OPINION “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” JOE WALTER T ires and their component parts are subjected to a broad range of temperatures during their post-production 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 separate steel beads per side in the critical ply turn-up region to alleviated with the introduction of cooler-running steel-belted facilitate shedding intense heat during severe service – while radials and double-dip polyester adhesives. But the ‘in-between’ limiting speeds to 6mph (10km/h) when fully loaded. belted-bias construction, foisted on the USA’s motoring Solid rubber tires are also speed and load limited due Thanks to public by the then-dominant Detroit-Akron auto alliance, to hysteretic heat build-up and possible thermal runaway upselling, even delayed full radial penetration into the domestic OE tire – potentially causing liquefaction of the interior rubber some economy cars are market by over a decade. as sometimes occurred on automobile tires before the available with V-speed Generally, a tire’s ability to absorb power and advent of cooler-running pneumatics early in the 20 th dissipate heat, not energy per se, controls temperature-century; but pneumatics for more heavily laden trucks rated tires capable of related failures at different speeds and wheel loads. PCR and farm tractors only followed decades later. Recently, tires, for example, become problematic at high speed with ‘thermal runaway’ has entered the vernacular during internet the onset of destructive traveling waves of absorbed power chitchat featuring videos of electric vehicle fires. Such fires, between 12-15hp (9-11kW) – the product of tire drag force and while infrequent and unexpected, are newsworthy due to the speed. Belatedly, in 1991, the tire speed rating symbol became spectacular long-burning nature of combustible lithium batteries. part of the tire’s sidewall service description in the US. Obviously, Compelling data has existed for decades indicating that the speed capability of any tire should always exceed that of the rubber degrades and tires age more quickly in warmer climates vehicle. Thanks to upselling, even some economy cars are now than in cooler ones. This partially explains NHTSA’s difficulty in available with V-speed rated tires capable of 150mph (240km/h) developing a meaningful tire aging standard (or use-by date) that operation – an incongruous situation! does not penalize consumers in northern states while targeting On the other hand, OTR tires, mainly earthmover, are power the requirements of southern regions – where tires may age out limited at much lower speeds and rated by a related metric, before they wear out. ton-miles per hour (TMPH) or an equivalent (TKPH), to prevent Then, there is the afterlife of spent tires – burned as a source overheating during operation. Every OTR tire, regardless of of energy, recycled into highway asphalt, depolymerized into application, has a specific maximum load and speed rating. For chemical constituents, and so forth. But that’s the never-ending example, a large bias-ply 70/70-57 earthmover tire weighing quest for more efficacious methods of harnessing the latent 15,000 lb (6,800kg) might feature 44 nylon plies and four energy – thermal and otherwise – of discarded tires. 150mph 14 www.tiretechnologyinternational.com March 2024 Illustration: Phil Hackett