a bicycle that has only one gear (= a device that controls how much power goes to the wheels of a vehicle) and that cannot move unless the pedals are being turned, often used for getting around a city

Andrew has ridden a fixed-gear bike on the streets of Chicago for years. 安德鲁多年来一直骑着一辆单速自行车穿梭于芝加哥街头。

On a fixed-wheel bike, you have to keep your legs moving at the same speed as the wheels.