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 and motorized 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.
Airless tires are attractive to cyclists, as bicycle tires are much more vulnerable to punctures than motor vehicle tires. However users of airless bicycle tires have complained of difficulty of mounting the tires on wheel rims (caused by tight tires) or tires slipping\rolling on rim when in use (caused by loose tires). Other common complaints are increased rolling resistance, increased wheel weight and a harsh ride (leading to rider fatigue and broken spokes).
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), which operates entirely without air. Michelin claims its “Tweel” has load carrying, shock absorbing, and handling characteristics that compare favorably to conventional pneumatic tires. The automotive engineering group of the 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. and is also the first group to make a commercially available mass-produced airless tire after their acquisition by Polaris, albeit, only as coupled with their vehicle.
Bridgestone is developing the Bridgestone Air-Free Concept Tire, which is similar to the Tweel, and can hold 150 kg 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.
Big Tyre Pty Ltd in Australia is developing a “non-pneumatic, non-solid wheel”, which is designed to handle high working loads, such as those found in underground mines. The wheel utilizes multiple arrays of concentric leaf springs to distribute force evenly across the wheel. A prototype of the wheel was built in 2011, and has been tested on an Eimco 936 underground loader.