A semi-trailer is a trailer with its axle placed behind the vehicle's center of gravity (when the vehicle is uniformly loaded) and equipped with a coupling device that can transmit horizontal and vertical forces to the towing vehicle. A semi-trailer is generally a three axle semi-trailer, which can be divided into many types such as eleven meter compartment semi-trailer, thirteen meter compartment semi-trailer, low flatbed semi-trailer, etc. It is a heavy-duty transportation vehicle that is connected to the semi-trailer head through a towing pin.