Reinventing the wheel – the airless tire
November 19, 2008 One of the more fascinating developments in the history of the automotive tire is the modern concept of the airless tire. Dunlop produced the first pneumatic tire for bicycles in 1888 and Michelin did likewise for cars in 1895, and for the last century, pneumatic tires have ruled. Michelin announced its airless Tweel technology three years ago and won the Intermat Gold Medal for Innovation in 2006, though we have yet to see a commercially available automotive product from the French giant. Now a new airless tire using a flexible, honeycomb-like internal structure could again prove to be a disruptive technology in one of the world’s largest industries. With development funded by the U.S. DoD, the initial aim of the project was to replace the Achilles heel of the military vehicle, but now the technology looks like going commercial for the rest of us.
Wisconsin-based start-up Resilient Technologies LLC is working on a four-year, US$18 million project with the U.S. DoD (Department of Defense) and the University of Wisconsin-Madison to research and develop a non-pneumatic tire for use on heavy-grade military vehicles such as Humvees. Tires have proven to be the weak link in Humvees, which can be immobilised by the scourge of the 21st century urban battlefield, the IED (Improvised Explosive Device). Once the mobility of a Humvee is compromised in a hostile environment, its inhabitants’ life expectancy closely approximates zero. As neither Michelin’s Tweel nor Resilient’s Airless tire do not have a tire-like pressurized air cavity, they cannot be punctured, thus assuring continuous mobility.
"You see reports all the time of troops who were injured by an IED or their convoys got stranded because their tires were shot out," says Mike Veihl, general manager of Resilient. "There's all sorts of armor on the vehicle, but if you're running in the theater and get your tire shot out, what have you got? You've got a bunch of armor in the middle of a field."
The company has made remarkable strides in just over two years of operations, cycling through literally hundreds of prototypes, developing subscale airless tires for lawn tractors, and finally the featured product: In April, Resilient installed a set of its creations on a Wausau-based National Guard Humvee, where it is undergoing rigorous on- and off-road tests.
Company reps say that Resilient's partnership with UW-Madison's Polymer Engineering Center (PEC) has played a major role in setting the high-speed development pace. The center serves as a subcontractor in the project and provides two graduate students under the general direction of mechanical engineering professor Tim Osswald.
In addition to conducting basic polymer research, the PEC works with dozens of companies, big and small, on materials testing and product development, says Osswald. The Resilient project presented one of the more complicated challenges his lab has seen, given the complete rethinking taking place in the design and the high levels of performance the tire must meet.
The Wisconsin design breakthrough, first developed by Resilient's in-house design and development team, takes a page from nature. "The goal was to reduce the variation in the stiffness of the tire, to make it transmit loads uniformly and become more homogenous," Osswald says. "And the best design, as nature gives it to us, is really the honeycomb."
Osswald and graduate students Nick Newman and Eric Foltz ran tests and simulations that helped Resilient confirm the quality of its unique design concept. They also studied other airless tire designs, including Michelin's "Tweel," to determine their properties compared to the Resilient design.
The patent-pending Resilient design relies on a precise pattern of six-sided cells that are arranged, like a honeycomb, in a way that best mimics the "ride feel" of pneumatic tires. The honeycomb geometry also does a great job of reducing noise levels and reducing heat generated during usage - two common problems with past applications.
"We definitely brainstormed," says Foltz. "We wanted to create more of a matrix of cells within the tire, and it seemed kind of natural to go with the honeycomb's hexagon shape. We tried some other shapes, such as diamond shapes, and they didn't perform as well."
Adds Newman: "Now that we've done this, it's amazing to think that we were going from literally sketching designs on a piece of paper in June 2006 to having actual Humvees riding around on prototype tires in April 2008. In under two years, really functional tires were created."
Veihl says Resilient has developed into a full-service operation, with in-house facilities that can develop new materials and run them through a battery of physical and environmental tests. This month, the company is in the process of installing a massive flywheel device called an Akron Standard road wheel, which can inflict wear and tear on prototype tires, simulating hundreds of thousands of miles at interstate speeds.
But the UW-Madison experience is vital to the continuing project, Veihl says. The group holds weekly teleconferences and Osswald has spent time at the company providing high-level polymer course work for Resilient's engineers. Many of the same tests done in Wausau were done in parallel at UW-Madison to further validate outcomes.
"Number one, you have students who are fresh thinkers, they have no preconceived notion about what is the right answer or wrong answer," he says. "And (Osswald) has given us the ability to dive into a lot of things much more quickly, because he's seen so many things that we haven't in working with these materials."
With Foltz and Newman graduating, Osswald will continue the partnership with two new graduate students. One project will be to evaluate sidewall designs, which will give the tire a more conventional look.
Veihl shares a strong enthusiasm for the company and what it could mean to the Wisconsin economy. While the military application is the most urgent primary market, Veihl says the tire has potential for virtually any vehicle where a flat tire causes significant headaches. That includes ATVs, mining equipment, farm machinery and construction equipment. The passenger automobile industry may be on the horizon as well.
But right now, Veihl is concentrating on his customers at the National Guard, many of whom have seen tours of duty in Iraq and offer invaluable advice. "They will tell you the real deal, and they're not shy about it. If we can develop a product that satisfies their requirements, then we've done our job."
U.S. Rep. Dave Obey, D-Wis., was instrumental in getting Department of Defense support for the company, which is affiliated with another company in his district, WADAL Plastics.
"Developing tires that will allow vehicles to continue to roll no matter what is thrown at them - even roadside bombs - is a real priority for the military and could be the difference between life and death for our troops in the field," says Obey.
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This design looks much better than Michelin\'s. The Tweel relies on its spokes compressing and stretching, causing lots of heat buildup and fatigue. In contrast this design uses a gentler bending deformation, relying more on support from below than tension from above to support the load.
Compare to a spoked bicycle wheel. The load is actually suspended by the spokes from the top of the wheel. The spokes have little strength in compression. The Tweel and conventional tires operate on the same principle, their spokes and sidewalls deform easily under compression.
Resilient Technologies\' design is closer to a wooden spoked wheel in concept. Those had spokes with a high compression strength that directly supported the load from the rim at the bottom up to the hub. Metal tired versions on animal drawn wagons are dished slightly and the spindles are angled down slightly so as the spokes rotate to the bottom they\'re vertical. The dishing increases the sideways strength of the wheel.
The early \'artillery wheel\' designs for motor vehicles weren\'t dished but used more massive spokes and hubs plus bolts to withstand side forces.
I\'ve some ideas on possible improvements for Resilient Technologies\' designs, if they want to contact me.
People are going to think that these wheels will be bullet-, pothole-, cut-, delamination-, fire-, and pavement-scratch-proof, which they won't be. They will be puncture-proof but, other than that, will not be the final answer to all things rolling.
I'm sure the current composite technology and design capabilities is adequate to handle as many fatigue cycles as can be thrown at the wheel as well as any resulting variations in temperature and rigidity but there will be realistic limitations. I would still go for this in a heartbeat :-)