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 and vertical forces to the tractor. Semi trailers
are generally three axle semi trailers, which are divided into 11 meter warehouse fence semi-trailer, 13 meter warehouse fence, low flat semi-trailer and so on. It is a heavy transport vehicle connected with the head of semi-trailer through traction pin.
（semi-trailer)Compared with the "single" vehicle, the semi-trailer can improve the comprehensive economic benefits of highway transportation. The transportation efficiency can be increased by 30-50%, the cost can be reduced by 30-40%, and the fuel consumption can be reduced by 20-30%. More importantly, the use of semi-trailer can also promote the organizational form of logistics in China to a certain extent.（semi trailer)