noun
a mechanism for changing gears on a bicycle, typically consisting of a chain, sprockets, and a mechanism to move the chain from one sprocket to another
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The derailleur is a crucial component of a bicycle's drivetrain system.
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I had to adjust my derailleur after it started making a strange noise.
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My derailleur got all messed up on that bumpy trail.
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His bad decisions acted like a derailleur, causing his life to go off track.