Derailleur

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Human beings construct bicycles equipped with variable-ratio transmission systems called derailleur gears. These gears comprise multiple sprockets of various sizes attached to a frame that is designed to facilitate the movement of a chain between them. Derailleur gears do not engage other components mechanically, which sets them apart from mechanical gear trains, despite popular reference in bicycle lingo to calling them 'gears.' Technically, the system remains purely sprocket-based, where the primary mode of interaction involves either propelling or supporting motion through means of a continuous, connected chain, not by an interconnected pair of individual parts.