A backup device for limiting the rotation of converter dollies about a trailer hitch is presented. A dolly mounted, laterally extending moveable bar is selectively pressed against the vertical members of the trailer's underride guard in order to prevent rotation of the dolly relative to the trailer. The bar is retracted from the trailer to allow significant rotation of the dolly during normal forward operation. The backup device is operable with many makes and models of trailers because the laterally extending bar provides a large area that the trailer underride guard may contact. In addition to being operable with many models of trailers, the present invention does not require specific features to be added onto a trailer pulling the converter dolly. Additionally, the large contact area of the laterally extending bar allows the device to function with dollies that are significantly rotated about a trailer hitch. The backup device is also robust, aerodynamic, and self contained. Although manual operation of the device is possible, by connecting the device to the pressurized trailer brake line, the electrical circuitry of the trailer, or other means, the backup device may be operated from within the tractor cab.