Tires | Research & Encyclopedia Articles

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Tire Summary

Tires

Historical Development

The first commercially successful pneumatic tire was developed in 1888 in Belfast by the Scottish veterinarian John Boyd Dunlop primarily to improve the riding comfort of bicycles. Dunlop also showed, albeit qualitatively, that his air-inflated "pneumatic" took less effort to rotate than did the solid rubber tires in use at that time. His qualitative tests were the first known rolling resistance experiments on pneumatic tires. Due to this significant reduction in rolling loss, many professional cyclists in Britain and Ireland adopted air-inflated tires for their bicycles by the early 1890s. Pneumatics for the nascent automobile industry soon followed.

Tires, like everything that rolls, encounter resistance. The resistance encountered by the tire rolling across a surface is a major factor in determining the amount of energy needed to move vehicles. Since Dunlop's original efforts, a considerable number of tire design improvements have been made that have tended to cause a decrease in tire power consumption. For example: separate plies of cotton cord were introduced in the early 1900s to replace Dunlop's square-woven flax (as early tires failed from fabric fatigue before wearing out); in the late 1950s the fuel-efficient radial-ply construction was commercialized in Europe to replace the bias-ply tire; this change also improved vehicle handling and increased mileage. (The radial tire features one or more layers of reinforcing cords or plies disposed perpendicular to the two beads plus a steel belt in the tread region, while the bias construction is built-up with an even number of plies arrayed at alternating, opposing angles between beads without a belt.) There has been a trend toward using larger-diameter tires, which are more rolling-efficient for comfort, appearance and safety reasons as vehicles were downsized in the United States during the 1980s. Elimination of the inner tube by tubeless tires, the use of fewer but stronger cord plies, and the production of more dimensionally uniform tires have each made small but measurable reductions in tire energy loss, although each of these changes was instituted primarily for other reasons.

By the 1990s, automobile tires in the industrialized portions of the world were often taken for granted by the motoring public because of the high level of performance they routinely provide in operation. This casual attitude toward tires by a large segment of the driving public can be partially explained by the sheer number of units manufactured and sold each year. In 1996 about one billion tires of all sizes and types were marketed worldwide with a value of more than $70 billion; this includes 731 million passenger car tires and 231 million truck tires with agricultural, earthmover, aircraft, and motorcycle tires constituting the remainder. Three regions of the world dominate tire production and sales: North America, Western Europe and Japan. Among them, these mature markets are responsible for more than three-quarters of all passenger car tires and one-half of all truck tires. Bicycle tires are now largely produced in less-developed countries, so an accurate number is hard to assess.

The pneumatic tire has the geometry of a thin-walled toroidal shell. It consists of as many as fifty different materials, including natural rubber and a variety of synthetic elastomers, plus carbon black of various types, tire cord, bead wire, and many chemical compounding ingredients, such as sulfur and zinc oxide. These constituent materials are combined in different proportions to form the key components of the composite tire structure. The compliant tread of a passenger car tire, for example, provides road grip; the sidewall protects the internal cords from curb abrasion; in turn, the cords, prestressed by inflation pressure, reinforce the rubber matrix and carry the majority of applied loads; finally, the two circumferential bundles of bead wire anchor the pressurized torus securely to the rim of the wheel.

However, it is the inelastic properties of the cord and rubber components of the tire that are responsible for the heat buildup and energy dissipation that occur during each tire revolution. This loss of energy results in a drag force that impedes tire rotation. The cyclic energy dissipation of tire materials is a mixed blessing for the tire development engineer. It is required in the contact patch between tire and road to produce frictional forces that accelerate, brake, and/or corner the vehicle, but it must be minimized throughout the tire in the free-rolling condition so as not to adversely impact fuel economy.

Depending on the specific car model and service conditions, tires can consume 10 to 15 percent of the

total energy in the fuel tank, and in this respect the power loss of the four tires is comparable in magnitude to the aerodynamic drag opposing the forward motion of the vehicle during urban driving at about 45 miles per hour.

Rolling resistance coefficient is defined as the nondimensional ratio of drag force retarding tire rotation to wheel load—with lower being better. For most passenger car tires freely rolling on smooth, hard surfaces, this coefficient varies between 0.01 and 0.02, but may increase to 0.03 or higher at increased speeds, at very high loads, at low-inflation pressures, and/or on soft surfaces.

Typical ranges for the rolling resistance coefficients of passenger car and truck tires in normal service (vs. a steel wheel) are given in Table 1.

Truck tires are operated at about four times the inflation pressure of passenger car tires, which principally accounts for truck tires' lower rolling resistance coefficients.

Power consumption is the product of drag force and speed—and four tires on a typical American sedan consume approximately 10 horsepower of engine output at 65 miles per hour.

The radial passenger tire introduced in North America during the 1970s had a 20 to 25 percent improvement in rolling resistance compared to the bias ply tire then in use. This was a major improvement that resulted in a 4 to 5 percent gain in vehicle fuel economy under steady-state driving conditions—that is, an approximately 5:1 ratio between rolling resistance reduction and fuel economy improvement. By the 1990s, evolutionary advances in tire design and materials continued with the general use of higher operating pressures, but an 8 to 10 percent improvement in rolling resistance now translates into only a 1 percent gain in fuel economy.

Trade-offs between rolling resistance (and therefore fuel economy) and safety occur in tire design and usage just as with vehicles. For example, the least hysteretic polymers and tread compound formulas that lower rolling resistance tend to compromise wet grip and tread wear. Also, worn-out tires with little or no remaining tread depth are much more fuel-efficient than new tires, but require greater distances to stop when braking on wet surfaces.

	Rolling Resistance Coefficient
Radial passenger tire	.008 – .015
Bias passenger tire	.016 – .025
Radial truck tire	.005 – .007
Bias truck tire	.008 – .010
Railroad wheel (steel)	.002 – .003

Energy for Manufacturing

The energy required to produce a tire is only about 10 percent of that consumed while in use on an automobile overcoming rolling resistance during 40,000 miles of service. The majority of the manufacturing energy expended, 60 to 70 percent, is for the mold during the vulcanization process.

At the end of its useful life, the tire, with its hydrocarbon-based constituents, is a valuable source of energy, with a higher energy value than coal. By the mid-1990s, approximately 80 percent of the tires worn out annually in the United States were being recycled, recovered, or reused in some fashion, with three-fourths of these serving as fuel for energy production in boilers and cement kilns.