear.[2]