Michelin Tweels.
In 2005, Michelin unveiled their "Tweel" concept - a word made up of the combination of Tyre and Wheel. After decades of riding around on air-filled tyres, Michelin would like to convince us that there is a better way. They're working on a totally air-less tyre. Airless = puncture proof. The Tweel is the creation of Michelin's American technology centre - no doubt working with the sound of the Ford Explorer / Bridgestone Firestone lawsuit still ringing in their ears.
The Tweel is a combined single-piece tyre and wheel combination, hence the name, though it actually begins as an assembly of four pieces bonded together: the hub, a polyurethane spoke section, a "shear band" surrounding the spokes, and the tread band - the rubber layer that wraps around the circumference and touches the road. The Tweel's hub functions just like your everyday wheel right now - a rigid attachment point to the axle. The polyurethane spokes are flexible to help absorb road impacts. These act sort of like the sidewall in a current tyre. But turn a tweel side-on and you can see right through it. The shear band surrounding the spokes effectively takes the place of the air pressure, distributing the load. Finally, the tread is similar in appearance to a conventional tyre. The image on the right is my own rendering based on the teeny tiny images I found from the Michelin press release. It gives you some idea what the new Tweel could look like.
One of the basic shortcomings of a tyre filled with air is that the inflation pressure is distributed equally around the tire, both up and down (vertically) as well as side-to side (laterally). That property keeps the tire round, but it also means that raising the pressure to improve cornering - increasing lateral stiffness - also adds up-down stiffness, making the ride harsher. With the Tweel's injection-molded spokes, those characteristics are no longer linked. Only the spokes toward the bottom of the tyre at any point in its rotation are determining the grip / ride quality. Those spokes rotating around the top of the tyre are free to flex to full extension without affecting the grip or ride quality.
The Tweel offers a number of benefits beyond the obvious attraction of being impervious to nails in the road. The tread will last two to three times as long as today's radial tires, Michelin says, and when it does wear thin it can be retreaded. For manufacturers, the Tweel offers an opportunity to reduce the number of parts, eliminating most of the 23 components of a typical new tire as well as the costly air-pressure monitors now required on all new vehicles in the United States. (See TPMS below).
Another benefit? No spare wheels. That leaves more room for boot/trunk space, and reduces the carried weight in the vehicle.
Reporters who took the change to drive an Audi A4 sedan equipped with Tweels early in 2005 complained of harsh vibration and an overly noisy ride. Michelin are well aware of these shortfalls - mostly due to vibration in the spoke system. (They admit they're in extremely-alpha-test mode.) Another problem is that the wheels transmit a lot more force and vibration into the cabin than regular tyres. A plus point though is cornering ability. Because of the rigidity of the spokes and the lack of a flexing sidewall, cornering grip, response and feel is excellent.
There are other negatives: the flexibility, at this early stage, contributes to greater friction, though it is within 5% of that generated by a conventional radial tyre. And so far, the Tweel is no lighter than the tyre and wheel it replaces. Almost everything else about the Tweel is undetermined at this early stage of development, including serious matters like cost and frivolous questions like the possibilities of chrome-plating. Either way, it's a promising look into the future.
Tweels are being tested out on the iBot - Dean Kamen's (the Segway inventor) new prototype wheelchair, and by the military. The military are interested because the Tweel is incredibly resistant to damage, even caused by explosions. Michelin hope to bring this technology to everyday road car use, construction equipment, and potentially even aircraft tyres.
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