tilt can be achieved however a preferable ratio of minor diameter to major diameter is 1 to 1 7 and the height from the center line of the bumper guard 27 to ground surface 13 is approximately one third of the distance from the major axis to said ground surface for a safe angle of tilt of approximately 25 u00b0 from the verticle therefore when bumper guard is rocked in contact with ground surface 13 the reactive force of impact acts on body 10 to urge it upwardly towards an upright position a modified form of the present invention is shown in fig4 and 6 referring to fig5 it is seen that the somewhat irregular rectangular opening consists of a first section 28 and a second section 29 the first section 28 has an upper short side 30 and a pair of spaced long sides 31 sloping downwardly to an open end juncture with left and right incremental sides 32 of the wider second section 29 second section 29 has a pair of spaced short sides 33 sloping downwardly from the incremental sides 32 to a lower connection to a long side 34 at the front end 15 of body 10 suspended downwardly within the first and second sections is a body rest 35 comprising a pair of spaced side walls 36 connected at their upper edges 37 to long sides 31 of the first section 28 then to incremental sides 32 and the downwardly sloping short sides 33 of second section 29 at the juncture of the first and second sections side walls 36 are formed at right angles and.