Tire Technology International - March 2024 Issue
Joe Walter
2024-04-06 09:02:32
“My first real-world experience with possible thermal runaway occurred during the late 1960s when first-generation PET cords were used in hot-running, belted-bias tires featuring fiberglass belts”
Tires and their component parts are subjected to a broad range of temperatures during their postproduction journey from birth to death. Energy loss as heat within the tire results from the hysteretic stress-strain cycles occurring repetitively with each wheel revolution. Conductive, convective and radiative processes transport spent energy (including tire-road friction) to ambient and internal air and the road surface. When heat generation remains larger than heat dissipation, tire component and air cavity temperatures steadily increase – and ‘thermal runaway’ is a possibility, which can cause rapid, and sometimes explosive, tire failure. If you meandered through thermodynamics, you may dimly recall that thermal runaway is characterized by an exponential increase of a substance’s temperature when heat rates increase more quickly than removal rates, causing an out-of-control exothermic reaction.
My first real-world experience with possible thermal runaway occurred during the late 1960s when first-generation polyester (polyethylene terephthalate or PET) cords were used in hot-running, belted-bias tires featuring fiberglass belts. These tires incurred high adjustment rates especially in the extreme temperature zones of the US – polyester thermal degradation, loss of adhesion and occasional cord melting happened in the scorching American Southwest on large Cadillac-size (L78-15) tires, as did fiberglass impact breaks in the frigid North. While PET cords possessed the requisite mechanical properties and competitive production costs, poor rubber adhesion remained troublesome in belted-bias constructions. The situation was alleviated with the introduction of cooler-running steel-belted radials and double-dip polyester adhesives. But the ‘in-between’ belted-bias construction, foisted on the USA’s motoring public by the then-dominant Detroit-Akron auto alliance, delayed full radial penetration into the domestic OE tire market by over a decade.
Generally, a tire’s ability to absorb power and dissipate heat, not energy per se, controls temperature-related failures at different speeds and wheel loads. PCR tires, for example, become problematic at high speed with the onset of destructive traveling waves of absorbed power between 12-15hp (9-11kW) – the product of tire drag force and speed. Belatedly, in 1991, the tire speed rating symbol became part of the tire’s sidewall service description in the US. Obviously, the speed capability of any tire should always exceed that of the vehicle. Thanks to upselling, even some economy cars are now available with V-speed rated tires capable of 150mph (240km/h) operation – an incongruous situation!
On the other hand, OTR tires, mainly earthmover, are power limited at much lower speeds and rated by a related metric, ton-miles per hour (TMPH) or an equivalent (TKPH), to prevent overheating during operation. Every OTR tire, regardless of application, has a specific maximum load and speed rating. For example, a large bias-ply 70/70-57 earthmover tire weighing 15,000 lb (6,800kg) might feature 44 nylon plies and four separate steel beads per side in the critical ply turn-up region to facilitate shedding intense heat during severe service – while limiting speeds to 6mph (10km/h) when fully loaded.
Thanks to upselling, even some economy cars are available with V-speed rated tires capable of 150mph
Solid rubber tires are also speed and load limited due to hysteretic heat build-up and possible thermal runaway – potentially causing liquefaction of the interior rubber as sometimes occurred on automobile tires before the advent of cooler-running pneumatics early in the 20th century; but pneumatics for more heavily laden trucks and farm tractors only followed decades later. Recently, ‘thermal runaway’ has entered the vernacular during internet chitchat featuring videos of electric vehicle fires. Such fires, while infrequent and unexpected, are newsworthy due to the spectacular long-burning nature of combustible lithium batteries.
Compelling data has existed for decades indicating that rubber degrades and tires age more quickly in warmer climates than in cooler ones. This partially explains NHTSA’s difficulty in developing a meaningful tire aging standard (or use-by date) that does not penalize consumers in northern states while targeting the requirements of southern regions – where tires may age out before they wear out.
Then, there is the afterlife of spent tires – burned as a source of energy, recycled into highway asphalt, depolymerized into chemical constituents, and so forth. But that’s the never-ending quest for more efficacious methods of harnessing the latent energy – thermal and otherwise – of discarded tires.
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