A semi-trailer is a trailer whose axle is placed behind the center of gravity of the vehicle (when the vehicle is uniformly loaded) and is equipped with a coupling device that can transfer horizontal or vertical forces to the tractor. Part of the total weight of the trailer is borne by the tractor. The load capacity of the trailer shall not exceed the on-board weight of the main vehicle. Its characteristics are: it has no power, is carried together with the main vehicle, and is driven by the main vehicle.
In order to adapt to loading different materials, the semi-trailer also has two forms: breast board container and flat pallet. The front part of the semi-trailer is equipped with a support device that is usually suspended, which can be placed on the ground when taking off and hanging to make the trailer stop stably. Semitrailers have large carrying capacity and good stability when driving. They are increasingly used in countries with developed automobile transportation, and are developing towards heavy-duty and specialization.
