Anyone who has ever driven a car run on a flat tire, but what if your tires could never go flat? In recent years a number of companies and inventors have been working on creating airless tires that would be impossible to puncture. Non-pneumatic tires (NPT), or Airless tires, are tires that are not supported by air pressure. They are used on some small vehicles such as riding lawn mowers and motorized golf carts. They are also used on heavy equipment such as backhoes, which are required to operate on sites such as building demolition, where tire puncture is likely. Tires composed of closed-cell polyurethane foam are also made for bicycles and wheelchairs. The main advantage of airless tires is that they cannot go flat, but they are far less common than air-filled tires.
The most well known design in this field is the Michelin Tweel, a combination wheel and tire. The design was one of the first to emerge, bringing the idea of non-pneumatic tires to the public's attention. But Michelin has been slow to roll out the technology beyond the test phase.in light of this; a company called resilient technologies has also been working on an airless tire. The company recently announced that prototypes of their honeycomb-like tires will ship in 2011 for use in the US military. This will no doubt help the airless tire field, as will other startup companies working on the task like Britek.
Non-pneumatic tires (NPT), or Airless tires, are tires that are not supported by air pressure. Airless tires generally have higher rolling friction and provide much less suspension than similarly shaped and sized pneumatic tires. Other problems for airless tires include dissipating the heat buildup that occurs when they are driven. Airless tires are often filled with compressed polymers (plastic), rather than air.
The Tweel (a portmanteau of tire and wheel) is an experimental tire design developed by the French tire company Michelin. The tire uses no air, and therefore cannot burst or become flat. Michelin is currently developing an integrated tire and wheel combination, the "Tweel” that operates entirely without air. Automotive engineering group of mechanical engineering department at Clemson University is developing a low energy loss airless tire with Michelin through the NIST ATP project. The Tweel would be the most radical change in the tire industry since the radial tire was invented nearly 60 years ago. Other changes include no more deadly blowouts. And in time, no more used-tire mountains or the need for as many shredding and recycling stations.
The Tweel is durable. Forget a nail, an armored vehicle with Tweel can go over an exploding landmine and keep moving. But for the car, the Tweel is still just a concept. On a test drive on tweels, a sedan handles well enough. The biggest problem is noise, once the speed hits about 50 mph. The noise gets worse the faster you go.
When the Tweel is put to the road, the spokes absorb road impacts the same way air pressure does in pneumatic tires. 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 tire's pneumatic properties. The tread and shear bands deform temporarily as the spokes bend, then quickly spring back into shape. Tweels can be made with different spoke tensions, allowing for different handling characteristics. More pliant spokes result in a more comfortable ride with improved handling.
The lateral stiffness of the Tweel is also adjustable. However, you can’t adjust a Tweel once it has been manufactured. You’ll have to select a different Tweel. For testing, Michelin equipped an Audi A4 with Tweels made with five times as much lateral stiffness as a pneumatic tire, resulting in very responsive handling.
Michelin reports that the Tweel prototype is within five percent of the rolling resistance and mass levels of current pneumatic tires. That translates to mean within one percent of the fuel economy of the tires on your own car. Since the Tweel is very early in its development, Michelin could be expected to improve those numbers.
THE FUTURE OF AIRLESS TIRES
The first large-scale applications may be in the military where a flat-proof tire would be advantageous. Military testing has indicated that the Tweel deflects mine blasts away from the vehicle better than standard tires 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 Tweel does have several flaws (aside from the name). The worst is vibration. Above 50 mph, the Tweel vibrates considerably. That in it might not be a problem, but it causes two other things: noise and heat. A fast moving Tweel is unpleasantly loud. Long-distance driving at high speeds generates more heat.
Another problem involves the tire industry. Making Tweel is quite a different process than making a pneumatic tire. The sheer scale of the changes that would need to be made to numerous factories, not to mention tire balancing and mounting equipment in thousands of auto repair shops, presents a significant (though not insurmountable) obstacle to the broad adoption of airless tires.