An analysis of parameters related to the directional instability of rear caster wheelchairs

A five degree of freedom dynamic model which uses a quasi-static approximation for lateral load transfer effects has been developed as a means of investigating the inherent directional instability associated with rear caster wheelchairs. Using a treadmill and test cart, the dependence of lateral wheelchair tire cornering force on variables such as forward speed, inflation pressure, camber angle, slip angle, and vertical load have been experimentally determined. A model has been developed which predicts wheelchair tire-road forces for different operating conditions. A simulation program which utilizes the tire force data to solve the equations of motion written for a body fixed reference frame attached to the wheelchair has been tested. Predictions from the program for trajectory and yaw velocity response are in good agreement with theory and observation for both rear caster and conventional type wheelchairs.

A parametric study of several design variables has delineated the conditions under which a rear caster wheelchair exponentially diverges to a state of uncontrolled and potentially unsafe motion upon being subjected to a slight disturbance. Center of gravity position and other geometric parameters, along with forward speed, have been shown to have a dominant influence on directional control, while tire selection has been shown to play a less important role. The effects of camber, toe, inertial properties, and caster friction have also been investigated.

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