An internal bicycle hub is an intricate planetary gear mechanism and is made of dozens of small parts. See, long before the derailleur became a staple in racing bikes in the 20th century, internal hub gears ruled the roost. In those days, these beautiful devices offered the first practical ways to shift gears on the fly. What a godsend!
In order to fully understand this beautiful system from a technical standpoint, we need to study the kinematics. or motion of the gearing action.
Click to zoom up the following one page article which explores the dynamics of the gearing in a 3 speed Sturmey Archer planetary gear hub. It explains how "gear ratios" are obtained in a planetary hub system by following some simple 'rolling contact' principles of gear motion. I borrowed it from my one of my favorite engineering textbooks. I hope this will make you appreciate the science behind bicycle transmissions. Also, when you get a chance, open up a hub and check it out for yourself!
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