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Tires and Passenger Vehicle Fuel Economy: Informing Consumers, Improving Performance - TRB Special Report 286 2 Background on Passenger Tires This chapter begins with an introduction and overview of basic terminology and trends pertaining to passenger tires and their use in the United States. The introductory discussion is followed by background on the development of tires, the structure of the tire industry, and tire regulations and standards. TIRE TERMINOLOGY AND TRENDS Pneumatic, or air-filled, tires are used on vehicles as diverse in form and function as airplanes, bicycles, tractors, and race cars. Accordingly, they encompass a wide range of sizes, designs, materials, and construction types. Nevertheless, structural elements that are common to all of these tires are the casing, bead, and tread band. The casing—often called the carcass—is the structural frame of the tire. It usually consists of directionally oriented cords banded together by rubber into layers, called plies, which give the tire strength and stiffness while retaining flexibility. The number of plies is determined by tire type, size, inflation pressure, and intended application. Plies oriented mainly from side to side are “radial,” while plies oriented diagonally are “bias.” In the area where the tread is applied, the plies in the radial casing are usually covered by a relatively stiff steel belt or a steel belt covered by a circumferential nylon cap ply. The steel belt is made by using fine wire twisted into cables as cords. For the inflated tire to be retained on the wheel rim, the plies are anchored around circumferential hoops made of multiple strands of fine, high-tensile wire located at the inner edges of the two sidewalls where they mate with the rim. These two hoops, called beads, are pressed against the rim flange by inflation pressure,
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Tires and Passenger Vehicle Fuel Economy: Informing Consumers, Improving Performance - TRB Special Report 286 thereby seating and sealing the tire on the rim. Encircling the tire is the tread. This is a thick band of rubber that forms the tire surface, from its crown (its largest radius) to its shoulders (the areas in which the tread transitions to the sidewalls). The tread is the only part of the tire that comes in contact with the road surface during normal driving. The tread band consists of a grooved section on top of a base. The tread’s design, including its grooved pattern, helps in the removal of road surface water and other contaminants from under the tire while maintaining an adequate level of frictional adhesion between the tire and road to generate torque, cornering, and braking forces under a wide range of operating conditions. For most passenger tires, the grooves start out 9/32 to 13/32 inch deep. Tires are normally considered worn when only 2/32 inch of tread remains. Most steel-belted radial passenger tires weigh more than 20 pounds, and they can exceed 50 pounds. The steel typically makes up about 15 percent of the total weight, the cord material another 5 percent, and the rubber compound in the carcass and tread about 80 percent (Modern Tire Dealer 2006, 51). Most of the rubber compound’s weight is from natural and synthetic polymers and reinforcing fillers. Other materials added to the compound during processing, such as oils, can contribute 3 to 25 percent of its weight. Because these compounding materials can account for about half of a tire’s total production cost, fluctuations in material prices can have important effects on tire retail prices (Modern Tire Dealer 2006, 46). The largest application of pneumatic tires is on highway vehicles, which consist of heavy and medium trucks, commercial light trucks, and cars and light trucks used as passenger vehicles. Heavy and medium trucks range from buses to tractor-trailers and construction vehicles. Their tires are designed for heavy workloads, long-distance travel, and rough terrain. Commercial light trucks include many full-size pickups and vans, as well as some SUVs. Their tires are designed mainly for rough terrain and heavy loads. Cars and light-duty trucks used for passenger transportation are the most common vehicles on the highway. Their tires are designed mainly for ride comfort, traction, handling, and wear life, as well as appearance and affordability. The focus of this study is on tires used on passenger cars and light-duty trucks. The federal government defines and regulates these passen-
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Tires and Passenger Vehicle Fuel Economy: Informing Consumers, Improving Performance - TRB Special Report 286 ger tires in the Federal Motor Vehicle Safety Standards (FMVSS), which are described later in this chapter. All cars are equipped with passenger tires, which usually contain the prefix “P” before their metric size designation molded into the tire sidewall. Even though they are classified as light trucks by the federal government, most SUVs, pickups, and vans used as passenger vehicles are equipped with passenger tires. The kinds of light- and medium-duty trucks used in commercial service, including full-size pickups and vans, have a gross vehicle weight rating of more than 6,000 pounds. These vehicles are usually equipped with tires having the letters “LT” molded into the sidewall. Designed for heavy loads and rough terrain, the LT tires are regulated separately by the federal government and are not part of this study. As a practical matter, the focus is on P-metric tires. Passenger tires are supplied to automobile manufacturers as original equipment (OE) and to motorists in the replacement market. Statistics on annual shipments of passenger tires for both OE and replacement uses are shown in Figure 2-1. More than 250 million passenger tires were FIGURE 2-1 Passenger tire shipments in the United States replacement and OE markets, 1990–2004. (SOURCE: RMA 2005, 11–12.)
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Tires and Passenger Vehicle Fuel Economy: Informing Consumers, Improving Performance - TRB Special Report 286 shipped in the United States in 2004, including about 199 million replacement tires and 53 million OE tires.1 Thus, replacement tires account for about 80 percent of passenger tire shipments. According to tire dealer data, Americans spent about $20 billion on replacement passenger tires in 2005 (Modern Tire Dealer 2006, 42). Tire shipment statistics reflect the changing size, age composition, and patterns of use of the U.S. motor vehicle fleet. The number of passenger vehicles in the fleet rose by 21 percent from 1990 to 2002. It was boosted by the addition of 14 million to 17 million new vehicles sold each year and a tendency for vehicles to remain in service longer (Davis and Diegel 2004, 4-5, 4-6). Passenger vehicles are driven an average of 12,000 miles per year, which is an increase of nearly 10 percent since 1990 (Davis and Diegel 2004, 4-2, 4-3). The combination of a growing fleet, vehicles lasting longer, and vehicles being driven more miles has fostered growth in the tire replacement market, which experienced a 33 percent increase in shipments from 1990 to 2004. HISTORY OF TIRE DEVELOPMENT The history of passenger tire development is punctuated by innovations and improvements in tire designs, materials, and manufacturing techniques. Three major periods of development merit attention: (a) the early era coinciding with the mass introduction of the automobile from the early 1900s into the 1930s; (b) the middle of the 20th century, when synthetic rubber became common and major design innovations such as tubeless and radial-ply tires came about; and (c) the period since the mass introduction of radial tires in North America beginning in the 1970s.2 1 Data on tire shipments are provided by the Rubber Manufacturers Association (RMA) and do not include shipments by companies that are not members of the association. RMA estimates that 79 million tires were imported in 2004 and that 68 million of them were manufactured by RMA companies (RMA 2005, 18). This differential suggests that about 11 million tires were imported by companies that are not members of RMA. Presumably, most of these 11 million tires were sold in the replacement market. The 11 million are not reflected in Figure 2-1. 2 Historical information in this section was derived from the following sources: T. French 1989; Tomkins 1981; RMA 2005; M. French 1989; Rajan et al. 1997; Lindemuth 2005; and Moran 2001.
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Tires and Passenger Vehicle Fuel Economy: Informing Consumers, Improving Performance - TRB Special Report 286 Early Tire Developments In the 1840s, Charles Goodyear invented the rubber mixing and curing process known as vulcanization, which was critical in making natural rubber a useful material for a wide range of products. John Boyd Dunlop patented the pneumatic tire for use on bicycles in the 1880s, and by the end of the century, Michelin in France, Goodrich in the United States, and others had adapted the pneumatic tire to the automobile. Within a few years, many companies with now familiar brand names were making tires, including B. F. Goodrich, Firestone, General, Goodyear, and U.S. Rubber (later Uniroyal) in the United States and Continental, Dunlop, Michelin, and Pirelli in Europe. By World War I, tens of thousands of cars, trucks, and buses were being mass produced each year in the United States, which created a burgeoning demand for tires and many other rubber products such as hoses, belts, and gaskets. New mixers, conveyor systems, and other time- and labor-saving equipment enabled tire production to keep pace with the growing output of automotive assembly lines. Nevertheless, the rapid changes in automobile technologies, new road surfaces, and faster and more frequent driving created new performance demands on tires. In this fast-changing environment, tire companies were forced to learn much about tire design and construction. Seeking a competitive advantage, tire companies began to invest more in research and development. They found that by replacing the rubber-coated and cross-woven canvas in the tire’s casing with plies of rubberized and directionally oriented fabric, the tire’s fatigue life was greatly extended. They also found that adding reinforcing agents, such as carbon black powder, to natural rubber greatly increased its resistance to abrasion and allowed tires to operate thousands of miles, rather than hundreds, before wearing out. The discovery of many other valuable rubber additives followed and further extended tire service life by slowing degradation from oxygen, heat, ultraviolet radiation, ozone, and moisture. The gains in tire wear life were accompanied by gains in operational performance, as understanding grew about the tire’s central role in vehicle steering, handling, and braking. Aided by improvements in tire molds and rubber compounding, tire makers introduced better gripping and more durable tread patterns during this period. The bias-ply construction,
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Tires and Passenger Vehicle Fuel Economy: Informing Consumers, Improving Performance - TRB Special Report 286 in which plies are oriented diagonally and at alternating angles, became common. This construction, along with the introduction of the steel rim, allowed the tire to support more weight—and thus enabled cars to become larger and heavier during the 1920s and 1930s. Midcentury Developments When Japan gained control of Asian rubber plantations during World War II, the United States imposed strict controls on rubber consumption by sharply curtailing the production of tires for nonmilitary purposes and by rationing motor fuel and thus driving activity. At the beginning of the war, the federal government estimated that rubber production could be sustained to meet wartime needs for only about 3 years; hence, it called on the nation’s chemical companies and research institutions to accelerate the development and introduction of synthetic rubbers made from petroleum and natural gas. This major research and development effort was highly successful and resulted in the annual production of hundreds of thousands of tons of synthetic rubber by 1944.3 Having gained experience with synthetics on military tires, tire companies adapted them to passenger tires after the war. When used in tread, synthetic rubber was found to have elasticity characteristics helpful in improving traction. Impermeable synthetic rubbers could be molded into tire inner liners, which allowed the development of tubeless tires. They improved tire puncture resistance by retaining air when damaged and were much easier to mount. By the 1950s, more than two-thirds of the rubber used in tires was synthetic (RMA 2005, 10). Another important development in tire technology in the decade after World War II was the advent of the steel-belted radial-ply tire and its commercial introduction in Europe by Michelin. Radial-ply tires differed in several respects from bias-ply tires. Whereas the cords in biasply tires run diagonally, the carcass cords in radial-ply tires run more directly from bead to bead, perpendicular to the tire’s circumference— an orientation made possible because the tread is stabilized by a stiff cir- 3 A history of this period of the tire industry’s development is given by Morawetz (2002) and is recounted in the video Modern Marvels—Rubber aired by the History Channel and available at www.historychannel.com.
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Tires and Passenger Vehicle Fuel Economy: Informing Consumers, Improving Performance - TRB Special Report 286 cumferential belt. Today, the belt plies are usually reinforced by small cords made of fine steel cable. The radial-ply tire offered two critical advantages: a much more stable tread foundation and a more flexible sidewall. These advantages translated into the practical outcomes of longer tread life, better wet and dry traction, improved puncture resistance, and reduced rolling resistance and energy consumption. Modern Radial Era As American motorists began driving foreign vehicles and some U.S. models equipped with radial-ply tires during the 1970s, they began demanding these tires in larger numbers. By the beginning of the 1980s, radial tires had become the standard construction type for both OE and replacement tires. Radials accounted for about 60 percent of passenger tire shipments in 1980, 97 percent by the end of the 1980s, and 99 percent in 2005 (Modern Tire Dealer 2006, 51). Tire wear life was a key selling point for radials, because average tire wear life increased by thousands of miles. In addition, tire companies marketed “all-season” tires made possible by the stability of the steel belt as a structural foundation, which prevented tread cracking in the required cross-groove pattern for winter traction. This development brought an end to the practice among many North American motorists of switching to specialized snow tires during the winter months. Radials also offered improved handling, which led to a growing array of tires designed and marketed as “performance,” “high performance,” and “ultra-high performance.” Starting in the 1980s, tire manufacturers started rating more tires in North America according to their designed maximum operating speed. The desired speed rating affected the choice of materials and construction of the tire. For instance, tires with higher speed ratings required stronger steel belts and belt compounds covered by a nylon cap ply. The speed rating letter is printed on the passenger tire’s sidewall after sizing information.4 The most common speed rating 4 The rating is based on laboratory tests during which the tire is pressed against a 1.7-meter-diameter metal drum to reflect its appropriate load and is run at ever-increasing speeds (in 6.2-mph steps in 10-minute increments) until the tire’s rated speed is met.
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Tires and Passenger Vehicle Fuel Economy: Informing Consumers, Improving Performance - TRB Special Report 286 TABLE 2-1 Common Speed Ratings for U.S. Passenger Tires Speed Rating Symbol Speed (mph) Speed (km/h) Example Applications Percentage of Total OE Tire Shipments in 2004 Percentage of Total Replacement Tire Shipments in 2004 S, T 112–118 180–190 Family sedans and vans 83 74 H, V 130–149 210–240 Sport sedans and coupes 15 22 W, Y, Z >149 >240 High-performance sports cars 2 4 SOURCE: RMA 2005, 22. symbols, maximum speeds, and typical applications for U.S. passenger tires are shown in Table 2-1. While tire manufacturers do not recommend driving at the top speeds for each speed-rated tire, they use the ratings as one means of distinguishing tires with different performance capabilities. In general, tires rated for higher speeds will also be designed to offer superior performance in a number of respects other than speed, such as handling and steering response. The ratings help motorists maintain vehicle speed capability when they replace speed-rated OE tires. Figure 2-2 displays the information molded in the passenger tire sidewall, including the size designation that usually follows the tire’s FIGURE 2-2 Passenger tire sidewall information and major dimensions. (SOURCE: www.tireguides.com.)
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Tires and Passenger Vehicle Fuel Economy: Informing Consumers, Improving Performance - TRB Special Report 286 name. The tire’s section width (in millimeters) is the first number in the size designation, followed by its aspect ratio, which is calculated by dividing the tire’s section height by its section width and multiplying by 100. Rim diameter (in inches) is the last number in the series, after “R” for radial. Hence a passenger tire with size designation P215/65/R15 has a section width of 215 millimeters, an aspect ratio (or profile series) of 65, and an inner circumference to fit a rim 15 inches in diameter. Tire industry survey data indicate that eight of the 10 most popular OE tire sizes for Model Year 2005 passenger vehicles fit 16- and 17-inch rims. Because it takes 3 or more years for OE sizing trends to make their way to the replacement market, tires with 15-inch rim sizes remained common among replacement tires in 2005 (Table 2-2). The OE data in Table 2-2 show the growing popularity of tires with larger section widths and lower aspect ratios—trends that have also become more evident in the replacement market with the availability of “plus-size” custom wheels to replace the original wheel and tire combination. With regard to possible future trends in the replacement market, tires with specially reinforced sidewalls, known as run-flat tires, have grown in popularity in the OE segment. Although they accounted for less than TABLE 2-2 Passenger Tire Size Popularity, 2005 OE Tire Size Percentage of Total OE Tires Shipped Replacement Tire Size Percentage of Total Replacement Tires Shipped P215/60/R16 6.0 P232/60/R16 6.4 P205/65/R15 5.2 P235/75/R15 6.0 P265/70/R17 5.0 P205/65/R15 4.7 P245/65/R17 4.6 P215/70/R15 4.0 P235/70/R16 4.3 P205/70/R15 3.7 P195/60/R15 3.5 P195/65/R15 3.4 P245/70/R17 3.2 P185/65/R14 3.1 P205/60/R16 3.0 P195/60/R15 2.7 P225/60/R17 2.8 P195/70/R14 2.7 P265/65/R17 2.6 P205/55/R16 2.4 Total, top 10 40.2 Total, top 10 39.1 SOURCE: Modern Tire Dealer 2006, 45.
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Tires and Passenger Vehicle Fuel Economy: Informing Consumers, Improving Performance - TRB Special Report 286 1 percent of replacement sales in 2005, their rate of growth will be influenced by OE acceptance (Modern Tire Dealer 2006, 46). These air-filled but partially structure-supporting tires are designed to operate with the loss of inflation, down to zero inflation pressure for speeds up to 55 mph for a distance of up to 50 miles. Originally developed for two-seat sports cars with little room for spare tires and jacks, run-flat tires can now be found on other passenger vehicles. They are marketed for their convenience and safety in the event of a flat in a remote or hazardous location. As noted later in the report, run-flat tires weigh more than conventional radial tires—which increases their material and production cost—and they tend to exhibit higher rolling resistance. TIRE INDUSTRY STRUCTURE The tire industry is international and driven by competition. The majority of OE and replacement tires sold in the United States are produced by several large domestic and foreign manufacturers, all operating internationally, including Michelin (France), Goodyear (United States), Bridgestone/Firestone (Japan), Pirelli (Italy), Cooper (United States), Toyo (Japan), Kumho (South Korea), Continental (Germany), Hankook (South Korea), Yokohama (Japan), and Sumitomo (Japan). Potentially adding to the competitive mix in the replacement market is the growing number of passenger tires produced by companies based in China, Taiwan, India, and other industrializing countries (Modern Tire Dealer 2006, 51). Tire manufacturers supply the two distinct—albeit related—markets: OE and replacement. Automobile manufacturers buy in large volumes that give them influence over tire prices and specifications. They demand tires with characteristics that suit their vehicle designs, marketing strategies, and production schedules. In turn, OE orders allow tire companies to keep their production facilities operating at efficient volumes. The OE business also can help generate future sales of replacement tires. By linking its tire lines with a specific vehicle make or model, a tire company can draw on the brand loyalty of motorists. Because four times as many replacement tires as OE tires are sold, such brand loyalty can be valuable to the tire manufacturer.
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Tires and Passenger Vehicle Fuel Economy: Informing Consumers, Improving Performance - TRB Special Report 286 Like makers of many other consumer goods, tire manufacturers seek to distinguish their products from those of competitors through branding. Most sell under heavily advertised manufacturer (or national “flag”) brands as well as associate and specialty brands, some acquired through mergers and acquisitions of well-known tire companies. Goodyear, for instance, sells under its own name and several other nationally recognized brands; it owns Dunlop (in the United States) and Kelly. Likewise, Michelin has acquired the BFGoodrich and Uniroyal brands in the United States, and Bridgestone also sells tires under the Firestone and Dayton brand names. These nine brands accounted for 51.6 percent of the replacement tire consumer market in 2005 (Modern Tire Dealer 2006, 39). Most major tire companies supply both the OE and the replacement markets. They typically use their flag brands for the former and a combination of flag and associate brands for the latter. An exception to this practice is Cooper Tire, which concentrates on serving the replacement market. It sells tires under its own brand name and under associate brands such as Starfire, Dean, and Mastercraft. In addition, most tire makers supply replacement tires to retailers selling under private labels, such as the Sears Guardsman, Wal-Mart Douglas, and Pep Boys Futura. In these cases, the retailer creates and controls the brand, often contracting for supplies from one or more tire makers offering the lowest price or other valued attributes such as supply reliability. OE Market OE tires outfitted on a specific vehicle are usually developed and supplied by one or two preselected tire makers. From the standpoint of the automobile manufacturer, it can sometimes be advantageous to engage at least two OE tire suppliers to ensure an ample and timely supply and to foster competition. As part of the development process, experimental tires are usually submitted to the automobile manufacturer by the tire maker, along with various test measurements. The tires are evaluated, and further refinements are made as needed. Most automobile companies have in-house tire testing facilities and expertise to assist in tire evaluation and specification. OE tires are usually specified in both quantitative and qualitative terms. The OE specification sheet will define the tire’s physical dimensions, such as mass, width, and diameter within the parameters of tire
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Tires and Passenger Vehicle Fuel Economy: Informing Consumers, Improving Performance - TRB Special Report 286 and rim standards. Because the tire is integral to the vehicle’s suspension, steering, acceleration, and braking, the automobile maker will also set precise and quantifiable targets for properties such as force and moment (cornering coefficient, aligning torque coefficient, etc.); deflection (spring rate); and traction (friction coefficients) in wet, dry, and snow conditions. Other quantifiable properties that are usually specified include electrical conductivity (resistance to static shock), speed endurance (suitable to the vehicle’s speed capability), tire wear resistance, and rolling resistance (rolling resistance coefficient).5 In addition, the automobile manufacturer will define several other tire attributes, sometimes through more qualitative means, such as the tire’s expected noise and vibration levels, sidewall appearance, and tread image. Some OE tire specifications are governed by FMVSS such as those covering tire structural safety and rim selection. These apply to all passenger tires. Other OE specifications are strongly influenced by the federal safety standards and other regulations applying to motor vehicles. For example, OE tire designs are influenced by federal standards for passenger vehicle brake systems and motor vehicle fuel economy. Replacement Market The logistics of tire manufacturing, inventorying, and distribution in the replacement market are focused on serving the complete market. Most replacement tires are designed to perform on the wide range of vehicles in the fleet, including vehicle models dating back many years. Hence, whereas the OE market is characterized by the supply of large quantities of select tire types and sizes, suppliers competing in the replacement market must offer a wide variety of tire sizes and types, generally produced in smaller quantities. As a result of market competition, evolving consumer demands and preferences, and changing tire dimensions and specifications introduced in the OE segment, the spectrum of replacement tire sizes and types is continually expanding. At any one time, replacement tires from hundreds of brands and lines are for sale in the marketplace, which consists of tens of thousands of individual products, or stock-keeping units, when size variability is taken 5 See Lindemuth (2005) for a more detailed listing of performance criteria and measures.
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Tires and Passenger Vehicle Fuel Economy: Informing Consumers, Improving Performance - TRB Special Report 286 into account. Consumers may choose among a handful to several dozen tire lines for their replacement needs. The choices range from national Internet and mail-order companies to tire dealers, manufacturer outlets, and retail department stores (Figure 2-3). Typically, the tires bought in the replacement market are balanced and mounted by the tire dealer, who adds about $50 to the cost of purchasing a set of four tires (Modern Tire Dealer 2006, 55). TIRE SAFETY AND CONSUMER INFORMATION STANDARDS Even as they market their products to differentiate among tire brands and lines, tire companies recognize the value of standardization. Early FIGURE 2-3 Distribution channels for replacement tires in the United States. (SOURCE: RMA 2005, 13.)
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Tires and Passenger Vehicle Fuel Economy: Informing Consumers, Improving Performance - TRB Special Report 286 in its history, the tire industry suffered from excessive product differentiation, especially in tire dimensions. Tires designed and configured for just one vehicle proved costly and difficult to replace when damaged or worn. Automobile manufacturers therefore advocated common size designations to promote interchangeability and competition in supply. Today’s passenger tires must conform to a number of standards. Some are required by government, while others are adopted voluntarily by industry and developed through national and international standard-setting bodies. Tire speed ratings, as previously discussed, are an example of a standard developed and implemented by industry. The following subsections describe those standards for passenger tire safety and consumer information that are required by the federal government.6 Federal Safety Regulations for Passenger Tires Between 1966 and 1970, Congress passed several acts defining and expanding the federal government’s role in regulating motor vehicle safety and creating the National Highway Traffic Safety Administration (NHTSA) under the U.S. Department of Transportation to implement them. NHTSA promulgated a series of FMVSS affecting various systems and components of the motor vehicle, such as interior displays and controls, brakes, and occupant protection devices. The rules governing tires cover two main areas: tire structural integrity and fitment. With regard to structural integrity, the regulations prescribe a battery of tests that must be passed demonstrating Tread plunger strength (a round hub is pressed against the tread with a given force to test strength), Resistance to bead unseating, High-speed performance at constant load and variable speed, and Endurance at constant speed and variable load. After passage of the federal TREAD Act of 2000,7 a low-pressure tire endurance test was developed for introduction, along with additional 6 See Walter (2005) for a more detailed review of government and industry standards and regulations pertaining to passenger tires. 7 The Transportation Recall, Enhancement, Accountability, and Documentation (TREAD) Act (Public Law 106-414) was signed into law on November 1, 2000.
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Tires and Passenger Vehicle Fuel Economy: Informing Consumers, Improving Performance - TRB Special Report 286 requirements for the testing of tire endurance. These requirements are scheduled to take effect in 2007. More additions to the regulations are anticipated in response to the TREAD Act as NHTSA examines tests for tire aging. With regard to tire sizing and fitment, the federal regulations require that all tires conform to standards for size, load, and pressure relationships developed by standard-setting bodies such as the U.S. Tire and Rim Association, the European Tire and Rim Technical Organization, and the Japan Automobile Tire Manufacturers Association.8 NHTSA requires tire makers to print sizing information on the tire sidewalls. Tires in compliance with the federal safety standards are marked with the “DOT” symbol (for U.S. Department of Transportation), along with additional information such as the location and date of tire production, maximum pressure, and tire material and construction type. Other FMVSS regulations influence tire design and construction, including braking standards for motor vehicles. Recently, NHTSA adopted a new rule that will require tire pressure monitoring systems to be installed on all new passenger cars and light trucks starting with 2007 vehicle models. Federal Consumer Information Requirements for Passenger Tires Separate from the federal tire safety requirements are federal requirements intended to provide consumers with information for making tire purchases. The Uniform Tire Quality Grading (UTQG) system applies to all passenger tires with the exception of winter tires and compact spares. In its current form since 1980, the UTQG system consists of grades for tread wear, wet traction, and temperature resistance. Manufacturers typically test one or more tire models from a tire line or grouping to establish the grades for each of the three qualities, which are then molded on the tire sidewall. 8 Other bodies include the Deutsche Industrie Norm, the British Standards Institution, the Scandinavian Tire and Rim Organization, and the Tyre and Rim Association of Australia.
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Tires and Passenger Vehicle Fuel Economy: Informing Consumers, Improving Performance - TRB Special Report 286 Tread Wear Grade The UTQG tread wear grade is a comparative rating generated from the results of an outdoor highway test course in which the subject tire is run in a convoy with several standardized “course-monitoring” tires. After 7,200 miles, the subject tire’s wear rate is compared with that of the monitoring tires. The tire manufacturer assigns a tread wear grade on the basis of extrapolations of measured wear rates. The ranking scheme suggests that a tire rated 200 should wear twice as long as a tire rated 100 on the government test course. The relative performance of tires, however, depends on the conditions of use, and therefore it may depart significantly from the norm because of variations in operating conditions and maintenance. The 2,371 rated passenger tire lines have the following distribution of tread wear grades according to information on NHTSA’s website:9 200 or lower, 11 percent; 201 to 300, 21 percent; 301 to 400, 33 percent; 401 to 500, 22 percent; 501 to 600, 8 percent; above 600, 5 percent. Neither NHTSA nor tire manufacturers are willing to associate expected mileage levels with particular grades because of the variability in wear that can occur on the basis of vehicle operating conditions, road conditions, tire maintenance, and individual driving patterns. Traction Grade UTQG traction grades are based on a tire’s measured coefficient of friction when it is tested on wet asphalt and concrete surfaces. The subject tire is placed on an instrumented axle of a skid trailer, which is pulled behind a truck at 50 mph on wet asphalt and concrete surfaces. The trailer’s brakes are momentarily locked, and sensors on the axle measure the longitudinal braking forces as it slides in a straight line. The coefficient of friction is then determined as the ratio of this sliding forced to the tire load. Grades of AA, A, B, and C are assigned according to the criteria shown in Table 2-3. Traction grades are intended to indicate a tire’s ability to stop on wet pavement. The UTQG traction grade does not take into account other aspects of traction, such as peak traction, traction on dry or snow-covered surfaces, or cornering traction. NHTSA website data indicate that of the 9 www.safercar.gov/tires/pages/Tires2.cfm. Results reported to NHTSA are not sales weighted.
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Tires and Passenger Vehicle Fuel Economy: Informing Consumers, Improving Performance - TRB Special Report 286 TABLE 2-3 UTQG Traction Grades Traction Grade Wet Asphalt Sliding Friction Coefficient Wet Concrete Sliding Friction Coefficient AA >0.54 >0.38 A >0.47 >0.35 B >0.38 >0.26 C <0.38 <0.26 2,371 rated passenger tire lines, 4 percent were graded AA, 78 percent A, and 18 percent B and C.10 Temperature Grade A tire operating at normal speeds can achieve internal temperatures in excess of 180°F. The UTQG temperature grade indicates the tire’s resistance to the generation of heat during operation at elevated speeds. Sustained high temperature can cause the material of the tire to degrade and reduce tire life, while excessive temperature can lead to sudden tire failure. Tires are tested under controlled conditions on a high-speed laboratory test wheel. The focus is on speed effects of properly loaded and inflated tires. Underinflation and overloading, which can cause heat buildup at normal speeds, are not tested. Tires are rated A, B, or C, with A being the highest grade. Tires graded A completed a 30-minute run at 115 mph without failing; tires graded B completed a 30-minute run at 100 mph, but not 115 mph; and tires graded C failed to complete a 30-minute run at 100 mph. According to NHTSA website data, 27 percent of the 2,371 rated passenger tire lines have an A grade, 59 percent a B grade, and 11 percent a C grade.11 SUMMARY Most vehicles used for personal and family transportation, including the growing number of vehicles designated as light trucks and multi-purpose passenger vehicles (i.e., vans, SUVs), are equipped with tires 10 www.safercar.gov/Tires/pages/Tires2.cfm. The data are undated. 11 www.safercar.gov/tires/pages/TireRatTemperature.htm. The data are undated.
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Tires and Passenger Vehicle Fuel Economy: Informing Consumers, Improving Performance - TRB Special Report 286 that are regulated by the federal government as passenger tires. Passenger tires make up the large majority of OE and replacement tires in the light motor vehicle fleet. Today’s passenger tire is a complex engineering composite that has evolved over the past century to function as a crucial structural and dynamic component of the vehicle. Its main structural components, as in all pneumatic tires, are the casing, tread, and bead. All of the components have been the subject of major advances in designs, materials, and construction methods. The most significant development in recent decades was the mass introduction of radial-ply tires starting in the 1970s in the United States. The radial-ply construction has had substantial positive effects on the durability, handling, and energy performance of passenger tires. About 250 million tires are shipped each year in the United States, and about 80 percent are replacement tires. The number and type of tires shipped reflect the size and composition of the passenger vehicle fleet. Growing sales of light trucks (vans, pickups, and SUVs) have led to an expanding array of sizes and performance capabilities in OE tires, which have evolved in the replacement market. The tire industry serves two distinct, albeit related, markets: OE and replacement. OE tires are developed for specific vehicles and are designed to work closely with the vehicle’s suspension, steering, and braking systems and to meet other automobile maker goals for their tires such as appearance, noise, durability, and rolling resistance. Replacement tires, in contrast, are designed to perform on a much wider range of vehicle brands and models. Variations in tire sizes, models, and types, as well as required years of availability, mean that there are tens of thousands of unique replacement tire products in the marketplace. Passenger tires must conform to a number of government and industry standards. All passenger tires must pass federal tests for structural integrity, which are aimed at preventing rapid loss of pressure, unseating, and loss of structural form that could cause a driver to lose control of the vehicle. In consumer-oriented regulations separate from its safety requirements, the federal government also requires passenger tires to be graded for traction, tread wear, and temperature resistance. The grades, which are molded into the tire sidewall, are not safety minima but are
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Tires and Passenger Vehicle Fuel Economy: Informing Consumers, Improving Performance - TRB Special Report 286 intended to provide consumers with information for making tire purchases. The tire industry has established its own standards for tire sizing and fitting and for rating a tire’s speed capabilities, which are also used by consumers in selecting tires suited to their particular vehicles and driving patterns. REFERENCES Abbreviation RMA Rubber Manufacturers Association Davis, S. C., and S. W. Diegel. 2004. Transportation Energy Data Book: Edition 24. Report ORNL-6973. Center for Transportation Analysis, Oak Ridge National Laboratory, Oak Ridge, Tenn. French, M. 1989. Manufacturing and Marketing: Vertical Integration in the U.S. Tire Manufacturing Industry, 1890s–1980s. Business and Economic History, Vol. 18, pp. 178–187. French, T. 1989. Tyre Technology. Adam Hilger, Bristol, England. Lindemuth, B. E. 2005. An Overview of Tire Technology. In The Pneumatic Tire (J. D. Walter and A. N. Gent, eds.), National Highway Traffic Safety Administration, Washington, D.C., pp. 1–27. Modern Tire Dealer. 2006. Modern Tire Dealer’s Facts Issue. www.moderntiredealer.com. Jan. Moran, T. 2001. The Radial Revolution. Invention and Technology, Spring, pp. 28–39. Morawetz, H. 2002. Polymers: The Origin and Growth of a Science. Dover Phoenix Editions, New York. Rajan, R., P. Volpin, and L. Zingales. 1997. The Eclipse of the U.S. Tire Industry. Working paper, National Bureau of Economic Research Conference on Mergers and Productivity, March. RMA. 2005. Factbook 2005: U.S. Tire Shipment Activity Report for Statistical Year 2004. Washington, D.C. Tomkins, E. S. 1981. The History of the Pneumatic Tyre. Eastland Press, London. Walter, J. D. 2005. Tire Standards and Specifications. In The Pneumatic Tire (J. D. Walter and A. N. Gent, eds.), National Highway Traffic Safety Administration, Washington, D.C., pp. 655–669.
Representative terms from entire chapter: