Roof rack bar with reduced vortex

The bar (14) has a central rod (18) covered by a case (20) having two longitudinal semi-shells (22, 24) mounted on both sides of the rod and having upstream longitudinal edges (26, 30). The semi-shells are maintained at a closed position in which the edges are joined, when the bar supports luggage, and at an open position in which the edges are moved apart to allow air flow between them, when the bar is free from the luggage.