In a previous article, I wrote about the position of the rear wheels of a manual wheelchair and its effects on centre of gravity and weight distribution between the front casters and rear wheels. This month, let's focus on how the position of the front casters effects the load distribution between the rear wheels and front casters.
Some models of manual wheelchairs allow the position of the housing for the front casters to be changed relative to the frame. The caster housing can be in either a leading position or a trailing position. In the below photo, to the left, the caster housing is shown in a trailing, or rearward, position; that is, the caster housing is posterior to the frame of the wheelchair. The below photo, to the right, shows the caster housing in a leading, or forward, position; that is, the caster housing is anterior to the frame of the wheelchair.
The position of the caster housing affects the load distribution between the rear wheels and front casters. When the caster housing is in a leading position, as compared to the trailing position, the load through the casters is decreased. Recall from previous Clinical Corner articles that when the load through the casters is decreased, it becomes easier to turn and to maneuver the wheelchair. When an increased percentage of weight is distributed through the front castors, which is what happens when the housing is in a trailing position, rolling resistance may be increased and the wheelchair may be more difficult to propel and to maneuver.
An additional benefit of having the front casters in the leading position is that the stability of the wheelchair is increased anteriorly. By increasing the wheelbase length in front of the wheelchair, the wheelchair is less likely to tip forward as the base of support is lengthened through the front of the wheelchair. (This is similar to the concept of the position of the rear wheel affecting posterior stability, which was discussed in the article, Centre of Gravity and Manual Wheelchairs, which can be found here.)
Of course, there are other things to consider when choosing between a trailing and a leading caster housing position. The combination of the caster housing position and the selection of caster size, fork geometry, hanger angle, and footplate position influence the relative position of the caster and the footplate. When the caster housing is in a leading position, there is less clearance between the caster and the footplate. Sometimes, interference between the caster and the footplate can be caused depending upon their relative size and angle. Caster interference may be observed when the caster fork swings when the wheelchair is being reversed and the caster gets "caught" on the footplate.
The potential for caster interference can lead to other choices being made to negate the interference, such as selecting smaller casters or selecting hangers with angles to position the feet further from the casters. These choices also have consequences. The size of caster selected will influence the rolling resistance of the wheelchair and the ability to climb obstacles as a caster can climb an obstacle half its size. With respect to hanger angles, sometimes a 60 degree hanger may be chosen to position the feet further from the casters. If a person has tight hamstrings or requires greater knee flexion, this will not be an optimal choice and may create positioning issues for the individual. If a person can indeed tolerate a 60 degree hanger angle the result of this choice is that the overall turning radius of the wheelchair is increased as the hangers are further from the wheelchair. Below are photos of different hanger angles to provide a visual representation of how the hanger angle increases the distance from the wheelchair. From left to right, the photos illustrate hanger angles that are 90°, 60°, and 70°.
As we have seen, it is not a simple choice when deciding caster housing position. Many variables must be considered in order to find the optimal solution for an individual.