If Michelin North America Inc. executives are on the mark with their latest projections, the next generation tire won't be a tire at all-it will be a ``Tweel,'' a non-pneumatic integrated tire/wheel unit those same executives are touting as revolutionary.
For now, though, the revolution is starting rather modestly-fitted on a Segway-inspired stair-climbing wheelchair.
``Major revolutions in mobility may come along only once in a hundred years,'' Terry Gettys, president of Michelin Americas Research and Development Center (MARC) said at a press conference at the recent North American International Auto Show in Detroit. ``But a new century has dawned, and Tweel has proved 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,'' he said.
The Tweel uses a network of elastomeric polyurethane spokes fused to a wheel hub and a circular outer flat rim to replace the casing, beads and sidewall structures as the load-bearing element. The outer surface of the Tweel rim is covered with a more conventional rubber tread and an underlying reinforcing belt.
Without the air needed by pneumatic tires, Tweel still delivers pneumatic-like performance in weight-carrying capacity, ride comfort and the ability to ``envelop'' road hazards, Michelin claims. The design is covered by at least two patents.
The first larger-scale commercial application could be for skid-steer and similar civilian and/or military vehicles, but a first-generation passenger car prototype has shown considerable potential as well, Mr. Gettys said, prompting the firm to go public with it. A commercially viable car Tweel, though, could be at least 10 to 15 years away, he cautioned.
The Tweel was designed and developed by engineers at MARC in Greenville, S.C., based on lessons learned from work on the firm's Zero Pressure run-flat pneumatic tires, said Bart Thompson, the lead engineer on the project at MARC.
Separately, Michelin engineers in Europe have developed another non-pneumatic tire concept, dubbed ``Airless.'' This one uses a series of polymeric rings arranged radially around a wheel hub, to which is attached a reinforcing belt/tread package.
The idea for the Tweel concept is not new, several industry executives pointed out, but its execution appears to be novel.
Spring-type wheel designs date to origins of the tire industry in the late 1800s, said Joseph Walter, adjunct professor of tire mechanics and vehicle dynamics at the University of Akron and a former Bridgestone/Firestone executive. What's new, though, is advances in materials development and engineering computing power that allow today's engineers to make the idea of the elastic wheel practical.
Mr. Walter said he believes the Airless concept actually has more promise, at least for larger vehicles.
The Tweel fitment unveiled in Detroit was for the iBOT mobility systems invented by Dean Kamen, the inventor of the motorized, gyroscopic-controlled, two-wheeled Segway scooter. The six-wheeled iBOT mobility device has the ability to climb stairs and navigate uneven terrain, offering mobility freedom impossible with traditional wheelchairs. Mr. Gettys said tire inflation is the No. 1 maintenance issue with wheelchairs.
The iBot user can even operate the chair balancing on just two wheels, bringing the rider eye-to-eye with other standing pedestrians. Additionally, Segway L.L.C.'s Concept Centaur, a prototype that applies self-balancing technology to a four-wheel device, also has been equipped with Tweel to increase its performance potential.
The Tweel automotive concept, as demonstrated at MARC on an Audi A4, is a ``stretch application with strong future potential,'' Mr. Gettys said. ``Our concentration is to enter the market with lower-speed, lower-weight Tweel applications. What we learn from our early successes will be applied to Tweel fitments for passenger cars and beyond.''
The company said it also has found that it can tune Tweel performance characteristics independently of each other. This means, for instance, that vertical stiffness-which primarily affects ride comfort-and lateral stiffness-which affects handling and cornering-can both be optimized, pushing the performance envelope in these applications.
The Tweel prototype, demonstrated on the Audi A4, is within 5 percent of the rolling resistance and mass levels of current pneumatic tires, Michelin said, translating to within 1 percent of the fuel economy of a conventional pneumatic original equipment fitment.
Michelin also said it has increased the lateral stiffness by a factor of five, making the prototype ``unusually responsive in its handling.''
For now the drawbacks include noise-the open spokes generate considerable noise-weight and higher rolling resistance, Mr. Gettys said. But the company is confident it can overcome these.
While practical applications of the Tweel on a road-going vehicle are still at least a decade away, the performance and maintenance implications are numerous.
For instance, Mr. Thompson said, a Tweel could be designed with holes or channels in the tread that would channel water through the tread, improving aquaplaning resistance considerably. In addition, the enlarged footprint could allow car makers to use a narrower tread Tweel for better rolling resistance without sacrificing braking or handling.
The appearance of the Tweel is the ultimate in low-profile, with just a half-inch or so of rubber tread showing. At speed a car with a Tweel appears to be floating as the spokes ``disappear,'' revealing the brakes and associated suspension hardware. Since the Tweel's spokes are made of molded polyurethane, they could be made any color, even chromed, Mr. Gettys said, for appearance.
As for Tweel maintenance, there would be precious little, he said, since there would be no need for mounting and/or balancing, and the Tweel design eliminates the possiblity of sidewall damage, wheel strike-through or pinch shock.
Theoretically, the Tweel could be retreaded relatively easily-the tread package is attached mechanically to the wheel-and put back into service, Messrs. Gettys and Thompson said.
Regarding the skid-steer variant, Michelin showed a video clip of two skid-steers-one with Tweel and the other pneumatic tires-doing spins. The Tweel-equipped one spins relatively effortlessly while the pneumatic-equipped one starts to hop and skip and has to stop. The firm also showed a video clip of a skid-steer driving over a land mine and then driving away, demonstrating the extended mobility possibilities of the Tweel for military applications.