Alexander Allan, of Dunedin, asks :-

Why are the wheels on racing wheelchairs splayed?

Barrie Woods, a rehabilitation engineer with the Christchurch Spinal Injuries Unit, responded.

Disabled persons use wheelchairs for many different purposes including sports. Just as you would buy the right pair of shoes to play tennis or rugby, having the right design of wheelchair for sport is most important. In sports such as tennis, basketball and rugby the wheelchair needs to be manoeuvred quickly and precisely and must be very stable so as not to tip over.

There are two ways to maximise stability: keeping the centre of gravity low, or using a wide track (ie the distance between the wheels on the ground. Lowering the centre of gravity is not sensible if it means that the occupant is sitting lower to the ground; height is an advantage in most games. However widening the track to the optimum degree for stability means that the wheels are too far apart for the user to propel the chair using the push rims. Applying a high degree of camber overcomes this problem by keeping the tops of the wheels within reach, whilst keeping the bottom of the wheels a good distance apart. The ability to turn within a small space is important too, and this is achieved by using a wider track for the main propelling wheels than for the front castor wheels. This gives the chair a tighter natural turning circle when propelled on one side only. There is one more advantage of cambered wheels in that the hands are protected to some extent in the event of collisions with other wheelchairs or goalposts.

Wheelchairs with highly cambered (splayed) wheels are most often seen in sports such as tennis, basketball and rugby. High camber is not as important for track racing, though in general all sports wheelchairs will have some degree of camber to provide stability. Excessive camber is undesirable on wheelchairs that are not used for sports as it restricts the places the wheelchair can get to, and makes it difficult to get in and out of (for example when transferring from the wheelchair into a car).