Number of Wheels
A wheelchair remains stable as long as the center of gravity of the athlete plus the chair remains inside the wheelchair's base of support.
The base of support is the area of ground marked by the points at which the wheels contact the surface.
In four-wheeled designs, the base of support is rectangular with the base a little narrower at the front than at the rear, while in three-wheeled designs, the base of support is triangular.
This means that as the weight of the athlete moves forward (as he or she leans forward to cut down air resistance) the center of gravity gets nearer to the edge of the base of support of the three-wheeled chair and the chair becomes less stable.
This lack of stability can be a problem for less experienced athletes, but for those who can handle them, three-wheeled chairs are faster.
They are faster because there is less resistance to passage over the ground for three rather than four wheels (Higgs, 1992a), and three-wheeled designs also have considerably less wind resistance than four-wheelers under most wind conditions (Higgs, 1992b).