General remarks
Since the first decade of the 20th century, internal-gear hubs, usually 3-speeds, have offered a practical, reliable gear-shifting option to the cyclist. With the popularity of derailer gears taking off in the early 1970's bike boom, internal-gear hubs went out of style in most US markets. This is unfortunate, because many casual or utilitarian cyclists are best served by this type of gearing.
Internal-gear hubs are more reliable than derailer systems, and require much less maintenance. The step-up ratios of their top gears make oversize chainrings unnecessary on small-wheel folding bicycles. Unlike derailers, internal-gear hubs can shift at a stop, very nice in stop-and-go urban traffic.
Internal-gear hubs tend to be heavier than derailer systems, and are slightly less efficient in some gears. The direct-drive middle gear can be more efficient than in a derailer system, as there is no drag from derailer pulleys. Most internal-gear hubs cannot have a quick-release axle.
Internal-gear hubs work on the principle of "planetary gearing."
The basic 3-speed hubs have a single "sun" gear solidly attached to the middle of the axle. 3 or 4 identical "planet" gears mesh with and revolve around the "sun" gear. The "planet" gears are surrounded by (and mesh with) a "gear ring" with its teeth on the inside (an inside-out gear).
As the planet gears, attached to a part called a "planet cage", revolve, the gear ring revolves 4 times for every 3 times the planet cage goes around. (Some models have different ratios.)
Low Gear has the sprocket driving the gear ring, while the planet cage drives the hub. Thus, the wheel turns 3 times for every 4 revolutions of the sprocket, lowering the gear by 25%.
Middle Gear has the sprocket still driving the gear ring, but has the gear ring drive the hub. Thus, the drive is direct. Although the internal gears are always rotating when the sprocket is in motion, they don't have any effect on the drive in middle gear.
High Gear shifts the sprocket's drive to the planet cage, while the gear ring continues to drive the hub shell. Thus, the hub turns around 4 times for every 3 sprocket revolutions, giving a 33% gear increase.
