wheels 14 with the load balanced thereon to avoid any tendency for the support system to distort noting fig3 and 5 the two wheels 19 of the wheel set rotatably mount to the opposed sides of the lower axially aligned stem portion 15a by means of a transverse axle 16 engaged through and mounted within an aperture 16 39 in the lower stem section 15a once adjusted the upper portion of the stem 14 is fixed in position by a bolt 17a engaged through a vertical slot 17 in the stem 14 and threaded into the bolt receiving hole 8 in the back or inner wall of the recess 11 in order to vertically adjust the stem 14 and hence the wheel assembly a height adjusting bolt 13a is vertically extended through aligned holes 12 and 12 39 respectively in the parallel walls 6 and 11 39 of the chamber 7 the wall 11 39 being the upper wall of the elongate recess 11 the enlarged head of the bolt 13a is positioned within the upper chamber 5 to limit the adjustment of the bolt 13a and prevent its inadvertent withdrawal from the aligning openings 12 and 12 39 the lower or abutment end of the bolt 13a engages the upper edge of the stem 14 with vertical adjustment of the bolt 13a being controlled by a nut 13 confined within the chamber 7 and through which the bolt 13a is threaded thus arranged.