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Simple plans

The image of pouring chemically reactive liquids into a mold and extracting a cordless pneumatic tire of any color is compelling, but pursuit of this objective has proved to be a fool’s errand. Multiple quests in search of a simplified tire manufacturing process were apparently spawned by Goodyear in 1961. The cover of a popular US magazine, Life, featured translucent polyurethane (PU) tires illuminated internally with incandescent light bulbs blinking individually or in unison.

Urethane was a logical choice as the casting material. First synthesized by Otto Bayer at IG Farben in 1937, it had been used in roller skate wheels since 1949 and in shoe soles shortly thereafter. Goodyear, after a decade of unsuccessful urethane experimentation, lost interest in furthering its research – but its competitors had other ideas. For example, in 1970 Firestone was proclaiming the commercialization of cordless cast automobile tires within five years. After two decades of futile activity in search of a workable urethane, this project was finally terminated in the early 1980s.

At best, Firestone demonstrated it could produce a cast car tire with the performance characteristics of bias-ply constructions. In hindsight, the company should have invested more heavily in its nascent radial technology to avert the disastrous recall of more than seven million Firestone 500 steel-belted tires during 1977-78 due to tread separations.

However, even with the acknowledged setbacks of tire giants Goodyear and Firestone, the search for a simplified tire manufacturing process continued. During the 1980s, Polyair (Kitsee, Austria), with know-how in fabricating urethane shoes, produced liquid injection molded (LIM) passenger car tires – which failed German Automobile Club tests; a limited number of tractor tires were placed in service with unremarkable results.

During the early 2000s, Amerityre (Boulder City, Nevada), a producer of flat-free, urethane foam-filled bicycle and mower tires, embraced the venture with enthusiasm (see Tire, March 2006 issue), but exited without success after a decade of self-promotion.

The advantages of cordless automobile tires are superficially plausible: only one polymeric material and two steel beads are required; no ply endings or splices mean more uniform tires; zero ply steer and conicity forces are achievable; manufacturing complexity, plant size and overall product cost can be reduced; and decentralized factories co-located with vehicle production sites are possible.

Disadvantages of an all-urethane structure are many, including wet grip, dry braking, service growth and rapid air loss that can result from cuts or punctures. These drawbacks are principally due to the inadequate physical properties of urethane and the lack of cord reinforcement.

Three stages of cast tire development tend to occur: the first stage is straightforward, a monolithic polyurethane structure (which proves inadequate); secondly, an all-urethane casing plus a conventional rubber tread; and lastly, the addition of belt plies.

Urethane alone in the tread is quickly determined to be deficient due to its relatively low coefficient of friction and low melting point. On wet surfaces, urethane is rather slippery, while in locked wheel braking, the tread can be worn to a flat surface – and perhaps melted in the contact patch. These problems are addressed with the addition of a rubber tread requiring proprietary adhesives to adequately bond rubber and urethane.

Due to urethane’s stress relaxation and creep characteristics, tire diameter and section width increase unacceptably during highway service. During burst testing, even high-stiffness PU tires exhibit balloon-like behavior. Ultimately, diametrical growth is inhibited by inserting belt plies between the rubber tread and urethane casing, but sidewalls still widen under static and dynamic loadings.

Importantly, static casting, or pouring liquid polymer into a mold, produces unwanted microscopic cavities containing trapped air that act as crack initiation sites; these voids tend to propagate with each tire revolution due to the lack of crack arresting cords or crack blunting fillers. More expensive centrifugal casting does not totally remove these small voids, but does reduce their size and number – but at added cost.

The inability of PU alone to arrest fatigue crack growth in cordless tires remains a major impediment to commercial development. If the cordless tire concept ever proves successful, barriers to entering the business will be considerably reduced. Many ‘low tech’ rubber or plastic fabricators could easily develop the required production know-how, causing disastrous disruption to century-old tire giants.



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