Drive Wheel Position – Is There a Perfect Solution? Considerations and Benefits

Do you often default to prescribing a Mid Wheel Drive (MWD) Power Wheelchair (PWC) as it has the smallest turning circle and is often perceived as the easiest to drive? I have asked this question a lot over the last month to different therapists during training sessions, and many therapists have been brave enough to share that they often default to prescribing a MWD PWC as it is the base that they are most familiar with. This being the case, I thought it would be a great opportunity to look at the benefits and considerations of the three main types of PWC bases. Front wheel Drive (FWD), Mid Wheel Drive (MWD) and Rear Wheel Drive (RWD).

Front-Wheel Drive (FWD) PWCs:

The front-wheel drive chair has the drive wheel in the front and the casters in the rear.

Benefits of Front-Wheel Drive

• Completing 90-degree turns require less space in a FWD when hugging the wall/corner.
• Excellent obstacle climbing - this is because the front wheels are connected to the drive motors ensuring the casters are being pulled over obstacles, as opposed to pushing.
• Enhanced traction on uneven terrains.
• Weight distribution in FWD is forward, providing good balance when utilising power seat functions that shift a client’s weight posteriorly.
• The drive wheels located in the front allow for negotiating obstacles from any direction (the base does not need to be straight on in relation to the obstacle).
• Four wheels on the ground minimise the energy and forces that are distributed through the chair such as vibration.

Considerations of Front-Wheel Drive

• Some people feel uncomfortable with the rear casters turning in FWD (swinging rear end). This can take some time for users to learn and be aware of.
• Historically, FWD bases tracked poorly at higher speeds, often referred to as “fish-tailing”; however, current tracking technology has reduced this experience. 
• Due to the forward weight distribution, anti-tipper wheels may be required at the front, especially when descending ramps at higher speeds.

Rear-Wheel Drive (RWD) PWCs:

RWD PWCs were the first type of power wheelchair base configuration manufactured. They are still used today but are not as common for an everyday PWC. The drive wheels are at the back of the chair and the casters are at the front.

Benefits of Rear-Wheel Drive

• Typically these bases offer good suspension, minimising vibration.
• RWD bases provide good tracking at higher speeds.
• May be intuitive for users to drive as they can see where their feet are going. 

Considerations of Rear-Wheel Drive

• RWD chairs can have a larger turning radius which makes it less suitable for tight spaces such as indoor use
• The front casters must be straightforward to negotiate obstacles and turning into an obstacle may be challenging.
• The rear wheel is connected to the drive motor which means the casters are being pushed over obstacles and through various terrains. When casters are being pushed, the force generated is forward and downward, which can increase the likelihood of becoming stuck in certain situations. 
• Rotating front casters may limit lower extremity positioning options.

Mid-Wheel Drive (MWD) PWC:

MWD PWCs are the most recent technology developed for power wheelchair bases and are the most popular. It combines aspects of FWD and RWD into a hybrid product. MWDs provide the smallest overall turning radius but may have some challenges negotiating varied terrains. The closer the drive wheel is to the center of the wheelchair base, the smaller the turning radius, provided the overall base length is the same.  Caster size could also affect the performance of the chair and functional transfers.

Benefits of Mid-Wheel Drive

• The smallest 180° turning radius for indoor use.
• MWD driving may feel intuitive and easier to turn in tight spaces (this is because the person is seated over the drive wheel and can continue turning). 
• Six wheels on the ground offer great stability and weight distribution. This is beneficial when ascending/descending ramps.

Considerations of Mid-Wheel Drive

• With six wheels on the ground, there is a chance for the drive wheels to lose traction and not be able to move the base in certain situations (commonly known as high centering).  Newer technologies on complex bases provide some compensation for this risk by providing suspension adjustment and additional programming options.
• Six wheels on the ground results in an additional point of energy transfer from the ground to the chair (and ultimately the user). This may potentially limit ride comfort for the client and possibly elicit pain, abnormal tone, or fatigue.  However, the suspension system can improve ride comfort.
• Rotating front casters may limit lower extremity positioning options. Revolving front casters could collide with optimally positioned footrests.  If this happens, adjustments may be required to compensate for this.
  
  

Ultimately there is not one right or wrong PWC base, as the best base is the one which meets the individual's goals, enables them to access their environments and climb obstacles whilst maximising their participation.

Have a closer look at comparing turning circles and manouvering tight corners in our Clinical Resources page (permobil.com). If you do have any questions, or want to discuss the ins and outs of drive wheel configuration further email us education.nz@permobil.com.

A special thanks to my colleagues in the USA for contributing to this blog.

Author

Tilly Brook MOccThy (Hons) (OT)

Director Clinical Services