Tire Wear Rates

F. C. Brenner I Tire Wear Rates REFERENCE: Brenner, F.C., "Tire Wear Rates," Tire Science and Technology, Vol. 8, Nos. 1-2, Jan.-Jun. 1980, pp. 13-19. ABSTRACT: During the early stages of tire wear, the wear rate determined from groove depth measurements may reflect a dimensional change not associated with rubber loss, that is superimposed on the tread loss. The combined effect can be fit with an exponential decay curve relating groove depth to miles run. A method for separating any shape change effect from wear rate effects is suggested. KEY WORDS: Tires. tread wear, shape change, rubber loss Tread wear experiments usually consist of running tires repeatedly over a fixed course and periodical ly determining the amount of rubber worn away. The rubber loss is measured by one of two methods: the groove depth or the weight method. Stiehler, Steel, and Mandel [1], using both methods in the same experiment, found that the groove depth method showed a large loss during the initial period of use thereafter quickly decreasing to some rela- tively small, nearly constant value. In contrast, the weight method showed losses of the same order for all periods of service. As a result, wear rates deter- mined from weight data remained nearly constant throughout the life of the tire whereas the groove depth data showed an initially high wear rate which also decreased and became constant. Figure l taken from their paper illus- trates this. Spinner and Barton [2] comment ing on Fig. 1 suggest that " . . . there is some dimensional change, not associated with actual loss in rubber in the early stages of wear, that is superposed on the actual tread loss." Recent results of Brenner [3] support this suggestion, The conclusions derived from the weight method must be given pr imacy because it directly measures the rubber loss. The groove depth method in- directly assesses the loss by measuring a geometric dimension of the tire f rom which, in principle, the rubber loss can be estimated. However, this last method may be influenced by geometric changes which may give spurious wear loss data as Spinner and Barton conclude. The opinions, findings, and conclusions expressed in this publication are those of the author and not necessarily those of the National Highway Traffic Safety Administration. INational Highway Traffic Safety Administration, Washington, D.C. 13 14 TIRE SCIENCE AND TECHNOLOGY ~00 R O3 laJ J 25O r~ Z < 0 I 20O F - E ~ 150 ~> . . . . .q \ . . . . DEPTH - - WEIGHT I00 I0 1 1 1 1 I [ I 1 [ 8 I 2 3 4 5 6 1 8 PERIOD 3O bq ILl ...I 2S Z (,q O 20 J F IG . 1 - Comparative rates of wear for the same tire by groove depth and weight loss. Assuming that the physical properties of the rubber and the environmental conditions remain essentially constant, it may be that the expenditure of a specific amount of energy in a wearing process will result in the removal of a fixed amount of rubber. Even though the groove depth method gives anomalous results, it is the most popular because it is cheap and fast. The weight method requires that the tires be stored until they reach temperature and moisture equilibrium with the atmosphere in which they are weighed, a process which requires 12 to 24 h. In practice, one can avoid the production of what appears to be initially anomalous data by breaking in the tire prior to the test. For bias and belted bias tires, this process is economically feasible since these tires reach an equilibrium condition in less than one thousand miles. However, for radial tires, the break-in is about five thousand miles and cost becomes a factor. This paper reports an analysis of tire behavior in which such dimensional changes and the rubber loss effects would be separated. The present data were generated to test the tread wear grading procedure in the Uniform Tire.Quality Grading Standard [4]. The research was con- ducted on the prescribed test course in the vicinity of San Angelo, Texas, on radial tires. The grading procedure requires that one set of four Course Monitoring Tires (CMT) be run with no more than three vehicles containing candidate tires of the same construction type. The effects of the uncontrolled factors are adjusted for under the grading procedure by using the ratio of the CMT base course wear rate (BCWR) to the BRENNER ON GROOVE DEPTH CHANGES WITHOUT WEAR 15 360 34O 320 - - �9 300 - - �9 280 - - �9 260 - - 240 - - �9 22( - - �9 200 * * �9 180 �9 4 8 12 16 20 24 28 32 36 Mileage (k-miles) FIG. 2 Average groove depth for Jour CMT*" versus mileage run. average wear rate of four CMTs in the test. The BCWR for the radial tire is 4.44 mils per thousand miles, based on numerous test results [5] The product of the obsered wear rate of the candidate tire and this ratio gives its adjusted wear rate. Effect of Break-In Two sets of extended tread wear test data were analyzed. One comprised data on ten lines of radial tires some of which ran to 38.4 thousand miles after an 800-mile break-in; some aspects of those tests have been discussed previ- ously [5]. The other set, made avai lable by the Rubber Manufacturers Associat ion (RMA) [6], consisted of data collected on one radial tire line in a 20 thousand-mi le test with no break-in. The average groove depth for each set of four tires on the same vehicle was calculated. The data for the first 4.0 thousand miles after break- in was fit to an exponential decay equation -kM D = Do e (1) 16 TIRE SCIENCE AND TECHNOLOGY where D is the groove depth in mils at any mileage M, in thousands of miles, Do is the groove depth after the break-in, and k is a constant. Table 1 presents the average observed groove depths for four radial tires (Tires 101 to 104 [5]) out to 4,0 thousand miles. The best equat ion is -0.0182M D = 355.8e (2) Groove depths calculated from Eq 2 are shown in Table 1 along with the dif- ferences between the observed and calculated values. TABLE I - Average observed groove depths for four radial tires at various mileages and the groove depths calculated from the exponential equation (Eq 2). Mileage, Groove Depth, mils 10 3 miles Observed Calculated Difference, mils 0 358.2 355.8 2.4 0.8 349.4 350.7 -1.3 1.6 343.8 345.6 -1.8 2.4 340.1 340.6 -0.5 4.0 331.9 330.8 1.1 The data for the tires on each vehicle reported upon in Ref 5 were fitted to the same exponential equation. The values for Do and k are shown in Table 2. Tire data (supplied by RMA) [6] from zero to a total of 4.8 thousand miles were processed for each of three cars in the way described above, except that the break-in data were included. The constants for the equation are given in Table 3. The RMA tires were all radial CMT, duplicates of the 100 series tires TABLE 2 - Exponential equation (Eq 2) constants for tires of ref 5. Tire a Do, mils k, (10 3 miles) -1 101-104 355.8 0.0182 106-109 354.8 0.0177 111-114 355.6 0.0181 201-204 357.9 0.0188 206-209 345.5 0.0188 211,212,214 252.5 0.0152 301-304 338.4 0.0172 306-309 312.9 0.0178 311-314 306.9 0.0131 401-404 304.5 0.0263 406-409 331.1 0.0183 412,413,414 286.7 0.0249 a Tire numbers are as in Ref 5. Tires in the 100 series are radial CMT. BRENNER ON TIRE WEAR RATES 17 in Table 2. The k values for both sets of these tires are the same, demon- strating that the rapid wear rate after the first 800 miles is part of the break-in effect. Comparing the k values in Table 2, we find that all are of the same magnitude. The values of the exponential term at 4.0 thousand miles for the slowest (Tires 311-314) and fastest wearing (Tires 401-404) tires are 0.949 and 0.900, a difference of 5 percent. TABLE 3 - Exponential equation constants for RMA data [6]. Tire Identification Do, mils k, (103 miles) -1 Car I 368.6 0.0194 2 366.1 0.0190 4 368.0 0.0204 Upon completion of 4.0 thousand miles after 800-mile break-in, the groove depth vs. mileage data can be fit by a straight line. In Fig. 2, the average data for four CMT (Tires 101-104) are plotted. The first six points are fit by the plot of the exponential curve and the remaining points by the least- squares straight line. D -- 345.7 - 4.37 M (3) Discussion The wear rate during the initial period of use may combine the effect of wear and shape change. Any shape change appears to be almost complete at 4 thousand miles, and thereafter the data can be fit by a straight line: D = Ds rM (4) where Ds is the intercept at zero mileage (after break-in), r is the wear rate, and M is the number in thousands of miles run. Since the rate as determined by Eq 4 accounts mainly for the groove depth loss, we may separate the apparent rate in the initial wear period into any shape effect and the actual wear. The change in groove depth with mileage during the initial period of wear gives an apparent wear rate ra: 18 TIRE SCIENCE AND TECHNOLOGY -kM (5) ra = k Doe If r as defined in Eq 4 is substracted from ra in Eq 5, we have the apparent loss of groove depth caused by the shape change. -kM ra-r = k Doe - r (6) The BCWR 4.44 mils per thousand miles is a good estimate of r for the radial CMT. F rom Eq 4, we find at 4.0 thousand miles (after break-in) for the radial CMT the shape change effect on the rate: -0.0180• ra - r = 0.0180 X 355e - 4.44 = 5.94 - 4.44 = 1.50 (7) At 4.0 thousand miles about 25% of the apparent wear rate may be the result of a shape change. These experiments confirm the findings of Dudley, Bauer, and Reilly [7] who found on a "small amount of data on some belted bias t i res . . , an initial- ly 'exponent ia l ' wear rate region was fol lowed by a more-or- less l inear region . . . " Ref 7 then reports this effect for radial tires. Since the phenomenon was observed earlier on bias tires [1], it appears to be general and common to all types of tires. Conclusions During the early stages of tire. wear, there appears to be a dimensional (shape) change, not associated with rubber loss, that is super imposed on the tread loss. The groove depth data, which includes any shape change plus the actual wear, plotted against miles run is fit by an exponential decay curve. A method for separating such a shape change from the actual wear effects has been demonstrated. Acknowledgements The author is grateful for the advice and assistance of Dr. H. Weingarten and to Mrs. Rita Babb for preparat ion of the manuscr ipt and many helpful editorial suggestions. BRENNER ON TIRE WEAR RATES 19 References [1] Stiehler, R.D., Steel, M.N., and Mandel, J., "Factors Influencing the Road Wear of Tyres," Transactions of the Institution of the Rubber Industry Vol. 27, 1951, p. 298. [2] Spinner, S. and Barton, F.W., "Some Problems in Measuring Tread Wear of Tires," NBS Technical Note 486, 1969. [3] Brenner, F.C., "A Note on Groove Depth Changes Without Wear," Tire Science and Technology, Vol. 8, Nos. 1-2, Jan.-Jun. 1980, pp. 10-12. [4] "Consumer Information Regulations, Uniform Tire Quality Grading," Federal Register, Vol. 43, No. 137, July 17, 1978, pp. 30542-30556. [5] Brenner, F.C. and Williams, H., "Test of Tread Wear Grading Procedure - - The Course Monitoring Tire Adjustment on Radial Tire Wear Rates," NHTSA Docket 25, General Reference 107, NHTSA, Dept. of Transportation. [6] "RMA Road Test Evaluation of Course Monitoring Tires," NHTSA Docket 25, Notice 22, No. 014, June 3, 1977. [7] Dudley, E.A., Bauer, R.F., and Reilly, P.M., "Prediction of Tire Tread Wear Rate and Tread Wear Rate Differences," Tire Science and Technology, Vol. 7, Nos. 3-4, July-Dec. 1979, pp. 43-57.