Definition of a Wheelchair

A wheelchair is a device used for mobility by people for whom walking is difficult or impossible, due to illness or disability.

It typically consists of a seat supported on two large wheels on an axle attached towards the back of the seat and two small wheels near the feet, though there are often small additional features to prevent toppling or to assist mounting curbs. The person moves by pushing with his/her hands circular bars on the outside of the large wheels with a diameter that is slightly less than that of the wheels, or by actuating motors, usually with a joystick.

Experiments have also been made with unusual variant wheels, like the omniwheel or the mecanum wheel. These allow more directional movement options. Makes include Storm, Twister, Harrier and Spectra.

Disabled athletes use streamlined sport wheelchairs for racing and basketball. Electric wheelchairs can be used as part of adapted sports such as Wheelchair Soccer.

Adapting the built environment to make it more accessible to wheelchair users is one of the key campaigns of disability rights movements. For example, the construction of ultra low floor trams and buses is being encouraged whereas the use of paternosters in public buildings without any alternative method of transportation has been criticized.

Rolling resistance

Rolling resistance is the first thing to overcome to make a wheelchair roll easy. It depends on the surface the wheelchair is driving on, mass distribution on the wheels, wheel radius, total mass and specific tire characteristics

The most important external aspect in al of this is the surface on which the wheelchair is moving. Indoors it’s possible to make the floors hard and smooth. Outdoors there is not much one can do to decrease resistance as a result of the characteristics of the surface.

Slopes and obstacles

Other important elements are slopes and obstacles. Indoors on can make adjustments like using slopes and taking away obstacles. Outside this is a bit more difficult. Then one can only adjust the wheelchair itself, like making the wheelbase longer, so it will be easier to climb curbs.

Aspects in the wheelchair itself influencing the manoeuvrability and rolling resistance are weight, handrim, camber angle, the seat, back support and castors.

The weight of wheelchair and user together influence the amount of rolling resistance the user had to overcome. Mass distribution is also an important aspect. Most of the total weight should lie over the rear wheels, yet not as much as causing the wheelchair to tip backwards.

An optimal configuration of the wheels is an important factor in overcoming rolling resistance. A larger distance between rear wheels and castors decreases the pressure on the castors, resulting in a lower rolling resistance. It is also important to prevent toeing in and out of the rear-wheels. Another factor is the type of tires around the wheels.

Internal friction

Special attention should be given to internal friction of the wheelchair caused by, for example, loose bolts and nuts, sliding joints and non-elastic connections.

The handrim is an essential part of a wheelchair for it is used to propel, brake, steer, negotiate obstacles and manoeuvre. Important aspects in finding the optimal power transmission from hand to handrim are shape, size, diameter, material and profile of the handrim, and antropometry, squeezing force of the hand, (dis)abilities and special wishes of the user. It should be noted that propelling a wheelchair using handrims is physiologically the least efficient way of propelling a wheelchair.

A large diameter of the handrim results in a relative high mechanical efficiency and effective force. In case of propelling a wheelchair over a long distance it is energetically favourable to use a handrim with a smaller diameter.

The way of grabbing the handrim when propelling influences the mechanical efficiency greatly. Also the friction coefficient is of great influence. It should be as low as possible in order not to brake the wheels while propelling, but it should be high enough to make it possible to transmit a certain amount of power from hand to handrim.

A camber angle has a positive influence on the stability sideways, the power transmission from hand to handrim and the manoeuvrability.

When using a camber angle, there is more risk for toe-in or toe-out, more pressure on the rear wheal axle and the complete wheelchair becomes wider. In general is the camber angle for an ADL-wheelchair 2 to 4 degrees and for a racing wheelchair between 4 and 12 degrees.

Position of the user

The most important aspects of the seat of a wheelchair are the horizontal and vertical position of the user, because they greatly influence the energy needed to propel the wheelchair. In general it is best to position the centre of mass right above the rear-wheel axle (horizontal position). In the vertical direction the user should be positioned in a way that he can just touch the rear wheel axle with his fingertips.

The position of the seat influences the accessibility of the handrim, and therefore the efficiency of power transmission from hand to handrim and the mechanical efficiency.

Castors

Castors are sensitive to forces exerted sideways. It can cause them to shimmy. When castors are positioned in a vertical position it is the easiest to make turns.

Each user has his own characteristics influencing the efficiency of propelling a wheelchair and his own idea of comfort. These characteristics are age, gender, figure, physical health and (dis)abilities. In general it can be said that wheelchair users don’t have much muscle mass in their arms and shoulder girdle, which makes it extra hard to propel a wheelchair. Still, a wheelchair should not keep a user from being mobile and of his social life.

Wheelchair caster shimmy II: Damping

James J. Kauzlarich, PhD; Theodore E. Bruning III; John G. Thacker, PhD
University of Virginia, Mechanical Engineering Department, Charlottesville, VA; Compaq Computer Corporation, South Colorado Springs, CO

Address all correspondence and requests for reprints to: James J. Kauzlarich, PhD, University of Virginia, Mechanical Engineering Department, Charlottesville, VA 22903-2442; email: jjk@virginia.edu.

Abstract--The theory of shimmy damping is investigated including tire friction, spindle bearing friction, and hydraulic damping. A new theoretical improvement in hydraulic damping is presented. Experimental results are presented along with a discussion concerning the limitations due to the approximations used in the theory. The basic theory of wheelchair caster shimmy was published by the authors in 1984, and an examination of the sources of shimmy damping is corrected and updated in this paper.

INTRODUCTION

Self-excited vibration is one of the most interesting topics in the field of vibrations and is the science governing caster wheel shimmy. Caster wheel shimmy can be experienced in everyday equipment, such as wheelchairs, grocery carts, gurneys, teacarts, and the like, and is universally recognized. Self-excited vibration is characterized by vibration that is produced by the motion of the system (e.g., wheelchair speed) itself. The flutter due to the motion of turbine blades or aircraft wings is a good example of this instability. In addition, machine tool chatter, internal flow-induced vibration of piping, and cross flow-induced vibration of wires and structures are treated under this topic in modern vibration texts (1-3).


In 1984, the authors presented a paper on the subject of wheelchair caster shimmy and turning resistance (4), with both theory and experimental data. Since that time, manufacturers have considered several methods for shimmy prevention, and the problem of shimmy prevention is well in hand. However, the theory behind shimmy prevention is not well known.

This paper presents applications of the basic theory to show how shimmy prevention works in ultra-light and powered wheelchairs. In addition, the theory leads one to new designs. The theory explains why a trial-and-error design may or may not work, and suggests possible new solutions.