derailleur
noun [ C ] engineering specialized uk/dɪˈreɪl.jer/ us/dɪˈreɪ.lɚ/
（自行车的）换挡装置，变速器
a type of bicycle gear that works by moving the bicycle chain from one sprocket wheel (= a wheel with a row of tooth-like parts) to another
Set the rear derailleur in the lowest gear, with the chain on the biggest sprocket wheel at the back. 把后轮的变速器调到最低挡，就是让链条在后面连到最大的链齿上。
derailleur gears 换挡齿轮