Tarp enclosure system for flat bed trucks and trailers

A retractable enclosure system for covering a bed of a truck, trailer or the like is provided, which includes a number of generally U-shaped bows for supporting a flexible cover, and elongate rails provided along each side of the bed. The rails each have a flat uppermost surface along which the bows move to extend or collapse the flexible cover. The bows span the bed having two vertical legs connected at an upper end by a central bight, and a guide assembly provided at the lower end of each leg. The guide assembly includes one or more supporting wheels which rotatably engage the upper surface of the rails and a guide wheel which rotatably engages the side of the bed and/or a portion of the rail to limit lateral movement of the supporting wheels.