Most automotive technology enthusiasts will remember Michelin’s 2005 Tweel, the novel combination wheel/tire design that tried to take pressurized air out of the tire equation. Flexible polyurethane spokes in a wedge-pattern array supported an outer rim and took the role of shock absorber normally filled by the inflated tire tube.
Non-pneumatic tires would, of course, be immune to punctures. And although the flat-proof tire is an exciting notion, the concept’s initial incarnation reportedly suffered from excess vibration and noise at higher speeds.
Now two Japanese tire makers, Bridgestone and Yokohama Rubber, have taken up the challenge to develop airless tire designs. Most recently Bridgestone unveiled at the Tokyo Motor Show a prototype non-pneumatic car tire with recyclable thermoplastic ribs as load-bearing elements. A few months previously in the fall of 2011, Yokohama Rubber introduced its airless concept tire at a design expo in Japan.
Green prototype starts small
A spokesman for Bridgestone Corp. of Kurume, Japan, said that the concept tire offers comfort and performance suitable for golf carts, lawn mowers, vehicles for the elderly, and other low-speed buggies.
“We began our research by focusing on light loads so the initial exhibit uses an electric cart,” he said via an e-mail communication. “Golf cart applications are also being discussed.” The first prototype—a high aspect-ratio design—is, after all, only 9 inches in diameter.
“Looking ahead, we hope to develop the product for use on automobiles,” he said.
Besides invulnerability to air leaks, the prospect that these tires might be greener than standard tires was also cited. “Our ultimate goal is to create a viable, more environmentally friendly alternative to conventional tires,” the Bridgestone spokesman said, noting that the design team used thermoplastic ribs partly because that material is easier to recycle than the thermoset rubber used in conventional tires. Even the tire tread is made from a thermoplastic material.
“Furthermore, the tire can also be retreaded, keeping it on the road longer before end-of-life recycling is necessary,” he continued. And only the tread around the circumference would be disposed of when it gets worn out as opposed to an entire tire.
“We decided early in development to use thermoplastic resin,” the company spokesman said. “We chose it because thermoplastic resin is strong enough to handle the load while at the same time easy to mold via injection molding.”
The tires’ thermopolymer spokes radiate from rim to tread with two rows curving in opposite directions to the left and the right (as you look from the side) to support vehicle loads. The crisscrossing array forms what looks like a sort of rotary leaf spring suspension.
The Bridgestone spokesman said that “the spokes, which have a special curve, and the ring, which connects the spokes in the circumferential direction, support the load with their bending stiffness.” The resulting resilient structure provides flexibility equal to that of a pneumatic tire and high strength as well.
“To date in the current application, small carts, we haven’t found any notable issues related to drive, noise, or vibration,” he reported. “But as we move forward to develop a viable solution for automotive tires, there are many challenges that we will have to address including durability, both hot and cold temperature extremes, as well as handling and control at higher speeds.”
“Our sights are ultimately set on creating a viable, more environmentally friendly alternative to conventional tires,” he concluded, noting that “currently, we do not have a set date for when a product is expected to be launched. What’s most important is that when we come to market with a product, it is able to provide the same safety and performance attributes that come with conventional tires.”
Full-size airless tire under test
The other recent innovator in airless tire technology, Tokyo-based Yokohama Rubber Co., has developed a full-size (225/40R18) car tire. The “Youmyaku” prototype tire, which is said to have been inspired by youmyaku, the Japanese term for the vein network of a leaf, is part of the company’s “Nature Design” series of concept tires. A promotional video for the series that is available on YouTube shows the airless tire in operation on a car: www.youtube.com/watch?v=NIB6Nu8WSa4.
The design, whose configuration is more in the Tweel mode, features a relatively thin shock-absorbing annulus around a large center hub, making it resemble a low-profile tire. The annulus is filled with a honeycomb-like network of curving flexible ribs that absorb shocks. It is mounted on what looks to be a conventional tire rim.
The Yokohama design reportedly takes the airless concept a bit further by perforating the tread surface to enable water to evacuate not only in the plane of the road but radially inward toward the center of the wheel, thus significantly reducing or even eliminating hydroplaning hazards.
Although these two tire concepts clearly have a considerable way to go before seeing commercialization, the safety and convenience of never having to suffer from flat tires seems a market-worthy goal. Mechanical tires, in addition, could be designed to have high lateral strength to afford better handling with no loss in comfort, say designers, because the stiffness properties of the ribs in the vertical and lateral directions can be tuned independently.
But until then, the non-pneumatic tire may be limited to carts, buggies, and a few NASA planetary rovers. The first large-scale airless tire applications may well emerge in the ranks of the military, where a flat-proof tire—perhaps even a landmine-resistant one—would be very useful to troops everywhere.