A frequent conversation I’ve had in recent weeks has been around drive wheel configuration and working out what drive wheel configuration may best meet a person’s needs.
In this part of the world we frequently prescribe mid-wheel drive (MWD) power wheelchairs, with their manoeuvrability working well in a variety of indoor spaces and improvements in suspension making them more user friendly outdoors, making it a chair that can meet an assortment of needs. But sometimes a MWD chair doesn’t work as well as we hoped and we are left trying to problem solve whether we can adjust the chair to make it work, or whether we look at whether another drive wheel configuration might work better.
Some of the common issues that get raised with MWD chairs include difficulty positioning a person’s feet, the person reporting the chair is difficult to use outdoors, or that using the chair outdoors impacts on their sitting tolerance. Sometimes these issues can be improved by a change of set up, but other times it is the limitations of a MWD chair coming into play.
Let us start with looking at drive wheel configuration and castors, as the different base configurations have their benefits and challenges. A front wheel drive (FWD) chair has the drive wheel at the front and two trailing castors, this arrangement lends itself to good outdoor performance as the front drive wheel can easily climb small obstacles. Historically FWD chairs were prone to ‘fish tailing’ making the chair challenging to drive at faster speeds, however this has been overcome with improvement in drive tracking technology. Some FWD chairs may have small anti-tip wheels on the front depending on their speed setting or power seat functions used on the chair.
A MWD chair has its two drive wheels in the centre of the chair, with two castors sitting in front of the power wheelchair base and two castors at the rear providing stability to the base. Advancements in MWD technology have resulted in improved suspension, a feature of this being castors that have some ability to rise and fall, helping overcome the historical issue of ‘high centering’, or when the drive wheels become suspended when the chair traverses a steep road crossing.
Rear wheel drive (RWD) chairs have the large drive wheels towards the rear of the chair with two castors at the front, these chairs tend to be the easiest chairs to drive at higher speeds,but have a larger turning circle making them more challenging to use indoors. Historically RWD chairs have been popular with users who frequently drive their chairs outdoors.
Remember also that where the drive wheel is positioned also impacts on how the chair moves. Many users will have a drive wheel configuration preference based on how the chair drives and how well this works in the environments they need to access.
Coming back to some of the commonly encountered MWD issues. For wheelchair users who sit with their feet spaced apart, perhaps due to limitations in joint range of movement or oedema, how wide their feet can be positioned in a MWD (or RWD) chair is limited by the width between the two front castors. For some users, the position of their feet is simply too wide for them to sit between the two castors, while others they may have issues with the front castors knocking their ankle when turning, or they have difficulty keeping their feet on the footplate – placing them at risk of injury if their foot gets caught between the footplate and the front castor. For users who are hoist transferred and have good hamstring range, using 70 degree swing away hangers may overcome this problem, however for those who stand transfer in/out of their chair, changing to a FWD option may work best, however you may be limited by what power seat functions you use if you want to avoid the anti-tips wheels at the front.
The second issue that is often reported is the user reporting that using the chair outdoors increases their pain, or their tone increases resulting in a loss of positioning.
Simple but sometimes effective ways to improve ride quality can be to look at a person’s seating (air or gel based seating may offer benefit over foam) or swapping solid tyres out for pneumatic, but sometimes it is the two extra castor wheels in contact with the ground that is causing the issue. When a chair encounters a small obstacle, how the chair responds depends on its drive wheel configuration. Thinking about something like a low threshold to enter a home, for a RWD or FWD chair, only two sets of wheels need to manage the threshold, with the first set of wheels heading up and over then the second set following, much like how a standard vehicle would manage such a threshold. For a MWD chair managing that same threshold, the front castors would rise up and over the threshold, then the drive wheel (which is positioned under the user) and then the rear castors, this has a very different feel for the user, and for some users this subtle difference can be enough to cause issues. When looking at base options for ride quality, don’t forget about your suspension options – not all suspension is created equal, and suspension options vary between model of chair.
A third and less common issue that can arise with a MWD chair relates to where a user’s weight is distributed on a chair and how the chair performs, particularly outdoors. On a MWD chair the user’s weight ideally needs to be distributed over the power wheelchair base – with their pelvis positioned either above or slightly behind the drive wheel.
For children and people of short stature, it may not be possible to position them over the drive wheel as their thigh length is too short, meaning they sit towards the front of the chair. For this group of people, a MWD chair may feel more like a RWD chair to drive, as the drive wheel is positioned behind them, or it may be that the chair struggles to get traction on softer surfaces such as grass or gravel as the person’s bodyweight is towards the front of the chair, not down through the drive wheel. Changing to a front wheel drive can help overcome these issues and can also result in a base that has a shorter footprint which can be more in keeping with the person’s size.
For adults, their bodyweight may be focused towards the front of the chair for a couple of different reasons. If a person has shortened hamstrings, they may be positioned towards the front of the seat pan to keep their feet away from the power wheelchair base itself, meaning their pelvis is sitting in front of the drive wheel. Or it may be that the person has significant lower limb oedema and carrying extra fluid in their legs, or has extra body weight that is carried in front of them, so while their pelvis may be in line with the drive wheel, their body weight is distributed towards the front of base. In these situations, having extra bodyweight towards the front of the chair can mean the suspension on the front castors is already loaded by the person’s bodyweight, hence when the chair is used outdoors, the castors have a limited ability to move and hence road crossings can become more challenging. For some chairs they may also be more sluggish to turn on carpet, as more force is required to turn the castors which are carrying more load than intended. Changing to a FWD chair typically overcomes these issues, as the drive wheel is now positioned where the castors previously were, with the drive wheel being more suited to carry this load. As a side note, positioning a person towards the front of the seat pan can also influence how the power tilt actuator functions on some chair models, with the actuator prone to early failure if the setup is not ideal.
Not sure if one of these situations is relevant to a person you are prescribing a chair for?
Reach out to your local technician/dealer/supplier and seek a second opinion, with a front and side on photo of the person in the chair to share if possible. Getting the drive wheel configuration right for a person can make a big difference in how well a power wheelchair functions for a person, maximising their mobility and what they are able to achieve.
Rachel Maher
Clinical Education Specialist
Rachel Maher graduated from the University of Otago in 2003 with a Bachelor of Physiotherapy, and a Post Graduate Diploma in Physiotherapy (Neurorehabilitation) in 2010.
Rachel gained experience in inpatient rehabilitation and community Physiotherapy, before moving into a Child Development Service.Rachel moved into a Wheelchair and Seating Outreach Advisor role at Enable New Zealand in 2014, complementing her clinical knowledge with experience in NZ Ministry of Health funding processes.