Aerodynamic devices for moving vehicles

A vehicle assembly for transporting freight, comprising:

• a frame of the vehicle assembly including a first side, a second side, and a longitudinal axis positioned between the first side and the second side;
• a frame member mounted to one of the first side or the second side of the frame at a location outboard from one or more rear wheels, the frame member including a slot oriented parallel to the longitudinal axis of the frame and having an opening facing outwardly from the one of the first side and the second side of the frame;
• and
• at least one undercarriage skirt formed of a pliable material and including a length and a coupling element, the coupling element configured to be slidably received in the slot such that the at least one undercarriage skirt is movable between a first position in which the at least one undercarriage skirt is positioned laterally outward from the one or more rear wheels, and a second position in which at least a portion of the one or more rear wheels are exposed,
• wherein the at least one undercarriage skirt is retractable in a direction parallel to the longitudinal axis of the frame to move from the first position to the second position.