A semi-trailer has an elongate bed with skirts depending there from adjacent to either side of the bed. The skirts are laterally flexible and can also flex vertically to overcome obstacles and objects. Elongate plates are mounted at one end to the underside of the bed adjacent one or the other of the depending skirts. The elongate plates extend inwardly beneath the bed and then diagonally downwardly to the adjacent skirt displaced below the attachment to the bed. Leaf springs are superimposed on the underside of the inward extensions of the elongate plates to preload the plates toward the bed of the semi-trailer. The skirts extend from landing leg assemblies on the semi-trailer to longitudinally adjacent the forward outboard tires of the wheel assemblies.