Tires and Passenger Vehicle Fuel Economy Informing Consumers, Improving Performance -- Special Report 286 (2006) / Chapter Skim
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4 Rolling Resistance, Traction, and Wear Performance of Passenger Tires
4 Rolling Resistance, Traction, and Wear Performance of Passenger Tires
Pages 77-104
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From page 77... ...
A complex picture of the numerous factors affecting tire rolling resistance and fuel economy emerged. Among the factors are tire geometry, tread compounds, inflation pressure, alignment, operating temperature, load, and tire construction type.
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From page 78... ...
When the tread wears and traction capabilities are diminished beyond a point deemed acceptable for safe operation, especially in wet and snow conditions, the tire is normally scrapped -- and thus becomes a candidate for the waste stream. Years of tire testing and experimentation have helped tire manufacturers understand the chemical and physical relationships that affect tire traction, wear resistance, and rolling resistance.
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From page 79... ...
Nevertheless, in combination with data on other tire properties, such as tread depth, their analysis can be helpful in identifying potential relationships and highlighting factors warranting further examination. EFFECTS ON TRACTION AND SAFETY PERFORMANCE Most data on the involvement of tires in motor vehicle crashes cover tire structural failures, as opposed to the safety role of specific tire operating characteristics such as traction.
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From page 80... ...
They generate all the forces that control its motion and direction, and a tire's properties clearly could be a factor in motor vehicle crashes and their avoidance. However, at what point a change in tire traction characteristics will lead to measurable changes in crash incidence and severity is unknown.
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From page 81... ...
Tires are graded for wet traction, temperature resistance, and tread wear. However, these grades are not safety ratings, and NHTSA has not studied how they relate to tire and motor vehicle safety performance in the field.
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From page 82... ...
Nevertheless, the committee believes that the combined data sets offer greater analytical opportunity for a general investigation of possible relationships. UTQG Traction Grades and Rolling Resistance Chapter 2 describes how passenger tires are tested by the UTQG system for wet traction and assigned a grade of AA, A, B, or C
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From page 83... ...
The utility of the UTQG traction grades for exploring possible rela tionships with other tire characteristics such as rolling resistance is diminished by the wide range of friction coefficients within each grade, which leads to a preponderance of tires across a wide array of sizes and types receiving a grade of A Without access to the measured friction coefficients underlying the grades assigned to individual tires, the rela tionships between traction and other characteristics cannot be estab lished precisely.
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From page 84... ...
Thus, whether achieving a low RRC and a wet traction grade of A is more difficult for tires designed for 13-, 14-, and 15-inch rims than it is for tires with larger rim diameters would be useful to determine. Figure 4-2 suggests that low RRCs are less common among the smaller tires with an A traction grade.
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From page 85... ...
. In summary, the data suggest the difficulty of achieving both an AA traction grade and very low rolling resistance, even among tires having larger rim diameters in the current market.
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From page 86... ...
It is of interest that passenger tires with a wet traction grade of AA -- which are disproportionately tires with speed ratings of W, Y, or Z -- are more likely to be used on high-performance sports cars than are tires with A or B traction grades. Few studies associating tire traction and crash incidence and severity have been undertaken.
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From page 87... ...
"Low" is a relative term. Differentials of 25 percent or more in RRCs can be found today among replacement tires having the same UTQG traction grades and other characteristics.
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From page 88... ...
A review of discarded tire samples by Michelin revealed that tread wear, both normal and abnormal, accounts for between two-thirds and three-quarters of discarded tires.5 Factors affecting tread wear and life span are therefore important not only from the standpoint of the motorist, who must buy tires more often if they wear out sooner, but also from the standpoint of society's interest in controlling scrap tire populations.
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From page 89... ...
licensing or registration requirements for scrap tire haulers, processors, and some end users; (b) manifests for scrap tire shipments and controls concerning who can handle scrap tires; (c)
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From page 90... ...
Some also support research to assess the environmental effects of using tires in various ways, including analyses of emissions from tire-derived fuels, leaching from tires used as fill, and the disposition of residue from tire processing. According to RMA data, 130 million of the 290 million scrap tires generated in 2003 were reused as tire-derived fuel in vari ous industrial facilities and about 100 million were recycled into new products (RMA 2005, 48)
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From page 91... ...
Nevertheless, the abatement of these stockpiles adds supply to the scrap tire markets, which complicates efforts to find economical uses for the millions of new scrap tires generated each year. UTQG Tread Wear Grades and Rolling Resistance Tires are rated for tread wear as part of UTQG.
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From page 92... ...
suggests the possibility of a relationship between rolling resistance and UTQG tread wear grade, which warrants more data for thorough statistical analysis involving more explanatory variables. Explaining Variability in RRC and Tread Wear Grades Multivariate analysis can help determine whether there is a relationship between RRC and UTQG tread wear rating and other variables among the 196 tires in the combined data.
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From page 93... ...
tires with speed rating of S or T (continued on next page)
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From page 94... ...
, · UTQG tread wear rating, and · Market [replacement or original equipment (OE)
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From page 95... ...
are included as dummy variables.6 The results of the two bestfitting regression models are presented below.7 The first model seeks to explain variability in RRC. The second seeks to explain variability in UTQG tread wear rating.
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From page 96... ...
These results imply that to obtain a 10 percent reduction in RRC, an average tire's tread depth would need to decrease by about 22 percent. The relationship between RRC and traction is more difficult to explore because most tires are graded A for UTQG wet traction (out of a possible AA, A, or B)
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From page 97... ...
Tread wear ratings of tires having speed ratings of H or V are lower by 50 to 130 points. As might be expected, there is also a statistical relationship between a tire's tread wear rating and tread depth.
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From page 98... ...
They can be cumbersome and costly to perform because of the many factors influencing rolling resistance. The multiple regression results are consistent with findings from previous experimental studies showing that RRC can be lowered by reducing tread depth, as discussed in Chapter 3.10 The overall results indicate that tread depth must be reduced by slightly more than 2/32 inch to achieve a 10 percent reduction in RRC, if tread reduction is the only change made.
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From page 99... ...
The regression results do show that reductions in UTQG tread wear ratings are explained in part by lower tread depth. The 18 percent reduction in tread depth (about 2/32 inch)
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From page 100... ...
Yet even as average tire life has been extended, the constantly expanding fleet of passenger vehicles and increases in vehicle travel have resulted in increasing numbers of tires being sold. Fifty million more passenger tires are shipped in the OE and replacement markets today than were shipped in 1990 (RMA 2005)
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From page 101... ...
. Hence, a comparable set of tires would need to be replaced every 3.75 years, which would generate 1.07 scrap tires per year for each of the 220 million passenger vehicles in the fleet, or about 235 million scrap passenger tires nationally.
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From page 102... ...
Indeed, the existence of numerous tires having both low RRCs and an A grade for wet traction suggests the potential to reduce rolling resistance in some tires while maintaining the most common traction capability as measured by UTQG. RRC differentials of 20 percent or more can be found among tires of the same size, speed rating, and UTQG traction grade.
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From page 103... ...
At the same time, the likelihood of shorter wear life for tires designed with reduced tread depth or with less wear resistance for any other reason works against controlling the growth in scrap tires caused by escalating motor vehicle travel. The potential for such adverse outcomes suggests the importance of exploring means of reducing tire rolling resistance that do not degrade wear life.
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From page 104... ...
Gent, eds.) , National Highway Traffic Safety Administration, Washington, D.C., pp.
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