At the Tokyo Motor Show late last year, Bridgestone introduced a non-pneumatic, or airless, concept tire. Comprised of a strip of rubber tread bonded to a structure of thermoplastic resin spokes, the airless tire is similar in concept to the Michelin Tweel introduced several years ago. An obvious benefit of these airless tires is that they are immune to punctures, and Bridgestone is promoting its version as being 100 percent recyclable and much better for the environment than conventional tires. Additionally, airless tires offer lower maintenance (no more checking pressure or fixing flats) and cheaper replacement costs as only the tread itself need be replaced when it wears out.
In addition to those benefits, the Bridgestone and Michelin airless tires both offer several performance improvements over pneumatic tires. Compromises are usually made in a conventional tire's construction because it's difficult to change stiffness in one direction -laterally or vertically - without affecting stiffness in the other. Make the tire stiff laterally for performance, and comfort suffers; reduce stiffness vertically for comfort and compliance over bumps, and the tire flexes too much laterally for performance. With the Bridgestone or Michelin airless tires, however, stiffness can be varied in any direction - the tire can be stiff laterally for stability, but pliable in the vertical direction for compliance and comfort. Michelin showed its Tweel on an Audi A4, and reported that the car was "unusually responsive in its handling." The Bridgestone airless tire adds another dimension to this targeted stiffness with the curved format of its spokes. By interlacing spokes with curves in different directions, the amount of twist in the tire can also be controlled- and again, separately from vertical or lateral stiffness.
Both the Tweel and the Bridgestone airless tire are limited to lightweight, low-speed vehicles for now. At the Tokyo show, Bridgestone showed a four-wheel mobility scooter with its airless tires, while Michelin's Tweel was at one time available on the iBOT, a four-wheel stair-climbing wheelchair which is no longer in production. Where do motorcycles fit in? At the Milan show in 2005, Michelin showed an "airless scooter" that used airless tires with a similar construction of rubber tread bonded to elastomer composite spokes set in a radial pattern. While the airless scooter tire is different in its construction compared to the Tweel or Bridgestone airless tire, the concept and performance benefits are similar.
On a motorcycle, we are also concerned with a tire's compliance when the motorcycle is leaned over in a corner and the suspension has difficulty functioning. Here the tire acts as the suspension, and an airless tire could be constructed to be nicely compliant at lean without sacrificing overall stiffness and stability. The performance benefits could be quite significant for motorcycles once the basic technology becomes widespread and further developed. It appears that the current struggle with airless tires is making the spokes rigid enough to carry the weight of the vehicle while keeping them flexible enough to absorb shocks. Still, both Bridgestone and Michelin are hoping to make airless tires available for passenger cars in the near future, and certainly motorcycles factor into those plans as well
At the Tokyo Motor Show, Bridgestone showed its airless tires mounted to a four-wheel mobility scooter; this video shows the tires in action and explains their construction in more detail.