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In motor vehicles and other wheeled vehicles, pneumatic tires create the necessary adhesion (traction) between the wheels and the surface of the road and cushion the dynamic loads on the wheels that result from the motion of the vehicle. The tires ensure vehicle control, traction under difficult road conditions, stability, and riding comfort. They substantially affect the braking distance, fuel consumption, and many other performance and economic characteristics of the vehicle.
Pneumatic tires are classified according to purpose as passenger-car tires (for passenger cars and light trucks), truck tires (for all other trucks, buses, trolleybuses, and trailers), agricultural tires (for tractors and other agricultural vehicles), off-the-road tires (for construction, reading, and hoisting and conveying machinery), motorcycle tires (for motorcycles, motor scooters, and mopeds), and bicycle tires. Special-purpose tires include those for aviation, monorail transportation, and so on.
Pneumatic tires are multilayer rubber-and-textile products. Their manufacture accounts for about 50 percent of the rubber consumed and a substantial portion of chemical fibers and other reinforcing materials. In 1976 approximately 7 million tons of rubber was used to produce 780 million tires throughout the world.
Construction. The common element in all tires is the casing (Figure 1), which maintains a specific shape for the tire under the influence of the internal (inflation) pressure. The carcass, or body, plies are the foundation of the tire; they impart strength and resilience. They consist of several layers of rubberized textile or, in some cases, metal cord (cord fabric). In bias-ply tires the cords in adjacent plies cross one another, and the angle between the direction of the cords and a plane passing through the axis of rotation is 50°–55°; in radial-ply tires the cords of all the plies lie in this plane. The number of plies in radial-ply tires is about half that of bias-ply tires, so that the carcass is more flexible.
The breaker is designed to provide a strong bond between the carcass plies and the outermost rubber layer (the tread); it is made of several layers of rubberized textile or metal cord (less often, of rubber). In bias-ply tires the cords of the breaker run at the same angle as the carcass plies; in radial-ply tires they run at an angle of 70°–85°, thus forming an inextensible belt that carries the major portion of the forces acting on the tires as a result of inflation pressure and external loads. Because of the combination of flexible carcass plies and a stiff breaker, radial-ply tires have a longer life, consume less power in overcoming rolling friction, and possess other service advantages over bias-ply tires.
The tread, which is formed as a single unit with the sidewalls, protects the carcass plies from mechanical damage and the effects of moisture. The thickest part, the section in contact with the road, has patterned projections and recesses of various sizes and shapes; the tread pattern determines the adhesion between the tire and the road, rolling resistance, resistance to abrasion, performance under difficult road conditions, running noise, and ease of control. The bead of the casing ensures that the tire is firmly seated on the wheel rim. The carcass plies are secured by bead wires.
An important construction feature is the method used to seal the tire, illustrated by tube-type and tubeless tires. The cavity of tube-type tires contains a ring-shaped inner tube with a valve, which retains the compressed air in the tire. In tubeless tires the air is retained by using chafers, by seating the tire on the rim under great tension, and by applying a rubber sealing layer to the inside of the casing. The advantages of tubeless tires are the high operating safety resulting from insignificant air leakage from a puncture, low weight, and ease of maintenance and repair. However, the necessary airtight fit on the rim requires special mounting equipment and greater precision in the fabrication of rims. The choice of tire construction depends on the application: for example, tubeless tires are best for passenger cars, where operating safety and a comfortable ride at high speeds must be ensured.
Operating characteristics. When stationary, a tire is subjected to the forces of the inflation pressure and the static load on the wheel; a rolling tire is additionally subjected to dynamic loads and loads arising from the redistribution of the vehicle’s weight between the axles and wheels. Under the influence of these forces, a rolling tire undergoes continuous cyclical deformations that differ in magnitude and direction in the various zones of the tire. The total number of deformation cycles that occur during the life of a tire may reach several million. Such deformations cause spontaneous heating (heat buildup) in the tire to temperatures of 60°–90°C.
The most important service characteristics of tires are load-carrying capacity, service life, traction, rolling resistance, and shockabsorbing capacity.
The load-carrying capacity (the maximum permissible static vertical load on the tire) depends mainly on the tire’s dimensions, inflation pressure, number and type of carcass plies, and operating conditions. The service life is defined as the tire mileage accumulated until the projections on the tread pattern become worn (for safe driving and for protection of the carcass from damage, the minimum height of the projections should be 0.5 mm for truck tires and 1.5 mm for passenger-car tires). The service life is reduced when road and climatic conditions are more severe, the load-carrying capacity is exceeded, the speed is increased, or the inflation pressure deviates from the optimum value for a given set of operating conditions (the inflation pressure ranges from 0.1 meganewtons per m2, or 1 kg/cm2, for automobile tires to 2 meganewtons per m2, or 20 kg/cm2, for aircraft tires). When the inflation pressure is reduced, the amplitude of the tire’s deformations and the heat buildup are increased, which accelerates the onset of fatigue failure. When the pressure is increased, the stresses in the tire are increased, the danger of a rupture (blowout) in the carcass when driving over an obstacle is greater, and the tread wear is accelerated because of the higher contact pressure on the road. The average mileage for passenger-car tires ranges from 40,000 to 60,000 km, and for truck tires, from 60,000 to 100,000 km.
Traction is one of the most important factors affecting the driving safety of a motor vehicle. Inadequate traction is the cause of 25–40 percent of highway transportation accidents on wet roads and 5–10 percent on dry roads. The properties of the rubber in the tread as well as the tread pattern affect this characteristic.
The rolling resistance determines to a significant degree the fuel consumption in vehicular transportation and also affects the dynamic characteristics. It depends appreciably on the weight and construction of the tire and on the materials used in manufacture. The shock-absorbing capacity establishes a tire’s properties as an element of the vehicle’s suspension, which damps dynamic loads. Optimum shock-absorbing capacity in an automobile depends on the proper combination of tire shock absorption’ and shock-absorption characteristics of the other suspension elements.
Materials and production technology. General requirements for the rubber used in tires include long fatigue life and low heat buildup; in addition, the rubber used for treads must resist abrasion and weathering. Tire carcass must be very resilient, breakers must be heat resistant, and inner tubes must be airtight. The principal types of raw rubber used for tires are styrene-butadiene, stereoregular butadiene, synthetic isoprene, and natural rubber; the most important reinforcing materials are polyamide and rayon cords.
Tire manufacture includes the following production processes: preparation of the rubber stocks in mixers, processing of the cords (impregnation with synthetic latexes, heat stretching, stabilization, and coating with rubber on calenders), preparation of components (cutting out the rubberized cord, assembling the pieces, coating the pieces with layers of rubber, shaping the tread stock with extruders, making the sidewall components), assembly of the casing on special machines, shaping and vulcanization of the casings on a single machine, and processing of stock for inner tubes with extruders and vulcanization in compression molds. Many of the operations are performed on automatic transfer lines, including preparation of the rubber stocks and assembly and vulcanization of the casings. (See alsoRUBBER, SYNTHETIC; RUBBER, NATURAL; and RUBBER, VULCANIZED.)
REFERENCESPnevmaticheskie shiny (issledovaniia po probleme povysheniia kachestva): Sb. st. Moscow, 1969.
Pnevmaticheskie shiny. Moscow, 1973.
Saltykov, A. V. Osnovy sovremennoi tekhnologii avtomobil’nykh shin, 3rd ed. Moscow, 1974.
Enlsiklopediia polimerov, vol. 3. Moscow, 1977.
V. F. EVSTRATOV