How Tread Loss Affects Rolling Resistance

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14 Rubber & Plastics News ● May 5, 2014 www.rubbernews.com Technical How tread loss affects rolling resistance

By Walter H. Waddell, more closely resemble tires when used in R. Christopher Napier Executive summary service, a new multistep roadwheel test ExxonMobil Chemical Co. procedure was investigated for six tires Six tires with known inflation pressure loss rates (IPLR) by ASTM F1112 (stat- Larry R. Evans with differing values of inflation pressure ic) and ISO 28580 rolling resistance (RRC) values were selected in order to devel- Transportation Research Center Inc. loss rates and rolling resistance.17 James A. Popio, op a dynamic IPLR value obtained during operation on a 1.707-meter roadwheel. Douglas Rice, After each incremental operation on the smooth drum, the inflation pressure Experimental and resulting rolling resistance values of the six tires were measured, allowing Madhura Rajapakshe One tire of six models for which the Smithers Rapra North America calculation of a Dynamic IPLR value. As a follow-up, an indoor treadwear test was IPLR values had been determined at developed on the 3.0-meter laboratory roadwheel in order to study the IPLR and It has been reported that more than 21°C and at 60°C using the ASTM F1112 RRC values of the tires when operating while the tread was being lost. 20 percent of the fuel consumed by a ve- protocol11 was used for treadwear testing. After each increment of the test on the textured drum, the inflation pressure and hicle is used to overcome the rolling re- Dynamic IPLR values had also been de- rolling resistance of tires were measured, along with detailed treadwear measure- sistance of the tires,1 even though the termined for these tire models during op- ments that included laser profiling. A final experiment was undertaken to study rolling resistance of tires has improved eration on a smooth-surfaced roadwheel.16 the rolling resistance of uniformly-worn tires at specific inflation pressures. by up to 50 percent in the last 40 years. In order to determine inflation pres- Using the information from the change in rolling resistance with the decrease in It is estimated that 25 percent of all en- sure values, a SmarTire sensor18 was in- inflation pressure, the competing effects of pressure decrease and tread loss were ergy consumed is used by motor vehicles. stalled inside each tire, and the tire was calculated for each of the six tires modeled during tire operation. This means that 5 percent of all the ener- inflated and checked for leaks. Modeling showed that the magnitude of rolling resistance increase due to infla- gy consumed in the U.S. still is being used Rolling resistance force was measured tion pressure loss was significantly greater than that of decreased rolling resist- to overcome the rolling resistance of tires.2 at 210 kPa according to ISO 28580 pro- ance due to tread loss. The predictions of the model were verified by testing tires The effect of tire inflation pressure on tocol19 at Smithers Rapra (Ravenna, with varying degrees of remaining tread at different inflation pressures. rolling resistance and therefore on vehicle Ohio). Without re-inflating, tires were fuel consumption has been shown to be run on a 3.0-meter diameter roadwheel significant.3 It was estimated that if tires the tire manufacturer and model. and average 2.6 times higher than IPLR using a drum surfaced with 80-grit maintained their proper inflation pres- IPLR values have been measured for values measured under static conditions sandpaper under 100 percent loads and more than 300 models of passenger car at room temperature, but were not as at 120 km/h until 2,900 km was reached. TECHNICAL NOTEBOOK tires12-13 with values ranging from a low high as when measured under static Each 2,900 km segment used a simu- Edited by Harold Herzlich of 0.86 percent-loss per month to a high conditions at 60°C. For each 10 kPa of lated driving cycle of straightaways and of 4.4 percent-loss per month. inflation pressure loss, tire RRC values left/right turns.20 Rolling resistance was sure, more than 900 million gallons of fuel Tire IPLR values also have been increase ~2.5 percent. re-measured according to ISO 28580, could be saved in the U.S. each year.4 found to be dependent upon the temper- Tires also continuously lose tread during but without re-inflating to 210 kPa. The Although all tires lose inflation pressure ature at which it is measured.14-15 For ex- normal operation. Since approximately 50 tire weight and tread depth was meas- continuously during normal operation, the ample, when measured at 70°C, IPLR percent of the rolling resistance energy for ured using the Bytewise CTWIST sys- use of halogenated butyl rubber as the values are 10-19 times higher than a passenger tire is consumed by the hys- tem21 and recorded. polymer in the tire innerliner provides for when measured at room temperature.14 teresis response of the tread compound, as This cycle was repeated until a total of low inflation pressure loss rates (IPLR) Previous studies measured the per- the tread mass decreases the rolling resist- about 17,400 km had been reached. At and low intracarcass pressure (ICP) val- formance of six tire models during opera- ance of the tire decreases as well. this point all tires had at least some por- ues. Maintaining proper inflation pres- tion on a 1.707-meter diameter laboratory In order to examine the opposing effects tion of the tread worn past the wear in- sure impacts the tire’s rolling resistance, roadwheel with smooth drum by stopping on the rolling resistance of tires as a re- dicators. The time interval between handling, treadwear and durability, lead- every 4,838 km to re-measure the ISO sult of the loss of both inflation pressure roadwheel and rolling resistance experi- ing to improved vehicle performance.5-10 28580 RRC values, without re-inflating and tread mass under conditions that See Tread, page 16 Tire IPLR determined in a tempera- the tire to the specified 210 kPa pressure. ture controlled laboratory at 21°C ac- All six tire brands produced similar Fig. 1. Example tire with tread removed to wear indicators. cording to the ASTM F1112-06 proce- inflation pressure decreases and rolling dures11 and expressed as percent-loss resistance increases after each cycle be- per month are found to be dependent yond an initial break-in period that was upon the tire type, such as passenger dependent upon the tire brand.16 car versus medium truck tire, and on Dynamic IPLR values were calculated

Fig. 2. IPLR and RRC values of the six tires selected for study. P015_RPN_20140505.qxp 4/30/2014 3:14 PM Page 1

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tire technology and product testing for ciate at PPG Industries and a technology Rubber Co., which later became Conti- The authors butyl polymers. manager for J.M. Huber Inc., where he nental, where he Walter H. Waddell is a senior re- He began his career with Westing- was responsible for global research and conducted all as- search associate in polymers technology house’s polymer elec- implementation of silica for the rubber, pects of tire testing at ExxonMobil Chemical Co. trical products divi- plastics and paint industries. with a focus on He has his doctorate in chemistry sion and has 25 years Evans has been granted 12 patents, is rolling resistance, from the University of Houston and of experience in the the author of 60 scientific publications endurance and tread- bachelor’s degree in rubber and plastics and has presented at more than 70 tech- wear. chemistry from the industry. He has his nical seminars around the world. Rice became a University of Illi- degree from Le- James A. Popio is vice president and project engineer at nois at Chicago. tourneau University. general manager of Smithers Rapra & Smithers, and his Waddell was a re- Napier is active in Smithers Pira Ltd. in the United Kingdom. primary job is man- search associate at the Rubber Division, He has a doctorate in engineering and Rice aging the produc- Columbia Universi- serving as chairman a master’s in mechanical engineering from tion at the tire and ty, where he received of the Best Paper Napier the University of Akron. He was general wheel test facility in Ravenna, Ohio. a research fellow Committee in 2009- manager and technical director at Madhura Rajapakshe is a research award from the U.S. 10 and as a member of the Program Plan- Smithers Rapra North America, a staff de- and development project engineer for tire National Institutes ning Committee. He is a member of the velopment engineer testing at Smithers Rapra North America. of Health, and an as- ASTM F09 Tire Committee. at Goodyear, and a He specializes in tire rolling resist- Waddell sociate professor of Larry R. Evans recently retired as a re- senior engineer at ance, durability, treadwear, traction, chemistry at Carnegie-Mellon University. search analyst assigned to the National Bridgestone. footprint pressure, He also was a section head in research at Highway Traffic and Safety Administra- Popio has been an forces and moments. Goodyear and senior scientist, silica tech- tion of the U.S. Department of Transporta- active participant Madhura has a nology at PPG Industries. tion. His primary job was to analyze data at ITEC, Tire Soci- doctorate in trans- Waddell has received numerous from NHTSA’s tire ety, Rubber Divi- portation engineer- awards, including the Distinguished and vehicle testing sion, Clemson Tire ing and a master’s Corporate Inventor from PPG; the programs. Industry Confer- in mechanical engi- Sparks-Thomas Award and the Melvin Evans has had a ence, Future of Tire neering from the Mooney Distinguished Technology varied background Technology Confer- University of South Award from the ACS Rubber Division; in the tire industry, ence, Carbon Black Popio Florida. and Award of Appreciation from the beginning at Good- World, ASTM, SEMA, IANA and SAE. He also has a de- ASTM F09 Committee on Tires. year, where he con- Douglas Rice is a production manag- gree in mechanical Rajapakshe He has published 150 papers, present- ducted research on er at Smithers Rapra North America. engineering from ed more than 147 seminars globally and rubber compound- He has had a long career in the tire in- the University of Peradeniya in Sri Lan- is an inventor with 40 U.S. patents. ing and fabric to dustry beginning at General Tire & ka. Christopher Napier is a research Evans rubber adhesion for engineer in polymers technology at tires and industrial ExxonMobil, where his primary focus is products. He was a senior research asso-

Fig. 3. Average measured inflation pressure of tires during testing.

Fig. 4. Tire weight loss (kg) during testing on the textured roadwheel. P016_RPN_20140505.qxp 4/30/2014 3:15 PM Page 1

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coefficient (RRC) values (Fig. 2). For example, the initial rolling resist- The same tires were then buffed again As tires were run on the textured drum, ance was approximately 15 percent high- and smoothed until the tread was uni- Tread the inflation pressure and tread depth er than previous tires measured for this formly worn to the treadwear indicators were both systematically decreasing. The model. Tire AI also had significantly less (Fig. 1). Tire rolling resistance at 210 Continued from page 14 inflation pressure decreased linearly at weight loss during these tests than did kPa and 150 kPa were remeasured. See ments was not a constant due to the dif- different rates for the six tire models in the other five tire models. data in Table I. There is a systematic de- fering work backlogs on each machine relation to their IPLR values (Fig. 3). In addition, while all tires experi- crease in rolling resistance of approxi- over the six-month period to complete The treadwear of all tires was linear enced significantly more wear in the mately 10 percent as the tread is re- all measurements. during the experiment but varied signif- shoulder region than in the tread center, moved from the tire. Tire inflation pressures during road- icantly by tire model. Fig. 4 shows the Tire AI was worn to the wear indicators However, the increase in rolling re- wheel conditioning and rolling resistance average tread loss in kg, as measured by at the shoulder but was less than 25 sistance when the inflation pressure is measurements were downloaded from the loss of weight of the tires. The measured percent worn at the tread center (Fig. reduced from 210 kPa to 150 kPa is 25 SmarTiresensor directly into a computer. rolling resistances of the tires, that are a 8). Tire AI also had the highest rate of percent or more, regardless of how much In a separate experiment, two tires of result of the competing phenomenon, inflation pressure loss during testing.16 tread remained on the tire. See Table each of the six tire models were meas- are shown in Fig. 5. II and Fig. 9. ured for rolling resistance according to Direct measurement of tires with the ISO 28580 protocol when inflated to Separating the effects of inflation worn tread Effect of IPLR and tread loss during 210 kPa and at 150 kPa. These tires pressure and treadwear These experiments supported the hy- real-world operation were then buffed, removing approxi- From the dynamic IPLR experiments pothesis that the competing effects on Assuming the correct initial inflation of mately one-third of the tread depth, and using these same tire models,16 we can rolling resistance of inflation pressure the tire on the vehicle, the inflation pres- run on the textured roadwheel until the calculate the change in rolling resist- loss and treadwear are simultaneously, sure that a tire experiences during nor- tread depth was uniformly worn at 50 ance as the tires lose inflation pressure but independently, occurring during mal operation is a function of both the percent of the original tread depth. while running on a smooth-surface road- normal tire operation. IPLR value of the tire and of the mainte- The rolling resistance at 210 kPa and wheel without any tread loss. However, the significantly higher nance given of the tire by the motorist. 150 kPa was remeasured. The same tires Subtracting this incremental change wear of the tire shoulder during the sim- Numerous studies have shown that were then buffed and smoothed until the at each step from the measured values ulated drive cycle, which was exacerbat- the average driver of a passenger vehi- tread was uniformly worn to the tread we can calculate the change in rolling ed by the reduction in the inflation pres- cle does not do a good job of maintaining wear indicators. For example, see Fig. 1 resistance that is solely due to tread- sure, made further study necessary the inflation pressure of their tires. These tires were then remeasured for wear. See for example the calculated since irregular wear patterns have been Recently, the Rubber Manufacturers rolling resistance at 210 kPa and 150 kPa. values for Tire A in Fig. 6. shown to affect tire rolling resistance.23 Association released a study showing The effect of tread loss for all tire To study the effect of treadwear and that only 1 in 6 of the drivers in the U.S. Results and discussion models is shown in Fig. 7. For five of the inflation pressure directly, two tires of were aware of the proper tire inflation Six tires previously studied as part of six tire models, the change in rolling re- each of the six tire models were meas- procedures. 24 the 11th Worldwide Tire Survey22 were sistance with loss of tread is similar. ured for rolling resistance according to Using data from the present study and selected for this study because of their A loss of 1 kg of tread mass produces a the ISO 28580 protocol when inflated to also from the Dynamic IPLR results pre- differing values of inflation pressure change in rolling resistance of approxi- 210 kPa and also at 150 kPa. viously reported,16 a calculation can be loss rate (IPLR). Tire N was selected mately 1 kg/t. This lends support to the These tires were then buffed, remov- made of the effect of both the tread loss due to a low IPLR = 1.4 percent-loss per conclusion that the reduction in rolling re- ing approximately one-third of the tread and the inflation pressure loss expected month, and Tire AI had a high IPLR = sistance due to tread loss is a result of re- depth and run on the textured-surface for a driver who re-inflates their vehicle 3.2 percent-loss/month. ducing the hysteretic mass of the tread roadwheel until the tread depth was uni- tires every three months if the tires have Tires from different manufacturers have compound in the tire. formly worn to 50 percent of the original IPLR values of 1.5 percent, 2.5 percent IPLR = 2.2 percent-loss per month (Tires A Although they were self-consistent, tread depth. The rolling resistance at and 3.5 percent loss per month. and B) and IPLR ~ 2.7-percent loss per the rolling resistance results for Tire AI 210 kPa and 150 kPa was remeasured. Tire rolling resistance continually month (Tires E and O). These tires also were anomalous throughout this experi- had differing ISO 28580 rolling resistance ment when compared to previous work. Fig. 6. Calculated rolling resistance due to inflation pressure loss and treadwear: Tire A. Fig. 5. Rolling resistance during testing on the textured roadwheel.

Fig. 7. Calculated change in rolling resistance due to tire weight loss. Fig. 8. Wear of tires at the shoulder and at the tread center during testing on the textured-surface roadwheel. P017_RPN_20140505.qxp 5/1/2014 3:47 PM Page 1

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would increase due to pressure loss but tire models studied have an IPLR value rienced by the driver is nearly 15 percent had been previously tested for Dynamic would immediately decrease upon prop- greater than 3.5-percent loss per month greater than the original rolling resist- IPLR16 allowing the calculation of the er re-inflation (Fig. 10). In this case, it and 5.5 percent of the tires have IPLR val- ance of the tire when properly inflated. expected increases in rolling resistance is shown that a tire would have an aver- ues less than 1.5-percent loss per month, These average rolling resistance values without loss of tread. age rolling resistance over its life that but 44.2 percent of tires have IPLR values are shown in Fig. 11. For comparison, the When corrected for the effect of infla- would be similar to the original rolling greater than 2.5-percent loss per month.13 values for tires re-inflated every three tion pressure, the tires decreased in resistance, assigned a value of 100. Alternatively, if a driver only re-in- months are shown as the dotted lines. rolling resistance by approximately 1 kg For the tire with a 1.5 percent-loss per flates the tires when the TPMS acti- per t for each kilogram of tread removed month IPLR value, the average values vates at 25 percent underinflation the Summary from the tire. (see blue straight line) ranges from 103 average rolling resistance is dramatical- Six models of tires were tested to simu- The rolling resistance of two tires each percent to 93 percent over the tire lifetime. ly increased. For tires with an IPLR val- late the effects of treadwear and inflation of the six tire models was measured at For the tire with an innerliner afford- ue of 1.5-percent loss per month, the av- pressure loss during tire operation. In the the ISO 28580 inflation of 210 kPa, and ing an IPLR of 3.5-percent loss per erage rolling resistance is only slightly first set of experiments, tires were tested at a reduced inflation of 150 kPa. month, the average values range from higher than the original value. without re-inflation for 17,400 km using a One-half of the tread was removed by 107 percent to 97 percent over the life of However, for tires with an IPLR value simulated driving cycle of straight and buffing and smoothing, and the rolling the tire (see green line). of 3.5-percent loss per month, the aver- left/right turns on a 3.0-m roadwheel tex- resistance was remeasured at the two Only 3.5 percent of the passenger car age rolling resistance that would be expe- tured with 80-grit sandpaper. pressures. The tread was then uniform- The inflation pressure of the tires was ly removed to the wear indicators and monitored using internal sensors and the rolling resistance again measured at Table II. Average rolling resistance values and ratios. the tires were stopped each 2,900 km for the two pressures. measurement of tread loss21 and rolling All tires showed a reduction in rolling resistance.19 Tires of the same models See Tread, page 18

Table I. Rolling resistance of tires measured at different pressures and degrees of tread loss.

Figure 9. Average RRC values normalized to original tire RRC.

Fig. 11. Calculated average rolling resistance for tires with three different IPLR values over tire lifetime if re-inflated only when the vehicle TPMS activates.

Fig. 10. Calculated average rolling resistance for tire with three different IPLR values over the tire lifetime with a re-inflation time period of every three months. P018_RPN_20140505.qxp 5/1/2014 4:06 PM Page 1

18 Rubber & Plastics News ● May 5, 2014 www.rubbernews.com Technical

experience average fuel efficiency ap- [Online] [Cited: Aug. 1, 2013] 11. “Standard Test Method for Static Testing of proximately equal to that expected from 3. F. Wicks and W. Sheets, “Effect of Tire Pressure Tubeless Pneumatic Tires for Rate of Loss of Infla- and Performance Upon Oil Use and Energy Policy tion Pressure” ASTM F 1112 – 06, ASTM Standards Tread the original rolling resistance of the tire. Options”, Boston, MA, American Nuclear Society, on Disc, Rubber, volume 09.02 (2007) However, for tires with a high IPLR 1991. Proceedings of the 26th Intersociety Energy 12. J.D. MacIsaac, L.R. Evans, J.R. Harris and E. Continued from page 17 value of 3.5-percent loss per month, the Conversion Engineering Conference Terrill, “The Effects of Inflation Gas on Tire Labora- resistance of 10 percent at each of the average fuel efficiency of the vehicle 4. L.R. Evans, “Tire Inflation and Fuel Consump- tory Performance”, presented at 2008 ITEC, Akron, tion”, Rubber & Plastics News, p 16-18, April 5, 2013 OH, Sept. 16-18, 2008 pressures when the tread was removed. would be expected to equal that of tires 5. D.S. Tracey and W.H. Waddell, Rubber & Plastics 13. R.C. Napier, W.H. Waddell and D.F. Rouckhout, However, the increase in rolling resist- with initial rolling resistance values that News, 34, 14, May 30, 2005 Rubber & Plastics News, July 15, 2013, p 16-18 ance when inflation pressure was re- are as much as 15 percent higher if the 6. W.H. Waddell, S.W. Botfeld, R.C. Napier, D.F. 14. J.R. Harris and J.D. MacIsaac, “The Effect of duced from 210 kPa to 150 kPa was sig- tires were to only be re-inflated when the Rouckhout and D.S. Tracey, Rubber World, 233(4), 33- Temperature on Static Rate of Loss of Inflation 38 (2006) Pressure in Tubeless Pneumatic Tires”, paper #18 nificantly greater, averaging 28 percent. TPMS system activates on the vehicle. 7. W.H. Waddell, D.S. Tracey and D.F. Rouckhout, presented at the Fall Technical Meeting of the Rub- Combining all data along with the Dy- Kautschuk Gummi Kunststoffe, 59(12), 659-665 (2006) ber Division, ACS, Cincinnati, OH, October, 2006 namic IPLR experiments,16 the effects of References 8. W.H. Waddell, R.C. Napier, D.F. Rouckhout and D.S. 15. W.H. Waddell, S.W. Botfeld, R.C. Napier, D.F. both tread loss and inflation pressure 1. Michelin, “Fuel Savings”, The Tire Digest. [Online] Tracey, Rubber World, 236(2), 36-38, 40-41, 43-44 (2007) Rouckhout, and D.S. Tracey, Rubber World, 233(4), 33- 9. W.H. Waddell, R.C. Napier, D.F. Rouckhout, D.S. Tracey 38 (2006) loss over a tire lifetime were calculated. [Cited: Aug. 1, 2013.] http://thetiredigest.michelin.com/performance-fuel-savings and U. Karmarkar, Rubber World, 238(6), 21-27 (2008) 16. W.H. Waddell, R.C. Napier, L.R. Evans, J.A. Popio, For tires with an IPLR value of 1.5- 2. U.S. Energy Information Administration. Analy- 10. H.J. Herzlich, Rubber & Plastics News, p 17-18, D. Rice and M. Rajapakshe, “The Effect of Pressure Loss percent loss per month, a driver would sis and Projections. Annual Energy Outlook 2013. 21, May 6, 2013 under Simulated Driving Conditions: Dynamic Tire In- flation Pressure Loss Rates”, paper #77, presented at the Rubber Division, ACS, Cleveland, OH, Oct. 22, 2013 17. J.A. Popio, D. Rice, M. Rajapakshe, L.R. Evans, W.H. Waddell and R.C. Napier, “Development of New Test Protocols to Study Tire Rolling Resistance using Simulated Driving Conditions”, paper #76, presented at the Rubber Division, ACS, Cleveland, OH, Oct. 22, 2013 18. SmarTire Active Pressure Monitoring Systems, for example see: www.smartire.com/company/technology 19. “Passenger car, truck and bus tyres—Methods of measuring rolling resistance—Single point test and cor- relation of measurement results”, ISO 28580, INTER- NATIONAL STANDARD, First edition, 2009-07-01 20. J.A. Popio, “An Approach to Indoor Wear Test- ing”, paper #101, presented at the Rubber Division, ACS, Milwaukee, WI, Oct. 14, 2010 21. M. Harris, “CTWISTTM System Delivers Fast and Accurate Tread Wear and Uniformity Measure- ments”, http://www.starrett.com/docs/other-down- loadable-resources/3011—-starrett-bytewise-preci- sion-tread-measurement-(lo-res).pdf?sfvrsn=7 22. W.H. Waddell, R.C. Napier, and D.F. Rouck- hout, “11th Worldwide Tire Survey: Tire Rolling Re- sistance”, presented at the 17th China Tire Technol- ogy Symposium, Zhangjiajie, China, Sept. 22-24; and the 2012 Tire Label Legislation & Marketing Conference Brussels, Belgium, 3-4 October 2012 23. L.R. Evans, J.D. MacIssac, J.R. Harris, K. Yates, W. Dudek, J. Holmes, J. Popio, D. Rice, and M.K. Salaani, DOT HS 811 154 - NHTSA Tire Fuel Efficiency Consumer Information Program Develop- ment: Phase 2—Effects of Tire Rolling Resistance Levels on Traction, Treadwear, and Vehicle Fuel Economy. Washington, DC: USDOT-NHTSA, 2009 24. RMA Tire Inflation Survey, Modern Tire Dealer online at www.moderntiredealer.com/channel/performance/news/story/2013/06/rma-survey-only-1-in- 6-check-pressure.aspx?prestitial=1 Products Otech Corp. said it has improved the design of two custom thermoplastic elas- tomer compounds by releasing its OTE2089, a thermoplastic olefin compound used in the manufacturing of recoil pads. Otech said the pads, used in competi- tion rifles and sporting goods firearms, has been developed to exhibit shock ab- sorption properties as well as a good compression set. The OTE2089 has the look and feel of vulcanized rubber, but it is lighter weight. Otech also released its OTR 7019, a formula designed specifically for the manufacturing of car mats. Otech said the compound improvements increase car mat abrasion resistance by 50 percent, but still has flexibility. Visit www.otechcompounds.com for more information. 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