Derailleur gears
Derailleur gears are a variable-ratio transmission system commonly used on bicycles, consisting of a chain, multiple sprockets of different sizes, and a mechanism to move the chain from one sprocket t...
Derailleur gears - Wikipedia
Electronic gear-shifting system
An electronic gear-shifting system is a method of changing gears on a bicycle, which enables riders to shift with electronic switches instead of using conventional control levers. The switches are con...
Electronic gear-shifting system - Wikipedia