A Review of Important Factors Affecting Treadwear

Abstract

Abstract For any specific tire use condition, treadwear performance is influenced by three main factor categories: (1), tire construction; (2), tread materials; and (3), environmental and vehicle use conditions. Tire construction factors are—generic type (bias, belted-bias, radial), tread pattern groove void level, and geometric shape, i.e., aspect ratio. The relative importance of nominal variations in each of these factors for treadwear performance is 100, 46, and 39, respectively. Performance improves for a change from bias to radial; high to low groove void; and high to low aspect ratio. The combined influence of generic type, aspect ratio, and other internal construction features (e.g., belt stiffness) can be described by one parameter, the ratio of the treadband edgewise bending stiffness, KBo and the carcass (spring) stiffness, Kc. Treadlife is a direct linear function of this ratio. Treadwear compound or material performance is a function of the rubber glass-transition temperature (weighted avg. for blends), and the degree of reinforcement which is dictated by the carbon-black structure, surface area, and surface chemistry, in addition to the amount of black in the compound. The effect of each of these is a complex function of (i), the severity of tire use (e.g., cornering intensities) and (ii), the long term (seasonal) and short term (daily) environmental factors of pavement microtexture (0.01 mm scale) and ambient temperature. Precipitation directly influences microtexture level through a chemical etching of the pavement aggregate particles. Increased Tg and carbon-black reinforcement can improve or degrade treadwear performance depending on the external factors of pavement microtexture and ambient temperature and also on the general severity of tire use. Treadwear performance is also influenced by the degradation characteristics of the tread compound. Degradation propensity is influenced by crosslink structure and general susceptibility to oxidation. High wear rates are encountered for compounds cured with high-sulfur cure systems (high crosslink polysulfide content) and with low levels of antioxidant. Substantial evidence exists to support a “two-mechanism” theory of rubber abrasion. Mechanism 1 is predominant when the rubber tread element experiences highly elastic surface deformations induced by frictional contact with the pavement asperities. Rubber particles are removed by a tear-tensile rupture process. Mechanism 2 is predominant when the rubber experiences a plastic or rigid body type of contact with the pavement asperities. This contact exists on a smaller scale (reduced deformation domain) and particles are removed by an abrasive-cutting action. Mechanism 1 is called “E-Wear”; Mechanism 2 is called “P-Wear”. E-wear is favored by high temperatures, low microtexture pavements, soft (low Tg) compounds with low reinforcement levels. P-wear is favored by high microtexture, low ambient temperatures, hard (high Tg) compounds with high levels of reinforcement. The confusing treadwear performance frequently encountered for compounds—when tested at different locations, at different times, with substantial treadwear index changes, and outright reversals—can be rationally explained on the basis of a shift of the predominant mechanism. These shifts are due to changes in the environmental factors and tire-use severity as tires are tested at different locations over varying seasonal periods. Microtexture follows a seasonal cyclic pattern; high in winter and low in summer. Ambient temperature follows an opposite cyclic pattern. Short term changes (daily ) in both microtexture and temperature occur within the long-term seasonal periods. These changes have to be accommodated in interpreting treadwear performance.