Tires slip or skid on road surfaces when braked. Surprisingly, rolling tires also slip even while presumably in contact with dry roads. While zero longitudinal slip theoretically results when the tire’s effective rolling radius equals its loaded rolling radius, lateral squirm of tread elements occurs along with three-dimensional micro-slip on road asperities. Even in a cornering maneuver, vehicle dynamicists differentiate between fore-and-aft zones in the tire footprint that grip, and then subsequently slip.
All of these motions between sliding surfaces create friction, and the resulting frictional forces produce wear. Not only are roadways responsible for tire wear, tires are responsible for pavement wear, evidenced by microscopic inspection of roadside debris. And as pavements wear, they become polished, reducing road friction and compromising safety. This is the principal reason why the US Federal Highway Administration encourages (but does not require) that all state highway agencies implement programs to monitor and manage pavement friction on public roads – and repave them when skid resistance falls below a safe level.
Pavement surface texture promotes tire-road friction, especially during wet-weather driving. In the tire industry, texture has traditionally been defined at two levels, with microtexture dimensions being as small as 0.01mm (the size of ultra-fine sandpaper grit) and macrotexture in the range of 1-3mm (the size of coarse beach sand to fine gravel). While texture boundaries and dimensions are somewhat arbitrary, there is a distribution of texture levels on any road surface. Macrotexture promotes surface water drainage, enabling microtexture to make rubber contact, which improves traction and increases tread wear. For their own needs, highway engineers have developed several qualitative methods for measuring pavement texture, including the outflow meter and the sand patch test. Various road profiling devices exist to quantify longer wavelength road properties (megatexture and roughness). The International Roughness Index (IRI) derived from such equipment is globally accepted as the recognized measure of surface conditions for assessing road maintenance needs.
Pavements are not only polished by tires, they are also damaged by tires. Pavement impairment often appears as rutting and fatigue cracking. Trucks and buses have been long recognized to cause more damage than passenger cars. However, it is not vehicle weight per se that produces predictable damage, but rather the weight distribution reflected in axle loadings. As a consequence, vehicles with the same weight but different weight distributions produce different amounts of impairment. And dynamically applied loads produce more damage than stationary axle loads – which increases at an exponential rate with loading. For example, the commonly employed ‘4th power law’ predicts that a truck overloaded by 10% causes pavement damage about 45% greater than the same truck at legal load limits.
Studded snow tires have been responsible for much roadway rutting. Ruts form due to the abrasive action of the metal studs protruding from the tread rubber elements. Rainwater trapped in ruts is a common factor contributing to hydroplaning. And ruts increase tire rolling resistance. Thus, many states limit studded tire use during non-winter months and some states have outlawed them completely. In any case, such tires are being used less frequently due to the improved performance of studless winter tires and the increasing popularity of all-wheel and four-wheel-drive vehicles in the northern regions of the USA.
Evidence suggests that any increase in vehicle suspension compliance will lessen pavement stress and strain. When pneumatic truck tires were introduced in the 1920s, they produced less highway damage than the solid rubber tires in use, due to reduced stiffness and more uniform road contact pressure. Air springs fitted to trucks seem to cause less road damage than leaf springs or walking beam suspensions. First generation wide-base tires (385/65R22.5), with their smaller contact area, induced more pavement distress than dual fitments. And the latest generation wide-base singles (445/50 R22.5), with a wider section-width and larger footprint, generally perform the same as duals in contributing less to roadway damage.
Tires and roads have much in common: both are subjected to temperature extremes and fatigue loadings; both feature matrix and filler constituents; their life can be extended by retreading or repaving; and both are highly engineered structures vital to today’s consumer and commercial road transport demands.