Directional instability of rear caster wheelchairs

Although less common than conventional front caster wheelchairs, rear caster wheelchairs are still in use for several reasons. Many people find manual rear caster wheelchairs easier to maneuver indoors at slow speeds. This is especially true when the user attempts to maneuver the wheelchair very close to an object, such as a table. Electric wheelchairs often have rear casters to accommodate for front wheel drive. If the larger drive wheels are located at the front of the wheelchair, obstacles such as a curb can be negotiated much more easily. However, a major disadvantage of rear caster wheelchairs is that they are generally known to be directionally unstable, especially at high forward speeds. This paper presents the results of a study to determine specific measures that can be employed to improve the stability of this type of wheelchair. The instability of rear caster wheelchairs is due primarily to road forces that act on the tires when the wheelchair is displaced from its line of motion by a bump or other irregularity in the road surface. The paper discusses the experimental investigation of these road forces as well as a dynamic model used to study the instability problem. The results of a computer simulation program used to perform a parametric study of different design variables are discussed. Center of gravity position, caster trail distance, and caster pin friction are found to have a dominant influence on rear caster wheelchair directional control. Several simple but significant design recommendations are presented.