Abstract: A mechanism for assisting the rolling or unrolling of a tarp on a roll bar over the top opening of a trailer, container or similar structure. The mechanism includes an elongated, C-shaped housing, pulleys mounted at opposite ends of the housing, an elastic cord that is secured to the housing and passes about the pulleys, and a strap attached to the opposite, free end of the elastic cord for engaging an end of the roll bar. The housing, pulleys and cord are arranged so that the cord is extended when the tarp is in the open position, urging the active end of the crank to the closed position during the rolling step. The elastic cord pulls against movement at the active end of the roll bar when the bar is unrolled from the closed position to the open position.