Building the Best Ultralight

Industry Experts Share Their Blueprints for Success

In recent issues, Mobility Management has discussed the media used to build ultralightweight chairs (Clash of the Titans: Aluminum vs. Titanium, March 2010) and the two main types of ultralight frame designs (Clash of the Titans II: Rigid vs. Folding, March 2011).

But ultralightweight performance is determined by more than just the frame. Choosing the right components — from wheels to pushrims to suspension and tires — plays a critical role in a chair's propulsion, handling and overall efficiency. So we asked experts on ultralightweight performance to share how components can make or break a good ride. — Ed.

Pushrims: To Propel & Control

Q: Describe the basic function of the pushrim. How does its design impact its function?

David Boninger, Three Rivers: The most basic function of the pushrim is as a surface that one uses to propel and control the movements of their wheelchair: propelling the chair forward, slowing it down, turning, etc. The pushrim is one of the primary interfaces between the user and the wheelchair. In a typical day, people contact or push on their handrim 2,000-3,000 times.

Design impacts function in many critical ways. Perhaps most importantly, how a handrim is designed impacts how it is used and impacts the consequences of that use (e.g., pain, fatigue, secondary injuries, etc.). For example, the standard handrim is a small, round tube, and research has shown that over 50 percent of wheelchair users grab their tire in addition to their handrim while propelling their wheelchair. This is because the standard round-tube design provides a small gripping surface that is often insufficient for the user, so they grab their tire as well.

The small round tube requires what is called a “pinch grip” — gripping the fingers tightly around the small tube — that exerts additional stress on the hands and wrists.

Q: How can changes in design or materials impact a pushrim's efficiency or an end-user's clinical outcomes?

DB: Handrims that go beyond the standard round-tube design offer a whole host of advantages. For example, research has shown that an ergonomic design — a handrim designed to match the grip of the hand — increases performance and efficiency on every push.

Ergonomically designed handrims also eliminate pushing on the tire and enhance control when braking. Perhaps most importantly, research has also demonstrated that ergonomic handrims help to reduce hand and wrist pain often associated with the repetitive stress of wheelchair propulsion. Hand and wrist pain among long-term manual wheelchair users is extraordinarily common: As many as 70 percent experience pain. Thus, ergonomic handrims that provide a superior grip (and eliminate the “pinch grip”) are an important option for addressing this high prevalence of pain.

Q: What are indications that a change in pushrim could be beneficial to an end-user? What clinical signs should providers and clinicians look for during the evaluation?

DB: The quick answer is that if therapists see their clients pushing on the tire, then they have an indication that their clients' current standard handrim is not sufficient for their needs. Of course, if the client is already experiencing pain in the hands and wrists, then a change in pushrim may be beneficial.

More generally, it is important to keep in mind that research on ergonomics in repetitive tasks (like pushing on a handrim) and on the management and treatment of pain associated with carpal tunnel syndrome suggests that a change in handrim — from a standard rim to an ergonomic rim — should be considered by all those who use a manual wheelchair as their primary means of mobility. New users should consider ergonomic handrims as a means of reducing stress during propulsion and as a first line of defense against the onset of pain in the hands and wrists. For veteran users, if there are already indications of pain in the hands and wrists, then ergonomic handrims should be considered as one of the possible steps toward reducing that pain.

Suspension: Dissipating Vibration & Spinal Compression

Q: What are suspension components on an ultralight chair designed to accomplish?

Janet Chelgren, Frog Legs Inc.: There are two types of suspension: front suspension and rear suspension.

The front suspension is designed to dissipate road vibration so it does not enter the frame of the chair and transfer to the rider. It must also absorb the higher impact energy created when the front caster wheels encounter obstructions while rolling over everyday surfaces.

A study conducted by the Department of Health & Human Performance, Iowa State University, determined that 80 percent of all the vibration within the frame of a wheelchair originates from the front casters.

Rear suspension is designed primarily to reduce spinal compression experienced during curb drops.

Q: How can changes in caster design or materials impact the caster's efficiency? Can a better-designed suspension help to prevent, for example, end-user injury or fatigue?

JC: A RESNA study (012700) conducted by Gerald Weisman and Dryver R. Huston of the Vermont Rehabilitation Engineering Research Center at the University of Vermont, Burlington, Vt., studied whole-body vibration as “wheelchair quality” and concluded that it is conceivable that the wholebody vibration a wheelchair user is exposed to may create a condition of fatigue-decreased proficiency after only one hour when outdoors.

A study conducted by Rory Cooper, Ph.D., Human Engineering Research Laboratories, VA Pittsburgh Healthcare System, argued that the vibrations and shocks individuals encounter on a daily basis while propelling their wheelchair may be sufficient to cause injury and concluded, “Suspension caster forks reduce the shock and vibration exposure to the user of a manual wheelchair." Although the impact on users of wheelchairs has not been extensively studied, secondary injuries due to whole-body vibration in workers in the trucking, aircraft, helicopter, maritime, and construction industries is well defined, and these studies have correlated vibration exposure with the risk of diskogenic back injuries.

It is certainly reasonable to assume that vibrations that are left under attenuated may be absorbed by tissues of the body. These tissues include skin, muscle and vessels in parts of the body in contact with the chair; bone, ligaments and intervertebral discs in the spinal column; and any active muscle tissue used to maintain head and trunk posture.

Areas in which suspension helps a wheelchair rider (include) spasticity; secondary injuries; fatigue and muscle tightness; crystallization of bodily fluids (kidney stones); neuropathy, predominately in the feet; pressure sores from shearing of the seat; (and) longevity of chair and other components.

Q: What are indications that a change in suspension could be beneficial to an end-user?

JC: Increased use of drugs to deal with (discomfort or pain); lack of motivation to travel by wheelchair; overall concerns about comfort; recent injuries from being launched forward because of an obstacle.

Asking about daily routines should also include questions about cobblestones, carpeting, how long they can sit comfortably and when or if they get fatigued or sore. Even if suspension is not required, I strongly recommend high-quality front and rear wheels.

Wheels: Attaining Energy Efficiency

Q: Ideally, what do we want ultralight wheelchair wheels to accomplish in terms of performance and ride?

Geoffrey Falkner, Topolino: In an ultralightweight wheelchair, the function of the wheels is even more critical than on a conventional chair.

Obviously, the wheels need to be lightweight, as you don’t want to negate the weight savings of the frame. Weight savings in wheels offer two advantages: less translational momentum, which means less effort to move the wheelchair and user (i.e., forward and backward). But you also save energy in terms of rotational momentum, or the energy it takes to spin or rotate the wheels themselves.

Even if a wheel is not attached to anything, it takes more energy to roll and stop a heavier wheel than a lighter wheel. Think about how many pushes a typical user goes through in an average day, starting and stopping, and the energy savings add up quickly! In other words, lightweight wheels are more efficient and require less energy to use than standard wheels.

However, ultralightweight wheels also have other demands placed on them. They need to be durable and capable of withstanding the rigors of everyday use. This includes not just rolling on rough surfaces or taking curbs and stairs, but also breaking down the chair and wheels. An everyday wheelchair wheel needs to be able to handle a wide variety of conditions.

Wheels also need to be comfortable to use, and deliver a smooth and responsive ride. A typical user cannot ride a wheel that is very harsh and stiff. A ride or wheel that is too cushiony is just as bad: As much energy is wasted going up and down, rather than forward or back.

End-users should consider upgrading to ultralightweight wheels if they show signs of excessive difficulty or fatigue in everyday maneuverability. This includes difficulty breaking down the wheelchair, taking the wheels off and loading them while entering/exiting their car. Shoulders and arm joints of wheelchair wheel users are extremely prone to wear and tear — especially the rotator cuff. A half pound of weight savings over many years of use can mean delaying or foregoing the onset of many repetitive-use injuries.

Tires: Treads for All Seasons & Terrains

Q: On an ultralightweight chair, what functions do tires serve?

Charles Li, Kenda USA: In order to make a lightweight tire, traditionally you have to have tire construction with higher tread per inch (TPI) on its nylon cord. This way, the tire can hold much higher air pressure to make rolling your wheels easier.

Usually, higher TPI tires will be lighter in weight. But at the same time, there is risk of getting either a pinch fl at from hard compacts, or puncture flats from thorns or other hard objects because higher air pressures make the inner tube thinner.

The other way to make tires lighter is to use kevlar bead instead of wire bead. The kevlar bead tire can be folded to be easy to carry, too.

As people are going out for outdoor activities more often today, there are more aftermarket tire choices and tire tread designs for various applications, including street tread (for home and street use), racing/slick tread (roads and hard ground surfaces), semi-slick tread (beach, sandy conditions), all-terrain tread (off-road) and knobby tread (trails; grassy, muddy or loose rock conditions; rural roads; snowy or icy conditions).

A proper change in tires not only benefits an end-user by helping him to save energy and prevent injury, but also provides functional advantages to make the ultralightweight chair more efficient, with better performance.

It's time to change tires when the tire tread is worn out and the nylon cord is showing. But it's also time to make a tire change when you see the tire is wobbling or not rolling straight.


A Provider’s Perspective

Michael S. Dueñas, ATP
Active American Mobility

Q: What benefits can ultralightweight manual wheelchair users enjoy if their chairs are “customized” with such components as wheels, tires, pushrims and suspensions that have been chosen especially for them and their lifestyles?

Michael S. Dueñas: First, let’s talk about types of ultralightweight manual wheelchairs and the various manufacturers, such as Quickie, Invacare, TiLite, Quantum Rehab & Colours Wheelchair, just to name a few.

What’s great about these types of wheelchairs is that they are truly designed to outperform standard wheelchair frames in several areas. These ultralightweight chairs have materials that not only weigh less, but also are stronger than conventional wheelchair frames. The weight, or lack thereof, within the frame provides a lighter wheelchair, allowing the user to propel further and use much less physical energy throughout the day.

Several case studies have shown the overall benefits ultralightweight users have vs. standard and basic frames. Along with these benefits come the adjustability and placement of wheels, which affects center of gravity and weight and balance issues. Front-loading or unloading casters and even the camber setting of the rear wheels can be readily adjusted; these are key points obtained during the evaluation process that will carry over to the final product.

The balance of a wheelchair means everything to an end-user. You can hit all your marks with the seating system, colors, timeline of delivery and price points, but if the user cannot function in their new chair because of improper balance, you might as well pack it up and let another provider take your place. That may sound a bit harsh; however, as an ATP, it’s the job we signed up for. There are other accessories that can be adjusted as well: armrests, lower-extremity footrest, seat depth, back cane angle and height. That’s a snapshot about the wheelchair frame and a few of its components.

Q: Why can it be beneficial to make changes to those components?

MSD: Each of us will experience changes physically as we naturally age, regardless of if we are injured, have a neuromuscular condition or a birth defect.

Because of these changes, wheelchair users will need mobility equipment that can change and/or keep up with them and those changes. Custom wheelchairs can be built with future changes in mind. If the chair was totally rigid in every area, then you must be expecting your client to never change during the life expectancy of the wheelchair.

Most rigid wheelchairs still allow adjustments with caster wheels, back frame angle, wheel lock placement and rear wheel camber. It just makes sense to have the ability to change or exchange components to better meet your client’s needs for the next few years until a newer replacement chair is needed.

Some clients may elect to have two different sets of rear wheels and camber tubes. They might also have two different seat cushions, one set for everyday use and the other for sports activities.

Q: From a seating perspective, how can moving the position of the seat on the base and/or adjusting the seat angle maximize performance for an ultralightweight chair user?

MSD: Function rules the road here.

If my client has a low-level spinal cord injury (SCI), the seat depth and angle may not need to be as radical because their core strength is better, and as long as their pelvis is secure, they are safe and highly functional while using their wheelchair.

If the level of injury is higher (cervical SCI), this client may require the most stable seating surface and angles (dump) to further maintain posture while self-propelling or just sitting in place while in the chair.

Seat-to-floor heights must also be addressed to help confirm function. If your client has areas within their home or place of business they need access to, make sure the wheelchair base has the ability to change if needed or is set up from the start.

And finally, let’s not forget center of gravity. This type of positioning dictates how stable the chair is, how easily the user can wheelie, and how much load the front casters are taking. This adjustment is a must for each and every ultralight wheelchair user.

Q: As an ATP, what factors do you consider when positioning that seat or choosing that seat angle?

MSD: I mentioned balance in the beginning. If function is dictated by positioning while using the wheelchair, then find what best meets the majority of those needs and capture it within the new wheelchair specifications.

Know that you may need to compromise in a few areas because no one product can meet every exact need for each and every client. One example may be that the client cannot yet modify their bathroom and can only access from a certain angle or point. This in turn affects the lateral transfer ability. This issue needs to be considered because they live by themselves with no daily caretaker to assist.

The actual seat cushion that's required is the biggest determining factor. If your client has a history of pressure ulcers, you might need to consider how thick and what profile that cushion will have and how it will affect the final outcome.

Communicating to all persons involved during the wheelchair clinic is the best practice. It’s another way of showing your clients and referral sources just how dedicated you are at providing the best product and customer service possible.


A Consumer’s Perspective

Josh Anderson, VP of Marketing
TiLite

Q: What benefits can ultralightweight manual wheelchair users enjoy if their chairs are “customized”?

Josh Anderson: Component selection is just as important as frame selection. Just as a poorly customized frame will decrease function, so will the selection of the wrong components.

Every component on the chair will have an effect on the chair's performance and therefore, the user's function. As an example, let’s look at rear wheels and handrims.

They are the interface to the chair. Selection of the wrong-sized wheel and handrim can limit access to the rear wheel and decrease the length of a push stroke. Having a wheel and handrim that are too small make the person have to reach further to access the rim and prevent them from having the appropriate handrim contact. Wheels and handrims that are too large force them to flex their elbows excessively and as a result, limit handrim contact. This can increase push frequency and put the individual at a greater risk for repetitive-use injury, while decreasing their control over the chair, because of the limited access to their handrims.

Weight of the rear wheels and handrims is also important, not just for transport, but also because a person has to overcome the weight of these components to accelerate and decelerate the chair, unlike the frame weight, which is mostly static. When someone buys a set of high-end wheels like Spinergy’s or Topolino’s, they are paying for ultralightweight, performance-oriented wheels. These wheels typically weigh a half-pound less per wheel than your standard wheel. That’s a big deal when you’re talking about rotational weight.

When it comes to handrims, weight is secondary to what provides the most efficient push. Ergonomic handrims like the Natural-Fits are big advancements because they give you more surface area for greater control of the chair and are specifically designed to decrease forces at the carpal tunnel, reducing wrist pain and injury.

Q: As an ultralightweight chair user, what customizations have you made to your chair?

JA: In a word, everything.

To me the most important part of building “the best ultralight chair” is having options. Because no two people are alike, being able to choose everything from the frame material to components is the only way you are going to achieve the best ultralight chair.

Because I’m 6'9", every aspect of my seated position has been modified to function in a world that was not designed around someone my height — never mind the wheelchair. Just to fit underneath a table at a restaurant, I had to sweep my legs back and lower my footrest to within 1.5" from the ground. My level of injury (C5-C6 incomplete) makes my seated position that much more challenging. Out of my chair, I’m like a bowl of Jell-o, so proper positioning and support of my feet, legs, hips and back are critical to my independence.

I have sensation, so I am able to tell what is comfortable and what is not or what triggers my spasms. That has helped me over the last 25 years to develop a seat position that is extremely effective for me. That doesn’t mean it's right for everyone, but certain aspects seem to work very well for taller individuals with a similar injury level.

For example, I try to sit as low as possible. That way, I can fit underneath tables, and my chair is as close as possible to the height of the objects I transfer on and off of everyday. That also allows me to position myself at the right distance from my rear wheels to optimize my push stroke using 25" (559mm) rear wheels. I have 6" of seat slope in my TiLite ZR, which helps me to feel really stable while I'm moving. It also shortens up my overall frame length, which is important because I travel a lot, and accessibility around the world has very different meanings.

Another option that makes a big difference for my mobility is soft roll front wheels. I don’t really like the term caster because that makes me think of the cheap wheels you find on a shopping cart, and the wheels you find on ultralight chairs have come a long way from that. I use a two-piece performance soft roll front wheel. The performance comes from the added machining of the aluminum hub that lightens the wheel up considerably, allowing it to roll easier. That, combined with the soft roll tire, allows me to move faster even over rough surfaces without triggering my spasms.

These are just a few of the customized options that I’ve done to my chair.

This article originally appeared in the June 2011 issue of Mobility Management.

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