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01/01/2005
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If Michelin’s radical Tweel ever reaches the production road vehicle, then it will be in the distant future — certainly not for at least 15 years. But in reality that’s only three car ‘generations’ away and well within the period covered by the industry’s most forward-thinking designers and engineers. Roger Bishop reports
The pneumatic tyre has been around for almost as long as the motor car itself and, during that time, new knowledge, materials and industry demands have dramatically changed its characteristics and performance. But will it always be the best solution? Technology purists would argue to the contrary, perhaps finding it more remarkable that it has lasted this long.
That’s what makes the airless Tweel (tyre and wheel) development so interesting; it challenges convention with a technology that has the potential to re-write the rules of traction, stability and comfort. It’s puncture-proof, too. But known, safe, conventional engineering exerts a powerful influence in the automotive sector and Michelin will have to make its case a very convincing one.
Developed at Michelin Americas Research and Development Centre in Greenville, South Carolina, the Tweel has for the first time been fitted to a production car—an Audi A4—and engineers report very promising performance.
Essentially the Tweel replaces the tyre and wheel with a composite reinforced tread band, connected to a wheel that is both flexible and deformable via rectangular polyurethane spokes. The structure has the weight-carrying ability, shock absorption, ride comfort and rolling resistance characteristics of a pneumatic tyre but also has suspension-like properties that are said to greatly improve handling.
The flexible spokes and wheel rebound with exceptional ease and development engineers have found that they can tune and optimise vertical stiffness (which primarily influences ride and comfort) and lateral stiffness (which affects handling and cornering) independently. This is impossible with an inflated tyre and opens up new opportunities for chassis engineers.
The rolling resistance and mass of the Tweel prototype fitted to the A4 are said to be within 5 per cent of current pneumatic tyres, delivering a fuel economy that is within 1 per cent of the OE fitment. However, lateral stiffness is increased by a factor of five making the prototype vehicle “unusually responsive”.
Terry Gettys, president of the Greenville centre, says the Audi automotive application is a “stretch application with strong future potential”.
“Our concentration will be on entering the market with lower-speed, lower-weight Tweel applications. What we learn from our early successes will be applied to fitments for passenger cars and beyond.”
The Tweel is in production and available as an enhancement for future iBOT mobility systems. Invented by Dean Kamen, iBOTs have the ability to climb stairs and navigate uneven terrains, massively extending the capability and usage of wheelchairs. Additional ongoing projects include construction skidsteers and a variety of military vehicles.
“Tweel enables us to reach levels of performance that quite simply aren’t possible with today’s conventional pneumatic technology,” says Gettys. In the meantime, the lessons learned from the project are being applied to improve conventional tyre programmes.
Michelin has been busy in some other areas of wheel, tyre and suspension development as our panels show.
Wheel integrates traction and active suspension
The 850kg, four-seater Hy Light fuel-cell-powered concept car shown at Challenge Bibendum features another Michelin technology first seen in Paris — the Active Wheel. Two are fitted to the front of the vehicle.
Each Active Wheel has an integrated active suspension system, disc brake and 40bhp permanent magnet motor. There are no mechanical connections to the energy source, allowing development engineers to remove a number of components including the clutch, gearbox, transmission, anti-roll bar and shock absorbers. Anti-pitch and anti-dive geometry are part of the design.
According to Michelin, vehicles fitted with this technology will be able to maintain a stable chassis while turning because body and wheel angles can adopt motorcycle geometries. Body height can also be adjusted between 10 and 35cm.
Treaded ‘fly trap’ arches support vehicles
Like Tweel, the Airless tyre — first seen at the 2004 Paris Motor Show — also abandons the use of pressurised air. Instead, it uses a technology nicknamed ‘fly trap’ invented in France by Michelin research engineer Marius Mignol.
Described as a “radial structure made of high-performance composite materials onto which a rubber tread is bonded”, it crucially includes metallic arches spaced every 15mm around the tyre. The supporting structure is designed to last the lifetime of the car itself. The tread, when it due course it wears down, is stripped off and a new one bonded in place.
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Author
Roger Bishop
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