roll u03b1 t gi 0 u03b1 r the device 2 also presents a certain torsional stiffness but in this case the coupling mechanism 5 permits variation of this torsional stiffness as will be explained below with reference to fig3 the coupling mechanism 5 comprises a case 30 of generally circular cylindrical shape the axis of which coincides with the axis a the case 30 for example has a diameter of between 100 and 250 mm and a length of between 200 and 800 mm the half bar 3 is firmly fixed to one of the ends of the case 30 at its opposite end the case 30 is coupled moveably in rotation to the half bar 4 the half bar 4 is fixed firmly to a circular plate 32 arranged inside the case 30 thus relative rotation of the half bars 3 and 4 about the axis a causes relative rotation of the case 30 relative to the plate 32 about the axis a in the example shown the half bar 4 enters the case 30 through a bore 31 at which the case 30 and the half bar 4 are coupled by a connection of pivot type in accordance with a modified embodiment the connection of pivot type may be situated between the plate 32 and the case 30 the case 30 presents a transversal wall 33 arranged parallel with the plate 32 between the transversal wall 33 and the plate 32 the case 30 includes three flexible rods 34 the rods 34 are made of an elastically deformable material for example.