Semi trailer is an articulated vehicle consisting of a towing engine and a semi-trailer that carries the freight. A semi-trailer does not trail completely behind the towing vehicle, but is attached at a point that is just forward of the rear-most axle of the towing unit, So that some fraction of the weight of the trailer is carried by the prime mover, with most of that at the rear axle (s) of the prime mover, but a small amount at the front axle (s).