WHEEL ASSEMBLY

A wheel assembly (10, 110), comprising: a wheel (16 , 80, 116) having an axle (86); and a constant force mechanism (48, 50) coupled to the wheel (16, 80, 116), the constant force mechanism (48, 50) comprising: a horizontal support (52); a horizontal carriage (56) associated with the horizontal support (52); a vertical support (54) oriented perpendicular with respect to the horizontal support (52); a vertical carriage (58) associated with the vertical support (54); and a rigid arm (70) that is pivotally coupled with the horizontal carriage (56), the vertical carriage (58), and the axle (86), wherein the rigid arm (70) is coupled with the vertical carriage (58) at a point (62) intermediate where the rigid arm (70) is coupled with the horizontal carriage (56) and the axle (86), wherein the horizontal carriage (56) is capable of being urged along the horizontal support (52), and wherein the vertical carriage (58) is capable of being urged along the vertical support (54).