A vehicle rack system including a stabilized cargo box attached to a pair of crossbars. In some embodiments, the stabilized cargo box includes a container defining a long axis and having a bottom portion hinged to a lid portion to form an enclosure to hold cargo. A plurality of clamp devices may mount the container to the pair of crossbars attached to a roof of a vehicle. The bottom portion may include a housing member defining a groove and also may include a stiffening tube disposed in the groove and providing first and second tubular regions that are laterally spaced from each other and that each extend at least generally parallel to the long axis. The first and second tubular regions may be connected to each other by a third tubular region of the stiffening tube that extends at least part way across a nose of the container.