106 support bracket 106 supports the main transverse support beam 108 which is substantially parallel to the hitch mount 102 but spaced upward therefrom the main support beam 108 supports the bike mount devices 110 112 114 116 each of which are identical seat mount assembly 120 is shown mounted on support 116 seat mount support beam 122 is screwed into bike mount device 116 using screws 124 125 near one end 122 the other end 128 includes a tightenable clamp 130 which allows selectively tightening and loosening pressure between the clamp 39 s first surface 134 and its second surface 138 second surface 138 includes an inner surface 140 which substantially of the same shape as the outer surface of an upwardly extending part 142 similarly a fixed portion 144 146 of the clamp is attached to the transverse support beam 122 and includes inner surfaces 147 that are substantially the same shape as the outer surface of upwardly extending part 142 in operation the clamp 130 can be tightened and loosened when tightened the clamp is in the position shown in fig1 the upwardly extending portion 142 is rigidly held between the parts 138 144 and 146 with the two fixed pieces 144 146 on one side of the upward extending portion 142 and the movable portion 138 on the other side hence the part 142 cannot move the clamp 130 can be loosened to allow the piece 138 to be retracted from the fixed surfaces 144 146 when moved sufficiently far away the part 142 can be similarly moved away and can move beyond the outer edge surfaces 148 of the fixed portions 142 144 of the clamp this allows the.