Non-pneumatic tires (NPT), or Airless tires, are tires that are not supported by air pressure. They are used on some small vehicles such as riding lawn mowers andmotorized golf carts. They are also used on heavy equipment such as backhoes, which are required to operate on sites such as building demolition, where tire punctures are likely. Tires composed of closed-cell polyurethane foam are also made for bicycles and wheelchairs. The main advantage of airless tires is that they cannot go flat, but they are far less common than air filled tires.
Airless tires generally have higher rolling resistance and provide much less suspension than similarly shaped and sized pneumatic tires. Other problems for airless tires include dissipating the heat buildup that occurs when they are driven. Airless tires are often filled with compressed polymers (plastic), rather than air.
Michelin is currently developing an integrated tire and wheel combination, the “Tweel” (derived from “tire” and “wheel,” which, as the name “Tweel” suggests, are combined into one new, fused part), that operates entirely without air. Michelin claims its “Tweel” has load carrying, shock absorbing, and handling characteristics that compare favorably to conventional pneumatic tires. Automotive engineering group of mechanical engineering department at Clemson University is developing a low energy loss airless tire with Michelin through the NIST ATP project.
Resilient Technologies and the University of Wisconsin–Madison’s Polymer Engineering Center are creating a “non-pneumatic tire”, which is basically a round polymeric honeycomb wrapped with a thick, black tread. The initial version of the tire is for the Humvee and is expected to be available in 2012. Resilient Technologies airless tires have been tested and are used by the U.S. Army.
Bridgestone is developing the Air-Free Concept Tyre which is similar to the Tweel and can hold 150 kgs per tire.
The Energy Return Wheel has the outer edge of the tire connected to the inner rim by a system of springs. The springs can have their tension changed to vary the handling characteristics.