Joe Walter assesses how to accurately predict tire wear – and concludes that a simpler, universally recognized technique is needed to replace current lab test machines.
Newly enrolled engineering students are somewhat confounded when I state that there are pros and cons associated with the dissipation of thermal energy from rolling tires. Specifically, one attempts to maximize energy losses at the tire-road interface to increase frictional grip during vehicle braking. At all other times when driving, one strives to minimize rubber hysteresis to reduce rolling resistance and improve fuel economy. Today, polymer scientists resolve these conflicting requirements by exploiting the viscoelastic properties of rubbers behaving uniquely in the different temperature and frequency regimes of tire operation.
It was not so easy when I joined Firestone in the mid-1960s. Fuel was inexpensive and plentiful, so rolling resistance and fuel economy, which are so important today, were secondary issues. While not then a priority test, the definitive device for evaluating the internal friction of new tire materials was the rebound test developed late in the 19th century. The rebound height of a small rubber ball divided by its drop height varies directly with its resilience and inversely with hysteretic loss. The rebound test, simple and cheap, provided the rubber industry with its first quantitative measure of material damping – percent rebound.
Recall that chemists and compounders dominated the R&D centers of the major US tire manufacturers during the first half of the 20th century. These skilled scientists and artisans excelled at developing rubber recipes that met the requirements of the bias-ply tires of that era. Mileage and wet traction were controlling factors in many tire development programs. Laboratory samples of new polymers could be characterized in a directionally correct manner for wear and traction by measuring their so-called glass transition temperature (Tg) before committing to an experimental tire build or scaled-up polymer production. The glass temperature regime, below 0°C (32°F), indicated where the material changed from brittle to flexible. For example, low-Tg polymers such as Polybutadiene were good for wear resistance but poor for grip; conversely, high-Tg polymers, such as butyl or its halogenated variants, behaved in the opposite manner. Natural rubber and SBR are usually positioned between these two extremes.
Additionally, tire wear and traction properties were directly influenced by the compounding ingredients in the tread formulation – especially the amount and type of carbon black. Tire companies generally relied on the British Portable Skid Resistance Tester to characterize such compound effects before committing to further studies. This device measured the friction coefficient between a laboratory rubber sample affixed to a moving pendulum and an immobile road surface. It was originally developed at the UK’s Road Research Laboratory (now known as TRL) in the 1960s to test new highway materials, but then extended to test the frictional characteristics of aircraft runways, floor tiles, and finally tire tread compounds.
The US situation changed dramatically with the OPEC oil embargo in 1973 and the imposition of CAFE regulations in 1975. Rolling resistance and fuel economy were no longer secondary issues. The retail price of US gasoline had more than doubled by 1979. In due course, the fuel-efficient radial tire displaced the bias construction. Researchers now addressed the development of low-hysteresis polymers and tread formulations to meet new and pending requirements. In 1982 Dunlop promoted its Elite tire line in Europe, featuring a high-vinyl S-SBR jointly developed with Royal Dutch Shell. At the same time Bridgestone, with JSR, introduced its functionalized tin-coupled polymer in Japan. Both materials provided a better balance between wet grip and rolling resistance by maximizing loss properties at 0°C (32°F) and minimizing the same at 60°C (140°F). These loss properties were characterized by the so-called tangent delta (tan δ) – the ratio of loss modulus to elastic modulus. This concept was further exploited by Michelin’s 1992 ‘green tire’ revolution, with silica-silane in the tread compound as a complement or replacement for carbon black – with some loss in tread wear.
Today we take for granted the ability to screen experimental rubber formulations for wear, traction and rolling resistance, among other properties. However, wear resistance behavior is not amenable to analysis using lab techniques based on viscoelastic principles such as dynamic mechanical spectroscopy. The industry ought to develop similarly simple, universally recognized, protocols for predicting tread wear to replace the multitude of laboratory rubber abraders and proprietary tire wear machines now in service.