Join Date: Jun 2005
Bikes: Raleigh, Rudge, James 3spds., and a cast of many
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This is from Sheldon Brown, at this page: http://www.sheldonbrown.com/sturmey-archer.html
(Thanks for asking a great question)
"...Internal-geared 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. A set of 3 or 4 identical "planet" gears mesh with and revolve around the "sun" gear. The "planet" gears are surrounded by (and mesh with) an inside-out "gear ring".
As the planet gears, attached to a part called a "planet cage" revolve, the gear ring revoles 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..."