What You Should Know About Wheelchair Camber

Camber is the angle made by the wheels of the chair. It is the angle between the vertical axis of the rear-wheels and the vertical axis of the chair when viewed from the front or rear of the chair. It is used in the design of steering and suspension. If the bottom of the wheel is farther out than the top (away from the axle), it is called negative camber. Tilted-rear-wheels, a popular feature on racing wheelchairs is being seen more and more on wheelchairs that are used for activities of daily living. One of the advantages of camber is that it provides better lateral (sideways) static stability for the wheelchair as a result of the greater distance between the low, bottom-points of the two wheels. This is especially advantageous when frequent sideways movements are needed as in wheelchair basketball. It can be described as an angling that brings the top of the two, major drive- wheels closer to each other.

In a study conducted in Holland, eight non-impaired wheelchair users participated in an exercise (test) using a motor-driven treadmill in order to study the effects of rear-wheel camber on wheel-chair movement for six shoulder muscles which was the major part of the study. The test consisted of four runs with rear wheels at 0°, 3°, 6° and 9°-degrees camber and at speeds of 2, 3, 4 and 5 km/hr. There were no significant changes and/or results on oxygen-usage, heart-rate or mechanical efficiency.

Camber angle alters the handling qualities of the suspension design; negative camber improves the grip when cornering. This is because it places the tire at a more optimal angle to the road, transmitting the forces through the vertical plane of the tire, rather than through a force across it. Another reason for negative camber is that a tire tends to roll on itself while cornering. If the tire had zero camber, the inside edge of the contact point would begin to lift off the ground, thereby reducing the area of contact. By applying negative camber, this effect is reduced, maximizing the contact point. Note that this is only true for the outside tire during the turn; the inside tire would benefit most from positive camber.

On the other hand, for maximum straight-line acceleration, the greatest traction is attained when the camber angle is zero and the tread is flat on the road. Proper management of the camber angle is a major factor in suspension design and must incorporate not only geometric models but also, real-life behavior of components such as flexing, distortion, elasticity, etc. What was once an "art" in design is now become a more exact and scientific technique through the use of the computer. They can optimize ALL of the variables, mathematically, instead of relying on the experiences and intuitive estimations of engineers. As a result, the improved handling of wheelchairs has been dramatic in recent years.

However, excessive camber angle can lead to increased tire wear and impaired handling. As camber is increased, the width of the wheelchair across the bottom increases. This adds lateral-stability to the system by increasing the wheelchair footprint. If too much camber is added, there may be a problem getting through doorways. The amount of camber required for ordinary, daily use is not the same as that that is required for wheelchair sports. The amount of camber introduced is measured in degrees where the higher the number, the more camber or angling. Some more expensive models of wheelchairs have methods of adjusting the camber.

Besides the greater stability of cambered wheelchairs, cambered rear-wheels provide an easier reach to the hand-rims and less hampered arm movements during push-and-recovery movement leading to a less strenuous propulsion technique. Thus, camber would be more efficient with less arm spreading and stabilization. A study on chair characteristics during the 1980 Paralympics showed a trend of increased success of the athlete with increasing camber. The most successful athletes had wheelchairs with a camber.

Off-road vehicles such as agricultural tractors generally use positive camber. In such vehicles, the positive camber-angle helps to achieve a lower steering effort. Also, some single-engine aircraft that are primarily meant to operate from unimproved surfaces such as bush-planes and crop-dusters have their main wheels equipped with positive-cambered wheels to better handle the de-flection of the landing gear as the aircraft settles on rough, unpaved airstrips.

Caster is the tilting of the steering-axis either forward or backward when viewed from the side of the chair. A backward tilt is positive and a forward tilt is negative. Caster influences directional control of the steering (in an automobile) but does not affect the tire- wear and is not adjustable. On an auto-mobile, caster is affected by the vehicle height. Overloading the vehicle or a weak or sagging rear spring will affect caster. Since we are talking about a wheelchair, caster does not come into play.

PROS

• Lateral stability is increased with a wider footprint by having a camber greater than zero.

• The ride is softer with a camber greater than zero.

• The push-rims are placed in a more positive position for pushing with a camber angle greater than zero since it is more natural to push down and outward.

• The hands are more protected when pushing in tight areas since the bottom of the wheels will make contact first with walls and door-frames.

• The strain on the shoulders is diminished since the plane of the wheel is closer to that of the shoulder.

• Turns are made quicker.

• The wheelchair is given a sportier look.

CONS

• The wheelchair is made to be wider by angling the wheels at the bottom.

• The cost of the chair is increased.

• The wheels may rub against the armrest(s), against the side-panels or the user due to an excessive camber angle.

• Traction may be diminished and wear on the tires, uneven, by comparison to a conventional tire.

NOTE: A conventional tire has treads placed in the center of the tire. Cambered tires ride on the inside edges of the tire. To compensate for cambering, some manufacturers make their tires with offset treads.