Hub gear

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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.

In this simple epicyclic gear mechanism, the inner gear or "sun gear" (green) provides the input rotation. The two "planet gears" (blue) rotate freely about the planet gear carrier (yellow) which is fixed. As the planet gears rotate about the sun gear, they propel the outer ring gear or "annulus" (red), which provides the output rotation

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.

History

Hub gears were invented in 1903 by the English company Sturmey Archer, and by the 1930s they had become common on bicycles across the world. Since the 1950s their popularity has been diminished by derailleur gears which have 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). Hub gear systems have in the past also been used on motorcycles, although this is now rare.

14-speed hub cutaway diagram

Advantages

• Because the mechanism is enclosed within the hub, it is not exposed to dirt or weather. Hub gears thus need less maintenance than derailleur gears and are more reliable, making them suitable for utility bicycles.
• The gear can be shifted when the bike is stationary, which makes them suitable for riding in city traffic with many stops and starts.
• Internal hub gears are less vulnerable to damage in shipping than derailleurs.
• Shifting is dramatically smoother than in external rear derailleurs.
• As the chain does not move sideways, it can be covered, protecting the rider's clothing from grease and damage, another feature useful for utility cycling.

Disadvantages

• Because of the limited space available in the hub, they have a 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 with as many as thirty-eight speeds.
• They are less efficient than derailleurs, at least in the unusual case of both types being clean and little worn.
• They are heavier than a derailleur system.

See also

• bicycle gearing
• Rohloff Speedhub 14-speed hub gear

External links

• Flash Animation of Hub Gear
• Sturmey Archer's website
• The Speedhub at the Rohloff GmbH website
• Sram, German manufacturer of gearhubs and other bicycle parts
• Shimano, Japanese manufacurer of gearhubs and other bicycle parts
• Sheldon Brown's overview of gear ratios and ranges for existing internal gear hubs
• Tony Hadland's Bicycle Gear History and Technical pages