Development of Folding Three-Wheeled Wheelchair
Frame for the Developing World
by
MA-SSACHUSETTS INST UTE
OF TECHNOLOGY
Amanda Joy Maguire
SEP 16 2009
Submitted to the Department of Mechanical
Engineering in Partial Fulfillment of the
Requirements for the Degree of
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Bachelor of Science
at the
Massachusetts Institute of Technology
June 2009
© 2009 Amanda Joy Maguire
All rights reserved
The author hereby grants to MIT permission to reproduce and to
distribute publicly paper and electronic copies of this thesis document in whole or in part
in any medium now know or hereafter created.
Signature of Author.............................
...................
Department of Mechdnical Enginefing
May 11, 2009
Certified by...................................
Ass
Anette Hosoi
ate Professor of Mechanical Engineering
Thesis Supervisor
Accepted by........................................
Professor J. Lienhard V
Collins Professor of Mechanical Engineering
Chairmen, Undergraduate Thesis Committee
Development of Folding Three-Wheeled Wheelchair
Frame for the Developing World
by
Amanda Joy Maguire
Submitted to the Department of Mechanical Engineering
on May 11, 2009 in Partial Fulfillment of the
Requirements for the Degree of Bachelor of Science in
Mechanical Engineering
ABSTRACT
Currently, wheelchairs in developing countries are supplied both through donations from NGOs
and local wheelchair workshops in these countries. A popular type of wheelchair manufactured by
these workshops is a three-wheeled wheelchair, which has a long wheel base and third castor wheel
in front of the chair. These wheelchairs perform better in the rough terrain of the developing
world, but do not have a folding frame, and thus cannot be transported. This severely diminishes
the ability of the wheelchair's user to travel far distances and generate income. Because of this
problem, this project aims at developing a frame for the three-wheeled wheelchair that folds into
small dimensions similar to a traditional four-wheeled wheelchair.
The major considerations when designing this wheelchair were weight, cost, and local availability
of materials. All materials used for the wheelchair can be found in the majority of developing
countries, as they consist of mild steel, bicycle parts, ABS plastic, and canvas for the seat. After
evaluation of designs, a double L-brace, which very closely resembles the x-braces used in
traditional four-wheeled wheelchairs, was decided upon. Being similar to these wheelchairs will
increase user acceptance of this wheelchair. Analysis optimized the dimensions of the L-brace, and
found all forces and moments on the wheelchair, so that the size of mild steel piping could be
minimized, thus reducing both the weight and cost. The maximum moment in the center rod of the
wheelchair was found to be 236.0 Nm and in the L-brace the maximum moment was 315.9 Nm.
With the application of a safety factor, these values lead to the selection of mild steel piping with a
diameter of 48.25 mm and a thickness of 1.5 mm for the center rod and a diameter of 60 mm and a
thickness of 1.2 mm for the rest of the frame. Using these materials give a final material cost of
$89.01, significantly less than the current cost of $230. The final weight is estimated to be 55.1
pounds, which is much higher than the current weight of only 45 pounds. This will be an
important feature to look at in future work, and hopefully this new design can be reduced to about
45 pounds.
Thesis Supervisor: Anette Hosoi
Title: Associate Professor of Mechanical Engineering
Contents
1 Introduction
1.1 Purpose
1.2 Wheelchair standards
1.3 Donated wheelchairs
1.4 Three-wheeled wheelchairs
1.5 Mobility
2 Design Requirements
2.1 Volume
2.2 Weight
2.3 Materials
2.4 Production
2.5 Use
2.6 Stability
2.7 Cost
3 Concepts
3.1 Review of current designs
3.2 Potential concepts
3.3 Evaluation of concepts
3.4 Decision on final design
4 Analysis
4.1 Forces on wheelchair
4.2 L-brace geometry
4.3 Moments around joints
4.4 Center rod alignment
4.5 Stability
5 Final Design
5.1 Dimensions of wheelchair
5.2 Materials
5.3 Total weight
5.4 Total cost
5.5 Production methods
6 Conclusion
6.1 Review of prototype
6.2 Testing of prototype
6.3 Unfinished work
6.4 Possible improvements
7 Acknowledgements
8 References
1 Introduction
1.1 Purpose
This report documents the redesign of the three-wheeled wheelchair manufactured in Eastern
African so that it has the capability to fold up into small dimensions similar to a folded traditional
wheelchair. Because the chair is for the developing world, there are several important factors to
take into consideration, including cost, local availability of materials, durability, ease of use, and
the aesthetic of the chair. This design was developed as part of the Wheelchair Design in
Developing Countries class in Spring 2008, and this semester there are two major improvements
which will be focused on - reducing the weight of the chair, and well as improving the joints and
pivots. This will involve completely determining all of the forces and moments throughout the
chair in order to chose the best material based on material properties.
1.2 Wheelchair standards
In 2008, The World Health Organization set guidelines on wheelchairs in the developing world.
These standards outline how wheelchairs should be designed and manufactured, as well as how
they should be disseminated, fit to their users, and then ultimately repaired. Before going into
some of these guidelines, it is important to define all of the parts that make up a wheelchair.
Figure 1 below displays all of the parts of a traditional four-wheeled wheelchair, along with the
names of each part. Although the design of a three-wheeled wheelchair is not exactly the same, it
is still comprised of basically the same parts.
.
nush handles
-....
rear wheels
oush rines
brakes
Sseat
Icalf
backrest
..
--
armrest
- cushion
frame
frame
strap
otrest
castor wheels
Figure 1. Parts of a wheelchair
In designing all of these parts of the wheelchair, safety of the user is of utmost importance and
number one priority. There are several parts of the wheelchair which are more susceptible to
causing injury and have been identified by the World Health Organization. It is these areas of the
wheelchair where the greatest care must be taken to make sure that injury is avoided. One of the
most prevalent injuries associated with wheelchair use is pressure sores. Inadequate or nonexistent
cushioning is the cause of these sores, which can become infected and lead to even more serious
injury or death. This is why cushions should be required on all wheelchairs. Stability is a major
concern, especially in developing countries where the wheelchairs will rarely be used on flat or
paved terrain. The wheelchairs must avoid tipping even on bumpy or treacherous terrains, so that
the user stays in the wheelchair, and does not fall out, which can lead to further injuries. An
obvious concern would be any sharp parts of the wheelchair, which can cut the user. Also, any
potential pinch points should be covered or redesigned so that the user does not get any part of
their body caught in the chair. Fit is another important, often overlooked, part of designing and
constructing a wheelchair. If the wheelchair does not properly fit the user, it can actually cause
more injury for the user. One example of this is potential shoulder injury from a wheelchair that is
too wide. While trying to maneuver using the push rings, if the chair is too wide, unnecessary
stress will be focused on the user's shoulders, causing injury. This is only a short list of potential
safety concerns that must be addressed in wheelchair design, but each is crucial and must be
considered in the design process (WHO, 41).
To test the design of a wheelchair to make sure that it fulfills these guidelines, the International
Organization for Standardization (ISO) has developed standards for all wheelchairs. These
standards, ISO 7176, are not required for the wheelchairs in each country, as each government has
the ability to develop their own national standards. However, the World Health Organization has
suggested that these countries either adapt the ISO's standards, or at least use them as a basis in
developing their own. Also, because the wheelchairs in developing countries are a mixture of
locally manufactured wheelchairs and donated ones, it is recommended that these standards be
applied to all wheelchairs within the country, regardless of its origin. The tests used to evaluate
wheelchairs are broken into three categories: functional performance; seating and postural support;
and strength, durability, and stability (WHO, 66).
Functional performance encompasses many aspects of the wheelchair when it is in use. These
standards focus on how the chair performs in different environments for different uses. The main
tests for functional performance are as follows:
*
static stability
*
dynamic stability
*
rolling resistance
*
ability to repair/availability of components
*
overall dimensions, mass, and turning space (WHO, 46,56).
Seating and postural support focuses on all of the places on the wheelchair which are in contact
with the user's body. The important aspects of these standards are the comfort of the user, along
with prevention of pressure sores. To evaluate a wheelchair's effectiveness in seating and postural
support, there are essentially two areas that should be reported on. These two elements are:
*
seating dimensions and adjustability
*
cushion type and characteristics (WHO, 56, 62).
Finally, the third category of wheelchair evaluation - strength, durability, and safety - gives an
idea of the reliability of the wheelchair, as well as its lifespan. These test involve learning about
the ability of the wheelchair to handle wear and tear due to static forces, impacts, and fatigue
stresses from use over time. Unlike the other two types of tests, evaluation of these criteria will be
a continuous process, as the wheelchair will need to be tested at all stages of its lifespan. These
tests will also take place in all potential environments for the wheelchair (WHO, 62-65).
1.3 Donated wheelchairs
There are several NGOs in the United States or Europe which focus on supplying wheelchairs to
developing countries. These charities pride themselves of their ability to transport a large quantity
of wheelchairs to developing countries. And while the intent of these NGOs is excellent and there
is some good that comes from their generosity, these wheelchairs still have some severe flaws that
make them inadequate from the developing countries where they are being distributed. There are
two major NGOs, Wheelchair Foundation and the Free Wheelchair Mission, which distribute a
majority of wheelchairs, and will be the focus of this section.
Figure 2. Wheelchair distributed by Wheelchair Foundation
The wheelchair above is the type that is distributed by Wheelchair Foundation. There are several
positive qualities of these wheelchairs which make them a gift to those that receive them. First of
all, they are much less expensive than locally manufactured wheelchairs, and for the end user who
receives the wheelchair, they are free. Many wheelchair users in developing countries cannot
afford to spend hundreds of dollars on a wheelchair, especially since their disability often has
hindered their ability to earn any income. Therefore, a free donated wheelchair may be their only
option to receive a wheelchair. Also, if the user only wants to use the wheelchair in and around
their home, these chairs are fairly sturdy and will last about 5-10 years.
However, there are several negative aspects of these chairs as well. Because they are produced in
the United States and Europe, these wheelchairs are very similar to hospital wheelchairs or
wheelchairs that are appropriate for flat and paved terrain. If a user wants to travel far in these
wheelchairs, they will be unable to, as the wheelchair is not durable enough. Another major issue
with these wheelchairs are the distribution methods. While there are three sizes of wheelchair,
little is done in terms of correctly fitting a wheelchair to its user. As mentioned in the above
section on standards, fit is extremely important in wheelchair design. Using a wheelchair that is
not fit to the user many times can end in more injuries and potentially be worse that not having a
wheelchair at all. Another issue with these wheelchairs is that because they are not manufactured
locally, there has been no consideration for locally available materials. This makes it very difficult,
if not impossible, for these wheelchairs to be repaired. Finally, these wheelchairs provide very
difficult competition for local wheelchair workshops, so these workshops are having a harder time
staying in business (Winter, 75).
Figure 3. Wheelchair distributed by the Free Wheelchair Mission
Like Wheelchair Foundation, the Free Wheelchair Mission also supplies wheelchairs to people in
developing countries. The same issues faced by Wheelchair Foundation wheelchairs exist with
these wheelchairs. They are not suitable for rough terrain, are ill-fit to users, and are not made of
locally available materials. However, there is another danger with this wheelchair, as stability was
not considered in its design. This wheelchair can tip very easily, which is obviously a huge peril to
its user. There are also many pinch points and sharp edges on the wheelchair. While the Free
Wheelchair Mission focused on producing a wheelchair as cheaply as possible, they did not
consider all of the safety issues that could arise, and ultimately designed a fairly dangerous, though
inexpensive, wheelchair.
1.4 Three-wheeled wheelchairs
Figure 4. Three-wheeled wheelchair manufactured in Tanzania
The above three-wheeled wheelchair is similar to those designed and manufactured in many
wheelchair workshops such as KCMC and Mobility Care, both in Tanzania. The important
distinguishing factor for these wheelchairs is the center rod with the castor wheel attached. The
long wheel base, along with the three points of contact, give this wheelchair greater stability than
traditional four-wheeled wheelchairs. This makes them a better fit for the rough terrain of
developing countries. Also, because these wheelchairs are produced in local wheelchair
workshops, they are constructed of only locally available materials, meaning that they can also be
locally repaired.
Although there are many positive attributes of these three-wheeled wheelchairs, they do have their
downside as well. First of all, the frame is built in such a way that the wheelchair cannot fold.
This means that it is unable to fit in a car or bus or other type of transportation, so the user cannot
take this wheelchair if they want to travel a far distance. Also, the current set up of local
wheelchair workshops cause these wheelchairs to be relatively expensive. At such a high price,
very few wheelchair users in developing countries can afford to purchase them, so even if they are
better for the terrain, they are often not actually used.
1.5 Mobility
In order to earn some type of income, many wheelchair users will have to travel a reasonably far
distance from their home. Usually this entire trek cannot be made without some type of alternate
transportation, such as a bus. For this reason, it is important that a wheelchair is transportable. In
designing for transportation, size and weight are the main components. Each design choice has
both advantages and disadvantages and portability is no different. Reducing the weight of a
wheelchair may make it easier to carry and move, but it will also potentially reduce the durability
of the wheelchair. To decrease the size, the wheelchair can have built in folding mechanisms.
However, additional mechanisms will also increase the weight of the wheelchair. Parts can also be
removable to reduce size and prepare for transport, but this increases the likelihood that parts will
get lost or broken. Also, the mechanisms required to removed parts, such as a push-button, are
relatively expensive and might not be locally available in all developing countries. All of these
design options must be considered with advantages and disadvantages weighed against each other
to optimize the wheelchair's design (WHO, 55).
2 Design Requirements
2.1 Volume
The important volume for this wheelchair is folded dimensions. Because the actual chair
dimensions are based off of the three-wheeled wheelchairs built at KCMC and Mobility Care in
Tanzania, these dimensions for the new chair will be consistent. In its functioning form, the
wheelchair will have a length of 43.5 inches, a width of 24 inches, and a height of 33.5 inches.
When folded, this chair should assume dimensions similar to that of a traditional wheelchair, such
as the donated four-wheeled wheelchairs, so that the three-wheeled wheelchair can fit in the same
storage spaces. To accomplish this, the three-wheeled wheelchair needs to have dimensions of
35x5x35 inches or less.
2.2 Weight
A huge concern with changing the frame design of the three-wheeled wheelchair is the additional
weight that could be added by increasing the amount of material used. Additional weight is
undesirable for several reasons. First, it will make it harder to actually use the wheelchair, as it
will require more human power to move. Secondly, the purpose of this chair is to be portable and
transportable. Increasing the weight significantly defeats the purpose, for it could potentially make
the chair too heavy to pick up, fold, and transport. This is why the chair must not be heavier than
the current three-wheeled wheelchairs, which weighs 45 pounds.
2.3 Materials
Because this design is meant for production in developing countries, it needs to be built entirely
from locally accessible materials for those areas. This limits the use of material to mainly mild
steel pipes, which are available in many sizes, as well as bicycle parts. There is also limited
availability of foam and fabric for the seat, as well as PVC or other plastics for armrests, bearings,
or other parts of the wheelchair. To ensure that all sizes of mild steel piping used are actually
available in these locations, the list of available materials at a Tanzanian steel company, Doshi
Hardware, was consulted.
2.4 Production
As well as only using locally available materials, it is also important that any designs can be
manufactured in the workshops found in developing countries. This requires limiting construction
processes to pipe bending, arc welding, cutting that can be done with a hand saw, and using other
hand tools. Also, it is important to minimize any required accuracy as much as possible, so that
tolerances are not exact to much less than a millimeter.
2.5 Use
There are two major requirements that will make this wheelchair user friendly. The first obvious
specification is the ability of the chair to actually hold a person. The chair should definitely be
able to support the weight of a person weighing 200 pounds, which translates to roughly 90
kilograms. To add in an additional safety factor, the chair will be designed to support the weight of
100 kilograms. There will be additional safety factors added in when materials are selected for the
wheelchair, but this is simply the first small safety factor included.
The second specification requires the wheelchair to be easily operated. It should require no more
than one person to fold up into its transportable dimension. This also entails the design being
intuitive, with a very small learning curve required to work the chair. There should be no
confusion as to how the wheelchair works when looking at it, so that how it operates is very
transparent to the end user.
2.6 Stability
Stability is another important aspect of the design, requiring the wheelchair to be perfectly
constrained. This means that the wheelchair should not have the tendency to want to close when
someone sits on, or to open too much. It should have a desirable equilibrium with the weight of an
individual on the wheelchair. Another important aspect of stability requires the wheelchair to have
a very centralized center of mass, so that it will not easily tip over in an direction, especially when
someone is sitting on it. This is an important safety feature of the wheelchair, so that it does not
actually cause more injury to its occupant.
2.7 Cost
While the addition of steps and/or material to the wheelchair may increase the cost some, this chair
must still remain affordable, so that wheelchair users are actually able to purchase and use it.
Current three-wheeled wheelchairs designed and built in workshops in developing countries
typically cost about $230. It is important to keep the cost of this wheelchair within 5% of that cost,
so the price of this chair must be no more than about $240.
3 Concepts
3.1 Review of current designs
From the Wheelchair Design in Developing Countries class at MIT, there have been two previous
attempts at designing and building a new folding wheelchair frame, which helped to develop this
design. The first design was developed in Spring 2007 and focused only on folding in the front of
the center rod on a three-wheeled wheelchair. This design consists of a hinge and two sections of
steel pipe for the center rod.
(b)
(a)
(c)
Figure 5. Hinge design to fold in center rod of three-wheeled wheelchair
As seen in Figure 5 (a) through (c), the hinge allows the rod to stay rigid like past center rods, but
can also cause the center rod to fold in half underneath the chair portion of the wheelchair. When
folded, the dimensions of this chair are similar to the unfolded dimensions of a traditional fourwheeled wheelchair (Wheelchair Design in Developing Countries, 2007).
Figure 6. Tricycle wheelchair designed and produced in Africa
The second design was developed by Lindsay Todman for her thesis in 2008. This time, a folding
mechanism was developed for a tricycle style wheelchair. This style of wheelchair, seen in Figure
6 above, is also supported on three wheels. However, unlike the three-wheeled wheelchair focused
on for this design, the tricycle wheelchair is hand-powered. It is driven by a hand crank in front of
the user, as opposed to using push rims around the wheels, like a traditional wheelchair. After
working through several potential designs for a folding model of the tricycle, Todman's final
design can be seen below in Figure 7, in both the unfolded and folded form.
(a)
(b)
Figure 7. Prototype of Lindsay Todman's folding tricycle wheelchair, in both the folded and
unfolded form.
To fold in the seat area of the wheelchair, a double x-brace was implemented under the seat. For
the fork stem, or the front rod that attaches the steering system to the rest of the wheelchair, a bend
was added to the original design. This allows the entire front area to rotate 180 degrees, so that it
can be folded into the chair. Overall, this design reduces the area consumed by a traditional
tricycle wheelchair by about 30%, which significantly improves the wheelchairs ability to be
transported if necessary (Todman).
3.2 Potential concepts
In developing several designs to satisfy the aforementioned design requirements, it was important
to look at every way in which both the seat could be folded to minimize the width of the chair and
the front castor wheel could be brought in to minimize the length of the chair. To satisfy these and
the other specifications, six very different designs were considered, as seen in Figure 8 below.
Figure 8. All potential design concepts for folding three-wheeled wheelchair
17
The first design involves using an x-brace underneath the seat of the chair just like a traditional
four-wheeled wheelchair. This will allow the seat part of the chair to fold in the same fashion as
the these four-wheeled wheelchairs. The front castor wheel in this design is attached to a rod that
attaches to one side of the chair. When being folded, the front rod will swing into the folded seat.
The second design again involves the seat part of the wheelchair folding together with an x-brace,
just like a traditional four-wheeled wheelchair. However, in this design, the front castor wheel is
connected to the chair on both sides and is taken off when folding up the chair. Each of the two
front rods then fold up similar to footrests, and fold into the seat.
The third design develops a modified x-brace to fold up the seat section. This L-brace is found at
both the front of the seat and the back, and the center rod which attached the front castor wheel is
directed through the center of both of these braces. The center rod then has a hinge on it, which
allows it to fold up underneath the seat. When a person is sitting in the wheelchair, that force will
hold the seat in tension and lock the bracket.
The fourth design requires the two larger wheels to be removed from the wheelchair frame. With
cloth armrests, the seat part can be folded in half, so that it is almost completely flat. Like the
above design, the center rod will be hinged and fold underneath the seat. There will then be places
for the wheels to be reattached to the top and bottom of the folded frame. This will then make the
folded chair almost completely flat, much like a pancake.
The fifth design has a truss system on the back of the seat. The bars in this truss can collapse in, so
that the seat can be folded together. Underneath the bottom of the seat is a pivoting bar. When the
chair is in use, the bar is perpendicular to the wheels, but by pivoting it ninety degrees, the bottom
of the seat is also able to fold. This allows both parts of the seat to fold in, and the center rod with
the castor would again be hinged, so that it can fold underneath the seat.
The sixth and final design is based on a walker, and has a V-shape. The frame would be made up
of two bars, shaped like a v, with the front castor wheel attached at the point. There would also be
a bar in the back for support. To fold, the back bar would have a joint, allowing it to collapse, and
the two legs of the v would come together.
3.3 Evaluation of concepts
In order to compare all six of these designs, seventeen important parameters were chosen, with five
being decided as most important and given a weight of two in the decision making process. The
parameters, as well as their weight and what they measure, are listed below in Table 1.
Parameter
Weight What This Parameter Measures
Cost
2
Takes into account total number of parts, complexity of
parts, and manufacturing steps. Goal is for wheelchair to be
within 5% of the cost of current wheelchair cost.
Folded Size
1
Want new design to fold to within same dimensions as fourwheeled folding wheelchairs
Robustness/Durability
2
Measurement of how resistant the chair is to damage. Also
takes into account the number of moving parts, which
should be minimized if possible.
Functionality/Effectiveness
1
Independent of the new folding mechanism, does it still
function as a working wheelchair?
Ease of Use
2
The wheelchair must be easily operated by one person. This
parameter looks at both operation as a wheelchair and
operation of folding the wheelchair.
Length of Life
2
Measurement of the longevity of the wheelchair and how
long it can safely be used before it must be replaced.
Weight
1
Should be no heavier than the current design, so that
transportation of the wheelchair is not difficult.
Modularity/Adaptability
1
How easily will it be for local wheelchair workshops to
apply this new design to their current wheelchair frames?
Can they use any existing frames or do they need to
completely re-do all of their jigs and tooling to build this
wheelchair?
Maintenance
1
Maintenance will need to be as minimal as possible and if at
possible, should be able to be performed by nearly anyone.
If the wheelchair needs to be taken into a wheelchair
workshop for repairs often, it will likely just not be used by
its owner.
Safety/Stability
2
The design must not tip and it should stay open when in use.
Complexity/Elegance
1
Design should be as simple as possible, using a minimal
number of only necessary parts.
Availability of Materials
1
Materials need to be locally and easily available in
developing countries, where the wheelchairs will be built.
Reliability
1
Important for wheelchair to consistently work in the way it
is designed.
Use in Different
Environments
1
Measurement of ability of wheelchair to perform on asphalt,
mud, sand, dirt, grass, and any other possible terrain.
Ease of Design
1
Can manufacturing processes used be replicated in the
wheelchair workshops in developing countries?
User Acceptability
1
Wheelchair needs to be transparent for the user, as well as
look like other wheelchairs they have seen, so that they are
more willing to use this new wheelchair.
Loose parts are more likely to be lost, so it is important to
minimize the number of parts that need to be removed in
order to fold the wheelchair.
Table 1. All important parameters used to decide between six different designs.
1
Loose Parts
Using these parameters, each was compared against an earlier design for a folding three-wheeled
wheelchair, which is mentioned in the above section on previous work. This earlier design merely
involved folding the front castor wheel under the chair. This design provided a datum, which was
a good measuring tool to differentiate between all of the remaining designs. The results of this
comparison, organized into a Pugh Chart, can be seen below in Table 2.
lFolding 3-wh
ee
Evaluation Criteria
lchair
jDATUM
Tish's Original
Detachable Castor L-Shaped
Single Side
Cost (number of parts, manufacturing steps, n
Folded Size
Robustness/Durability
Functionality/Effectiveness
Ease of Use
0
0
0
0
0
Length of Life
0
0
0
Weight
Modularity/Adaptability
Maintainance
Safety/Stability
Complexity/Elegance
Availability of Materials
Reliability
Use in Different Environments
Ease of Design
User Acceptability
Loose Parts
0
0
0
0
0
0
0
0
0
0
0
-1
-1
0
-1
1
0
-2
-1
-1
-1
0
-1
1
0
-1
0
0i
-2
0
-2
-2
-2
TOTALS
0
-1
1
-1
1
-1
-12
1
1
-1
1
-2
-12
Pancake Truss and IV-Shaped
o
-1
2
2
2
-2
-1
o
-1
1
-1
0
0
-1
1
2
0
0
-1
0
2
0
-1
-1
0
0
0
2
1
-1
0
-2
-1
-1
0
-2
-1
0
o
o
0
0
0
0
0
-1
0
0
-2
0
0
0
-7
-4
Table 2. Pugh Chart used to compare the six designs for a folding three-wheeled wheelchair.
From this chart, the results for each wheelchair design were compared, leading to the selection of
the final design.
3.4 Decision on final design
Based on the results from the Pugh Chart, it appears obvious that the L-bracket design is the best
of the potential concepts. Not only did it perform best overall in the comparison, it also performed
very well in some important categories. Ease of use was a parameter given a weight of two, and in
this category, the L-bracket performed exceptional well. This is because the will fold essentially in
the same way that current four-wheeled folding wheelchairs do. The concept was also given high
scores for folded size, functionality and effectiveness, and complexity and elegance. One final
reason that the L-bracket is chosen is because while it did receive high scores for several
parameters, it also did not receive any exceptionally bad scores for any of the parameters. For all
of these reasons, going forth, the chosen design for the folding three-wheeled wheelchair is the Lbracket design.
4 Analysis
In order to set the dimensions and materials for this new wheelchair, quite a bit of analysis was
required. This analysis included solving for all of the forces and moments on the wheelchair,
which allowed for a material to be selected that is strong enough to support a person, but is also
lightweight so that the wheelchair maintains its portability. The geometry of the L-brace will both
be important for the calculation of amount of material needed, but also to determine the final
folding dimensions of the wheelchair. Explanation of all of the analysis done for this wheelchair
can be found in each of the sections below.
4.1 Forces on wheelchair
In order to solve for all forces on the wheelchair, the first step was to find the center of mass on the
chair both without a person and with a person in it. Figure 9 (a) and (b) show the free body
diagrams to find both of these centers of mass.
Figure 9. Center of mass free body diagrams
For the chair that is being designed, r, the radius of the wheel, equals 13 inches. The lengths of 11
and 12 are 25 inches and 6 inches, respectively. In the free body diagram with a person, h equals
29 inches, 13 is 23 inches, and 14 is 8 inches. Also, x stands for the mass of the person and W is the
mass of the chair. Summing the forces in each scenario gives Equations 1 through 4 below, which
are the center of mass equations.
-25F +12F 2 =0(1)
F,+2F 2 = Wg (2)
2F 2 q 1-Fq
2 =0 (3)
F,+2F2 =g(W+x) (4)
In these equations, ql and q2 are defined by Equations 5 and 6.
(6W+8x)
q= (W+x)
q2
(13W+23x)
(W+x) (6)
(W + x)
The next forces that must be balanced are those on the center rod. Figure 10 below shows its free
body diagram.
Figure 10. Center rod free body diagram
Balancing these forces gives Equation 7 and finding the finding the moment balance gives
Equation 8. In Equation 8, 15 equals 18 inches and 16 is 14 inches.
F- F 3+ F 4=0 (7)
-F315+ F 4( 5+1 6 )=0 (8)
Next, the forces on the seat of the wheelchair are balanced. Although the weight of a person will
cause forces on all parts of the seat uniformly at various angles, it is easiest to model as one body
force straight down. There are four main contact points between the frame and seat. The free body
diagrams for both the front and back of the seat that were used to calculate these forces can be seen
below in Figure 11, where Figure 11 (a) is the front of the seat and Figure 11 (b) is the back.
Figure 11. Seat free body diagrams - (a) is the front of the seat and (b) is the back
To simplify the force balance, it is assumed that the force is evenly distributed, so that all four
reactionary forces in the frame are equal. Given this assumption, the force balance in the seat is
defined by Equation 9.
F-
(xg)
-(4cos 0 )
(9)
.
In this equation 0 is the angle that the seat makes when sat on. This angle will vary with the
weight of the person and the material selected for the seat. For these force calculations, an angle of
45 degrees was used.
The next set of force
s in
the frces in the L-braces. Again, there are two separate
free body diagrams, one for the front L-brace and one for the back. These diagrams can be seen in
Figure 12 (a) for the front L-brace and Figure 12 (b) for the back one.
Figure 12. L-brace free body diagrams
Balancing these forces give Equations 10 and 11 for the front and back L-braces, respectively.
F 3+2F 6 -2F 5cos 0=0 (10)
F4+ 2F 8-2F 5 cos =0 (11)
The last set of forces and equations needed to completely solve the forces in the wheelchair are the
internal reactionary forces in one bar of the L-braces. Figure 13 shows all of the forces in each of
the bars.
Figure 13. Reactionary forces in L-brace bars
To find all of these reactionary forces, equilibrium will need to be imposed in the x-direction and
y-direction. The moments in the bar will also need to be balanced. Equation 12 gives the force
balance in the x-direction, Equation 13 in the y-direction, and Equation 14 is the balancing of
moments.
R 1 +R 3 -Fsinca=0 (12)
F 6+
F3
R 1 l-F
-F 5cosa=0 (13)
6 1-F 5 L=0 (14)
In Equation 14, 1and L are based on the geometry of the L-brace, which is solved for in the next
section.
Using all of these equations, all forces in the wheelchair are solved for. For these calculations, a
value of 100 kilograms (220 pounds) was used for the mass of a person. The wheelchair was
estimated to have a mass of 20 kilograms, which is just under 45 pounds. With these values and all
other determined dimensions of the chair, all forces were solved for. Table 3 below gives the
values for all of these forces.
Fl = 516.1 N
F5 = 346.7 N
R1 = 459.3 N
F2 = 330.3 N
F6 = 344.7 N
R3 = 31.1 N
F3 = 1179.7 N
F7 = 346.7 N
R4 = 245.2 N
F4 = 663.6 N
F8 = 86.65 N
Table 3. All forces on this three-wheeled wheelchair
4.2 L-brace geometry
Figure 14. Unfolded L-brace geometry
In the above diagram, it can be seen that there are many unknowns that must be solved for to
completely solve the geometry of the L-brace. From this diagram, several important geometric
relationships help simplify the number of variables. Initially, the variables can be simplified as
they are below.
W = 21cos =2Lcos c
L
W
W
(2cos a)
(15)
H
2H
H
-x =tan 1( 2H
(sin o)
W
1=
(16)
(17)
(2cos3) (17)
h= sin
(18)
To fully solve this geometry, one more equation needs to be added, which comes from the folded
dimensions of the L-brace. The important dimensions for this geometry can be seen in the figure
below.
Figure 15. Folded L-brace geometry
From this figure, another relationship between the variables is found.
w= 21cos(rr/2-oa+) (19)
Plugging in Equations 16 and 17 into Equation 19 gives
W=
W
(cos3)
2H
cos(rr/2- tan- 1( 2H )+0) (20)
W
From here, there are a few dimensions that can be plugged in, based on the geometry of current
three-wheeled wheelchairs and folding four-wheeled wheelchairs. In order to reach the same
folded dimensions as current folding four-wheeled wheelchairs, the folded dimension of the chair
must be no larger than 3.5 inches. This sets w equal to 3.5. Also, based on current three-wheeled
wheelchair geometry, the center rod is 11 inches off of the ground. In this design, the center rod
will attach through the center of the L-brace. This then means that h must be less than 11, or the
bottom of the L-brace will hit the ground. There also needs to be some clearance, so that the chair
does not get stuck when going over small hills or bumps in the terrain. D defines the height of the
seat of the wheelchair, which based on current geometry is 17 inches. This will be kept consistent
for the new design. With these constraints, the rest of the parameters can be found using the about
equations. Here are all of the dimensions in Table 4.
W = 8 inches
w = 3.5 inches
H = 6 inches
1= 8.87 inches
D = 17 inches
d = 7.2 inches
Table 4. Final dimensions for L-brace
h = 3.8 inches
a=30 degrees
L= 10 inches
= 25.4 degrees
4.3 Moments around joints
To determine all of the moments along the frame of the wheelchair, the forces found in section 4.1
first had to be broken into components parallel and perpendicular to all of the tubing of the frame.
Once this was done, the moments could be found using the forces normal to the frame. There were
four major moments found in the frame, two along the center rod, and two in the L-brace. Figure
16 below shows the location for each of these moments.
Figure 16. Location of all four moments on the center rod and L-brace
Four equations were derived to find the values for these four moments. They are as follows, with
the same variables used as in the sections above.
M , =-(F6sinf +RcosP)x, (21)
M 2=-(F6cos(oa-3)+R,sin(a -f))(l+x 2 )-(M 3 =-F
M 4= F 3
4 -F
F
F
sin(90-a)+ R 3 cos (90- a))x 2 (22)
4 x 3 (23)
4 (1 6+
4)
(24)
In these equations, xl varies from 0 to 8.87 inches, x2 varies from 0 to 10 inches, x3 varies from 0
to 14 inches, and x4 varies from 0 to 18 inches. Solving these equations for the maximum
moments, or when the x values are at their maximum, give the four moments found below in Table
5.
M1 = 50.5 Nm
M3 = 236.0 Nm
M2 = 315.9 Nm
M4 = 13.3 Nm
Table 5. Maximum moments along center rod and L-brace
4.4 Center rod alignment
The alignment of the center rod is important for several reasons. Any amount of error in the
alignment will be magnified in the alignment of the castor wheel. Even a small error in alignment
of the rod could result in the chair tending to one side and not rolling straight, which will
ultimately cause wear on one side of the wheel, and propagate the issue. With construction of the
wheelchair occurring in workshops in third world countries, it is difficult to obtain very tight
tolerances. The best that can be reasonably reached is about 2 millimeters. What will determine
the alignment of the center rod and the castor wheel will be where and how the stops for the Lbrace are attached to the chair. The figure below sets up the necessary geometry to minimize error.
Figure 17. Geometry of error found in center rod alignment
O, is the amount of angular error in the center rod, E is the tolerance, and R is the distance away
from this error that alignment is obtained. This distance will be defined by how far from the center
rod the stops for the L-brace are attached. Because
0
, is a very small angle, small angle
approximations can be used to give Equation 25 below.
9E=
(25)
Small angle approximations can also be applied to find the relationship between the angle of error
and the width of the castor wheel off of the ground, given in Equation 26.
,=E
= (26)
In this equation, x is the width of the wheel that is not touching the ground and L is the distance
between the center rod and the ground.
Combining Equations 25 and 26 shows how R is dependent on x. This can be seen in Equation 27.
E x
-=--+
R= (EL) (27)
R L
x
From Figure 17 w is the width of the wheel, which is 2 inches. Minimizing x will be important,
but x can absolutely not be any larger than 1 inch, although less than 0.5 inches is ideal. As
previously mentioned, the best expected tolerance is about 2 millimeters, or about 0.0787 inches.
In other wheelchairs, the distance from the center rod to ground is about 11 inches, so L will be set
to 11.
Given x=0.5 inches, R is 1.73 inches. Therefore, to ensure that at least three-fourths of the castor
wheel is on the ground, the stop must be at least 1.73 inches from the center rod. A possible design
for the stop places it at the very end of the L-brace, so that it is attached to the armrests. As the Lbrace slides down the armrests as the wheelchair is opening, it will stop once it hits these stops,
made of a strong rigid material, likely ABS or another type of plastic. Figure 18 shows a drawing
of this design.
stop for L-brace
Figure 18. Placement of plastic stops for L-brace
From this design, R is set to equal 10 inches. With R =10, x becomes 0.087 inches, which leaves a
very small amount of the castor wheel off of the ground. By having such a small amount of the
wheel off of the ground, the front castor wheel should be more aligned and the wheelchair should
move forward straight, not off to an angle. Not only will this help the user maneuver the
wheelchair, but this should also reduce wear on the front castor wheel, extending its lifetime.
4.5 Stability
One of the potential problems with this design could be the stability of the seat portion. Without
properly constraining the seat, one of two failure modes could exist. The first possible issue would
be that the weight of a person in the chair could cause the chair to continue opening too far.
However, the stops mentioned in the center rod alignment section should alleviate this problem, as
they will stop the chair from opening any further when the L-brace hits them. The other problem
that could arise would be that the chair will actually collapse inward when someone sits on it. This
too is resolved in the design, as the tension in the seat caused by the weight of a person is
counteracted by the reactionary forces in the frame of the chair. Because of this, the seat is
perfectly constrained, and will remain opened when sat on by the user.
5 Final Design
5.1 Dimensions of wheelchairs
Based on the analysis completed in the above section, Figure 19 shows the final design of the
frame of the new folding three-wheeled wheelchair. The length of this wheelchair is 32 inches, the
width is 24 inches, and the height is 32.2 inches.
Figure 19. Final design of folding three-wheeled wheelchair
5.2 Materials
The size of mild steel used in this design is dependent on the moments solved for in the previous
section, as this will determine the strength needed in the wheelchair. A safety factor of 5 is also
added to these values, just to make sure the wheelchair is very safe and durable, and so that in its
regular use it never approaches failure mode. For this wheelchair, two types of mild steel will be
used. One diameter and thickness will be used for the center rod, and another for the rest of the
frame. For the center rod, the largest bending moment found was 236.0 Nm. Applying the safety
factor gives a moment of 1190.0 Nm. To handle this, a pipe with a diameter of 48.25 millimeters
and a thickness of 1.5 millimeters will be used. For the rest of the frame, the largest bending
moment was 315.9 Nm, or 1579.5 Nm once the safety factor is applied. To support these
moments, the best choice of pipe is mild steel with a diameter of 60 millimeters and a thickness of
1.2 millimeters.
Besides the mild steel tubing for the frame, there are several other parts that are necessary for the
completion of this folding three-wheeled wheelchair. First of all, it will need three wheels. The
two larger wheels can consist of 26 inch bicycle wheels. The third will be a castor wheel. Because
bicycle parts are fairly easy to acquire in developing countries, especially in Africa, it is
advantageous to use as many bicycle parts as possible in manufacturing. This is why all bearings
in the wheelchair will also be bicycle bearings. The armrests and stops for the L-brace will be
made of a sturdy plastic, such as ABS or acrylic. Finally, fabric is needed for the seat and backrest.
To maximize the life of the seat, a sturdy fabric should be used, like canvas.
5.3 Total weight
Based on the material selection in the above section, the approximate total weight of this folding
three-wheeled wheelchair design can be determined. For all of the available sizes of mild steel,
Doshi Hardware in Tanzania lists the weight for 10 feet of tubing. Using these weights, the 32 inch
long center rod will weigh 2.768 kilograms. The weight for all of the mild steel for the rest of the
frame will be approximately 19.617 kilograms. These components will comprise the majority of
the weight of the wheelchair, with the weight of the fabric for the seat being negligible. The other
important weights to add in are the weights of the wheels as well as the ABS plastic that will
comprise the arm rests. Each 26 inch wheel will weight about 1 kilogram, and taking the density
of ABS to be 0.037 pounds per cubic inch, this wheelchair will have about 1.295 pounds of ABS.
This leaves the total weight of the new wheelchair design, in pounds, to be 55.1 pounds. While
this is about 10 pounds higher than the current design, it is important to note that this can only be a
maximum bound, as when estimates had to be made, they were over estimates, and not under
estimates. It is likely that the final weight of the wheelchair will be less than this, but it is definite
that it will not be greater than 45 pounds.
5.4 Total cost
To find the material cost to produce one folding three-wheeled wheelchair, the above materials
were priced at Doshi Hardware in Tanzania. This ensures that all materials used are locally
available in at least one developing country, and that the prices used are appropriate for where the
materials would be bought. The mild steel necessary for the center rod will cost $3.20 in US
dollars for 32 inches. For the rest of the frame, mild steel costs will total about $26.94. Both of
the 26 inch bicycle wheels will cost $6.24, brakes will cost $2.73, and the rest of the necessary
axles and bearings should amount to approximately $5.00, all in USD. While unable to find an
exact local costs for the front castor wheel, the ABS for the armrests, and canvas for the seat, costs
could be found in the United States at McMaster-Carr. Though there are many different available
materials and sizes for castor wheels, the ceiling for cost is about $20.00. This number will be
used in the calculation as it gives the absolute maximum material cost. The ABS will cost about
$28.14 and $3.00 worth of canvas will be sufficient for both the seat and backrest. Totaled, this
gives a final material cost of $89.01, well under the goal cost of $240.00.
5.5 Production methods
To assemble this entire wheelchair, very simple methods of production are necessary. Cutting
pipes to the correct size can be done using a variety of machinery, from a miter saw to a band saw
to even a hand saw. To correctly attach the pipes that make up the L-braces, they will need a notch
in them, which can be done with any type of angle grinder or band saw. Finally, all parts of the
wheelchair are welded together. All wheelchair workshops, even in developing countries, should
have the capacity to use these methods, and thus, should be able to produce this chair. The most
difficult part of production will be the alignment of the L-braces to the correct angles that
minimize the folded size of the wheelchair, but a jig can be constructed to enable repeatable and
accurate construction for every wheelchair.
6 Conclusion
6.1 Review of prototype
Without constructing a complete prototype, it is not possible to determine how well the final design
met all initial design criteria, but up to this point in the design process, here is a breakdown of how
the design compares to the initial parameters used to compare all design concepts. Any parameters
that can not yet be evaluated are marked with N/A.
Parameter
Evaluation of Design
Cost
Material cost of wheelchair is well under goal
Folded Size
Frame should fold to a dimension of 3.5 inches, not including
the width of the wheels on each side. (see below section)
Robustness/Durability
N/A
Functionality/Effectiveness
N/A
Ease of Use
Folding mechanism is very similar to that of traditional fourwheeled folding wheelchairs, so there should be a very small
learning curve.
Length of Life
N/A
Weight
Upper bound on weight of new design is 55.1 pounds, which is
10 pounds heavier than current design.
Modularity/Adaptability
N/A
Maintenance
N/A
Safety/Stability
Wheelchair is perfectly constrained, so it should remain open
when in use.
Complexity/Elegance
Only additional parts over those used in folding four-wheeled
wheelchair are center rod and front castor wheel.
Availability of Materials
Constructed mostly from mild steel and bicycle parts, both
widely available in developing countries.
Reliability
N/A
Use in Different Environments
N/A
Ease of Design
All manufacturing processes can be replicated in wheelchair
workshops in developing countries. Some potential difficulties
with alignment, but a jig could be developed to improve
alignment.
User Acceptability
N/A
Loose Parts
Table 5. Evaluation of Design
No parts of wheelchair taken off to fold.
6.2 Testing of prototype
(a)
(b)
Figure 20. First prototype of folding three-wheeled wheelchair
Using the correct dimensions solved for in the L-brace, a prototype was constructed to test the
ability of this design to fold to the correct dimensions. It must be noted that when constructed,
analysis was not complete on the forces and moments in the wheelchair, along with the center rod
alignment. Therefore, this prototype does not use the correct sizes for the mild steel piping, and
does not incorporate the plastic stops design. A whole wheelchair was also not built, just the basics
of the frame, in order to prove the ability of the folding mechanism.
(a)
Figure 21. First prototype in folded configuration
(b)
As can be seen above in Figure 21, the prototype easily folds into a smaller dimension. When
completely folded, it reaches a width of 3.5 inches. While the addition of wheels on each side will
increase this width, this is still a significant decrease to the original width of the wheelchair of 24
inches, and make it much more portable.
6.3 Unfinished work
Due to the time constraints of the semester, along with the difficulty of solving for all of the
relevant forces and moments within this wheelchair, a full prototype of the wheelchair was never
able to be constructed. Using all of the analysis done so far, the next step will be to construct the
wheelchair itself. A bench level experiment was performed to verify that the L-brace dimensions
chosen are optimal, but it would be best to support this experiment with results from a wheelchair
made of the correct materials and with the correct dimensions. This prototype had the correct
dimensions, but the material selection had not been optimized, so the materials are not those that
would be used in the actual wheelchair. This type of prototype can also be tested for strength,
durability, and stability, again supporting all of the analysis done up to this point.
Once constructed and tested, the next step will be to bring the wheelchair to a wheelchair
workshop in a developing country. This will allow for feedback from potential end users, which
can result in improvements to the design. An important aspect of the design which can not be
evaluated until this point is user acceptance of the design. While it may be important to design and
build a sturdy wheelchair that performs well, if the end user does not like the wheelchair and does
not want to use it, the rest is useless. Testing this in a developing country will be crucial to the
design of the wheelchair.
After these steps happen and any changes are made in the design, the next step will be to prepare
for production. Although it appears that all materials for the wheelchair are locally available in
developing countries, there is no better way to test this then to actually build the wheelchair in a
developing country using only materials that are found and bought locally. Currently,
manufacturing has been optimized for the production of one wheelchair at a time. Preparing for
production will involve optimizing this process so that many wheelchairs can be built. This will
likely involve making a jig for the frame.
Once this is all complete, the wheelchair will be ready to be built and distributed in developing
countries.
6.4 Possible improvements
The aspect of the wheelchair that could use the most improvement is the weight, as this design is
10 pounds heavier than the currently produced design. Reducing the weight is always important
for several reasons. First of all, a lighter wheelchair improves mobility for the user. It is more
difficult to maneuver a heavy wheelchair, especially when going uphill or through difficult terrains.
Also, a lighter wheelchair is better for transporting, as it can more easily be lifted or carried.
Finally, a light wheelchair is likely made from thinner tubing, which is probably less expensive,
thus the entire wheelchair is less expensive. Cost is of utmost important in developing countries,
so any method to decrease cost should be used.
A possible way to decrease weight is to build the frame out of trusses, similar to a bicycle frame.
Bicycles are extremely lightweight because trusses can handle extreme forces and stresses without
failure.
7 Acknowledgements
In helping me complete this project, there are so many people that I must thank for their
participation in or support of my success. First and foremost, there is no way that I would be able
to have completed this project without the help of Amos Winter. For two years now Amos has
played a part in my interest and ultimate completion of this project. Since being one of my trip
leaders for my D-Lab trip to Tanzania, Amos ignited my interest in developing countries, and more
specifically, in wheelchair design in developing countries. Then, as lecturer and lab instructor for
my group in the Wheelchair Design class, he helped the progress of the first iteration of this
design. And finally, this year he served as an advisor for my thesis, and helped me complete all of
the necessary analysis required to improve the wheelchair's previous design. Throughout the entire
process the help I have received from Amos has proved crucial to the project, and I sincerely thank
him for all of the support along the way.
Secondly, I would like to thank Professor Anette Hosoi for acting as my official professor thesis
advisor. Without her support I would also not have been able to finish.
A lot of the initial work on this project was completed in a team for Wheelchair Design in
Developing Countries, and I must thank the rest of my group from that class for all of the hard
work they put into the project during that semester. Neeharika Bhartiya, Andrea Blakemore, and
Becca Hung devoted countless hours to this project in Spring 2008, and within this group we were
able to prove the ability of this L-brace design concept, which then allowed me to go ahead and
improve on the design over the following year. This was an amazing group to work in, and I truly
appreciate the time and effort put forth by these wonderful women towards this project.
While we may have a lot of knowledge and resources here at MIT about engineering and design
principles, a very important aspect of this project was interaction with wheelchair workshops in
developing countries. With help from people working there, I was able to get a better sense of both
the capabilities of these workshops, as well as expectations from the end user. Two people in
particular, Daniel Namkessa of Mobility Care in Arusha, Tanzania and Abdullah Munish of KCMC
in Moshi, Tanzania, provided their expertise in this area and helped push the project along. Not
only did I meet with both Daniel and Abdullah in Tanzania, and communicated with them over email, but they both also visited MIT in Spring 2008 to help with the design of this wheelchair.
Their advice and knowledge allowed for consideration of aspects of the wheelchair that would
have been potentially overlooked, and I want to thank them for their input and contribution.
For sparking and continuing my interest in international development, MIT lecturer Amy Smith
must also be thanked. Through all of her hard work developing the D-Lab program at MIT, I have
been able to take advantage of so many opportunities, such as visiting Tanzania, and without this
program, I would have likely never pursued this field for my thesis.
I would also like to thank all of the Mechanical Engineering Professors at MIT with whom I have
interacted over my four years at MIT. Each has taught me a great deal and helped make me the
engineer I am today, and I have a great deal of gratitude for them all.
Last, but certainly not least, I would like to thank my family and friends for all of the support they
have shown me. They have always believed in me and helped me become the person I am today.
Without them, I would have never been able to endure the last four years at MIT and accomplish
all that I have. Each and every one of them is in part responsible for my success, and I thank them
all from the bottom of my heart for always believing in me.
8 References
McMaster-Carr. (2009). Retrieved May 2009, from: mcmaster.com.
Todman, Lindsay. (2008). Design of a Mobility Aid for Developing Countries.
Wang, Nathan. (2008). Design and Prototyping of a Retrofittable Motorized Module for Hand
Powered Tricycles for Developing Countries.
Wheelchair Design in Developing Countries. (2007). Retrieved May 2009, from MIT:
http://web.mit.edu/sp.784/www/
Wheelchair Design in Developing Countries. (2008). Retrieved May 2009, from MIT:
http://web.mit.edu/sp.784/www/
Winter, Amos. (2005). Assessment of Wheelchair Technology in Tanzania.
World Heath Organization. (2008). Guidelines on the Provision of Manual Wheelchairs in Less
Resources Settings.