Hub gear

Hub gears or internal-gear hubs are a type of gear system used on bicycles. Hub gears are used mostly on utility bikes and various types of small wheeled bicycle, such as folding bikes. 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 relative to the rear wheel's sprocket depending on which gear is selected.

In the United States and United Kingdom, hub gears are less common than derailleur gears which are the dominant gear system on most modern bicycles in these countries. In most of continental Europe, however, hub gear systems are dominant. 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 or twist grip on the handlebars.

Hub gears were invented in 1903 by an English company Sturmey Archer, and by the 1930s they had become common on bicycles across the world. Their popularity has been diminished by derailleur gears since the 1950s, due to their lower price and wider gear range, although hub gears have undergone a small revival in recent years.

Though most hub gear systems use one rear sprocket, SRAM's DualDrive system combines an epicyclic hub with a multi-speed rear derailleur system to provide a wide-ranging drivetrain concentrated at the rear wheel. The system is useful for folding bicycles (where a multiple front chainset could foul the bike's folding mechanism) and in recumbent bicycles and freight bicycles (where small wheels and/or increased weight require a wider range of gears with smaller individual steps between each ratio).

Advantages

 * 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, making them very suitable for utility bicycles.
 * The other main 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.
 * Bicycles built with internal hub gears instead of derailleurs also are less vulnerable to damage in shipping.
 * Shifting is dramatically smoother than any external rear derailleur.
 * As the chain does not have to move, it can be covered protecting the rider's trousers from grease and damage, another feature useful for utility cyclists.

Disadvantages

 * The main disadvantage of hub gears is that, because of the limited space available in the hub, they have a far more limited range of gears than derailleurs. Traditional hub gear systems have only three or four speeds. Recently, however, seven and eight speed internal hubs have become common and models are available which have as many as thirty-eight speeds.
 * They are also claimed to be less efficient than derailleurs, however this is only when comparing with a clean derailleur system which has little wear.
 * They are extremely heavy compared to a derailleur system.