Houndmills Ind Est
Hants RG21 6YU
Tel: (01256) 477 333
Fax: (01256) 470 055
Booster Electric Vehicles
Holly Bank Mills
West Yorkshire HD3 3JB.
Tel: (01484) 643 444
Fax: (01484) 646 679
Meadow Lane Ind Estate
South Wirral L65 4TY.
Tel: (0151) 355 4393
Fax: (0151) 355 4171
17 Crosslands Road
Manchester M28 1JH.
Tel: (0161) 702 0238
Fax: (0161) 702 0238
4 Radley Road Industrial Estate
Oxon OX14 3RY.
Tel: (01235) 523 353
Fax: (01235) 531 019
Gerald Simonds Healthcare
9 March Place
Bucks HP19 3UG.
Tel: (01296) 436 557
Fax: (01296) 433 273
Bridgend Industrial Estate
Mid Glamorgan CF31 3PY.
Tel: (01656) 647 327
Fax: (01656) 649 016
Powys LD8 2UF.
Tel: (01544) 267 674
Fax: (01544) 260 287
Marshall Sports Chairs
Surrey. RH4 3AQ.
Tel: (01306) 877 653
Fax: (01306) 877 658
Garretts Green Lane
West Midlands B33 0SQ.
Tel: (0121) 785 0355
Fax: (0121) 783 5723
Unit 5a, Gatehouse Trading Estate
Walsall WS8 6JZ
Tel: (01543) 379 905
Fax: (01543) 376717
Ross & Bonnyman
Angus DD8 3DG.
Tel: (01307) 466 262
Fax: (01307) 468 119
Scandinavian Mobility UK
Weldon South Ind Est
Northants NN18 8AZ.
Tel: (01536) 267 660
Fax: (01536) 402 507
The Coach House
Dorset BH23 1EU.
Tel: (01202) 481 818
Fax: (01202) 476 688
Sunrise Business Park
West Midlands DY8 4PS.
Tel: (01384) 446 688
Fax: (01384) 446 699
Unit 1A, Standard Way
North Yorkshire DL6 2XE
Tel: (01609) 772 374
* Normally considered to be an indoor chair, but can be
used on a good, even, outdoor surface.
A wheelchair is a device used for mobility by people
for whom walking is difficult or impossible, due to illness or
It typically consists of a seat supported on two
large wheels on an axle attached towards the back of the seat
and two small wheels near the feet, though there are often small
additional features to prevent toppling or to assist mounting
curbs. The person moves by pushing with his/her hands circular
bars on the outside of the large wheels with a diameter that is
slightly less than that of the wheels, or by actuating motors,
usually with a joystick.
Experiments have also been made with unusual variant
wheels, like the omniwheel or the mecanum wheel. These allow more
directional movement options. Makes include Storm, Twister, Harrier
Disabled athletes use streamlined sport wheelchairs
for racing and basketball. Electric wheelchairs can be used as
part of adapted sports such as Wheelchair Soccer.
Adapting the built environment to make it more
accessible to wheelchair users is one of the key campaigns of
disability rights movements. For example, the construction of
ultra low floor trams and buses is being encouraged whereas the
use of paternosters in public buildings without any alternative
method of transportation has been criticized.
Rolling resistance is the first thing to overcome
to make a wheelchair roll easy. It depends on the surface the
wheelchair is driving on, mass distribution on the wheels, wheel
radius, total mass and specific tire characteristics
The most important external aspect in al of this
is the surface on which the wheelchair is moving. Indoors it’s
possible to make the floors hard and smooth. Outdoors there is
not much one can do to decrease resistance as a result of the
characteristics of the surface.
Slopes and obstacles
Other important elements are slopes and obstacles.
Indoors on can make adjustments like using slopes and taking away
obstacles. Outside this is a bit more difficult. Then one can
only adjust the wheelchair itself, like making the wheelbase longer,
so it will be easier to climb curbs.
Aspects in the wheelchair itself influencing the
manoeuvrability and rolling resistance are weight, handrim, camber
angle, the seat, back support and castors.
The weight of wheelchair and user together influence
the amount of rolling resistance the user had to overcome. Mass
distribution is also an important aspect. Most of the total weight
should lie over the rear wheels, yet not as much as causing the
wheelchair to tip backwards.
An optimal configuration of the wheels is an important
factor in overcoming rolling resistance. A larger distance between
rear wheels and castors decreases the pressure on the castors,
resulting in a lower rolling resistance. It is also important
to prevent toeing in and out of the rearwheels. Another factor
is the type of tires around the wheels.
Special attention should be given to internal friction
of the wheelchair caused by, for example, loose bolts and nuts,
sliding joints and non-elastic connections.
The handrim is an essential part of a wheelchair
for it is used to propel, brake, steer, negotiate obstacles and
manoeuvre. Important aspects in finding the optimal power transmission
from hand to handrim are shape, size, diameter, material and profile
of the handrim, and antropometry, squeezing force of the hand,
(dis)abilities and special wishes of the user. It should be noted
that propelling a wheelchair using handrims is physiologically
the least efficient way of propelling a wheelchair.
A large diameter of the handrim results in a relative
high mechanical efficiency and effective force. In case of propelling
a wheelchair over a long distance it is energetically favourable
to use a handrim with a smaller diameter.
The way of grabbing the handrim when propelling
influences the mechanical efficiency greatly. Also the friction
coefficient is of great influence. It should be as low as possible
in order not to brake the wheels while propelling, but it should
be high enough to make it possible to transmit a certain amount
of power from hand to handrim.
A camber angle has a positive influence on the
stability sideways, the power transmission from hand to handrim
and the manoeuvrability.
When using a camber angle, there is more risk for
toe-in or toe-out, more pressure on the rear wheal axle and the
complete wheelchair becomes wider. In general is the camber angle
for an ADL-wheelchair 2 to 4 degrees and for a racing wheelchair
between 4 and 12 degrees.
Position of the user
The most important aspects of the seat of a wheelchair
are the horizontal and vertical position of the user, because
they greatly influence the energy needed to propel the wheelchair.
In general it is best to position the centre of mass right above
the rear-wheel axle (horizontal position). In the vertical direction
the user should be positioned in a way that he can just touch
the rear wheel axle with his fingertips.
The position of the seat influences the accessibility
of the handrim, and therefore the efficiency of power transmission
from hand to handrim and the mechanical efficiency.
Castors are sensitive to forces exerted sideways.
It can cause them to shimmy. When castors are positioned in a
vertical position it is the easiest to make turns.
Each user has his own characteristics influencing
the efficiency of propelling a wheelchair and his own idea of
comfort. These characteristics are age, gender, figure, physical
health and (dis)abilities. In general it can be said that wheelchair
users don’t have much muscle mass in their arms and shoulder
girdle, which makes it extra hard to propel a wheelchair. Still,
a wheelchair should not keep a user from being mobile and of his
Wheelchair caster shimmy II: Damping
James J. Kauzlarich, PhD; Theodore E. Bruning III;
John G. Thacker, PhD
University of Virginia, Mechanical Engineering Department, Charlottesville,
VA; Compaq Computer Corporation, South Colorado Springs, CO
Address all correspondence and requests for reprints
to: James J. Kauzlarich, PhD, University of Virginia, Mechanical
Engineering Department, Charlottesville, VA 22903-2442; email:
Abstract--The theory of shimmy damping is investigated
including tire friction, spindle bearing friction, and hydraulic
damping. A new theoretical improvement in hydraulic damping is
presented. Experimental results are presented along with a discussion
concerning the limitations due to the approximations used in the
theory. The basic theory of wheelchair caster shimmy was published
by the authors in 1984, and an examination of the sources of shimmy
damping is corrected and updated in this paper.
Self-excited vibration is one of the most interesting
topics in the field of vibrations and is the science governing
caster wheel shimmy. Caster wheel shimmy can be experienced in
everyday equipment, such as wheelchairs, grocery carts, gurneys,
teacarts, and the like, and is universally recognized. Self-excited
vibration is characterized by vibration that is produced by the
motion of the system (e.g., wheelchair speed) itself. The flutter
due to the motion of turbine blades or aircraft wings is a good
example of this instability. In addition, machine tool chatter,
internal flow-induced vibration of piping, and cross flow-induced
vibration of wires and structures are treated under this topic
in modern vibration texts (1-3).
In 1984, the authors presented a paper on the subject of wheelchair
caster shimmy and turning resistance (4), with both theory and
experimental data. Since that time, manufacturers have considered
several methods for shimmy prevention, and the problem of shimmy
prevention is well in hand. However, the theory behind shimmy
prevention is not well known.
This paper presents applications of the basic theory
to show how shimmy prevention works in ultra-light and powered
wheelchairs. In addition, the theory leads one to new designs.
The theory explains why a trial-and-error design may or may not
work, and suggests possible new solutions.
Entire Thesis (External Site)