free wheel

free-wheel的词性：
n.（名词）

1. A power-transmission device that allows the drive shaft of a motor vehicle to continue turning when its speed is greater than that of the engine shaft.
空转轮：当机动车的主动轴的速度高于引擎轴的速度时使主动轴持续转动的一种动力转动装置

2. A clutch in the rear-wheel hub of a bicycle that permits the wheel to turn without pedal action, as in coasting.
飞轮：自行车后轮的毂上的离合器，（如在滑行时）不踩踏板而能使轮子转动