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Tweel
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.
Design
The 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.<ref>http://auto.howstuffworks.com/tweel-airless-tire.htm</ref>
Benefits and drawbacks
Potential benefits of the Tweel include the obvious safety and convenience of never having flat tyres. Also, the concept has the potential for true performance gains. Eventually it may be able to outperform conventional tyres since it can be designed to have high lateral strength (for better handling) without a loss in comfort since the design of the spokes allows the vertical and lateral stiffness to be tuned independently. Because only the tread around the circumference would be disposed of when worn as opposed to a whole tyre, the environmental impact should be less. The Tweel can also withstand a police 'stinger' spike strip, making it hard for law enforcement to catch a suspect in a vehicle equipped with Tweels.
The Tweel prototype, demonstrated on an Audi A4, is within five percent of both the rolling resistance and mass levels of current pneumatic tyres, which means the fuel economy should be within one percent of the OE fitment. Additionally, the test tyre on the demonstration Audi has lateral stiffness five times that of the OE tyre, providing exceptionally responsive handling.
The Tweel does have several flaws, however, the worst being vibration. Above 50 miles per hour (80 km/h), the Tweel vibrates considerably. That in itself is a big problem, but it also causes two other things: noise and heat. A fast moving Tweel is unpleasantly loud.<ref>http://www.cbsnews.com/stories/2005/01/27/eveningnews/main669888.shtml</ref> Also, long-distance driving at high speeds generates more heat than Michelin engineers would like, which might lead to premature failure.
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.<ref>http://www.inautonews.com/nasa-rover-vehicle-equipped-with-michelin-tires</ref> This has resulted in the Lunar Rover Initiative AB Scarab wheels.<ref>LRI AB Scarab wheels</ref>
