Stop distances for ten studless winter tires

Transcription

1 TECHNICAL REPORT Stop distances for ten studless winter tires Niclas Engström Henrik Andrén Roland Larsson Lennart Fransson

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3 Stop distances for ten studless winter tires October 9, 29, rev.9 Niclas Engström a Henrik Andrén b Roland Larsson a and Lennart Fransson b a Division of Machine Elements, Luleå University of Technology b Division of Structural Engineering, Luleå University of Technology Supported by: Kempe foundations, I2, CASTT and the Swedish Road Administration. Academic partners:

4 ISSN: ISBN Luleå 29

5 Preface This report contains stop distance measurements performed during winter 28/29 at Ice- Maker s test tracks on lake Kakel, Arjeplog. Luleå University of Technology, the Divisions of Machine Elements and Structural Engineering are thrilled that we have been given the opportunity to be a part of these tests. We would like to thank the Swedish Road Administration, Kempe foundation, I2 and CASTT for funding of these tests. We also would like to thank Mikael Nybacka and Janne Granström at LTU for their help with measuring systems. A special thanks to Carl-Henrik Ulegård at the Swedish Road Administration for all his help during the entire project. Luleå June 29 Niclas Engström, Henrik Andrén, Lennart Fransson and Roland Larsson

6 Contents 1 Executive summary 3 2 Introduction 5 3 Goals and objectives 6 4 Methodology Calibration Conditions during tests Data presentation Results Brushed old polished ice, March 1, Brushed old polished ice, February 11, Brushed old polished ice, February 12, Rough old system 2 ice, February 11, Discussion 19 7 Conclusions 21 List of Figures 23 List of Tables 24 A Tires 25 B Vehicles 28 B.1 Vehicles B.2 Vehicles C V-Box3i 3 C.1 Specification C.2 Certificates

7 D Deceleration 34 D.1 Brushed old polished ice, March D.2 Brushed old polished ice, Februrary D.3 Rough old system 2 ice, February E Shortest stop distances 37 E.1 Brushed old polished ice during March 1, E.2 Brushed old polished ice during February 11, E.3 Brushed old polished ice during February 12, E.4 Rough old system 2 ice during February 11, F Theory 42 F.1 Basic kinetics F.2 Deceleration measurements

8 Chapter 1 Executive summary In this report we highlight stop distance and roadgrip differences for ten sets of studless winter tires. There is a need to evaluate performance of winter tires and illustrate to the public that there are differences between winter tires. Some are made for northern Europe, some for central Europe and some for other parts of the world where special conditions and regulations apply. We also see a need to relate roadgrip measurements with different conditions. This to make distributed roadgrip information more clear to drivers related to their equipment. We must make drivers more aware that the tires on the vehicle are a very important factor when it comes to produce high and safe roadgrip. Stop distance tests are a well accepted method to measure performance of winter tires. In our opinion brushed old polished ice is a low grip surface that are relevant to test roadgrip on. The test track section with brushed old polished ice was roughly 1 m long and 1 m wide. The stop distance brake tests were performed from left to right. See Figure 1.1. Test track layout Brushed old polished ice Brake activation marker M Vehicle tire paths 1 m 5 m Figure 1.1: Test track Layout during stop distance measurements on Brushed old Polished Ice. Stop distance measurements were also performed on system 2 ice. This is a surface created with a grader equipped with system 2, system 2 is based on rounded hard metal teeth roughly 3 mm apart on the edge of a blade. This creates furrows in the ice, a rough surface. We found that roadgrip was high and very similar for all tires on system 2 ice. Stop distances were short and we do not think that a surface like that are dangerous, as long as the driver adapts speed and distances to the available roadgrip. 3

9 CHAPTER 1. EXECUTIVE SUMMARY Temperatures during all test ranged from -25 C to -2.5 C, however most tests were performed between -18 C and -6 C. If stop distance test becomes mandatory for winter tires and are made in a controlled environment then we recommend a temperature above -6 C. Roadgrip decrease rapidly above that temperature as ice surfaces becomes slicker. Speeds of 3 km / h, 5 km / h and some times 7 km / h were used during the stop distance measurements. Different sizes of vehicles were used during the stop distance measurements, Volvo XC9, XC7, C3 and V7. No difference in stop distance related to car size was found. Tires for the tests were selected based on tests for magazines. We selected a test winning tire, a tire considered very bad for winter conditions and a tire we generally use in our tests as a reference tire. In our tests the test winning tire was actually performing below the norm on brushed old polished ice. Furthermore we found out that the very bad tire tested in one magazine [7] was classified for summer use by the manufacturer. The winter tires from this brand performed to the norm on brushed old polished ice and was considered an adequate winter tire. One tire outperformed all others on brushed old polished ice, it was made in Japan, where studless winter tires are the only kind allowed. It created at least 4 % higher roadgrip than any other tire. Tires are an important factor on winter roads. Tire information regarding performance on ice should be available to the public. We recommend that performance data is measured and presented by an independent entity. Tread depth is one important factor when evaluating conditions on a winter tire, however correct rubber compound is much more important than tread depth. 4

10 Chapter 2 Introduction Correct winter tire selection is difficult for most vehicle owners, a correct choice depends on many factors. It has become even more important as studded tires are on a decline in many areas of Sweden, this is true, especially for highly populated areas around larger cities, see [6]. This could increase the risk if the vehicle owners have insufficient information to base their tire selection on. In the tire industry it is an established fact that tires are made for different tasks and/or areas of the world. This information is not readily available for most vehicle owners, the owner has to rely on information found in publications and recommendations from his local tire salesman. In general the information is not sufficient and there is a need for a classification system were tires are rated and recommended for a certain region. The most dangerous situation occurs when a vehicle owner purchase a tire made for a region south of his location. He will get a tire with significantly harder rubber compounds and thus a low level of hysteresis at cold temperatures, a.k.a. a glass transition point T g, significantly higher than local temperatures. This will reduce the roadgrip and possibly lead to an incident. There have been research done on differences between different tires, see [5] and the results are clear, wrong rubber compound leads to a drastic reduction in roadgrip on ice and snow. This information must be available for all vehicle owners. It is strongly recommended that an tire index is created and maintained. The European New Car Assessment Programme EURO NCAP is one example of support to the buyer. EURO NCAP is a voluntary safety assessment program, however there are no tire selection section giving points for correct tire selection to a new vehicle. This is a significant weakness in the safety assessment. This report is a step in the process of creating an independent index. We are testing the stop distance for ten sets of tires. They are from five different brands and are studless. The bulk of the tests were performed on smooth brushed old polished ice. Temperatures were well below freezing, resulting in higher roadgrip than one would see with temperatures close to C. We will see some dramatic differences as one set was made for a special region of the world where studded tires are banned, namely Japan. 5

11 Chapter 3 Goals and objectives The ultimate goal for this project is to decrease fatalities, injuries and damages on property in transportation activities during winter seasons. To accomplish this we need to increase the knowledge about winter tires and how they achieve roadgrip. One step toward the ultimate goal is to increase awareness about the importance of selecting winter tires. This is a goal that is possible to reach if the Swedish Road Administration, by themselves or with assistance of LTU, create a regulatory test that all winter tired must undergo to be approved for use in winter conditions, see [1]. Assistance from the Scandinavian Tire and Rim Association, AB Svensk Bilprovning, and VTI should be considered. Our objectives in testing studless winter tires were to build a case for a tire index and/or tire classification system, since there are significant differences in roadgrip depending on what type of winter tire one has mounted on a vehicle. We also wanted to see what impact vehicle weight has on stop distances. Tests on different surfaces were performed to find critical surface types where accidents are more likely to happen. 6

12 Chapter 4 Methodology The basis of our tests are stop distance measurements on brushed old polished ice. Measurements were made with a total of ten sets of studless winter tires. Three sets came from two brands, two sets from one brand and one set from two brands. There were also one set old studded tires tested. Tires from a specific brand were not always the same model, as the two larger test vehicles were of sport utility vehicle (SUV) type and subsequently higher than regular cars. Tire manufacturers make stiffer tires for high vehicles to reduce the risk of tipping. For further tire information see Appendix A. Brake distance measurements were carried out during two periods, the first from to and the second period from to During the first period we used three Volvo cars, models were: XC9, XC7 and C3. For detailed information about the cars see Appendix B. Stop distance measurements were performed with two GPS (global positioning system) based V-Box3i 1 Hz units, see Appendix C for detailed information. To increase accuracy in the measurements, inertial motion sensors named IMU2 were connected to the V-Box3i units. In the software for the V-Box3i units we used an option to set speed dependent triggers to start and stop distance measurements. This trigger function will, when activated, start the measurement as soon as the speed decrease below a set value. For these tests the speeds selected were 3 km / h, 5 km / h and sometimes 7 km / h. During the second period we used a Volvo V7 and we tested with one set of tires from the previous period and one set that we wanted to complement the tests with. Tests with a rented Ford Mondeo equipped with used studded tires, F5 were done to see how old studded tires compare with new studless tires on brushed old polished ice. The test track section with brushed old polished ice was roughly 1 m long and 1 m wide. See Figure

13 CHAPTER 4. METHODOLOGY Test track layout Brushed old polished ice Brake activation marker M Vehicle tire paths 1 m 5 m Figure 4.1: Test track Layout during brake tests on brushed old polished Ice. Driving direction is from right to left. Preparation on the old polished ice was done by a local entrepreneur during early morning hours. The surface was brushed with a radial rotating brush, pushing the debris forward in the longitudinal 1 direction of the track. A very strong fan blows the lose debris in the lateral direction, off the track. Before the tests we drove straight down the same path across the brake test area. This was done multiple times to get a polished surface with stable characteristics. To minimize the amount of debris on the test area, we drove in the same tire tracks outside the brake test area. See Figure 4.2. During the preparation phase we utilized higher speeds then during the test phase, this to ensure that conditions would be similar throughout the whole length of the test track. Figure 4.2: Clear smooth polished ice made over rough system 2 ice. During some parts of the test a light snowfall fell in the test area. To ensure stationary conditions we drove with all vehicles to keep any lose snow from accumulating on the test surface. 1 Longitudinal is in the track and vehicle direction, lateral meaning to the side. 8

14 4.1. CALIBRATION CHAPTER 4. METHODOLOGY Data that was not repeated during at least three stop distance measurements were discarded. Instructions to the test drivers during the tests were: Use cruise control to maintain a slightly higher speed than target speed. Drive along the same path as before. Make smooth and small directional adjustments to maintain the right direction throughout the brake sequence. Brake firmly at the marker and apply firm pressure on the brake pedal until the car comes to a complete stop. When the braking is completed drive away following initial direction without spinning the wheels. Follow a fixed path back to the start position to minimize debris in the test area. Repeat the sequence until at least three similar stop distances has been recorder for each speed. Tire changes during the first four test days were done at a local tire shop. This shop where located roughly five km from the test track. During the second test section of two days we changed wheels manually on lake Kakel. 4.1 Calibration Calibration of the V-Box3i is done by the manufacturer. For certificates see Appendix C. A stand alone V-Box3i with a GPS antenna has a position accuracy of 3 m 95 % Circle of Error Probable (CPE); this means that the V-Box position measurement will fall into a circle with diameter 3 m 95 % of the time. During start up one should turn on the V-Box unit and park the vehicle for at least 1 minutes in a position that has as few obstacles as possible blocking satellites, this is done to lock onto as many satellites as possible. Speed accuracy is.1 km / h, for further details see Appendix C. 4.2 Conditions during tests Conditions from to Tests were conducted in February, generally one of the colder months of the year, as is evident from Figure 4.3. Temperature and relative humidity was measured at ice level with a USB-52 RH/Temperature Data Logger, protected from direct sunlight by a screen. Measurement data was confirmed with a Oregon Scientific Professional Wireless Weather Station WMR1N. Figure 4.3 shows that measurements started as the sun was beginning to warm up the ice surface, reaching a peak, followed by a decline as the sun was setting. This data correlated well with the weather station data, despite the latter being located 1.7 m off the ground. The data logger naturally reported a slightly elevated relative humidity compared to the weather station. Despite that the dew point never exceeded temperature, hoar frost is formed, see Figure 4.4. As temperature is lower at the ice surface due to radiation. The last measurements on gave higher roadgrip due to hoar frost forming on the ice. Those results were removed from the data before analysis. 9

15 CHAPTER 4. METHODOLOGY Relative Humidity (%) Temperature ( C) 4.2. CONDITIONS DURING TESTS Feb Feb Feb 29 Figure 4.3: Temperature (blue line) and relative humidity (green line) together with dew point (red dotted line) in C during test times (white areas) and night times (gray areas). Vertical dotted lines represent midnight. Figure 4.4: Left: evidence of hoar frost growth during night. Right: a picture of ice crystals (hoar frost) as the sun settled. Conditions from In March the average temperature had gone up, still safely below C. Because of plowing and brushing we had to place the USB data logger in the surrounding snow. This made relative humidity appear higher, still comparable to weather station data. Nighttime temperature in Figure 4.5 previous to the tests were stable. During daytime temperature goes up, but stays well below zero. However note that ice is more slippery at warmer temperatures[2]. 1

16 4.3. DATA PRESENTATION CHAPTER 4. METHODOLOGY 1 9 Temperature ( C) Relative Humidity (%) 1 1 Mar 29 Figure 4.5: Temperature (blue line) and relative humidity (green line) together with dew point (red line) in C during test times (white area) and nighttimes (gray areas). 4.3 Data presentation Collected data is presented with graphs illustrating stop distances in [ m] and graphs illustrating rate of deceleration in [ m / s 2]. In the graphs each vehicle is represented by a certain symbol. Symbol size represents three speeds: 3 km / h, 5 km / h, and 7 km / h. Every symbol represents one measurement. Each tire brand is color coded, for examples see Figure 5.1 or Figure D.1. The four shortest stop distances were extracted for each speed, surface type, tire, vehicle and day. As a precaution the shortest of those four were removed. Data is presented as an average minimum stop distance with two standard deviations for the different tire types to produce 95 % confidence intervals. In the graphs we also can see many measurements other than the four mentioned above. These measurements were recorded during preparations and not during steady state conditions. Each set of three stop distances is assembled in a table, see Appendix E were time, car type, tire index, speed, braking distance, temperature and relative humidity are listed for the different days and surface types. 11

17 Chapter 5 Results Results will be presented according to the following list: Tests from 3 km / h and 5 km / h to km / h, during March 1, 29 on brushed old polished ice. Tests from 3 km / h and 5 km / h to km / h, during February 11, 29 on brushed old polished ice. Tests from 3 km / h and 5 km / h to km / h, during February 11, 29 on brushed old polished ice. Tests from 3 km / h and 5 km / h to km / h, during February 12, 29 on brushed old polished ice. Tests from 7 km / h to km / h, during February 12, 29 on brushed old polished ice. Tests from 3 km / h, 5 km / h and 7 km / h to km / h, during February 11, 29 on rough old system 2 ice. 5.1 Brushed old polished ice, March 1, 29 Stop distances for tires A2 and C2, were tested with a Volvo V7. Tests with used studded tires, F5 on a Ford Mondeo, were also performed to evaluate how they compare with new studless tires. See Figure 5.1 below. Temperatures during these tests were between -6 C and -2.5 C, see Figure 4.5 for condition data. 12

18 5.1. BRUSHED OLD POLISHED ICE, MARCH 1, 29 CHAPTER 5. RESULTS , braking distances on polished ice at 3km/h Braking distance [m] A C F V7 Ford A2 C2 F5 V7 Ford Mondeo Figure 5.1: Stop distances on brushed old polished ice at 3 km / h, March 1, 29. Bars represents average stop distances, horizontal lines represent two standard deviations Stop distances are similar in proportion at 3 km / h (Figure 5.1), compared to results at 5 km / h (Figure 5.2). Old studded tires maintain their advantage over new studless tires. Variations in breaking distance for studded winter tires are low. This is partially an artifact, as studless tires were used to clear the ice and stabilize conditions before tests with studded tires were performed. Studded tires had a lower total average , braking distances on polished ice at 5km/h 8 1 Braking distance [m] A C F V7 Ford A2 C2 F5 V7 Ford Mondeo Figure 5.2: Stop distances on brushed old polished ice at 5 km / h, March 1, 29. Bars represents average stop distances, horizontal lines represent two standard deviations Tire C2 was tested specifically as a magazine [7] claimed that this brand had horrible performance on ice. We found that the magazine [7] had a summer tire in their article and not a winter tire. This brand s studless winter tire has no significant weakness compared to tire A1 and tire C2 is better than average, in our tests. 13

19 5.2. BRUSHED OLD POLISHED ICE, FEBRUARY 11, 29 CHAPTER 5. RESULTS 5.2 Brushed old polished ice, February 11, 29 Figure 5.3 shows that tire A1 performs worse during February 11, 29 compared with results during measurements made March 1, 29, see Figure 5.1. Note that tires A1 and A2 are the same tires. The only difference of the testing equipment is what vehicle they were fitted on. This fact is interesting as ice temperature were higher during March 1, 29. Generally ice is more slippery at that temperature [4]. However low temperatures affect roadgrip as rubber compound stiffens, especially if the rubber compound was made for a warmer climate. Temperatures during these tests were between -18 C and -8 C. For conditions during February 11, 29 see Figure 4.3. There are several changes in test conditions that could have resulted in changed performance. It is however surprising that we lose performance, when a major factor suggested that we should have shorter stop distances during the brake tests made the February 11 th , braking distances on polished ice at 3km/h 8 1 Braking distance [m] A D E C3 XC7 XC A1 B1 D1 A3 B3 E3 A4 B4 D4 C3 XC7 XC9 Figure 5.3: Stop distances on brushed old polished ice at 3 km / h, February 11, 29. represents average stop distances, horizontal lines represent two standard deviations Bars Results in Figure 5.4 is interesting, as we can compare A brand tires on the Volvo C3 and the Volvo XC9. Loose particles on the ice resulted in longer preparation time to achieve stable conditions for tire A4, see Figure D.2 in Appendix D, when they were achieved, performance was significantly better on polished ice than for any other tires we tested. Stop distance at 3 km / h, for the Volvo C3 with A1 tires are 38 % longer, compared to stop distances measured for the Volvo XC9 with A4 tires. See Appendix E. The difference at 5 km / h was 48 %. These results are supported by Hjort [5] He found that the A4 tire was better than all but one of the new studded winter tire tested during his brake tests on ice at -3 C. One can also read that the shore A value at 2 C for tire A4 in Hjort s test was 43. This shore value is significantly lower than any other tire tested. Second lowest in Hjort s tests had a shore value of 55 and the highest value for a summer tire was 7. We were unable to find the Shore scale used, but assumed it to be A, as is typical for automobile tires. In We can also see what a huge impact debris has on the performance on smooth surfaces 14

20 5.3. BRUSHED OLD POLISHED ICE, FEBRUARY 12, 29 CHAPTER 5. RESULTS , braking distances on polished ice at 5km/h Braking distance [m] A D E C3 XC7 XC A1 B1 D1 A3 B3 E3 A4 B4 D4 C3 XC7 XC9 Figure 5.4: Stop distances on brushed old polished ice at 5 km / h, February 11, 29. represents average stop distances, horizontal lines represent two standard deviations Bars 5.3 Brushed old polished ice, February 12, 29 Figure 5.5 shows results of the same test that first was made the day before, February 11, 29. Temperatures are comparable, see Figure 4.3. This data has a minimum spread. The difference between tire A1 and A4 has increased to 91 %, as is evident in Table E.3. The only real indication is that measurements for the XC9 with A4 tires from February 11, 29, has two values close to each other and one significantly higher, see Table E.4. This indicates that the difference from the 11 th actually underestimated roadgrip with the A4 tire. Otherwise the D type tires perform similar and comparable to the tires A1 on the C , braking distances on polished ice at 3km/h Braking distance [m] A C C3 XC A1 B1 D1 A3 B3 E3 A4 B4 D4 C3 XC7 XC9 Figure 5.5: Stop distances on brushed old polished ice at 3 km / h, February 12, 29. represents average stop distances, horizontal lines represent two standard deviations. Bars In Figure 5.6 the same pattern can be seen at 5 km / h as for the results at 3 km / h previously. A brand tires outperform the rest, and the B and D brand give comparable results, even if the 15

21 5.3. BRUSHED OLD POLISHED ICE, FEBRUARY 12, 29 CHAPTER 5. RESULTS B1 tires do outperform the D , braking distances on polished ice at 5km/h Braking distance [m] A B D C3 XC A1 B1 D1 A3 B3 E3 A4 B4 D4 C3 XC7 XC9 Figure 5.6: Stop distances on brushed old polished ice at 5 km / h, February 12, 29. represents average stop distances, horizontal lines represent two standard deviations. Bars 7 km / h tests were only made with A brand tires as seen in Figure 5.7, since other tires failed to stop in the 1 m brushed old polished ice area. Once again, A4 tires outperform the A1 tires , braking distances on polished ice at 7km/h Braking distance [m] A1 B1 D1 A3 B3 E3 A4 B4 D4 C3 XC7 1 2 XC9 3 A C3 XC9 Figure 5.7: Stop distances on brushed old polished ice at 7 km / h, February 12, 29. represents average stop distances, horizontal lines represent two standard deviations. Bars Rough old system 2 ice, February 11, 29 Data from tests on rough old system 2 ice were stable and contained small differences in stop distances, see Figure

22 5.3. BRUSHED OLD POLISHED ICE, FEBRUARY 12, 29 CHAPTER 5. RESULTS , braking distances on rough ice at 3km/h Braking distance [m] A B C E C3 XC7 XC A1 B1 D1 A3 B3 E3 A4 B4 D4 C3 XC7 XC9 Figure 5.8: Stop distances on brushed old system 2 ice at 3 km / h, February 11, 29. Bars represents average stop distances, horizontal lines represent two standard deviations. At higher speeds, see Figure 5.9, it is interesting to note that tire D4 outperforms tire A4, whereas both tire A1 and D1 on the Volvo C3 perform very similarly. The reason for the good performance with D4, are that harder rubber compounds results in stiffer thread pattern, this will create a strong physical connection through gear interaction with the rough ice surface and thus higher roadgrip. Think about this as how well the thread pattern resist bending. Gear interaction is the largest difference between friction and roadgrip , braking distances on rough ice at 5km/h Braking distance [m] A B C E C3 XC7 XC A1 B1 D1 A3 B3 E3 A4 B4 D4 C3 XC7 XC9 Figure 5.9: Stop distances on rough old system 2 ice at 5 km / h, February 11, 29. Bars represents average stop distances, horizontal lines represent two standard deviations. Results are similar in Figure 5.1, D4 are slightly better than A4 on rough old system 2 ice. The combined results for A4 on brushed old polished ice and on rough old system 2 ice are much better then for tire D4, see Figure 5.6 for results on brushed old polished ice. 17

23 5.3. BRUSHED OLD POLISHED ICE, FEBRUARY 12, 29 CHAPTER 5. RESULTS , braking distances on rough ice at 7km/h Braking distance [m] A B C E C3 XC7 XC A1 B1 D1 A3 B3 E3 A4 B4 D4 C3 XC7 XC9 Figure 5.1: Stop distances on rough old system 2 ice at 7 km / h, February 11, 29. Bars represents average stop distances, horizontal lines represent two standard deviations. 18

24 Chapter 6 Discussion Testing a large amount of tires on different vehicles requires planning down to the smallest details, significant material resources and trained personnel. We are well on our way to collect experience to successfully perform winter tire tests in a professional manner. In the magazine Auto Motor & Sport [7] a summer tire (Goodride R-VH68) was tested and the brand Goodride was discredited. The magazine wrote that it was a dangerous winter tire that should not be allowed to be sold in Sweden. Goodride R-VH68 is not a winter tire, it is a summer tire. Tire C2 (Goodride SW61) is not dangerous at all. Further more, tire C2 are not on the list of approved winter tires and we had to get it from Europe. Goodride does not have any tire on the list of approved winter tires. We see no reason why their model SW61 should not be approved for winter use. Tire A4 significantly outperforms all other tires on brushed old polished ice, including other tires from the same brand. It is important to note that the high grip SUV tires A3 and A4, fitted on the Volvo XC7 and XC9, were manufactured in Japan. The rubber compound in tires A3 and A4 is softer [5] than rubber compounds in the European manufactured A1 and A2 tires. Our theory is that the soft rubber compound is the key to this high roadgrip on brushed old polished ice. The reason for the softer rubber compound are regulations in Japan that forbids studded winter tires. This drives the need to adapt the rubber compound such that it produce high roadgrip on dangerous surfaces. This is a clear indication that research should be made on different rubber compounds and on their characteristics. This will result in better winter tires and correct information to owners of vehicles such that they can purchase the right type of winter tires for their use. One objective was to measure if there was a difference in stop distance depending on the size or weight of the vehicle. We found no roadgrip differences originating from size and weight. During our measurements effects originating from the tires dominates the length of the stop distances, not the size or weight. Tires are manufactured specifically for different types of vehicles and are so different that no direct comparison is possible. No tire made for a small car should be used on a big, high SUV, as this type of vehicle is prone to rollover in many types of accidents. Roadgrip on rough old system 2 ice was good and even during these tests with new studless winter tires. During brake tests made March 19, 28, see technical report Road grip test in Arjeplog [3] it was found that used tires can have large differences in roadgrip on rough old system 2 ice. Rough old system 2 ice is a surface that together with good winter tires create relatively good roadgrip. The situation is not the same if the vehicle has bad winter tires. Rubber compounds made for warmer temperatures gets very glassy and stiff in low temperatures, this reduces the roadgrip and unless the wavelength of the asperities in the road surface are close 19

25 CHAPTER 6. DISCUSSION to the tread pattern and will result in low levels of roadgrip. Another important finding was how significantly roadgrip was affected by a low amount of lose snow and/or ice particles on the ice surface. Some of the tested tires collected snow on the thread pattern, and in the cold environment, snow to ice friction is quite low [2]. If the layer was thin enough and not all over the thread pattern, it seemed like snow crystals could act as glue between the tire and the ice surface and thus increase the roadgrip. Hoar frost, forming on the ice surface as the sun settled (Figure 4.4), also severely disturbed the measurements. Braking on a brushed old polished ice surface with freshly regenerated ice crystals increased the roadgrip compared with brake test in tracks that had been freshly polished by earlier brake tests. More research will be done to address the importance of rubber compound in these and other tires. Tire A4 was about average on rough ice see Figure 5.1. The advantage that the soft rubber had on smooth ice surfaces are gone, as the gear interaction forces are reduced compared to tires with harder rubber compound. According to Figure 5.1 and Figure 5.2 Old Studded winter tires outperforms new studless winter tires with 9% - 11% on brushed old polished ice. Measurements verify that studded tires maintain safe levels of roadgrip on hard ice surfaces as the tire age and wear. 2

26 Chapter 7 Conclusions Results show large performance differences between the tested tires, some with long stop distances and others with very short stop distance, like the A4 tire with surprisingly short stop distances, see Figure 5.4. This leads to the conclusion that there should be a classification system for winter tires. Our recommendation for classes are: Nordic winter tire European winter tire South European winter tire Results from tests on rough system 2 ice show low differences between all tires and high roadgrip. This in combination with the fact that there are less grip related accidents on surfaces with high grip, indicate that classifications should be performed on one or more low grip surfaces, such as brushed old polished ice. Another important road condition to consider is "black ice", which is asphalt covered with a thin sheet of clear ice. This condition is very difficult to visually detect for a driver and is therefor a threat to safe transportation. To ensure adequate traffic safety, road grip has to be measured [3]. The best way to measure roadgrip is by using real winter tires. The fundamental reason for this is that the rubber compound are similar to almost all winter tires used. A winter tire index for critical surfaces and/or winter tire classifications are needed to help vehicle owner when they select appropriate winter tires. Today it is close to impossible to determine what different winter tires are good at. This should be based on measurements with a standard roadgrip measurement unit and one or several standardized winter tire tests. Standardized tests should be performed under well controlled conditions. If one use full scale tests with a car, then we recommend an enclosed building with climate control. Getting comparable results from different field tests is difficult since conditions are unstable. No relation between stop distance and vehicle weight could be found. Tire model is the most dominant factor when it comes to roadgrip. Further investigations of rubber compounds is important to increase understanding of roadgrip. 21

27 Bibliography [1] S. R. Administration, Vägverkets författningssamling 23:22 kap. 9 3, N/A 23, address for hard copy Vägverket, Borlänge. By , distribution@vv.se. [2] G. Casassa, H. Narita, and N. Maeno, Shear cell experiments of snow and ice friction, Journal of Applied Physics, vol. 69, no. 6, pp , March [3] N. Engström, H. Andrén, R. Larsson, L. Fransson, and M. Nybacka, Road grip test in Arjeplog, Luleå University of Technology, Luleå University of Technology, Luleå, Technical report ISSN: , 28, test with several roadgrip measuring devices. [4] I. Golecki and C. Jaccard, Intrinsic surface disorder in ice near the melting point, Journal of Physics C, vol. 11, pp , May [5] M. Hjort, SUV-däcks väggrepp på is, Statens Väg- och transportforskningsinstitut, VTI, VTI, Linköping, Technical report 58-25, December 26, blizzak DM-Z3 is in the report. [6] D. Informationsråd, Undersökning av däcktyp samt mönsterdjup i Sverige, Däckbranschens Informationsråd, Slottsgatan 8, Varberg, Publikation 29:41, Januari/februari 29, beställd av Vägverket, kontaktperson Pontus Grönvall, Tel: [7] M. Ström, Ta kontroll, Auto motor & sport, no. 21, pp. 52 6, oktober 28, test of 21 studless and studded winter tires. 22

28 List of Figures 1.1 Test track Layout during stop distance measurements on Brushed old Polished Ice Test track Layout during brake tests on brushed old polished Ice. Driving direction is from right to left Clear smooth polished ice made over rough system 2 ice Temperatures and humidities on to Ice crystallizations on objects and test surface Temperatures and humidities on Stop distances on brushed old polished ice at 3 km / h Stop distances on brushed old polished ice at 5 km / h Stop distances on brushed old polished ice at 3 km / h Stop distances on brushed old polished ice at 5 km / h Stop distances on brushed old polished ice at 3 km / h Stop distances on brushed old polished ice at 5 km / h Stop distance on brushed old polished ice at 7 km / h Stop distances on rough old system 2 ice at 3 km / h Stop distances on rough old system 2 ice at 5 km / h Stop distances on rough old system 2 ice at 7 km / h B.1 Test cars XC9, XC7 and C B.2 Test cars V7 and Mondeo C.1 V-Box C.2 Certificate for V-Box3i C.3 Certificate for IMU D.1 Braking tests on brushed old polished ice on D.2 Braking tests on brushed old polished ice on D.3 Braking tests on brushed old polished ice on D.4 Braking tests on rough old system 2 ice on F.1 Deceleration curve at 1 Hz

29 List of Tables B.1 Test cars XC9, XC7 and C B.2 Test cars V7 and Mondeo C.1 VB3i Specification E.1 Shortest braking distances on polished ice E.2 Shortest braking distances on polished ice E.3 Shortest braking distances on polished ice E.4 Shortest braking distances on rough ice

30 Appendix A Tires Label Brand Model A1/A2 Bridgestone Blizzak Nordic WN-1 B1 Continental 25/55 R16 25/55 R16 25/55 R16 25/55 R Rotation Outside Outside Outside WTSLR WTSLR WTSLR WTSLR Europe Germany Indonesia China 94R 8.7 mm 94T 8.2 mm 94H 8.6 mm 91H 7.5 mm yes yes yes Not on list1 Viking Contact 5 D1 GT C2 GoodRide SW61 Champiro WT-AX (Snowmaster) Tire side Thread Size Manufacture date Mounting instruction Type Country of origin Load rating Thread depth Verified on STRO list 1 These tires were imported from Europe as they not are on the STRO-list of approved winter tires. 25

31 Label A3 B3 E3 Brand Bridgestone Continental Wanli Model Blizzak DM-Z3 4x4 Cross Contact Winter Snowgrip Tire side Thread Size 215/65 R16 215/65 R16 215/65 R16 Manufacture date Mounting instruction Rotation Outside Rotation Type WTSLR WTSLR WTSLR Country of origin Japan Germany China Load rating 98Q 98T 98H Thread depth 1.3 mm 8.5 mm 7.7 mm 2 Verified on STRO list yes yes yes mm in central furrow. 26

32 Label A4 B4 D4 Brand Bridgestone Continental GT Model Blizzak DM-Z3 Viking 4x4 WinterContact Savero WT Tire side Thread Size 235/65 R17 235/65 R17 235/65 R17 Manufacture date Mounting instruction Rotation Outside Rotation Type WTSLR WTSLR WTSLR Country of origin Japan Czech Republic China Load rating 18Q 14H 14T Thread depth 1. mm 8.6 mm 1.9 mm Verified on STRO list yes yes yes 27

33 Appendix B Vehicles B.1 Vehicles Table B.1: Test cars XC9, XC7 and C3 Make & Model Volvo C3 Volvo XC7 Volvo XC9 Reg no. EBS 546 HDW 88 JME 793 Color Light blue Light gray Gray Year Chassi no. YV1MK84- YV1BZ714- YV1CZ Type Sedan Sedan Sedan Transmission Manual Automatic Automatic Service weight 133 kg 182 kg 215 kg Total weight 175 kg 24 kg 275 kg Tire dim. 25/55 R16 91V 215/65R16 12V 235/65 R17 14V Rim dim. 6,5JX16X52,5 7JX16X5 Length 425 mm 495 mm 48 mm Width 179 mm 189 mm 191 mm Height 145 mm 161 mm 179 mm Figure B.1: Test cars XC9, XC7 and C3 28

34 B.2 Vehicles Table B.2: Test cars V7 and Mondeo Make & model Volvo V7 Ford Mondeo Reg no. DPS 4 GLC 58 Color Red Gray Year Chassi no. YV1BW694- WFGXXGB BG7U29312 Type Sedan Sedan Transmission Manual Manual Service weight 168 kg 158 kg Total weight 23 kg 228 kg Tire dim. 25/6 R16 96V 215/55 R16 9V Rim dim. 7JX16X5 6.5JX16H2OS5. Length 496 mm 485 mm Width 189 mm 189 mm Height 155 mm 147 mm Figure B.2: Test cars V7 and Mondeo 29

35 Appendix C V-Box3i Stop distance measurements were carried out with LTU:s V-Box3i and a V-Box3 from Artic falls. Both were equiped with inertial motion sensors, IMU2. Figure C.1: V-Box3 3

36 C.1 Specification Table C.1: VB3i Specification Make & Model RACELOGIC VB3i Distance Accuracy.5 % Update rate 1 Hz Resolution 1 cm Height resolution 6 m Velocity Accuracy.1 Km / h Update rate 1 Hz Maximum velocity 1 Mph Minimum velocity.1 Km / h Resolution.1 Km / h Latency 6.75 ms Absolute Positioning Accuracy 3 m 95 % CEP Update rate 1 Hz Resolution 1.8 mm Heading Resolution.1 Accuracy.1 Acceleration Accuracy.5 % Maximum 2 g Resolution.1 g Update rate 1 Hz 31

37 C.2 Certificates Figure C.2: Certificate for V-Box3i 32

38 Figure C.3: Certificate for IMU2 33

39 Appendix D Deceleration To compare different initial speeds and calculate roadgrip values, see Appendix F and Figure F.1, deceleration values are presented in addition to stop distance. Data is presented as averaged 1 Hz samplings of deceleration during the entire braking sequence. This is discussed further in Appendix F. D.1 Brushed old polished ice, March 1 Data in Figure D.1 reveals that the studded tires F5 on the Ford Mondeo had better grip on brushed old polished ice than studless tires tested that day. At the end of the day deceleration was greater for the A2 tire then for tire C2. It had been very close during the two earlier test sessions. Deceleration [m/s 2 ] V7 C2 3km/h V7 C2 5km/h Mondeo F5 3km/h Mondeo F5 5km/h V7 A2 3km/h V7 A2 5 km/h 9: 1: 11: 12: 13: 14: 15: 16: 17: Time of day [h] Figure D.1: Braking tests with studless tires A2 and C2 fitted on a Volvo V7 and studded tires F5 fitted on a Ford Mondeo during March 1, 29 on brushed old polished ice with an average temperature of -6 C. Temperature was stable during the entire day, snow was drifting onto the track especially in the beginning of the day. We had to brush the ice surface and drive over the ice surface to remove debris, these actions resulted in stabilized measurements. 34

40 D.2 Brushed old polished ice, Februrary It can be seen in Figure D.2 and Figure D.3 that tires A1 and A4 outperforms other tires on most runs. Tires B1 and B4 performs similarly to D1 and D4. Deceleration [m/s 2 ] C3 D1 5km/h C3 D1 3km/h XC7 E3 5km/h XC7 E3 3km/h XC9 D4 5km/h C3 A1 5km/h C3 A1 3 km/h XC9 A4 5 km/h XC9 A4 3 km/h 9: 1: 11: 12: 13: 14: 15: 16: 17: Time of day [h] Figure D.2: Decelerations on brushed old polished ice during February 11, 29 for tires A1, D1, A3, E3, A4 and D4 with Volvo C3, Volvo XC7 and Volvo XC9. Deceleration [m/s 2 ] : 1: 11: 12: 13: 14: 15: 16: 17: Time of day [h] C3 D1 5km/h C3 D1 3km/h XC9 D4 5km/h XC9 D4 3km/h C3 B1 5km/h XC9 B4 5km/h C3 A1 7 km/h C3 A1 5 km/h C3 A1 3 km/h XC9 A4 7 km/h XC9 A4 5 km/h XC9 A4 3 km/h Figure D.3: Decelerations on brushed old polished ice during February 12, 29 for tires A1, D1, A3, E3, A4 and D4 with Volvo C3, Volvo XC7 and Volvo XC9. D.3 Rough old system 2 ice, February 11 This is probably the least interesting surface, since most tires break very well and roadgrip is good. Interesting to note in Figure D.4 is that tires A4 performs in the lower parts of the spectrum, not significantly worse than any other tire. 35

41 Deceleration [m/s 2 ] : 1: 11: 12: 13: 14: 15: 16: 17: Time of day [h] C3 D1 7km/h C3 D1 5km/h C3 D1 3km/h XC7 E3 7km/h XC7 E3 5km/h XC7 E3 3km/h XC9 D4 7km/h XC9 D4 5km/h C3 A1 7km/h C3 A1 5km/h C3 A1 3 km/h XC9 A4 7 km/h XC9 A4 5 km/h XC9 A4 3 km/h Figure D.4: Decelerations on rough old system 2 ice February 11, 29 for tires A1, D1, A3, E3, A4 and D4 with Volvo C3, Volvo XC7 and Volvo XC9. 36

42 Appendix E Shortest stop distances In tables Table E.2 to Table E.1 the measurements are listed according to date, ice type, car, tire and speed. Only the second shortest to fourth shortest stop distance are represented 1 this as we found that very short stop distances could occur when hoar frost increased roadgrip. Many of the longer stop distances measured were due to snow and other lose particles on the track. E.1 Brushed old polished ice during March 1, 29 Table E.1: Shortest braking distances on polished ice. Average of a set of measurements are given together with one standard deviation, all in meters. Time Vehicle Tire Speed Dist. Dec. Temp. Relative HH:MM:SS type index ( km / h ) ( m) ( m / s 2) ( C) humidity 9:41:8 V7 2C % 9:39:32 V7 2C % 9:36:23 V7 2C % Average braking distance: Standard deviation:.66 9:32: V7 2C % 9:28:58 V7 2C % 9:26:2 V7 2C % Average braking distance: Standard deviation:.69 13:29:38 V7 2A % 16:33:41 V7 2A % 13:26:34 V7 2A % Average braking distance: Standard deviation:.12 1:34:15 V7 2A % 16:35:1 V7 2A % 9:59:19 V7 2A % Average braking distance: Standard deviation:.21 Time Vehicle Tire Speed Dist. Dec. Temp. Relative HH:MM:SS type index ( km / h ) ( m) ( m / s 2) ( C) humidity 1 If a fourth stop distance not are available, then only two measurements are presented 37

43 Time Vehicle Tire Speed Dist. Dec. Temp. Relative HH:MM:SS type index ( km / h ) ( m) ( m / s 2) ( C) humidity 14:52:11 Ford 5F % 14:56:14 Ford 5F % 14:54:3 Ford 5F % Average braking distance: 18.6 Standard deviation:.15 14:59:39 Ford 5F % 15:3: Ford 5F % 14:36:58 Ford 5F % Average braking distance: 5.33 Standard deviation:.23 Time Vehicle Tire Speed Dist. Dec. Temp. Relative HH:MM:SS type index ( km / h ) ( m) ( m / s 2) ( C) humidity E.2 Brushed old polished ice during February 11, 29 Table E.2: Shortest braking distances on polished ice. Average of a set of measurements are given together with one standard deviation, all in meters. Time Vehicle Tire Speed Dist. Dec. Temp. Relative HH:MM:SS type index ( km / h ) ( m) ( m / s 2) ( C) humidity 15:28:15 C3 1A % 15:23:7 C3 1A % Average braking distance: 26. Standard deviation: :53:21 C3 1D % 16:5:17 C3 1D % Average braking distance: Standard deviation:.94 11:12:47 XC7 3D % 11:1:41 XC7 3D % Average braking distance: Standard deviation: :4:13 XC9 4A % 15:45:11 XC9 4A % 15:35:26 XC9 4A % Average braking distance: Standard deviation: :32:28 C3 1A % 15:3:57 C3 1A % Average braking distance: Standard deviation: :32:19 C3 1D % 16:37:8 C3 1D % 16:35:1 C3 1D % Average braking distance: 77.9 Standard deviation: :8:26 XC7 3D % 11:4:23 XC7 3D % Average braking distance: 71.9 Standard deviation:.41 Time Vehicle Tire Speed Dist. Dec. Temp. Relative HH:MM:SS type index ( km / h ) ( m) ( m / s 2) ( C) humidity 38

44 Time Vehicle Tire Speed Dist. Dec. Temp. Relative HH:MM:SS type index ( km / h ) ( m) ( m / s 2) ( C) humidity 15:29:45 XC9 4A % 15:31:9 XC9 4A % 15:27:16 XC9 4A % Average braking distance: 35.5 Standard deviation: 1.8 1:36:59 XC9 4D % 1:42:55 XC9 4D % Average braking distance: Standard deviation:.44 Time Vehicle Tire Speed Dist. Dec. Temp. Relative HH:MM:SS type index ( km / h ) ( m) ( m / s 2) ( C) humidity E.3 Brushed old polished ice during February 12, 29 Table E.3: Shortest braking distances on polished ice. Average of a set of measurements are given together with one standard deviation, all in meters. Time Vehicle Tire Speed Dist. Dec. Temp. Relative HH:MM:SS type index ( km / h ) ( m) ( m / s 2) ( C) humidity 11:41:44 C3 1A % 11:31:4 C3 1A % 11:29:15 C3 1A % Average braking distance: Standard deviation:.74 14:56:36 C3 1D % 14:55:15 C3 1D % 9:26:31 C3 1D % Average braking distance: 25. Standard deviation:.45 11:44:4 XC9 4A % 11:39:26 XC9 4A % 11:42:54 XC9 4A % Average braking distance: Standard deviation:.18 14:59:54 XC9 4D % 14:56:16 XC9 4D % 14:47:18 XC9 4D % Average braking distance: Standard deviation:.2 11:54:16 C3 1A % 11:47:34 C3 1A % 11:49:43 C3 1A % Average braking distance: Standard deviation:.45 16:32:45 C3 1B % 16:39:52 C3 1B % 16:56:37 C3 1B % Average braking distance: Standard deviation: 2.99 Time Vehicle Tire Speed Dist. Dec. Temp. Relative HH:MM:SS type index ( km / h ) ( m) ( m / s 2) ( C) humidity 39