Life on Wheels (55 page)

The heavier your manual chair, the more energy you will use to push it. This might seem like a good strategy for keeping your weight down, but it’s more likely to fatigue you earlier in the day and limit your activities. A heavier chair is also more difficult for anyone assisting you, making it harder to push you a long distance, carry you up a stairway, or store your chair when you’re not in it. Although chairs are generally lighter in this modern era than the aluminum frames of the past, there is still considerable range in the weight of various products—especially when you consider (as you must) the addition of various options and the weight of whatever cushion is optimal for you.

As you make the many choices necessary to define the best chair for you, you will want to consider chair weight versus the advantages of features that will make it heavier. For example, spoked wheels are lighter than molded wheels but require more maintenance. You can choose such items as titanium axles to lighten weight further (though nominally), but they are considerably more expensive. An important safety issue with a light chair is that it has a longer braking distance. You’ll want to remember this while getting used to your wheels.

Titanium frames have become more common, since the company TiSport began marketing chairs in the late 1990s with introduction of the Cross-Sport model. Titanium is much lighter than other metals used for wheelchair frames and also has the qualities of being stronger, absorbing more vibration, and not rusting. TiSport is the American wholesaler of raw titanium, which helps them keep costs relatively lower, although titanium chairs are more expensive—and therefore less likely to be fully covered by insurers. The other major chair makers have since added their own titanium designs to their product lines.

Your optimal arm reach for pushing will be determined in part by the diameter of the wheels. Manual chairs typically use 24-inch wheels, but wheels are available as small as 20 inches and as large as 26 inches. The combined relationships of the size of the wheels, the height of the seat from the floor, how high you want your feet from the ground, and the depth of your cushion will determine the relationship of your arms to the handrims of the wheels. This is a crucial relationship, defining how effectively you can apply your strength to the wheels and how well your body will endure the repetitive use of your arms and shoulders in this manner.

If your arms are already extended (straight, with your elbows locked) at the beginning of a push—because the wheels are small or you are sitting too high—you will be pushing entirely from the shoulders, rather than being able to engage your triceps muscles, which are the ones between your elbows and shoulders that you use to straighten your arm (and so apply pushing force to the wheels). At the other extreme, if you begin the push with your elbows very bent and your shoulders raised because the wheels are large or you are sitting too low, you will be overstraining your arms and shoulders to get the chair to start moving.

Wheel diameter must be configured in conjunction with seat height and cushion thickness. The sequence of events leading to the decision for wheel diameter goes something like this: first you must ensure proper ground clearance for your footrests based on your leg length. Then you can determine minimum seat height, and from seat height you can determine seat angle. A seat lower at the back will bring your arms closer to the wheels and rims. From there the therapist will determine where your reach falls and determine wheel size from that. Only after you know how high and at what angle your seat will be can you determine your appropriate wheel size. If you want to use larger wheels—which can be easier to push—you might decide to choose a seat height that is greater than the minimum needed to ensure sufficient clearance from the ground for your feet.

Two important decisions for chair stability and optimal wheeling efficiency are where the wheel axle will be in relation to the back of the chair and the amount of angle, or camber, on the wheels. With most modern manual chairs, you can adjust the axle position forward or back in relation to the back of the chair. This is the center of gravity, the point of pivot where your body weight is applied against the back of the chair. When the wheel is moved forward on the axle, more of the chair weight is behind the axle, so it will tip more easily, lifting the front casters off the ground. At the least you want to be able to slightly lift the casters with a bit of extra push to lift the chair’s casters above bumps in the sidewalk, for instance. But if the casters lift, even slightly, during a normal push on a slight upward incline, then you are wasting energy because some of the force of your push is translating into the upward movement of the casters rather than to the forward movement of the whole chair.

You might learn the technique of doing wheelies to “jump” curbs. The axle position will control how efficiently you can perform these maneuvers without putting yourself at risk of falling backward in your chair. When you get a new chair, you might begin with a more stable rearward position of the main wheel axles and then move the wheels forward as you gain more experience and confidence driving.

Correct wheel position is also determined by your weight and how your weight is proportioned. For example, taller people have longer legs and so have more body weight forward in the chair. More weight forward in the chair allows for a more forward wheel position without risk of falling. If you typically wear heavy shoes or boots, you might take that into account by moving the wheels slightly forward. People with dual leg loss have much less forward body weight, so they must use a rearward wheel position. There are chairs that are specially designed to accommodate this particular type of weight distribution—some even have the axle placed behind the vertical line of the chair back.

If the wheels are too far back, they can be hard to reach for pushing, forcing you to pull your arms farther back. When the wheels are farther back, your chair will be more inclined to veer toward the street on sidewalks, which are always sloped toward the street for water drainage. You will have to push harder on the downhill wheel to compensate for gravity pulling the front end sideways. This is even more true with rigid frame chairs, which are considerably more nimble than folding frames and, so, are more sensitive to issues such as this.

Another feature to consider is the amount of “camber” you want on your wheels. Camber is the angle of the wheels toward your body as they rise from the floor (see Figure 4-4). Camber angle is specified when you order your chair, and most products offer you a range of choices. Some chairs have the ability to adjust the camber over a certain range or can be switched between one or two angles, which you would choose when you order. This is an issue to be considered for any manual chair, rigid or folding.

The greater the camber, the wider the wheelbase and, therefore, the greater the lateral stability of the chair as you turn corners or lean over. It also makes the chair more nimble, which is why athletic chairs all use considerable amounts of camber. However, more camber than is necessary for stability will add width at the floor, making it more difficult to pass through narrow spaces.

A different kind of wheel angle—which you want to avoid being improperly adjusted—is “toe-in” or “toe-out.” If your wheels were to roll independently of the chair, they would either roll toward (toe-in) or away (toe-out) from each other. Some wheel or caster adjustments can cause your wheels to be angled in one of these ways, making the chair harder to drive and, possibly, applying force on the frame and axles that can cause damage in the long term. Changing the camber of your wheels can impact the toe-in or toe-out, and some chairs have adjustments for this issue. In many chairs, the toe-in/toe-out issue will be automatically accounted for when you switch between available angles. Your technician needs to be sensitive to this issue.

The standard wheel has steel spokes. Some third-party companies have offered designs that use different approaches to the steel spoke. The goal being lighter and sportier looking.

The X-Core wheel has three “spokes,” though they are more like elegant “legs” made of carbon fiber composite. They are easier to clean than having to wipe down each individual spoke of a standard wheel, and it is sometimes beneficial to be able to grab a spoke as you’re wheeling or braking. Being a fixed, cast wheel, there is no need for any spoke adjusting.

Spinergy wheels (Figure 4-5) were designed by two recent high school graduates who were bicycle enthusiasts, looking for an alternative to the standard wheel. They discovered an ultra-lightweight material previously available only to the military. This material offered the additional benefit of some degree of shock absorption and the ability to retain its shape under stress. They used this material for the spokes, which do not “kink” when impacted from the side like a steel spoke does. The spokes retain their shape, so they need little tuning.

Since the original Spinergy design was created, the number of spokes has been getting smaller and smaller. The company’s newest wheel has only 12 spokes. The design of the wheel itself also offers benefits; the company has engineered it to reduce the “moment of inertia”—the amount of force it takes to cause the wheel to rotate—less force, less strain to the hands and arms.

Originally designed for bicycles, Spinergy wheels—though expensive—have become very popular among manual wheelchair users. Some manufacturers offer them as an option on their order sheet.

Two recent concepts in wheel design have sought to reduce the amount of force necessary to propel a manual wheelchair and, therefore, put this more lightweight and less costly choice within the range of more people with limited strength and stamina. One of the concepts involves a geared system with which you continue to push on the wheels and handrims in the normal manner. Another adds specialized arms to the chair and the wheels.

MagicWheels® are a geared set of wheels that can be added to most standard wheelchairs. They have two speeds—one that is a normal, 1:1 ratio, feeling the same as standard wheels. In the second gear, the wheels travel twice as far for the same amount of push. In second gear, the wheels also go into “hill-holding” mode so that you cannot roll backward, and, if going forward down a slope, the wheels provide some braking effect. They add approximately 10 pounds to the weight of the chair but are no wider than standard wheels (www.magicwheels.com).