fig8 each shroud element 50 51 is mounted on a spindle shaft and extends with a cylindrical projection 53 54 in hollow cylinder shape onto the outer periphery of each of these projections 53 54 the cage for a ball bearing 17 18 u2014 joined to the light rim 28 or 29 for each guide roller 13 14 u2014 is forcibly fitted the turning assembly is axially immobilized by traditional means the shroud assembly 52 is formed of two opposed shroud elements 50 51 each of which rotatably supports a guide roller 13 14 the shroud elements 50 51 are cross linked by mechanical linkages or make up a single piece mechanical shroud assembly preferably there are low front 55 and low rear 56 transversal linkages that rigidify the assembly against spacing and that may help to push away debris and objects by their lower edge 57 and 58 respectively being shaped like a spoiler these low cross linkages are preferably complemented by upper reinforcing linkages such as 59 in front and 60 in the rear in the rear the shroud element assembly 52 is joined to the ends of a swiveling cross shaft 61 that is elastically drawn downward and mounted onto a rear support 3 that is joined to guide arms 4 to 8 articulations needed for the swiveling displacement of the rear support 3 in relation to the elements are additionally provided in order to enable the assembly 39 s guide orientation movements for following the curves of the guide rail path this variation confers additional advantages such as simple construction and improved roller extraction resistance obviously the invention is not limited to the preferred embodiments described above and.