POWER WHEELCHAIRS – A NEW DEFINITION

Ian Denison PT ATP

Doug Gayton ATP

 

PRESENTATION SUMMARY

New power wheelchairs are being introduced at an incredible rate. New models have little in common with their forerunners. In their efforts to avoid patent infringements and to bring us unique products, manufacturers have made it difficult for us to pigeonhole chairs according to basic configuration. It used to be that a chair was either rear or front wheel drive. Not any more….

 

In an effort to identify the basic performance characteristics clinicians and clients at GF Strong Rehab Centre tested a number of power wheelchairs. The initial need was to create a new definition for the various configurations, a family tree if you like. This paper describes our attempt to make sense of the power wheelchair jungle.

 

OBJECTIVES

By the end of the workshop participants will:-

1.       Understand how the basic design of a power chair contributes to it’s performance envelope.

2.       Understand more about the factors to consider when selecting a power wheelchair.

 

OVERVIEW

Traditionally power chairs have been classified as Rear wheel drive (RWD), Mid wheel drive (MWD), or Front wheel drive (FWD).

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Our experience led us to classify chairs according to the drive wheel location relative to the system centre of gravity (chair and user). This classification makes it easier to understand and to predict how a chair performs.

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Horizontal Location of C of G

The horizontal centre of gravity location determines how the weight is distributed between the driving wheels and the auxiliary wheels. Regardless of the tire type or the surface being negotiated, reducing weight on the auxiliary wheels produces less drag. Increasing weight on the driving wheels produces more traction.

 

Weight distribution is critical in determining how the chair will perform for a given user in a given environment. When a wheel has more weight passing through it, the performance characteristics of that wheel are more significant in determining the chair’s overall performance. Whenever the chair is being driven it is desirable to have as much weight on the driving wheels as possible.

 

Skidding

The effect of weight distribution is evident when changing direction on slippery surfaces. Chairs with lots of weight on their casters will spin their drive wheels and continue skidding in the same direction for a few feet. Those with most of the weight on their driving wheels produce minimal wheel spin and a much quicker turning response. Wheelchair basketball players deal with skidding by popping wheelies prior to making a quick turn. This puts all the weight through the driving wheels eliminating the skid.

 

Tracking

If the centre of gravity is in front of the drive wheels (as in all RWD chairs) the chair wants to keep going straight. If the centre of gravity is behind the drive wheels (as in all FWD chairs) the rear wheels want to overtake the drive wheels requiring frequent corrections with the joystick.

 

These are just some of the characteristics discussed in this paper. The performance of each configuration indoors and outdoors is discussed in detail. The instructional session should leave you with a clear understanding of where new chairs fit in and an ability to predict their strengths and weaknesses.

 

 

POWER WHEELCHAIR TEST:

 

If you can answer these questions give this workshop a miss.

 

  1. Why can a chair that goes up a steep hill easily be uncontrollable down the same hill?

 

  1. Why do RWD chairs have a greater range than equivalent FWD chairs?

 

  1. Which chairs give the smoothest ride on uneven terrain?

 

  1. What does the expression turn in it’s own radius mean?

 

  1. Why can’t you turn a chair that has all its weight on its casters?

 

  1. In what circumstances can a LR RWD negotiate smaller areas than CWD chairs?

 

  1. Which way will a chair turn when it exceeds its critical side slope angle?

 

  1. Which configuration will most likely spin its wheels on a curb cut?

 

  1. How fast should a chairs top speed be?

 

  1. Which configuration is most likely to meet the needs of a specific client?

 

 

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