Abstract: In a tarp system used to cover an open container such as a truck, the tarp being connected on one longitudinal edge to a roll bar, an improved mechanism for assisting movement of the roll bar comprises one or more constant force springs, each spring attached at one end to a longitudinal side of the container and at the other end to a reel mounted on the roll bar. When the roll bar is at one side of the container, the tarp is rolled around the roll bar and the constant force springs are extended. As the bar rolls across to the opposite side, the tarp is unrolled to cover the container opening, and the constant force springs wind into a coil around the reel.