by the wall 610 the centre of the line of sensors 608 which is positioned perpendicular thereto when however the diametrical plane of the wheel is displaced from the vertical the liquid surface 614 inclines as shown by the dashed line in fig5 and the light plane strikes the line 608 in a position removed from the centre by a distance proportional to the angle of inclination of the wheel diametrical plane to the vertical with reference to fig3 each device 5 comprises a baseplate 52 comprising the already stated hole 50 for mounting on the pin 40 forming part of the bracket 4 for fixing the device to the wheel below the hole 50 the plate comprises on one side bosses 53 comprising a threaded hole against which the lugs 617 of a unit 6 are fixed on the opposite side to face the bracket 4 the plate 52 comprises two projecting parallel guides 54 between which there is supported a prismatic bar 55 which can be fixed to project in two symmetrical positions from one side or from the opposite side of the plate 52 to the free end of the bar 55 there is fixed a unit 6 positioned horizontally in a direction perpendicular to the unit fixed to the plate 52 the two units 6 of each device 5 are arranged to interact with the units arranged symmetrically on the adjacent devices 5 in accordance with the scheme of fig1 the signals collected by the lines of sensors 608 are firstly.