particular to fig6 the carriage frame members 106 108 and attached wheel assemblies 110 112 114 116 can be selectively locked into the lowered ground engaging position by interposing lock bar 152 across the free ends of carriage first frame member lever arms 120 122 the lock bar 152 is captured between the carriage first frame member lever arms 120 122 and the retaining collars 154 156 positioned on the carriage second member lever arms 136 138 the upstanding hooks 132 assist in maintaining the lock bar 152 in position over the carriage first frame member lever arms 120 122 it will be recalled that the terminal ends 166 168 of cross bar 146 are carried within channels 162 164 of carriage first frame member lever arms 120 122 the overall result is locking of the wheel assemblies 110 112 114 116 in the lowered ground engaging position referring to fig7 the carriage is shifted to its raised position by first applying downward pressure to at least one of the foot pedals 134 to depress the upstanding hooks 132 of carriage first frame member lever arms 120 122 below the lock bar 152 lock bar 152 can then be slid within retaining collars 154 156 so as to clear the.