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(自行车的)换挡装置,变速器
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。