EVS28 Seoul, Korea, May 17-20, 2015 Laboratory Alignment Procedure for Improving Reproducibility of Tyre Wet Grip Measurement Kwangki Jeon 1*, Yongho Yoo 1, Jinwoo Lee 1, Dohyun Jung 1, Seolhee Lim 2, Seongwoo Park 2 1 KOREA AUTOMOTIVE TECHNOLOGY INSTITUTE, 303 Pungse-ro,Pungse-myeon, Dongnam-gu, Cheonan City, Chungnam 330-912, Republic of Korea, kkjeon@katech.re.kr 2 KOREA ENERGY MANAGEMENT CORPORATION, Yongin City, Gyeonggi, 448-994, Republic of Korea Abstract Korea and EU governments are running the tyre labelling system from 2012. All tyres sold in Korea have to carry a label displaying information about two performance criteria such as rolling resistance and wet grip. The rolling resistance of tyres determines their fuel efficiency grading and the wet grip of tyres determines their braking safety grading. Measurement of rolling resistance and wet grip must be reproducible; tests on the same tyres in different laboratories must produce the same results in order to ensure a fair comparison between tyres from different suppliers. In addition, a good reproducibility of testing results prevents market surveillance authorities from obtaining different results from those of the suppliers when testing the same tyres. The laboratory alignment procedure of rolling resistance was developed and published as an EU Commission Regulation No. 1235 in 2011. But the laboratory alignment procedure of wet grip has not developed yet. So there are a lot of differences of wet grip test results among the test laboratories in the world. The proposed new procedure for the alignment of test laboratories with regard to the measurement of tyre wet grip can improve the reproducibility of the wet grip testing results. The proposed wet grip laboratory alignment procedure was verified through five laboratory alignment tests between KATECH and five test laboratories in the world. Keywords: WG(Wet Grip), Tyre Labelling System, Laboratory Alignment Procedure, Reproducibility, BFC(Braking Force Coefficient) 1 Introduction As an effort to reduce exhaust gas emission of the transportation, the improvement of tyre energy efficiency is required. Therefore, tyre manufacturers are trying to develop eco-friendly tyres [1, 2]. The tyre labelling system has been enforced by European Parliament and EU Council since November, 2012 [3] as well as by Korea government since December, 2012 [4]. European tyre label indicates the grade of three performance parameters such as rolling resistance, wet grip and outside rolling noise. All tyres sold in Korea have to carry labels with information about two performance criteria (rolling resistance, wet grip). EU, Japan and Korea s tyre label are shown in Fig. 1. Fuel consumption of cars is related to the rolling resistance of tyres. Wet grip is one of the most important safety characteristics of tyres and it EVS28 International Electric Vehicle Symposium 1
is displayed in red box of Fig. 1. Tyres with excellent wet grip will have shorter braking distance on wet road. Figure 1: Tyre Labels of EU, Japan and Korea Measurement of rolling resistance and wet grip for these tyre labels must be reproducible; tests on the same tyres in different laboratories must produce the same results in order to ensure a fair comparison between tyres from different suppliers. In addition, a good reproducibility of testing results prevents market surveillance authorities such as MOTIE and KEMCO of Korea from obtaining different results from those of the suppliers when testing the same tyres. The laboratory alignment procedure of rolling resistance was developed and published as an EU Commission Regulation No. 1235 in 2011 [5]. But the laboratory alignment procedure of wet grip has not developed yet. So there are a lot of differences of wet grip test results among the test laboratories in the world. In this study, new procedure for the alignment of test laboratories with regard to the measurement of tyre wet grip is proposed in order to improve the reproducibility of the wet grip testing results. The proposed wet grip laboratory alignment procedure was verified through five laboratory alignment tests between KATECH and five test laboratories in the world. 2 Uncertainty Factors of Wet Grip Test 2.1 SRTT Factor Analysis SRTT16 is very important because wet grip index is calculated by using the average peak BFCs(braking force coefficients) of candidate tyre and SRTT16. But there are no guidelines about use period of SRTT16 s in the regulations. Four SRTT16 s which have same DOT (production date) with several different braking times were selected and tested. When the use period of SRTT16 rise from 100 to 800 braking times, average PBFC is increased by 2.4% with some tread wear as shown in Fig. 2 [6]. Average PBFC 0.813 0.818 0.828 0.833 Figure 2: Effect of braking times of SRTT 2.2 Surface Friction Factor Analysis There is some variation of friction even in the same wet grip test track. So each braking test run should be made at the same spot in one test cycle according to the wet grip test regulation. In order to review the effect of surface friction factor on wet grip, the wet grip test is performed at several positions with different frictional properties of the same wet grip test track using four C1 test tyres as shown in Table 1. Table 1: Specification of Test Tyres ID Usage Size Test Tyre A Summer Tyre 225/40ZR18 Test Tyre B All Season Tyre P205/65R16 Test Tyre C All Season Tyre 205/65R15 Test Tyre D Summer Tyre 245/45ZR18 Average PBFC values of five test tyres are shown in Fig. 3. As friction of test surface increases, both average PBFC of SRTT16 and test tyres increase in proportion. But the change of wet grip index is very little. In the case of test tyre B of Fig. 3, wet gip index is increases by 2.5% [6]. Average PBFC 0.9 0.85 0.8 0.75 0.7 0.65 0.6 1.1 1 0.9 0.8 0.7 0.6 100 300 500 800 Braking Times Average PBFC Tread depth (mm) Average Braking Force Coefficient Position A Position B Position C Test Tire A Test Tire B Test Tire C Test Tire D SRTT 1 SRTT 2 Figure 3: Effect of Surface Friction on PBFC 9 8 7 6 5 4 3 2 1 0 Tread Depth (mm) EVS28 International Electric Vehicle Symposium 2
2.3 Surface Temperature Factor Analysis Wet grip performance of tyres is deeply related to air and surface temperature of test track. In order to review the effect of surface temperature factor on PBFC and wet grip, a lot of wet grip test were performed at several temperature conditions from 5 to 35 in the same position using a C2(light truck) SRTT and three C2 test tyres as shown in Table 2. When surface temperature rise from 5 to 35, average PBFCs of each tyre decrease with different slopes as shown in Fig. 4 because of each tread rubber s characteristics. Table 2: Specification of C2 Tyres ID Usage Size C2 SRTT All Season Tyre 225/75R16C C2 Tyre A Summer Tyre 195/70R15C C2 Tyre B Summer Tyre 185R14C C2 Tyre C All Season Tyre 185R14C Figure 4: The Effect of Wet Surface Temp. on PBFC 2.4 Tyre Factor Analysis For tyre break-in and conditioning, two braking runs shall be performed under the test load, pressure and speed as specified in wet grip test regulation. From the conditioning test results of three new C1(passenger car) tyres show that there is a transition area to be stabilized PBFC values for certain tyres as shown in Fig. 5. A few car makers have their own guidelines on tyre conditioning method using 80 braking runs at 32 kph for their tyre traction correlation test. 3 Laboratory Alignment Test Procedure of Rolling Resistance 3.1 Laboratory Alignment Test Procedure of Rolling Resistance The rolling resistance of tyres determines their fuel efficiency grading. The laboratory alignment procedure of rolling resistance was developed and published as an EU Commission Regulation No. 1235 ANNEX IVa Laboratory alignment procedure for the measurement of rolling resistance in 2011 [5]. A set of five or more alignment tyres shall be selected for the alignment procedure in compliance with the criteria below. One set shall be selected for C1 and C2 tyres together, and one set for C3(truck and bus) tyres. The set of alignment tyres shall be selected so as to cover the range of different RRCs of C1 and C2 tyres together, or of C3 tyres. RRC values of each alignment tyre of the set shall be spaced out and distributed uniformly as shown in Table 3. Measurement method of RR laboratory alignment test is summarized in Table 4. The reference laboratory shall measure RRC of each alignment tyre four times and retain the three last results for further analysis. 3.2 Laboratory Alignment Test Results of Rolling Resistance KATECH is the unique reference test body and its RR test machine is the unique reference machine in Korea tyre labelling system. From 2011, a lot of laboratory alignment test for RR have been carried out between KATECH and other candidate test laboratories in the world as shown in Fig. 6. Table 3: Tyre Selection of RR Lab. Alignment Test Min. number of Tyres Min. Range of RRC Interval of RRC Section Width Outer Diameter Five (5) 3 kg/t (for C1/C2) 2 kg/t (for C3) (1.0 ± 0.5) kg/t (for C1/C2) (1.0 ± 0.5) kg/t (for C3) 245 mm (for C1/C2) 385 mm (for C3) 510 ~ 800 mm (for C1/C2) 771 ~ 1143 mm (for C3) Figure 5: Effect of Tyre on PBFC EVS28 International Electric Vehicle Symposium 3
Table 4: Measurement method of RR Lab. Alignment Test No. of each RRC Test Four (4) No. of Retaining Test Results Last Three (3) RR Test Temp. (25 ± 1) Test Velocity 80 km/h (for C1/C2/C3 K-) 60 km/h (for C3 -J) Test Load 80 % of Max. Load (for C1) 85 % of Max. Load (for C2/C3) Thermal Min. 3 hours (for C1) of Test Tyres Min. 6 hours (for C2/C3) Warm Up Duration 30 min. (for C1) 50 min. (for C2/C3 LI 121) 150 min. (for C3 LI>121, D<22.5) 180 min. (for C3 LI>121, D 22.5) alignment tyre at least ten braking runs and retain the six braking runs results for further analysis. In order to minimize the effect of uncertainty factors to wet grip index, a lot of conditions should be controlled more tightly than those of current wet grip test procedure. Air and surface temperature should be within 10 and 30 for normal tyres. Test load should be (75 ± 1) % of maximum tyre capacity. Maximum braking times of SRTTs should be within 300. Braking zone of wet grip test track should be controlled within ± 2.0 m for longitudinal direction and ± 0.25 m for lateral direction. Maximum deviation of time-topeak of test trailer should be within ± 0.10 s for C1 tyres and ± 0.14 s for C2 and C3 tyres. Tyre conditioning should be carried out at least 40 braking runs at 32 kph for C1 tyres and 25 kph for C2 and C3 tyres. Table 5: Tyre Selection Requirements of WG Lab. Alignment Test Figure 6: Lab. Alignment Test Results of RR Min. number of Tyres Min. Range of WGI Interval of WGI Section Width Outer Diameter Five (5) 0.3 (for C1/C2) 0.2 (for C3) (0.1 ± 0.05) (for C1/C2/C3) 245 mm (for C1/C2) 385 mm (for C3) 510 ~ 800 mm (for C1/C2) 771 ~ 1143 mm (for C3) 4 New Laboratory Alignment Test Procedure of Wet Grip 4.1 New Laboratory Alignment Test Procedure of Wet Grip Tyre selection requirements for the wet grip laboratory alignment test are summarized in Table 5. A set of five or more alignment tyres shall be selected for the wet grip alignment procedure same as RR in compliance with the criteria below. One set shall be selected for C1 and C2 tyres together, and one set for C3 tyres. The set of alignment tyres shall be selected so as to cover the wide range of wet grip index. Wet grip index of each alignment tyre of the set shall be spaced out and distributed uniformly as shown in Table 5. Wet grip laboratory alignment test procedure using a trailer method is summarized in Table 6. The reference laboratory shall measure each 4.2 Laboratory Alignment Test Results of WG with Korean Test Laboratories To verify the proposed wet grip laboratory alignment procedure of 4.1, laboratory alignment tests between KATECH and three test laboratories in Korea. From Fig. 7 to Fig. 9 show the wet grip alignment test results only using C2 tyres between KATECH and three test laboratories. Because of using only C2 tyres, the set of alignment tyres cover quite small (from 0.2 to 0.3) ranges of wet grip index. Nevertheless, the alignment graphs of Fig. 7 to Fig. 9 show good linearity of R 2 values of 0.97, 0.96 and 0.95 respectively. 4.3 Laboratory Alignment Test Results of WG with EU Test Laboratories To verify the proposed wet grip laboratory alignment procedure of 4.1, laboratory alignment EVS28 International Electric Vehicle Symposium 4
tests between KATECH and two test laboratories in EU. Fig. 10 and Fig. 11 show the wet grip alignment test results using C1 and C2 tyres between KATECH and two test laboratories. Because of using C1 and C2 tyres, the set of alignment tyres cover wider (from 0.4 to 0.5) ranges of wet grip index in these test cases. The alignment graphs of Fig. 9 and Fig. 10 show good linearity of R 2 values of 0.96 and 0.98 respectively. Table 6: Measurement method of WG Lab. Alignment Test (Trailer Method) No. of Braking for each Tyre No. of Retaining Test Results Air/Surface Temp. Test Velocity Test Load Thermal Max. Braking Times of SRTT Braking Zone of Track Track Time-to-Peak Max. Deviation of Time-to-Peak Tyre Ten (10) Six (6) (20 ± 5) (for C1/C2/C3) (Reg. : 5~35 ) (10 ± 5) (for C1 Snow Tyre) (Reg. : 2~20 ) (65 ± 2) km/h (for C1) (50 ± 2) km/h (for C2/C3) (75 ± 1) % (for C1/C2/C3) (Reg. : (75 ± 5) %) Min. 2 hours (for C1/C2/C3) 300 ± 2.0 m for Longitudinal, ± 0.25 m for Lateral (Reg. : ± 2.5 m / ± 0.25 m) Min. 10 braking at 65 kph (for C1) Min. 10 braking at 50 kph (for C2/C3) 0.2 ~ 0.5 s (for C1) 0.2 ~ 1.0 s (for C2/C3) ± 0.10 s (for C1) ± 0.14 s (for C2/C3) 40 braking runs at 32 kph (for C1) (Reg. : 2 braking runs at 65 kph) 40 braking runs at 25 kph (for C2/C3) (Reg. : 2 braking runs at 50 kph) Figure 7: Lab. Alignment Test Results of Wet Grip (KATECH to Korean Test Lab. A) Figure 8: Lab. Alignment Test Results of Wet Grip (KATECH to Korean Test Lab. B) Figure 9: Lab. Alignment Test Results of Wet Grip (KATECH to Korean Test Lab. C) Figure 10: Lab. Alignment Test Results of Wet Grip (KATECH to EU Test Lab. D) Figure 11: Lab. Alignment Test Results of Wet Grip (KATECH to EU Test Lab. E) 5 Conclusion In this paper, a few uncertainty factors of wet grip index such as SRTT, surface friction, surface temperature, tyre conditioning were analyzed using a lot of experiments. And new procedure for wet grip alignment of test laboratories is proposed in order to improve the reproducibility of the wet grip testing results. In order to minimize the effect of uncertainty factors of the wet grip laboratory alignment test, a lot of test conditions such test tyre EVS28 International Electric Vehicle Symposium 5
selection, air and track surface temperature, test load, maximum braking times of SRTTs, braking zone of test track, maximum deviation of timeto-peak, braking runs for tyre conditioning are proposed to be controlled. The proposed wet grip laboratory alignment procedure was verified through five laboratory alignment tests between KATECH and five test laboratories in the world. Acknowledgments This work was supported by the Ministry of Trade, Industry and Energy of Korea. References [1] J. Oh, H. Lee, S. Kim, Review of Wet Grip Test Method for Eco-friendly Tyre Labelling, KSAE, 2011. [2] I. Kuwayama, H. Matsumoto, H. Heguri, Development of Next Generation Ecology Tire Technology, JSAE, 2013. [3] 2011 COMMISSION REGULATION (EU) No.228/2011 amending Regulation (EC) No 1222/2009 of the European Parliament and of the Council with regard to the wet grip testing method for c1 tyres. [4] The Ministry of Trade, Industry and Energy Notification Number 2011-237, 2014-227. [5] 2011 COMMISSION REGULATION (EU) No.1235/2011 amending Regulation (EC) No 1222/2009 of the European Parliament and of the Council with regard to the wet grip grading of tyres, the measurement of rolling resistance and the verification procedure. [6] Y. Yoo, K. Jeon, D. Jung, S. Hwang, Uncertainty Factor Analysis of Tyre Wet Grip Index for EU and Korea Tyre Labelling System, EVS27, 2013. YonghoYoo, Republic of Korea. He received the M.S. degree in mechatronics engineering at Sungkyun kwan University, Suwon, Republic of Korea, in 2015. His interests are vehicle dynamics, ADAS and tyre wet grip for tyre labeling system. Jinwoo Lee, Republic of Korea. He received the B.S. degree in mechanical engineering from Korea Polytechnic University, Siheung, Republic of Korea, in 2007. His research interests are fuel efficiency and tyre rolling resistance for tyre labeling system. DohyunJung, Republic of Korea. He received the Ph.D. degree in mechanical engineering from Korea Advanced Institute of Science and Technology, Daejeon, Republic of Korea, in 2001. His research interests are vehicle control for chassis systems and ADAS systems. Seolhee Lim, Republic of Korea. She received the B.S. degree in mechanical engineering from Incheon University, Incheon, Republic of Korea, in 2013. Her research interest is tyre labelling system. Seongwoo Park, Republic of Korea. He received the B.S. degree in public administration from Hanyang University, Seoul, Republic of Korea, in 1996. His research interests are tire energy efficiency, vehicle fuel economy and EV policies. Authors Kwangki Jeon, Republic of Korea. He received the Ph.D. degree in mechanical engineering from Seoul National University, Seoul, Republic of Korea, in 2013. His research interests are vehicle dynamics, tyre wet grip, ADAS and autonomous vehicle control. EVS28 International Electric Vehicle Symposium 6