A motor vehicle main chassis includes left and right main frame rails having extending along opposite lateral sides of the chassis. An extended rear frame member having left and right rear frame rails is disposed below and attached to the main frame rails, the rear frame rails extending horizontally in a plane situated below and parallel to the main frame rails. Left and right intermediate spacer rails are attached, respectively, to a bottom surface of the left and right main frame rails and extend rearwardly of the main frame rails. The left and right rear frame rails may be attached, respectively, to a bottom surface of said left and right intermediate spacer rails and extend rearwardly therefrom.