Authored by Ulf Sandberg*
Abstract
Conventional pneumatic (air-inflated) tires have dominated the tire market for over a century. However, several tire manufacturers have presented concepts or prototypes for airless tires which use rubber only in the treads, while the load-carrying structure is made of composite materials such as polyurethane, thermoplastics, fiberglass and epoxy laminates, but also eco-friendlier and bio-degradable materials have been proposed.
First, some early projects (1989-2008) are described in which a Swedish innovative airless tire concept was tested, resulting in so-called Composite Wheel prototypes. This reduced noise emission and rolling resistance substantially and had excellent wet skid resistance properties while also providing acceptable vehicle handling. However, in full-scale testing, it did not endure an extreme test on a “pot-hole” test track due to a design lapse.
Then, a number of concept or prototype airless tires suggested by tire manufacturers in the last 15 years are reviewed. In general, very sparse information have been provided, but they all promise (without scientific proof) significant improvements over pneumatic tires. Advantages are foreseen especially in terms of dramatic reduction in global use of raw materials, use of more eco-friendly materials and much less complicated production. Even 3D-printing can be used in some cases. One of the prototypes is currently tested for use on an electric vehicle in service, with a possible market introduction targeted in 2024.
It is concluded that airless tires have substantial potential for initiating a revolution in terms of use of raw materials of which many may be eco-friendly, in addition to significant advantages over pneumatic tires in the environmental footprint during operation. However, a wide use of airless tires will have dramatic consequences for the tire manufacturing plants, and significantly influence the vehicle industry, with a risk of recent investments not being fully paid-off. Therefore, a wide introduction of airless tires on the market might take a longer time than what is technically needed, regardless of all benefits.
The Pneumatic Tire, A Most Sophisticated Product
The pneumatic (or air-inflated) tire as a concept was invented in the 19th century and was conquering the market for tires in the first decades of the 20th century. Since then, this concept has completely dominated the tire market and is currently stronger than ever. The basic idea is to use compressed air contained in a toroid or torus as a cushion between the vehicle and the road pavement and to let the solid materials in the outer part of this toroid or torus transfer the forces in both transverse and longitudinal directions between the axle and the pavement. To this end a rubber tread is added around the torus. However, the most essential material in the tires is air which, in case of punctures, will make the tire non-useable. Trials have been made with inflation by other gases, primarily nitrogen, but air is used in almost all road vehicle tires.
The product we call tire and is used on all rolling vehicles, is thus pneumatic in its principle and relies on a proper inflation. From its invention until today, the main principle is the same, but today’s product has been improved continually and in several relatively small steps over the more than 100 years since it entered the market. It is now an incredibly sophisticated product; an average tire today contains well over 100 separate components of various materials. It is often claimed by experts that the tires are the most important and sophisticated part of a road vehicle. Rubber is the main raw material used in manufacturing tires, and both natural and synthetic rubber are used, where synthetic rubber constitutes 60-70% of the total rubber amount. Other materials include carbon black, various oils, sulfur, silica, textile and steel reinforcements, to name a few.
Manufacturing such a product is very complicated. This means that manufacturing plants and the materials chains get complicated too, which has led to very substantial investments by the manufacturers that must be paid off during decades of production. Thus, when the radial tire was invented by Michelin in the 1940’s, despite it soon was found to be technically much better than its predecessors (i.e., the diagonal or bias ply tires), it took until the 1960’s until the radial tire became dominant on the European market. However, in USA, the tire and vehicle manufacturers hesitated to accept the radial tires. It was not until about 20 years after Europe when radial tires started to dominate the market in USA. This was partly due to US cars with very soft suspension not being adapted to the new tires, partly due to the refusal to rebuild their tire plants, as radial tires required very costly retooling.
Other major concepts that have been tried including building air inflated tires with essentially only a plastic torus fitted with a rubber tread, or to replace the air with solid material, such as polyurethane. None of them have been developed further than into prototypes.
The Airless Tire - First Trials
In 1982, Goodyear was granted a patent for an Integral Wheel- Tire (IWT) for passenger cars, based on a single-sided wheel rim with an asymmetrical deformation behavior [1]. The IWT was made of composite materials, having a circumferential part covered with a rubber tread. An extensive report dealing with this IWT was presented in 1989 [2], indicating very good characteristics. Yet, nothing more happened, at least not publicly reported.
Without being aware of the Goodyear concept, another airless tire (with dimensions suited for cars) was designed by the Swedish inventor Mr. Hans-Erik Hansson [3]. The patent was granted in Sweden and applications were filed in the major industrial countries. This wheel, like the Goodyear IWT, was a non-pneumatic, highly flexible construction in composite materials giving unique characteristics. Relative to the Goodyear design it used a more sophisticated shape, potentially giving much more flexibility in the design.
This product was named the “Composite Wheel (CW)” due to its integration of rim with tire, using a composite material of glass-fiber/ polyester laminate. To avoid confusion, it should be noted that, in principle, a normal air-inflated tire is also made of composite materials, although many more and totally different ones. Figure 1 shows a cross-section through the first version of the Composite Wheel (Figure 1).
In a project run at Swedish National Road and Transport Research Institute (VTI), the tread and underlying “belt” of this CW design was perforated with holes that let air and water flow through the tread and structure. With this design a noise reduction of up to 10 dB (A-weighted) was measured compared to one of the most popular quality tires of the time (a Goodyear Eagle NCT70) [4]. A problem was that after some driving with normal loads at highway speeds some cracks appeared in the “belt” area. The design was apparently not durable enough, which had not been the purpose with this particular design. The purpose had been to study the potential for noise reduction of the concept and not to build a fully durable tire. Nevertheless, as intended, the noise reduction potential had been demonstrated. It should be noted that in today’s traffic, the noise emission of tires constitutes the major part of traffic noise, so a 10 dB reduction would mean a revolution to traffic noise mitigation.
The reasons for the noise reduction were identified to be essentially two-fold [5]:
For low frequencies, the torus of a normal air-inflated tire can be compared to a closed loudspeaker cabinet. The latter separates the sound waves from the rear of a loudspeaker and the sound waves from the front of the loudspeaker, in order to avoid that these waves will cancel each other (which happens if there is no loudspeaker cabinet or large baffle). In loudspeaker technology this is known as “the baffle effect”. In the CW design, the sound waves from the rubber tread and the sidewall are not separated from the waves of the rear side of the same; therefore, creating a case where these low-frequency waves are partly or fully cancelling each other.
For high frequencies, the so-called air pumping sound which is created by trapping the air in the interface between rubber tread (with its open or closed grooves) and pavement surface, results in high air pressure gradients, when air is “pumped” in and out of the interface. In the CW design, this mechanism is eliminated by holes constructed or drilled through the tread and belt where air in tread grooves or trapped in cavities in the pavement can easily escape from the traps. Further, the so-called “horn effect” (the shape of the front and rear of the tire circumference curved over the flat pavement surface technically resembles an acoustic horn which may amplify sound created in the throat of the horn) is eliminated as the tire tread is perforated.
In reality, the two noise-reducing mechanisms are partly overlapping between low and high frequencies and in combination they constitute an efficient “killer” of noise.
Later Trials with the Composite Wheel
Tires are required to meet several very important requirements, of which noise would be one, but other performances should not be impaired when noise is reduced. Consequently, later projects should aim at studying the CW concept from a holistic point of view, considering primarily safety, wear, durability, vehicle handling, energy consumption and CO2 emission (the latter two depending on rolling resistance). In a later international project [6], utilizing carbon- fiber and epoxy laminate as materials but retaining the basic principle shown in Figure 1, focus was on noise, rolling resistance, durability, and vehicle handling. With regard to safety, it was obvious that skid resistance or hydroplaning on wet roads would be improved by the CW design as long as the tread was “ventilated” by holes, so no measurements were needed to confirm it. In order to improve durability, some of the noise properties had to be sacrificed. Figures 2 and 3 show the prototype samples during the final testing session.
To read more about this article......Open access Journal of Modern Concepts in Material Science
Please follow the URL to access more information about this article
To know more about our Journals...Iris Publishers
No comments:
Post a Comment