Hub gears are less common than derailleur gears which are the dominant gear system on most modern bicycles. Hub gear systems have in the past, also been used on motorcycles although this is now quite rare.
Unlike derailleur gears, where the gears and mechanism are exposed to the elements, hub gears are enclosed within the hub of the bicycle's rear wheel. Gears are changed by a cable which is tightened or loosened by a lever on the handlebars.
The main advantage of hub gears is that, because the mechanism is enclosed within the hub, it is not exposed to dirt or weather. This means that, unlike derailleur gears, hub gears need very little maintenance and are very reliable. Bicycles built with internal hub gears instead of deraillurs also are less vulnerable to damage in shipping. The other advantage is that the gear can be changed when the bike is stationary, which makes them suitable for riding in city traffic with lots of stops and starts.
The main disadvantage of hub gears is that, because of the limited space available in the hub, they have only a limited range of gears, typically just three or four speeds. Recently, however, seven speed internal hubs have become common and models are available which have as many as fourteen speeds. The other main disadvantage is that they are heavier and more expensive than derailleur gears.
Hub gears work by internal planetary or epicyclic gearing, which means that the outer case of the hub gear unit (which is attached to the spokes) is made to turn at a different speed to the rear wheel's axle depending on which gear is selected.
Hub gears were invented in 1903 by an English company called Sturmey Archer, and by the 1930s they had become common on bikes across the world. Their popularity has been much diminished by derailleur gears which, since the 1950s, have become the most common type of gear system on bicycles, due to their lower price and wider gear range, although hub gears have undergone a small revival in recent years.
The Rohloff Speedhub is a 14 speed internal hub gearing system. It is considered by many as a great leap forward in gearing system for bicycles. 14 equally-spaced gears are operated by one twistgrip with no overlapping ratios. The gear range is as wide as a mountain bike's 27-speed derailleur system.