expansion shaft 20 and a crank arm 21 namely when the operational lever 2 is moved in the f b direction as shown in fig2 the driving yoke 3 pivotally rotates about the x x direction as a center the movement of the lever 2 in the r l direction permits the driving yoke 3 to pivotally rotate about the y y axis as a center in other words every movement of the operational lever 2 can be modified by the foregoing differential mechanism with the moving arm 9b and the torque arm 9a fig4 is a partially cutaway side view illustrating the state in which a forward backward clutch mechanism is connected with an expansion joint mechanism fig5 is a sectional view taken along the line a a of fig4 illustrating the state in which the clutch is actuated in fig2 a 4 5a and 5b annular grooves provided face to face in the cluth hub 11 rotating around a hub shaft 25 as a center fixed to a frame bracket 24 by screws have an outer spring 12 and an inner spring 13 fitted therein the frame bracket 24 24 39 is connected to frame 1 as shown in fig2 a and fig4 one end of each spring 12 and 13 is radially equipped with projections.