When Robert Thomson invented the pneumatic tire a century and a half ago, a rubber doughnut inflated with air was only one of several ideas the Scotsman proposed for softening the ride and reducing the power required to propel the carriages of his day. Thomson also suggested filling what he called his “aerial wheel” with sponges, springs, and/or horsehair to avoid flats caused by “concussion between the wheel and the roadway.”
In spite of the excellent performance provided by today’s tires, efforts persist to let the air out, once and for all. The goals are to eliminate flats, lengthen tread life, facilitate recycling, and—hold for the most tantalizing bit—improve handling.
The ultimate proof of an airless tire’s potential came in the early 1970s on NASA’s Lunar Roving Vehicle. Designed by Ferenc Pavlics and constructed by GM, these 9-by-32-inch tires consisted of steel-mesh toroids (doughnut-shaped coils) attached to aluminum wheels. V-shaped titanium treads provided traction on the dusty moon surface. We’re guessing that ride quality was less of an issue than in the Earth market.
Michelin brought the idea back home in 2005 with a tire-and-wheel combination cleverly called the “Tweel”. After spending years developing PAX run-flat tires, Michelin engineers concluded that eliminating a tire’s need for air made more sense. The Tweel consists of a thin rubber tread band reinforced by a composite-plastic belt and supported by resilient V-shaped polyurethane spokes. Introductory claims versus conventional pneumatic radials were two to three times the tread life and five-times-higher lateral stiffness with only a slight increase in rolling resistance. The Tweel’s combination of soft vertical compliance with stiff lateral resistance seemed like the answer to every handling engineer’s dreams. Michelin demonstrated Tweels on an Audi A4 and two Segway vehicles and announced that the first applications would be for military vehicles and skid-steer construction equipment. Time magazine called this one of 2005’s most amazing inventions.
Those who drove the Tweel-equipped Audi reported one shortcoming—excessive noise at high speed—which Michelin attributed to spoke vibration. Then the Tweel story changed from excess noise to utter silence. When asked for a progress report on its airless-tire research project, Michelin refused comment. Possible reasons why: The Tweel did not pan out as expected, work is under way on some top-secret military application, or Michelin is hesitant to share its findings with competitors.
Late last year, though, Bridgestone thickened the plot by presenting an airless tire concept, which it says was the result of three years of research, at the Tokyo motor show. Looking beyond eliminating flats, the company stressed the environmental benefits of using only readily recyclable materials in this tire’s design. Mimicking the Tweel, the airless Bridgestone consists of a thin rubber tread supported by flexible thermoplastic spokes and a rigid aluminum center section. Inner and outer spokes run in opposite directions to provide vertical compliance without twisting. Bridgestone claims that high-speed noise and vibration are not concerns, but there are other issues—such as how to avoid trapping debris in the spokes—before the concept can progress from light, low-speed applications to the highway.
We’re expecting that nonpneumatic tires are a decade away. Beyond their functional attributes, two things will propel them toward acceptance: Tire companies must address the growing mountain of bald tires defiling the landscape, and they desperately need a new business model. Selling advanced-technology wheels and tires has got to be more lucrative than losing money on original-equipment applications in hopes of earning a profit with replacement rubber.
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