THE WHEEL
retain
suf cient
lateral
stiffness.
The second has been fastening
the bendy material to rigid metal hub and
rim, for which both a mechanical xture
and adhesives are used.
The company, which only employs
seven people, also assembles the wheels
(production capacity: 100/month); the
springs are also made nearby. It has
bene ted from £264,000 of public
funding for product development and pay
for underpinning research, carried out
in 2017 with Strategic Simulation and
Analysis (for spring development testing)
and Composite Braiding, as well as two
crowd-sourcing Kickstarter campaigns in
2013 and 2015.
Another cushioning wheel, intended for
motor vehicles, also had some big news
this year, after originally being brought to
market in 2012. Michelin’s Tweel (right)
is made of a special rim, polyurethane
spokes and a more or less conventional
hub – but without a tyre.
In June, Michelin signed a major
development agreement with General
Motors of the USA to develop a version
of the Tweel, to be called Uptis, for an
autonomous car model from 2024 (main
picture). “The rapid gains in autonomous
electric vehicles are an interesting
opportunity for Uptis,” says Tweel product
development engineer Chris Mast. Uptis
promises never needing to be pumped up,
or run the risk of a at or a blowout. “The
auto-makers want a maintenance-free
unit. With electric cars, you don’t have
to change the oil; their brakes can last a
lifetime. Tyres are the last maintenance
wear item on the vehicle. If we can offer
a maintenance-free tyre, we round out
the whole package.”
Conventional spoked wheels
don’t bounce because the spokes
tie the rim to the hub, keeping
it rigid. Anyone who has played
with a plastic hula hoop knows
how a circular ring of material can
act like a spring; throw it on the
ground side-on, and it will bounce,
oscillating between a horizontal and
vertical oval. This is too bouncy to
make a good car wheel.
But the way a Tweel is built
enables it to perform in an entirely
different way, even though it acts like
a semi-rigid mechanical spring. Its rim
consists of a layer of rubber sandwiched
between more rigid materials on the inside
and outside of the rim. Wound around
the circumference of outer and inner rim
is a so-called inextensible membrane
that prevents the rim from stretching or
compressing. When the hoop is de ected,
it forces the interior rubber layer to shear,
or stretch, transferring the load and
bending moment throughout the hoop
structure. Since the hub hangs from the
top spokes, the lower spokes and contact
patch are free to compress or deform. That
reduces the contact pressure of the wheel
on the ground, and over obstacles, similar
to a pneumatic tyre (pictured, below).
The stiffness of this type of structure,
known scienti cally as a Timoshenko
Beam, depends on the shear strength of
the rubber and the thickness of the beam.
Varying other design elements such as
architecture, type of reinforcement and
number of spokes can match the Tweel
design to suit different applications. But,
once designed, its behaviour does not
change. Admits product development
engineer John Duty: “That’s the best and
the worst thing about the Tweel. We design
its air pressure for life. We can’t alter it for
more comfort.” And ‘life’ really could be
much of the operational use of the parent
vehicle. Tweels have been found to last
three times as long as pneumatic tyres in
lawnmowing applications in the USA.
Other Tweel models are used for golf
carts, skid steer construction vehicles. And
they could be used in many other types as
well. The biggest challenge, reports Chris
Mast, is matching the performance and the
price. Although the designs are long-lived,
they also cost twice or three times as
much to manufacture as pneumatic tyres.
Work continues on ways to try to bring
down costs.
Rigid or
deformable
hub
Thin
deformable
spokes
Shear
beam
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/www.ied.org.uk