so that it can match the top distal surface of the wheel support 3 which is also flat the support blocks 5 are in vertical alignment with each other each support block 5 has a safety hook 51 on one side thereof for latching a respective pin 41 extending from the wheel support 3 which rotatably supports a respective auxiliary wheel 21 22 so as to prevent said auxiliary wheel from slipping out of place when in use referring now to fig4 and 6 the structure and operation of the two way hinge 6 can be seen the two way hinge comprises an upper left plate 62 an upper right plate 61 a lower left plate 63 a lower right plate 62 four springs 81 82 83 and 84 and four rods 71 72 73 and 74 one of each of the upper plates 61 62 is fixed to a lateral side of the support block 5 and the other complementary plate 62 61 is fixed to a respective wheel support 3 the two way hinge 6 is biased to a 90 degree fold about its horizontal axis as seen in fig5 and is biased to a folded position its vertical axis as shown in fig6 in other words springs 81 and 83 are biased to a 90 degree fold between the upper and lower right plate 61 and 64 springs 82 and 84 on the other hand are biased to a completely folded position between the upper plates 61 and 62 and between the lower plates 63 and 64 respectively obviously when the.