A wheelchair comprising: • a base including a base frame which is comprised of opposing side tubes joined by a pair of longitudinally spaced, laterally extending tubes; • a plurality of wheels (16, 18) that is adapted to support the base relative to a supporting surface; • a seat for supporting an occupant, with the seat being an element of an adjustable seating system; • a seat frame being comprised of the adjustable seating system and comprised of opposing side tubes and curved tracks joined by a plurality of longitudinally spaced, laterally extending tubes, said tracks supporting the seat, said tracks, serving as a rolling or sliding surface that allows the seat to rotate with respect to the base, and said tracks having a constant-radius arc with a focal point that is adapted to be coincident with the center of gravity of the wheelchair occupant with the center of gravity of the wheelchair occupant capable of being moved fore and aft via the adjustable seating system to ensure that the center of gravity is located at the focal point of the constant-radius arc; • a backrest and a footrest assembly included by the seat frame in addition to the seat, all of which are adapted to be adjusted fore and aft with respect to the focal point, with the backrest comprising laterally spaced canes (26, 62) supported relative to the seat frame by couplings, the couplings including an assembly of plates having upper ends operatively attached to one another and lower ends attached to the side tubes of the seat frame and the lower ends of the plates moving relative to the side tubes of the seat frame while remaining operatively connected to the side tubes of the seat frame; • opposing low friction support assemblies supporting the seat frame and the seat relative to the side tubes of the base, the low friction support assemblies being adjustable to change an overall range of seat tilt by fixing the low friction support assemblies to the base at different angular orientations; and • lock assemblies for locking the said tracks in relation to the low-friction support assemblies, characterized in that the lock assemblies are supported by inner plates of the support assemblies and comprise protrusions that are adapted to be engaged with recesses in the tracks, the protrusions entering the recesses to lock the tracks into an angular position, and with the protrusions being retracted from the recesses the seat being rotatable to a different tilt angle relative to the base. • a base including a base frame which is comprised of opposing side tubes joined by a pair of longitudinally spaced, laterally extending tubes; • a plurality of wheels (16, 18) that is adapted to support the base relative to a supporting surface; • a seat for supporting an occupant, with the seat being an element of an adjustable seating system; • a seat frame being comprised of the adjustable seating system and comprised of opposing side tubes and curved tracks joined by a plurality of longitudinally spaced, laterally extending tubes, said tracks supporting the seat, said tracks, serving as a rolling or sliding surface that allows the seat to rotate with respect to the base, and said tracks having a constant-radius arc with a focal point that is adapted to be coincident with the center of gravity of the wheelchair occupant with the center of gravity of the wheelchair occupant capable of being moved fore and aft via the adjustable seating system to ensure that the center of gravity is located at the focal point of the constant-radius arc; • a backrest and a footrest assembly included by the seat frame in addition to the seat, all of which are adapted to be adjusted fore and aft with respect to the focal point, with the backrest comprising laterally spaced canes (26, 62) supported relative to the seat frame by couplings, the couplings including an assembly of plates having upper ends operatively attached to one another and lower ends attached to the side tubes of the seat frame and the lower ends of the plates moving relative to the side tubes of the seat frame while remaining operatively connected to the side tubes of the seat frame; • opposing low friction support assemblies supporting the seat frame and the seat relative to the side tubes of the base, the low friction support assemblies being adjustable to change an overall range of seat tilt by fixing the low friction support assemblies to the base at different angular orientations; and • lock assemblies for locking the said tracks in relation to the low-friction support assemblies, characterized in that the lock assemblies are supported by inner plates of the support assemblies and comprise protrusions that are adapted to be engaged with recesses in the tracks, the protrusions entering the recesses to lock the tracks into an angular position, and with the protrusions being retracted from the recesses the seat being rotatable to a different tilt angle relative to the base.