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Hello all!
There are many factors that influence the maneuverability of manual wheelchairs and I previously have written articles on many of these factors. Past articles have been written about rolling resistance, overall weight, centre of gravity and weight distribution between the front casters and rear wheels, caster housing position, rear wheel camber, and rear wheel position in standard wheelchairs. All of these articles are available on Clinical Corner and can be found by using the Search feature in the upper right corner or looking through the tag cloud in the lower right. This month, I would like to focus on one simple choice that affects turning efficiency and performance and that is the choice of the fork in an adjustable folding wheelchair.
Essentially, there are two different types of forks for casters – standard forks and multi-position forks. (I will not confuse the discussion with the inclusion of frog legs suspension forks.) The picture to the left (below) illustrates a standard fork while the photo to the right shows a multi-position adjustable fork. Both types come in different heights to accommodate different caster sizes. A longer fork is required for larger casters, while a shorter fork has fewer options for caster sizes; however, a shorter fork will have a smaller turning radius and greater turning efficiency than a longer fork.
The difference between the standard and multi-position adjustable forks relates to the relative position of the holes for attaching the casters. In the standard fork, the holes follow the angle of the fork, whereas in the multi-position fork, the holes for the caster are aligned perpendicular to the ground.
If we think about how the fork and caster swivel about the stem bolt during changes in direction, we can better appreciate the effect of the length of the fork, in addition to the effect of the difference in the positions of the holes between standard and multi-position forks. When travelling in a forward direction, the fork and caster trail the stem bolt. When reversing, the caster and fork swivel about the stem bolt and are in a leading position relative to the stem bolt. Therefore, the longer the fork, the greater the turning radius, whether one is using a standard fork or a multi-position fork.
In the above images, the yellow dotted line represents the centre of the stem bolt. In the image to the left, the pink circle represents a caster, the pink dotted line represents the centre of the caster through the top hole, and the pink arrow represents the trail when the caster is in the top hole. Notice how if the seat to floor height is changed and the bottom hole of the standard fork is used, the trail is increased, as represented by the blue arrow, with the blue circle representing the caster and the blue dotted line representing the centre of the caster in the centre of the lowest hole. The image to the right demonstrates that in a multi-position fork, there is no difference in the trail of the fork and caster no matter which hole is used for front seat-to-floor height, as demonstrated by the green circles representing the caster and the green dotted line aligning through the centre of all the holes on the caster fork.
Another factor to consider when selecting between a standard fork and a multi-adjustable fork is the potential for interference between the casters and the rear wheels and/or footplates. Because the trail is increased when using the lowest hole in a standard fork, depending upon the size of the caster used and the angle of the hangers and footplates, interference between the caster and foot plate can occur. With the multi-position adjustable fork, the distance between the caster and the rear wheels and/or footplate is maintained, no matter which hole is used for the front floor-to-seat height. The multi-position fork also has the effect of maintaining the same wheelbase no matter which adjustment hole is selected.
As I said at the beginning of this post, the choice of fork may be a simple decision; however, it's impact on turning efficiency and performance cannot be forgotten.