The subject of the invention is a guidance surface (4) designed to carry an endless conveyor surface, this surface being characterized in that the curvature of the guidance surface is determined by a horizontal roller (7) and two fixed lateral surfaces (8) located on either side of the aforementioned roller, where each lateral surface is extended beyond the bearing surface occupied by the belt, and, at least locally, at least one of the lateral surfaces is fixed to the chassis by means of rapid fixing devices so as to constitute an access door.