Abstract: A hand-propelled wheelchair comprising a frame element to which is mounted a pair of large drive wheels and a pair of front caster wheels, the wheelchair further comprising a seat unit having a base and a back, the seat unit being slidably mounted on the frame in such a manner as to be manually slidable and arrestable in a plurality of desired positions along the frame element by the user while seated in the seat of the wheelchair, to vary the center of gravity of the user and the seat unit with reference to the axis of the drive wheels without disassembly of the drive wheels, the frame element or the seat unit from each other.