开拖拉机或卡车都需要double clutch来换挡．

因为它们都是用的unsynchronized transmission

The purpose of the double-clutch technique is to match the rotational speed of the input shaft being driven by the engine to the rotational speed of the gear you wish to select (directly connected to rotating wheels). Once the speeds are matched, the gear will engage smoothly. If the speeds are not matched, the dog teeth on the collar will "crash" or grate as they attempt to fit into the holes on the desired gear. A modern synchromesh gearbox accomplishes this synchronization automatically.