Rolling Loss of Pneumatic Highway Tires in the Eighties

Up to and including World War II, not more than a dozen papers had been written on the subject of tire rolling loss. The number quickly increased after the oil embargo in 1976 and reached a peak around 1980 with more than thirty papers published per year. Several symposia were held in these years on the subject, and a comprehensive review of rolling-loss research was written by one of the authors of this article. Other, briefer reviews were published at that time by Barson, who discussed various techniques of measuring tire rolling loss; Moore, who pointed out ways of reducing rolling loss and hence fuel consumption; Clark, who touched upon some physical phenomena of tire rolling loss; and Chang and Shackleton, as well as Levine, who reviewed the effects of tire construction and materials on rolling loss. The main concern at that time was to understand the mechanism of tire rolling loss so that it could be effectively reduced. Hence, accurate laboratory testing procedures were developed to identify the effects of major tire and service factors on rolling loss, such as tire type, geometry, reinforcement, materials, load, inflation pressure, speed, slip angle, temperature, and road texture. Concurrently, predictive models were developed ranging from simple empirical relations to complex finite-element programs, so that besides costly experiments, mathematical simulations could be employed as guidance toward lower tire rolling losses and thus meeting the stringent governmental fuel-economy standards. The efforts of the tire industry to reduce tire rolling loss have been very successful, and further progress is to be expected. Figure 1 gives an indication of the rolling loss improvements achieved over the past 15 years. The average rolling loss coefficient of a sample of radial passenger car tires manufactured in 1975 was 12.6 N/kN; its standard deviation was 1.3 N/kN. A similar sample of radial tires manufactured a few years later indicated an average coefficient of 11.0 N/kN with a standard deviation of again 1.3 N/kN. A check of recently manufactured (1989) radial passenger car tires revealed a further drop to 9.3 N/ kN with a standard deviation of only 0.4 N/kN. Hence, in the first years after the oil embargo, the average rolling loss was reduced by roughly 10%—without change of the rather broad dispersion, however. The following years led to further improvements of an additional 15%, this time with a drastic drop of variability. This indicates the effects not only of material and construction improvements but also of tighter controls in the production process.