AIRLESS TYRES
AIRLESS TYRESINTRODUCTIONFor more than 100 years, vehicles have
been rolling along on cushions of air encased in rubber. The
pneumatic tyre has served drivers and passengers well on road and
off, but a new design by Michelin could change all that the tweel
airless tyre .The tweel (a portmanteau of tyre and wheel) is an
experimental tyre design developed by the French tyre company
Michelin. The tyre uses no air, and therefore cannot burst or
become flat. Instead, the Tweel's hub connects to flexible
polyurethane spokes which are used to support an outer rim and
assume the shock-absorbing role of a traditional tyre's pneumatic
properties.
PROBLEM IN CONVENTOINAL TYRELOWER ROLLING RESISTANCE The
increasing concerns over the green-house effect will in the near
future require more attention to rolling resistance than ever
before; in fact from an already high attention to a very high
attention. The trend towards lower rolling resistance has been
obvious for many years. Significant progress was reported in the
recent Tyre Energy Efficiency Report in reducing rolling
resistance, as measured for new passenger tyres, over the past 25
years. More tyre models today, when measured new, have rolling
resistance coefficients below 0.009, and the most energy-efficient
tyres have coefficients that are 20 to 30 percent lower than the
most energy efficient radial models of 25 years ago [TRB 286,
2006]. Another trend is the increased popularity of run-flat tyres;
mostly having stiffer sidewalls or some material added that can
avoid running a flat tyre on the rim. The above-mentioned Tyre
Energy Efficiency Report concluded that run-flat tyres weigh more
than conventional radial tyres which increases their material and
production cost and they tend to exhibit higher rolling
resis-tance. This author thinks that this may turn the trend back
to more traditional designs, or turn the interest over into designs
which have run-flat capabilities without increased rolling
resistance. The increasing popularity and more frequent
governmental support for hybrid or electric veh-icles will also
require lower rolling resistance since this directly affects the
distance one can run in the electric mode. Finally, it shall be
mentioned that labeling of energy efficiency (in practice rolling
resistance) of tyres is likely to happen in the near future. The
intention is that consumers will use this informa-tion to their
selection of replacement tyres; per-haps even vehicle manufacturers
would use such information when deciding on OE tyres if this
information will be available for the full range of tyre brands and
dimensions and not only be determined by themselves for a few
tyres. A conference organized by the IEA in November 2005 [IEA,
2005] indicated a rather universal support for the labeling of
energy efficiency and also the Tyre Energy Efficiency Report
suggested this. INCREASING CONCERN FOR LOW NOISE AND ROLLING
RESISTANCE NECESSARY Both rolling resistance and noise emission are
expressions of energy losses in the rolling of tyres. It is not
surprising that these characteristics are at large positively
correlated; although exceptions exist. Nevertheless, it is this
author's conclusion that exterior noise and rolling resistance will
drive the tyre development to a large extent in the coming years
[Sandberg, 2003]. Probably, the present focus on high-speed and
high-power performance, which both are in some conflict with low
noise and rolling resistance (and thus air pollution), will at last
have to give in to the latter performances. Another present trend
is the high priority put on the visual appearance of tyres, as a
selling argument; in particular for "sporty" vehicles. The styling
trend was heavily criticized recently as being in conflict with
good technology by one of the foremost tyre experts in the world,
Dr Joe Walter, in a column in Tire Technology Interna-tional
[Walter, 2006]. It is likely that this trend will be broken when it
is in conflict with the increasing environmental demands. Vehicle
manufacturers will have to face the possible effects of this which
may be uncom-fortable to some.
OPTIONS AVAILABLE WITH TRADITIONAL TECHNOLOGY Using traditional
technology, the author suggests the following options as a few
examples for reduction of noise: Adapting winter tyres for all-year
use: The principles used in construction of winter tyres may be
partly adapted to summer tyres; in order that summer tyres may
obtain some of the favourable noise characteristics of winter
tyres; yet having handling and wet friction properties acceptable
for summer use. This may include using smaller tread elements, more
frequent siping and softer rubber compounds. some compromises like
these mentioned above are already seen in the all-weather designs
being so popular in the USA. Can some winter tyres even be used the
entire year? To answer this question, it is interesting that the
author knows some tyre experts working for tyre companies who use
"pure" winter tyres all the year. This is not to say that all or
most winter tyres would be suitable also for summer use, but it
suggests that at least some of them are so; probably with some
sacrifices, for example wear. Reducing the air/rubber ratio in the
tread pattern: In the SILENCE project one of the possibilities
being explored is the reduction of the air/rubber ratio in the
tread pattern; for example by reducing the width of channels in the
tread pattern. It has been found that a combination of softer
rubber and lower air/rubber ratio may influence tyre/road noise
emission on an ISO surface by about 6 dB(A). If, todays common
ratio of 30 % is replaced with 20 % this would give a potential
noise reduction of 3 dB(A). Work will continue; for example to see
how a reduction from 30 to 20 % may be combined with acceptable
hydroplaning characteristics (this may be difficult for
high-performance cars). Using softer rubber compounds: Typically,
winter tyres may have a Shore hardness of 55-60. It has been well
demonstrated that softer rubber compounds result in lower noise
emission, other things being equal. If tyres did not have to be
produced for such high speed categories as today, softer compounds
may be used. Softer tyre rubber compounds are already used in Japan
and in USA, but in Europe they are considered less acceptable due
to the high maximum speeds on certain motorways. If, for example,
the green-house effect will force also Europe to introduce maximum
speed limits on all motorways, the situ-ation might approach that
in Japan and USA. THE OPTIONS FOR LOWER ROLLING RESISTANCE The
examples above have potentially lower rolling resistance in common
to the lower noise emission. However, the rubber compound is of
extra importance here and additions such as silica mean progress to
this performance parameter. THE QUIET TYRE WITH NO MARKET An
example of a successful noise reduction design was presented in
[Saemann et al, 2001]. Dr Saemann and his colleagues had produced,
by means of traditional measures, a truck tyre that was equally
quiet as a slick tyre. However, al-though the tyre had fully
acceptable properties in other respects than noise, it was found
that this tyre was not desired or needed by the vehicle industry,
partly due to its visual appearance, partly due to that there was
no need for any quieter tyre by the vehicle industry. This author
thinks that such neglect of quiet designs will be impossible in the
future. UNCONVENTIONAL TECHNOLOGY AND INNOVATIONS The pneumatic
tyre provides a rolling performance in most important respects that
is amazing. Only a minor defect may demonstrate that this
performance is not a matter of course but a result of a sensitive
design. But this does not go without saying that the pneumatic tyre
is the only useful device that could provide a safe, quiet and
economic rolling for a vehicle. If a mere fraction of all the
resources spent on tyre development so far would be spent on, for
example, development of the composite wheel or the so-called TWEEL
(see below), what can one achieve then? An interesting editorial
appeared in Tire Techno-logy International recently. It was written
by the former Director of Research at Dunlop Tyres in the UK, Dr A.
R. Williams. He wrote [Williams, 2005-a]: What is standing behind
the corner? Are there some tyre innovations or unconventional
designs that may offer a breakthrough or at least a large step
towards lower noise emission and rolling re-sistance? The following
describes a few examples of such attempts currently being
explored.DESIGN OF TWEEL AIRLESS TYRESThe Tweel consists of a
cable-reinforced band of conventional "tyre" rubber with molded
tread, a shear band just below the tread that creates a compliant
contact patch, and a series of energy-absorbing polyurethane
spokes. The rectangular spokes can be designed to have a range of
stiffnesses, so engineers can control how the Tweel handles loads.
The inner hub contains a matrix of deformable plastic structures
that flex under load and return to their original shape. By varying
the thickness and size of the spokes, Michelin can generate a wide
array of ride and handling qualities. The tread can be as
specialised as any of today's tyres and is replaceable when
worn.
HOW IT WORKSThe Tweel doesnt use a traditional wheel hub
assembly. A solid inner hub mounts to the axle and is surrounded by
polyurethane spokes arrayed in a pattern of wedges. A shear band is
stretched across the spokes, forming the outer edge of the tyre. On
it sits the tread, the part that comes in contact with the surface
of the road. The cushion formed by the air trapped inside a
conventional tire is replaced by the strength of the spokes, which
receive the tension of the shear band. Placed on the shear band is
the tread, the part that makes contact with the surface of the
road. When the Tweel is running on the road, the spokes absorb road
defects the same way air pressure does in the case of pneumatic
tires. The flexible tread and shear bands deform temporarily as the
spokes bend, then quickly go back to the initial shape. Different
spoke tensions can be used, as required by the handling
characteristics and lateral stiffness can also vary. However, once
produced the Tweels spoke tensions and lateral stiffness cannot be
adjusted
Historically SignificantMajor revolutions in mobility may come
along only once in a hundred years," said Terry Gettys, president
of Michelin Americas Research and Development Center in Greenville,
S.C. "But a new century has dawned and Tweel has proven its
potential to transform mobility. Tweel enables us to reach levels
of performance that quite simply aren't possible with today's
conventional pneumatic technology."The most intriguing application
may be Michelin's early prototype Tweel fitment for passenger cars.
The mobility company released video of promising Tweel performance
on an Audi A4.The Tweel automotive application, as demonstrated on
the Audi, is definitely a concept, a stretch application with
strong future potential.The History of automotive tyre
development1844 - Charles Goodyear invents vulcanised rubber1845 -
Robert William Thomson patents vulcanised rubber pneumatic tyre but
was too costly and doesn't catch on1888 - John Dunlop invents
pneumatic tyres for bicycles.1889 - A Belfast Cycle Race was won on
pneumatic rubber tyres beginning public awareness. Unfortunately
the original tyre was glued to the wheel, making it difficult to
access the inner tube1890 - CK Welsh patents the design of a wheel
rim and outer cover with inextensible lip.1895 - Andre Michelin
uses pneumatic tyres on an automobile (unsuccessfully).1903 - Paul
Weeks Litchfield patents tubeless tyre. He rose to become the
chairman of Goodyear in the year 1940.1904 - mountable rims were
introduced that allowed drivers to fix their own flats.1908 - Frank
Seiberling invented grooved tires with improved road traction.1910
- B.F. Goodrich invents longer life tires by adding carbon to the
rubber.1911 - Philip Strauss invents the first successful
automobile tyre, which was a combination tyre and air filled inner
tube. Strauss' company the Hardman Tyre & Rubber Company
marketed the tires.1937 - B.F. Goodrich invents the first synthetic
rubber tires and patents a substance called "Chemigum"1948 -
Michelin patents the radial tyre1954 - first original equipment
tubeless tyre fitted to the now defunct Packard.
Advantages One of the greatest advantages of this technology
would be the fact that the tyre is service-free. No more air
pressure check, no more flat tires and no more blow-outs mean a lot
less to worry about when driving car. It is also conceived to last
longer. Also, the balancing between traction and comfort could
become a thing of the past. Thats because Michelin has found that
it can tune Tweel performances independently of each other, which
is a significant change from conventional tires. This means that
vertical stiffness (which primarily affects ride comfort) and
lateral stiffness (which affects handling and cornering) can both
be optimised, pushing the performance envelope in these
applications and enabling new performances not possible for current
inflated tires.It doesnt require maintenance and it is risk-free,
the Tweel tyre could be a good choice for special vehicles like
those used in the army, in the construction business or even in the
exploration of other planets. In 2009, Michelin has developed for
NASA a Tweel-based tyre to be used in the latest generation of
lunar rover vehicles. The Michelin Lunar Wheel maintains
flexibility and constant ground pressure, allowing the vehicle to
move through loose soil and craters. In addition, it combines low
mass and high payload capacity, making it 3.3 times more efficient
than the original Apollo Lunar Rover wheels. Its textile tread
enables the rover to maintain traction at very low
temperatures.Tweel technology could also penetrate the personal
mobility market. At the public demonstration of the Tweel, Michelin
placed prototypes on the iBOT, a personal mobility device for
physically impaired people, and the Segway Centaur, a four-wheeled
ATV-type vehicle that uses Segways self-balancing technology.
DISADVANTAGESIt is not the perfect tire. At least not yet. One
of its biggest flaws is vibration. Above 50 mph, the Tweel vibrates
considerably, thus generating noise and heat. A fast moving Tweel
is reportedly unpleasantly loud. Long distance driving at high
speeds generates more heat than Michelin engineers would like.
Thats why, for the moment, the first applications of the Tweel are
in low-speed vehicles, such as construction vehicles. The Tweel is
perfect for such use because the ruggedness of the airless design
will be a major advantage on a construction site. Michelin is also
exploring military use of the Tweel, which would be ideal in combat
situations, where conventional tyres are an easy target. Another
big obstacle in the Tweels way is the tire industry itself. Making
Tweels is quite a different process than making a pneumatic tire.
The retooling of the many tire factories, plus the equipment
necessary to service the new tire around the world represents also
an important obstacle to the broad adoption of airless tires.
Because of these drawbacks, Michelin is not planning to roll out
the Tweel to consumers any time soon.Last but not least, another
challenge for the Tweel could be the drivers themselves who would
see their beloved radial tires and rims replaced by a not so good
looking Tweel. Of course, Michelin could place some covers to hide
the spokes, but the psychological impact on the consumer should not
be neglected. It might be the inventor of the Tweel, but another
company is working on a similar project. Resilient Technologies is
developing their own airless tire, known as the NPT (non-pneumatic
tire). That company is using a more aggressive development and
marketing strategy aimed at military use. The NPT is based on a
different configuration of spokes, but the general idea is the same
as Tweel's.Applications
Given the high speed problems with the Tweel, the first
commercial applications will be in lower-speed, lower-weight
vehicles such as wheelchairs, scooters, and other such devices. The
iBOT mobility device and Segway's Concept Centaur were both
introduced with Tweels. Michelin also has additional projects for
Tweel on small construction equipment, such as skid steer loaders,
for which it seems well-suited.The first large-scale applications
may be in the military where a flat-proof tyre would be
advantageous. Military testing has indicated that the Tweel
deflects mine blasts away from the vehicle better than standard
tyres and that the Tweel remains mobile even with some of the
spokes are damaged or missing.NASA has contracted Michelin to
develop a wheel for the next generation Lunar Rover based on the
Tweel. This has resulted in the Lunar Rover Initiative AB Scarab
wheels.The first large-scale applications may be in the military
where a flat-proof tyre would be advantageous. Military testing has
indicated that the Tweel deflects mine blasts away from the vehicle
better than standard tyres and that the Tweel remains mobile even
with some of the spokes are damaged or missing.NASA has contracted
Michelin to develop a wheel for the next generation Lunar Rover
based on the Tweel. This has resulted in the Lunar Rover Initiative
AB Scarab wheels.Future of Tweel Technology:For Michelin, Tweel is
a long-term vision that represents the next step in a long path of
industry-changing innovations. Fifty years ago, Michelin invented
the radial tyre and there is no question that radial tyre
technology will continue as the standard for a long time to come.
Michelin continues to advance the performance of the radial tyre in
areas such as rolling resistance, wear life and grip.In the
short-term, the lessons learned from Tweel research are being
applied to improve those conventional tyre performances. In the
future, Tweel may reinvent the way that vehicles move. Checking
tyre pressure, fixing flats, highway blow-outs and balancing
between traction and comfort could all fade into memory.CONCLUSIONS
It is concluded that tyres featuring low noise and low rolling
resistance will be required in the near future and that the
interest in and need for im-proved characteristics in this respect
will receive much more attention and priority in the tyres of the
next 10 years than for present market tyres. If the climate changes
will force a sudden and dramatic change in transportation and
vehicle emissions policies, which is not an unlikely scenario, the
tyre and vehicle manufacturer who fails to consider unconventional
solutions may suddenly find itself in an inferior position to the
one who can see and actually explore the possibilities of new
technologies. There are possibilities to reduce noise and rolling
resistance further than today by traditional tyre design measures;
in particular if the extreme high-speed demands (speeds in excess
of 200 km/h) can be abandoned. It is further concluded that there
are several possi-bilities for a breakthrough in tyre design for
low noise and low rolling resistance within the next 10 years or
so, provided sufficient resources are spent on developing the
concepts presented above.REFERENCES Sandberg, U.; Ejsmont, J. A.;
Kropp, W. and Larsson, K. (2003): "Low noise tires A co-operation
project in northern Europe", Paper N494, Proc. of Inter-Noise 2003,
Seogwipo, Korea. Sandberg, Ulf (2003): "The road to quieter tires".
Article in Tire Technology International '03, Annual Review 2003,
UKIP Media & Events, Dorking, Surrey, U.K.
12 DEPARTMENT OF MECHANICAL ENGINEERING