Frequently Asked Questions
Would you please clarify the effect of pneumatic tires on rolling resistance?
According to many sources, pneumatic tires have lower rolling resistance compared to solid or airless insert tires. This is especially true for individuals who are bariatric, as solid tires demonstrate greater deformation as the load increases than pneumatic tires (pneumatic tires maintain their circular shape better under load). This means that a heavier individual will experience an even greater rolling resistance with solid tires than someone who weighs less. Of course, other factors have to be considered, such as tolerance to the risk of flats and maintenance of pneumatic tires. There is, however, a pneumatic tire with a Kevlar lining that reduces the risk of punctures.
How does the position of the rear axle far forward affect size of caster wheels?
If the rear wheel is large and the rear axle is in its most forward position, there can be interference with the front caster if the caster size is too large, particularly when working with shorter depth wheelchairs, especially in pediatrics. Of course, there are other things to consider as well, such as required seat-to-floor height and required foot positioning, etc. You will not typically run into a problem with interference, however, as most manufacturers have online configurators for vendors. The configurator will show a "no go" configuration so that you are not able to select a comnbination of rear wheel, caster, and center of gravity (CoG) setup that would cause the interference. The issue, however, could arise if CoG adjustments are made after the wheelchair had been configured, as I said, usually in shorter depth wheelchairs.
What is the effect of the width of the front caster and rear wheel?
There are a number of factors to consider, including terrain. A wider caster or rear wheel increases rolling resistance on smooth surfaces, but on soft ground the additional width helps to avoid sinking into the ground. Think about seeing a road bicycle with very thin tires and a mountain bicycle with very thick tires designed for the different terrains. In a wheelchair, a possible solution for mixed terrain may be to use a caster that is beveled so that only the center of the caster makes contact on smooth surfaces for reduced rolling resistance, but the additional front width from the beveling provides support to avoid sinking into soft ground.
I missed what you were saying about foot propulsion. Would you please repeat it?
When I was talking about foot propulsion, I explained that there are different patterns of foot propulsion and this will affect the finished seat-to-floor height. Most of us are familiar with taking the client's measurement from the popliteal fossa to the heel of the shoe the client normally wears and using that measurement for the finished seat-to-floor height. This will suffice for someone who uses a typical heel strike and pull-through pattern. However, there are individuals who use a shuffling foot pattern or even those who use the balls of their feet when propelling the wheelchair. For those who use a shuffling pattern, they likely do not have much dorsiflexion or plantarflexion available so this needs to be accommodated in the finished seat-to-floor height. The measurement is taken the same way (back of knee to heel of shoe), but about ¼" may be added to the finished seat-to-floor height to allow for the flat-footed movement of the feet so that they do not get "stuck" when trying to initiate movement.
Lastly, for the person who uses the balls of their feet, the measurement for seat-to-floor height should be taken from the back of the knee to the ball of the foot in the position that the client uses to propel a manual wheelchair. Depending upon the amount of plantarflexion the client is in, it could be that the seat-to-floor height is higher by about ½" to allow the forefoot to be in contact with the ground for propulsion. For someone who presents with very significant plantarflexion (e.g., someone with complex seating needs), the finished seat-to-floor height could be even higher.