Assistive Technology & Seating Service Vancouver Coastal Health GF Strong Rehab Centre 4255 Laurel Street Vancouver BC V5Z 2G9 Power wheelchairs..a New Definition As an Equipment Specialist working in a busy Rehab Centre one of my jobs is to help clients select an appropriate mobility device for use after discharge. Once a clients needs are determined we have to match them to a chairs performance. We no longer have the luxury of being able to trial all the chairs that the client likes the look of instead we short list the chairs based on our understanding of how the various basic configurations perform. There is not a "best chair", there is not a "best configuration". There is however a best chair for a specific client who wants to access a specific environment. 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 article describes our attempt to make sense of the power wheelchair jungle. Traditionally power wheelchairs have been classified as Rear wheel drive (RWD), Mid wheel drive (MWD), or Front wheel drive (FWD). www.assistive-technology.ca file:///g /Web%20Services/Sites-Production/atss/www/htdocs/scin.html (1 of 10)12/30/2006 3:41:07 PM
In an attempt to categorize power wheelchairs, we have created a family tree with three main branches. The names of the branches describe the location of the drive wheel relative to all the wheels in contact with the floor. This model allows us to predict with reasonable accuracy how the chair will perform in many different situations. For clarity we describe the performance characteristics of five different configurations, the reader can easily project the performance characteristics of a chair with a different set up e.g., a RWD with 75% of the weight on the driving wheels would perform somewhere between the High and Low ratio RWD chairs. Basic Definitions Low Ratio RWD The drive wheels are behind the centre of gravity ( c of g). The front wheels are casters. The chair may have basic anti tippers at the rear as a safety feature. High Ratio RWD file:///g /Web%20Services/Sites-Production/atss/www/htdocs/scin.html (2 of 10)12/30/2006 3:41:07 PM
The drive wheels are behind the c of g. The front wheels are casters. The anti tippers at the rear are large and designed to be in contact with the ground in some normal driving situations. CWD The drive wheels are directly below the c of g. The front and rear wheels are casters and designed to be in contact with the ground at all times The ratio of weight on the driving wheels for centre wheel drive chairs varies depending on the caster location and whether the chair has suspension. I have tested chairs with ratios between 50% and 90% on flat surfaces. In some instances it is relevant to consider CWD?s as a whole, in other instances they will be split into High Ratio Centre Wheel Drive (HR CWD) and Low Ratio Centre Wheel Drive (LR CWD). LR CWD have about 60% of their weight on the drive wheels and HR CWD about 85%. High Ratio FWD The drive wheels are in front of the c of g. The rear wheels are casters. The anti tippers, at the front are large and designed to be in contact with the ground in some normal driving situations. Low Ratio FWD The drive wheels are in front of the c of g. The rear wheels are casters. The chair does not have anti tippers. file:///g /Web%20Services/Sites-Production/atss/www/htdocs/scin.html (3 of 10)12/30/2006 3:41:07 PM
Factors Influencing Power Wheelchair Performance Our testing has given us an appreciation for the complexity of power wheelchair performance. To use an automotive analogy, if I tested a Ford Explorer and compared it to a BMW 318, it would not be fair for me to say that Ford vehicles have more cargo space and BMW?s are more responsive. It is also inappropriate to draw the conclusion that small cars are built by Germans and mid sized SUV?s are built in America. Cause and effect must be analyzed and understood, some characteristics of a specific chair can be attributed to it?s base configuration while others are due to the programming. Many factors play an overlapping role in defining wheelchair performance. Design factors (such as location of the drive wheels) are limitations inherent in the chairs configuration. These factors broadly define the chair?s performance characteristics. Adjustable factors (such as programming) fine tune exactly how the chair will perform within the envelope defined by the design factors. When looking at a specific chair it is important to consider these factors separately. We looked at performance indoor, outdoor and off road but this article will only cover maneuverability indoors. Manoeuverability Many people believe CWD and FWD chairs are more manoeuverable than an equivalent RWD. Each configuration has benefits and drawbacks; the most important task is to match a chair?s characteristics to the user?s needs and strengths. The consumer must identify the specific manoeuvering tasks they require and ensure the chair is capable of completing them. The bottom line is: It is imperative the chair characteristics match the driver?s physical and cognitive strengths as well as the environment in which it will be driven. Rate of turn There is definitely a feeling of great manoeuverability when driving front wheel drive chairs. I suggest this is partially an illusion created by the horizontal distance between the centre of rotation of the chair and the otolith organs in the ears which provide information to the brain regarding balance and changes in position. In RWD chairs, the ears are close to the centre of rotation of the chair and further away in a FWD chair. file:///g /Web%20Services/Sites-Production/atss/www/htdocs/scin.html (4 of 10)12/30/2006 3:41:07 PM
If all chairs are turned through a 360-degree arc in the same time, the driver of the FWD chairs otolith organs are displaced 3 to 5 times further than those of the operator of the RWD chair. The otolith organs, subjected to these differing accelerations create the perception of a fast turn. The Otolith Illusion Turning radius Chairs pivot about a point midway between the driving wheels; this is the centre point of the circle described by a chair turning about its axis. The turning radius is the distance between that point and whichever part of the chair is furthest from that point when viewed from above (usually the footrest or caster wheel). file:///g /Web%20Services/Sites-Production/atss/www/htdocs/scin.html (5 of 10)12/30/2006 3:41:07 PM
The closer the driving wheels are to the middle of a chair, the smaller the turning radius will be. 3 point turn (narrowest hallway a chair can be turned around in) All chairs tested were able to turn around in a hallway of the same width as the diagonal distance from the extreme left front corner of the chair to the extreme right rear corner of the chair. However, chairs having the driving wheels a distance away from the centre of the chair needed a longer hallway to turn around in. file:///g /Web%20Services/Sites-Production/atss/www/htdocs/scin.html (6 of 10)12/30/2006 3:41:07 PM
Performance Analysis Indoor Shape of the space This is a crucial factor in determining which chair will prove to be more manoeuverable for a given individual in a specific environment. When the available space is limited chairs are usually manoeuvered with the wheels counter-rotating or with one wheel fixed. The diagrams below show the shape of space required to execute a 90 degree turn. file:///g /Web%20Services/Sites-Production/atss/www/htdocs/scin.html (7 of 10)12/30/2006 3:41:07 PM
A RWD chair is the most suitable for situations where there is limited space behind the driver. If there is limited space in front and behind, then a CWD chair will prove to be more manoeuverable. If you have a confined area in front and more space behind, a FWD requires the least space in front to turn through a given arc. Performance Analysis Indoor Corridors and doorways Generally speaking a CWD or HR FWD chair provides a reasonable compromise between the space requirements of a FWD and RWD. There are times however when compromise is not acceptable. file:///g /Web%20Services/Sites-Production/atss/www/htdocs/scin.html (8 of 10)12/30/2006 3:41:07 PM
This diagram illustrates a narrow corridor with a door on one side at the very end. If the door is marginally wider than the chair, only a LR FWD chair will be able to make the turn to get from the corridor through the doorway. All other chairs require the corridor extend beyond the doorway in order to complete the turn. However, when you reach your destination and retrace your steps, only a LR RWD chair will be able to turn after passing through the narrow doorway into the corridor. In the lower diagram only the RWD chairs can negotiate the turn into the narrow corridor. (Note: that a HR RWD chair being reversed requires about the same space as an equivalent LR FWD chair being driven forward: and vice versa.) Performance Analysis Indoor Visual cues and ease of learning file:///g /Web%20Services/Sites-Production/atss/www/htdocs/scin.html (9 of 10)12/30/2006 3:41:07 PM
When initiating a sharp turn into a doorway scarcely wider than the chair the driver relies primarily on two cues - distance from the wall and position relative to the doorjamb. Both HR and LR RWD chairs must be driven out from the wall in which the door is located (distance varies depending on the size of the door opening) and the turn initiated after the driver?s head passes the doorjamb. CWD and HR FWD chairs can be driven closer to the wall and the turn initiated sooner, while the doorjamb is still in sight. LR FWD chairs can be driven at any distance from the wall and the turn initiated while the jamb is still in sight, however the back end requires a lot of space to complete the manoeuvre. People with poor judgement, low vision or cognitive dysfunction may find it easier to steer either a FWD or CWD chair through narrow doorways. Having said that, with a RWD chair the bulk of the chair is in front of the driver and may therefore be easier for some people to visualize the space available in narrow confines. file:///g /Web%20Services/Sites-Production/atss/www/htdocs/scin.html (10 of 10)12/30/2006 3:41:07 PM