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.