of the rear wall 38 of the support arm 34 include registered longitudinal slots through which the headed end portions of a pair of threaded bolts 56 are slidingly received the threaded bolts 56 are threaded into the opposite ends of an internally threaded sleeve 58 slidable in the support arm 34 and the sleeve 58 is yieldingly biased toward the lower ends of the slots 54 by an expansion spring 60 connected between the sleeve 58 and anchored relatively to the lower end of the support arm 34 the support arm 34 mounts a lower end transverse sleeve 62 on its right sidewall and the hitch bar 20 mounts a vertical sleeve 64 on the right side thereof see fig2 opposite ends portions of a right angle or l shaped pivot pin 66 are journalled in the sleeves 62 and 64 and the base end of the support arm 34 therefore pivotally connected to the hitch bar 20 for oscillation relative thereto about a vertical axis as well as a horizontal axis extending transversely of the lower end of the support arm 34 in this manner the support arm 34 may be swung between the lowered position thereof illustrated in fig4 and the raised operative position thereof illustrated in fig2 the hitch bar 20 also includes a z shaped clamp strap 70 removably mounted thereon by a threaded fastener 72 and the clamp strap when fastener 72 is tightened engages the transversely thickened and outwardly flaring rear end 74.