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Experimental investigation of pneumatic tire performance on ice: Part 2 – Outdoor study
Available online at www.sciencedirect.com
Journal of Terramechanics
ScienceDirect Journal of Terramechanics xxx (2015) xxx–xxx
www.elsevier.com/locate/jterra
Experimental investigation of pneumatic tire performance on ice: Part 2 – Outdoor study Anudeep K. Bhoopalam a,⇑, Corina Sandu a, Saied Taheri b a
Advanced Vehicle Dynamics Laboratory, Mechanical Engineering, Virginia Tech, Blacksburg, VA 24061, USA b Center for Tire Research, Mechanical Engineering, Virginia Tech, Blacksburg, VA 24061, USA
Abstract The concept of ‘Lab to the Road’ is very challenging, especially for tire operation on ice. The changing ambient and ice conditions necessitate the need for robust engineering for the safety of vehicles on icy roads. Test method has a direct effect in measuring tire behavior on ice, with differences in the test setup and ability to maintain constant test conditions. The purpose of design for each test method also varies; certain test methods are designed to understand absolute tire performance, as compared to studying relative tire performance. This paper examines the performance of tires on ice through field tests conducted at the Keweenaw Research Center. Part-1 (Bhoopalam et al., 2015) presented a detailed indoor test program conducted on the Terramechanics Rig at the Advanced Vehicle Dynamics Laboratory, Virginia Tech, including the ice preparation procedure, test method, and test results. The two tests procedures were performed to understand how each test method influences the test results. A comparison of the laboratory (Bhoopalam et al., 2015) and field test method is also presented in this paper, with reasons for the differences in the measured values presented. Ó 2015 ISTVS. Published by Elsevier Ltd. All rights reserved.
Keywords: Tire-ice friction; Indoor tire-ice testing; Outdoor tire-ice testing
1. Introduction Laboratory evaluation of tire performance on ice offers the advantage of repeatability of ice and ambient environmental conditions; however, testing is still conducted on artificially created ice. The advantage of testing in the field comes from observing the behavior of the tire in real world conditions. The disadvantage, however, is that repeatable test conditions might not be feasible due to changing weather conditions. Wind is a major concern when testing in the field; winds often carry dust which settle on the ice surface altering surface properties. Outdoor test programs are conducted when the weather conditions stay constant DOI of original article: 10.1016/j.jterra.2015.02.006.
for a certain time period, which makes scheduling outdoor test really hard. An extensive review of literature for both indoor and outdoor tests for the performance of tires on ice has been presented in Bhoopalam and Sandu (2014). This second part presents a complete description and analysis of the outdoor test program conducted at the Keweenaw Research Center and a comparison of laboratory test results with field test results. The next section of this paper describes the outdoor test program, the test procedure and the test conditions. A comparison of the indoor and outdoor test programs and the reasons for differences in measured friction levels are discussed in the third section. Finally, conclusions and future recommendations are presented in the last.
⇑ Corresponding author.
E-mail address: [email protected] (A.K. Bhoopalam). http://dx.doi.org/10.1016/j.jterra.2015.03.001 0022-4898/Ó 2015 ISTVS. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Bhoopalam, AK et al., Experimental investigation of pneumatic tire performance on ice: Part 2 – Outdoor study, J Terramechanics (2015), http://dx.doi.org/10.1016/j.jterra.2015.03.001
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2. Field testing Outdoor tests are an actual representation of the real world conditions, to understand the performance of the SRTTs outdoors, field tests were conducted in February 2014 at the Keweenaw Research Center in Calumet, Michigan, USA by Mobility Research Inc. The outdoor testing program included studying the performance of the same two physical Standard Reference Test Tires (SRTT) (Bhoopalam et al., 2015) on ice with dry friction, at the same three levels for normal load and inflation pressure. The outdoor tests were conducted using a traction truck in the month of February 2014, with the test tire on the right-rear wheel, as seen in Fig. 1. Testing was conducted
on split-l track as seen in Fig. 2, with the left wheels on asphalt and the right wheels on the ice surface. The test program commenced in the evening to have no effect of the solar load. 2.1. Conditions for outdoor test program Outdoor testing was conducted for a reduced experiment matrix compared to the one presented in Part-1 (Bhoopalam et al., 2015) of this paper; testing with different levels for the camber angle, toe angle, and ice temperature was not a part of the outdoor test program. All outdoor tests were conducted at 0° camber and toe, at an ice temperature of 13 °C and an ambient temperature of 18 °C. The design of experiment matrix for the outdoor testing program is as shown in Table 1. 2.2. Terminology
Fig. 1. Mobility Research Inc.’s traction truck employed to evaluate tire performance on ice. Reprinted with permission from Mobility Research Inc.
The tire performance evaluated in the field at the Keweenaw Research Center is expressed in terms of a driving coefficient. The outdoor test program was carried out as per the procedure ASTM-1805 (ASTM F1805-06). ASTM-1805 defines the driving coefficient as the ratio of longitudinal force and vertical load. In Part-I (Bhoopalam et al., 2015) of this study the results are expressed are expressed in terms of the normalized drawbar pull. According to the ISTVS standards (International Society for Terrain-Vehicle Systems Standards, 1977), the drawbar pull refers to the force available for external work in a direction parallel to the horizontal surface over which the vehicle is moving, which refers to force the longitudinal force measured in the x-direction. Thus the normalized drawbar pull (Bhoopalam et al., 2015; International Society for Terrain-Vehicle Systems Standards, 1977) is analogous to the driving coefficient (ASTM F1805-06); in the interest of keeping the terminology as per ASTM1805, the results from the field testing are expressed using the expression ‘driving coefficient’. 2.3. Test method-ASTM 1805
Fig. 2. Split-l ice track at the Keweenaw Research Center.
Field testing was conducted as per ASTM-1805 (ASTM F1805-06), with the buffed and treaded tires as the candidate tires; the treaded SRTT was also used as the control tire. The control tire was run every third condition, to follow the sequence of C (control), T1, T2, C, T3, T4, C, as specified in ASTM F1805-06. Initial test procedures before
Table 1 Design of experiment matrix for outdoor evaluation of tire performance on ice. Test condition
Number of levels
Level-1
Level-2
Tires Ice temperature Inflation pressure Normal load
2 1 3 3
Treaded SRTT 10 °C Dry friction 60% Nominal pressure 145 kPa 60% Load index 4000 N
Buffed SRTT 100% Nominal Pressure 242 kPa 100% Load index 7000 N
Level-3
120% Nominal pressure 276 kPa 120% Load index 8500 N
Please cite this article in press as: Bhoopalam, AK et al., Experimental investigation of pneumatic tire performance on ice: Part 2 – Outdoor study, J Terramechanics (2015), http://dx.doi.org/10.1016/j.jterra.2015.03.001
A.K. Bhoopalam et al. / Journal of Terramechanics xxx (2015) xxx–xxx
V tire 1 V vehicle
0.14
0.1 0.08 0.06
0.02 0 0
10
20
30
40
50
60
70
80
% Slip
Fig. 3. Variation in measured driving coefficient from 10 spin ups for the treaded SRTT. 100% inflation pressure of 242 kPa and 100% normal load of 7000 N.
ð1Þ
Vtire refers to the velocity of the tire and Vvehicle refers to the velocity of the vehicle. The velocity of the tire is measured with an encoder on the right-rear wheel. The results presented in next section have been converted in theoretical slip ratio to maintain uniformity between results, as presented in Bhoopalam et al. (2015) and in this paper.
0.12
0.04
0.16 0.14
Driving Coefficient
Practical Slip Ratio ¼
0.16
Driving Coefficient
the test program included spraying a thin layer of water (using a water truck) on the ice surface; a wide tire is then used to polish the ice surface once the water layer is frozen. Ten spin-ups were conducted for each test condition, as per Table 1, following the sequence of running the control tire every third test. Each spin-up lasted for about 3 s, during which the practical slip ratio was ramped up from 0% to 300% on the right rear wheel. The test vehicle was driven in a straight line maintaining a test speed of 8.0 ± 0.8 km/h, as specified in ASTM F1805-06. A two-axis wheel force transducer at the right-rear wheel center measures the forces in the x and z directions. The ratio of the x-force and the z-force is defined as the coefficient of friction in ASTM F1805-06. The practical slip ratio formulation as seen in Eq. (1), was used to report the findings from the outdoor test program.
3
0.12 0.1 0.08 0.06 0.04 0.02
2.4. Field test results
0 0
2.4.1. Repeatability of test results An acceptable repeatability of test data was observed for the field tests conducted at the Keweenaw Research Center. However, the variation of the measured driving coefficient was in a wider band when compared to the indoor test program (Bhoopalam et al., 2015). The curves for the 10 spin-ups can be seen in Fig. 3 for the treaded SRTT with 100% load and inflation pressure; a standard deviation of 0.006 was calculated for the measured peak values. Fig. 4 shows the measured curves for the buffed SRTT with 100% load and inflation pressure, a wider deviation was calculated for the at the peak value of 0.035 compared to the treaded SRTT. 2.4.2. Effect of normal load Outdoor tests were conducted for three levels of normal load of 60%, 100%, and 120% of the load index. At low slip ratios, the higher the normal load, the lower the friction coefficients were seen for both, the buffed and treaded
10
20
30
40
50
60
70
80
% Slip
Fig. 4. Variation in measured driving coefficient from 10 spin ups for the buffed SRTT. 100% inflation pressure of 242 kPa and 100% normal load of 7000 N.
0.16 0.14 0.12
Driving Coefficient
The following section describes the effect of the normal load, the inflation pressure, and the tread depth during operation on ice, as obtained from outdoor testing. A step jump is observed at very low slip ratios in field test results; this is due to the fact that the practical slip ratio is ramped up from 0% to 300% in a time period of 3–4 s. The range of the driving coefficient, as measured from the outdoor test program, is 0.054–0.19, as also reported by Martin and Schaefer (1996).
0.10 0.08 60% Load Index
0.06
100% Load Index
0.04 120% Load Index
0.02 0.00 0
10
20
30
40
50
60
70
80
% Slip
Fig. 5. Effect of normal load on drawbar pull for the treaded SRTT on ice with 100% inflation pressure- 242 kPa, from outdoor testing.
SRTT. The treaded SRTT exhibited this trend until a slip ratio of 30%, as seen in Fig. 5, and for the buffed tire until 17% slip ratio, as seen in Fig. 6. Above these slip ratios,
Please cite this article in press as: Bhoopalam, AK et al., Experimental investigation of pneumatic tire performance on ice: Part 2 – Outdoor study, J Terramechanics (2015), http://dx.doi.org/10.1016/j.jterra.2015.03.001
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A.K. Bhoopalam et al. / Journal of Terramechanics xxx (2015) xxx–xxx 0.16
0.16
0.14
0.14 0.12
Driving Coefficent
Driving Coefficient
0.12 0.10 0.08 60%Load Index
0.06
100%Load Index
0.04
120%Load Index
0.10 0.08 0.06
60% Pressure
0.04
100% Pressure 120% Pressure
0.02
0.02
0.00 0
10
20
30
0.00 0
10
20
30
40
50
60
70
Fig. 6. Effect of normal load on drawbar pull for the buffed SRTT on ice with 100% inflation pressure- 242 kPa, from outdoor testing.
50
60
70
80
% Slip
80
% Slip
40
Fig. 8. Effect of inflation pressure on drawbar pull for the buffed SRTT on ice with 100% normal load – 7000 N, from outdoor testing.
both, the treaded and the buffed SRTT, exhibited the same behavior and no trend was seen with variation in the normal load.
60% Load Index- Treaded SRTT
60% Load Index- Buffed SRTT
100% Load Index- Treaded SRTT
100% Load Index- Buffed SRTT
120% Load Index - Treaded SRTT
120% Load Index- Buffed SRTT
0.16
2.4.3. Effect of inflation pressure The effect of the inflation pressure on the friction coefficient was also studied through field tests. As seen from Figs. 7 and 8, a variation in the friction levels was seen at low slip ratios; however, a clear trend was observed. At higher slip ratios the effect of the inflation pressure was not noticed. For all levels of inflation pressure both, the treaded and the buffed SRTT, performed the same with friction versus slip ratio curves overlapping. 2.4.4. Effect of tread depth Comparing the buffed and the treaded SRTT at three levels of inflation pressure and three levels of normal load, the treaded SRTT provided better friction levels compared to the buffed SRTT all the time. An increase of 35% in friction levels was seen from the buffed SRTT to the treaded SRTT operating with a 100% load index and at the
Driving Coefficient
0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 0
20
40
60
80
% Slip
Fig. 9. Comparison of friction slip ratio curves for the treaded and buffed SRTT with 100% inflation pressure of 242 kPa at different normal loads, from outdoor testing.
60% Inf. Pressure-Treaded SRTT
60% Inf. Pressure-Buffed SRTT
100% Inf. Pressure-Treaded SRTT
100% Inf. Pressure-Buffed SRTT
120% Inf. Pressure-Treaded SRTT
120% Inf.Pressure-Buffed SRTT
0.16
Driving Coefficent
0.14
0.16
Driving Coefficient
0.14 0.12 0.10
0.12 0.10 0.08 0.06 0.04 0.02 0.00
0.08 60% Pressure
0
20
40
60
80
% Slip
0.06 100% Pressure
0.04
Fig. 10. Comparison of friction slip ratio curves for the treaded and buffed SRTT with 100% normal load of 7000 N with different inflation pressures, from outdoor testing.
120% Pressure
0.02 0.00 0
20
40
60
80
% Slip
Fig. 7. Effect of inflation pressure on drawbar pull for the treaded SRTT on ice with 100% normal load – 7000 N, from outdoor testing.
nominal inflation pressure, as seen from Figs. 9 and 10. The effect of inflation pressure and normal load on the tire was not predominant at high slip ratios, all the friction
Please cite this article in press as: Bhoopalam, AK et al., Experimental investigation of pneumatic tire performance on ice: Part 2 – Outdoor study, J Terramechanics (2015), http://dx.doi.org/10.1016/j.jterra.2015.03.001
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levels were seen to overlap after a slip ratio of 65%. Hence, at high slip ratios, the SRTT performs similarly, irrespective of the tread depth, inflation pressure, and the load on the tire during operation on ice.
This section compares the test results from the indoor and outdoor test programs. The effects of operational parameters, namely normal load and inflation pressure, as captured by the indoor and the outdoor programs, are explained in following sub-sections. The reasons for the differences in the performance measurements between the two test programs are explained in the next section.
120%Load Outdoor 120% Load Indoor
0.25 0.20 0.15 0.10 0.05 0.00 0
10
20
30
40
50
60
70
80
Slip Ratio %
Fig. 12. Comparison of drawbar pull/friction slip ratio curves for the buffed SRTT with 100% inflation pressure of 242 kPa at different normal loads, from outdoor and indoor test programs.
3.1. Effect of normal load The effect of normal load during the operation of the treaded SRTT on ice as measured in the laboratory was shown in Bhoopalam et al. (2015). From the indoor experiments, the higher the normal load, the lower the drawbar pull was measured. Whereas with higher normal loads, higher friction levels were measured in the field; this trend was observed until a slip ratio of 20%. Above 20% slip ratio, no clear trend was observed from the field tests and the curves for the three normal load cases were seen overlapping, as in Fig. 11. A comparison of the performance of the buffed SRTT on ice as measured on the Terramechanics Rig and on the outdoor ice track is shown in Fig. 12. The buffed SRTT tire behaved the same as the treaded SRTT until a slip ratio of 15% when tested in the laboratory. At slip ratios of 15% and above no clear trend was observed with variation of normal load. From the field testing at the Keweenaw Research Center, at low slip ratios until 12% higher friction levels were measured with higher normal load, in the mid slip ratio range no clear trend was observed. High slip ratios above 40% the 60% load case yielded the maximum friction.
60%Load-Outdoor
100%Load-Outdoor
60% Load-Indoor
3.2. Effect of inflation pressure Comparing the effect of inflation pressure as measured from the indoor and outdoor test program, from indoor studies for the treaded SRTT, after a slip ratio of 20%, with lower inflation pressure, a higher drawbar pull was measured, as seen in Fig. 13. After a slip ratio of 50%, a reverse trend is seen due to effect of heat generation in the contact patch. The results from the field tests show a lower friction level with a lower inflation pressure up to a slip ratio of 9%, after which the effect of inflation pressure is not captured by the outdoor test procedure and curves for the three inflation pressure cases are seen overlapping. The buffed tire exhibited no clear trend until a slip ratio of 15%, after which the tire with nominal inflation pressure showed the highest drawbar pull, as seen in Fig. 14. Both 60% and 120% inflation pressure cases measured a lower drawbar pull compared to the 100% inflation pressure, from indoor testing. The field tests for the buffed tire did not capture any effect of the inflation pressure; from Fig. 14 all the three curves are seen overlapping.
120%Load-Outdoor
100% Load-Indoor
120% Load-Outdoor
0.35
60% Pressure-Outdoor
100% Pressure-Outdoor
60% Pressure-Indoor 120% Pressure-Outdoor
100% Pressure-Indoor 120% Pressure-Indoor
0.25
0.30
Driving Coefficient / Normalised Drawbar Pull
Driving Coefficient / Normalised Drawbar Pull
100%Load Outdoor 100% Load Indoor
0.30
Driving Coefficient / Normalised Drawbar Pull
3. Comparison of indoor and outdoor test programs
60%Load Outdoor 60% Load Indoor
5
0.25 0.20 0.15 0.10 0.05
0.20 0.15 0.10 0.05 0.00
0.00 0
10
20
30
40
50
60
70
80
Slip Ratio %
Fig. 11. Comparison of drawbar pull/friction slip ratio curves for the treaded SRTT with 100% inflation pressure of 242 kPa at different normal loads, from outdoor and indoor test programs.
0
10
20
30
40
50
60
70
80
90
Slip Ratio %
Fig. 13. Comparison of drawbar pull/friction slip ratio curves for the treaded SRTT with 100% normal load of 7000 N with different inflation pressures, from outdoor and indoor test programs.
Please cite this article in press as: Bhoopalam, AK et al., Experimental investigation of pneumatic tire performance on ice: Part 2 – Outdoor study, J Terramechanics (2015), http://dx.doi.org/10.1016/j.jterra.2015.03.001
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A.K. Bhoopalam et al. / Journal of Terramechanics xxx (2015) xxx–xxx 60% Pressure Outdoor 60% Pressure-Indoor 120% Pressure Outdoor
90
100% Pressure Outdoor 100% Pressure-Indoor 120% Pressure-Indoor
80
Shore-A Hardness
Driving Coefficient / Normalised Drawbar Pull
0.25
0.20
0.15
0.10
70 60
Treaded-SRTT 50
Buffed-SRTT 40
0.05 30 -20
-15
-10
0.00 0
10
20
30
40
50
60
70
0
5
10
15
20
25
Temperature (¶C)
80
Slip Ratio %
-5
Fig. 15. Variation of tread hardness with temperature for the SRTTs.
Fig. 14. Comparison of drawbar pull/friction slip ratio curves for the buffed SRTT with 100% normal load of 7000 N with different inflation pressures, from outdoor and indoor test programs.
4. Reasons for difference in friction measurements – indoor versus outdoor test programs The previous section compares the friction measurements/tire performance from the indoor and the outdoor test programs. As one might have observed, the results from the two test programs are not similar. This section describes the investigations carried out to understand the reasons for the differences noticed in the friction measurements. Table 2, lists the differences in the calculation procedures and the test conditions. A difference by 3–5 °C in the ice temperature and by 30 °C in the ambient temperature existed; such large differences in temperature alter the mechanical properties of the rubber compound of the SRTT and thus the performance of the SRTT (a later subsection describes the variation of tread hardness of the SRTT with temperature). A difference by 2 times in the ice thickness existed between the two test programs; however, we consider that a 3 in. ice thickness in the laboratory would not likely cause a performance variation as compared to the tire performance on ice thickness of 6 in. Ice in the laboratory was created using water from the Town of Blacksburg, which is fit for drinking. Whereas at the Keweenaw Research Center ice was created using water from an underground well; one can expect a high iron or copper content in this water due the old mines present in the town of Calumet, Michigan. The chemical content of the water sample used in ice creation also alters Table 2 Comparison of test conditions and calculations for the indoor and outdoor test program. Parameter
AVDL
MRI
Slip ratio calculation Water source Ice thickness Ice temperature Ambient temperature Longitudinal velocity
Theoretical slip ratio Town of Blacksburg 2.5–3 in. 8 to 10 °C 11–12 °C 0.218 km/h
Practical slip ratio Underground well 6 in. 13 °C 18 °C 8 km/h
the performance levels of the tire; the review paper (Bhoopalam and Sandu, 2014) contains additional information about previous studies in this subject. There existed a difference of 40 times in the longitudinal velocity of the tire in the field versus the laboratory; an increased longitudinal velocity means increased heat generation in the contact patch, which definitely reduces the friction levels. 4.1. Ice resurfacing procedure The ice preparation and resurfacing procedures are explained in Bhoopalam et al. (2015) for the indoor and Section 2.2 for the outdoor test program, respectively. In the indoor investigations the tire runs on fresh ice for every test condition with resurfacing procedures followed between every test run. Whereas in outdoor tests, an initial water layer is sprayed on the ice after the water layer is frozen, a wide tire is used to polish the ice surface. After which the testing was conducted, with no resurfacing between the runs. The initial polishing and no resurfacing the ice between the runs leads to smoothing of ice surface and there by a reduction in the friction level. 4.2. Slip ratio control Part-1 (Bhoopalam et al., 2015) of this study and Section 2.2 describe the slip ratio control procedure followed while testing indoors and outdoors, respectively. For indoor tests the slip ratio is maintained at a steady-state slip ratio of time period of 20 s; during field testing the slip ratio is ramped up from 0% to maximum in the 3 s to 4 s. This difference in slip ratio control leads to change in heat generation in the contact phenomenon. The ramp slip ratio control and 8 km/h longitudinal velocity during filed testing lead to shorter adhesion times and thus a reduced friction level is seen when compared to outdoor testing. 4.3. Tread hardness versus temperature The hardness of the SRTT tread was measured using a durometer at different temperatures as in Fig. 15, a 20% increase in tread hardness was observed from the indoor
Please cite this article in press as: Bhoopalam, AK et al., Experimental investigation of pneumatic tire performance on ice: Part 2 – Outdoor study, J Terramechanics (2015), http://dx.doi.org/10.1016/j.jterra.2015.03.001
8000
7000
7000
6950
6000
Indoor
5000
Outdoor
6900 6850 6800
4000 6750 3000
6700
2000
Outdoor (N)
Indoor (N)
A.K. Bhoopalam et al. / Journal of Terramechanics xxx (2015) xxx–xxx
6650
1000
6600
0
6550 0
0.5
1
1.5
2
2.5
3
Time (s)
Fig. 16. Comparison of time histories of normal load, indoor testing versus outdoor testing.
laboratory temperature to the outdoor test temperature. This measurement also confirms the findings in Bhoopalam et al. (2015), a reduced drawbar pull is seen with a reduction in ambient temperature. The increase in thread hardness is also a reason for the reduction in friction levels from indoor to outdoor testing. 4.4. Normal load time histories The time histories of the normal load of the two test procedures were compared as seen in Fig. 16. The Terramechanics Rig is equipped with an active normal load control system as described in Bhoopalam et al. (2015), which maintains a contents level of load throughout the entire run. On the other hand, a variation of 150–200 N was observed during the 3 s of slip ratio ramp-up during outdoor tests. A change in normal load means a change in effective rolling radius and a change in the slip ratio. Maintaining a constant normal load during filed testing is a challenge with the traction truck experiencing a certain amount of bounce. We estimate that the variation in normal load is not a major, but a minor contributor to the difference friction levels measured in the laboratory and in the field. 5. Discussion: comparison with prior research The effect of test method studied by Shoop et al. (1994) measured the same peak friction coefficient with two test vehicles on the field. The measured peak friction value of 0.18 is close to tests conducted as a part of this study at the Keweenaw Research Center. However, Shoop et al. (1994) also report a different peak friction value with a third friction tester as a result of using a different test method, similar to obtaining different values in the laboratory versus field tests in this study. Shoop et al. (1994) and Coutermarsh and Shoop (2009) also report the ice condition affecting the magnitude of the traction coefficient and different shaped traction curves. Our study reports a similar observation based on the two test procedures, indoor and outdoor. Martin and Schaefer (1996) and
7
Hunter, 1993 classify winter surfaces into different types and report different friction coefficients for each of them. Differences in the ice surfaces between the lab and in the field will definitely lead to different friction coefficients. Roberts, 1981) observed an increase in the glass transition temperature and resilience of the tread compound with decrease in temperature, which further led to an increase in the friction coefficient value. The increase of hardness of the tread rubber, as seen in Fig. 15, together with changes in other mechanical properties of the tread (Roberts, 1981), explain the higher friction values from laboratory tests versus those obtained in field conditions. 6. Conclusions The outdoor testing program led to the understanding of the effect of inflation pressure, normal load, and tread depth during the operation of the SRTT on ice. The variations of friction levels with normal load were seen in an opposite trend compared to indoor testing at low slip ratios and at high slip ratios the effect of the normal load was not noticed. No increasing or decreasing trend of friction levels was observed with variation in the inflation pressure when tested as per ASTM-1805. However, the effect of the tread in clearly improving traction on icy surfaces was captured even with the outdoor testing program. Outdoor testing as per ASTM-1805 is mainly used by the industry for relative performance analysis of different tires, rather than a method to study the effect of different operational parameters, which should be kept in mind. The reasons for differences in the recorded friction levels were investigated, with the temperature, change in tire properties and test setup being identified as major factors causing this discrepancy. The design purpose of the test method should also be noted. The in-house developed indoor AVDL test method was successful in capturing the effect of different operational parameters, where ASTM-1805 was successful in capturing the effect normal load and inflation pressure. Certain test methods are widely used to study the relative tire performance with respect to the SRTT and minimal procedures exist to study the absolute tire performance. Hence, the demand of the day is the establishment of standard test setups and standard test methods for uniform evaluation of tire performance on ice. 7. Future recommendations Current regulations require the tire manufacturer to rate the braking performance of tires, thus experimental investigations to understand braking performance of tires on ice are necessary. Future directions could also include evaluation of cornering performance of tire on ice by varying operational parameters. A tire-ice model capable of simulating tire performance of ice with consideration of all the ambient, ice, and tire conditions would help to reduce the cost involved for experimental studies.
Please cite this article in press as: Bhoopalam, AK et al., Experimental investigation of pneumatic tire performance on ice: Part 2 – Outdoor study, J Terramechanics (2015), http://dx.doi.org/10.1016/j.jterra.2015.03.001
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Acknowledgments This study has been partially supported by the Center for Tire Research (CenTiRe), an NSF-I/UCRC (Industry/University Cooperative Research) at Virginia Tech and by the Advanced Vehicle Dynamics Laboratory (AVDL). The authors hereby wish to thank the project mentors and the members of the industrial advisory board of CenTiRe for their support and guidance. The authors also would like to extend their thanks to Mr. Paul Schultz, President, Mobility Research Inc., for conducting the outdoor testing program at the Keweenaw Research Center and to Michelin for providing the SRTTs for this study. References
Bhoopalam, A.K., Sandu, C., 2014. Review of the state of the art in experimental studies and mathematical modeling of tire performance on ice. J. Terrramech. 53, 19–35. Bhoopalam, A.K., Sandu, C., Taheri, S., 2015. Experimental investigation of pneumatic tire performance on ice: Part-1 Indoor Study. J. Terrramech. http://dx.doi.org/10.1016/j.jterra.2015.02.006. Coutermarsh, B., Shoop, S., 2009. Tire slip-angle force measurements on winter surfaces. J. Terrramech. 46 (4), 157–163. International society for terrain-vehicle systems standards, 1977. J. Terrramech. 14 (3), 153–182. Hunter, J., 1993. Reconstructing Collisions Involving Ice and Slippery Surfaces. SAE Technical Paper 930896. Martin, D., Schaefer, G., 1996. Tire-road Friction in Winter Conditions for Accident Reconstruction. SAE Technical Paper 960657. Roberts, A.D., 1981. Rubber–ice adhesion and friction. J Adhes 13 (1), 77–86. Shoop, S., Young, B., Alger, R., Davis, J., 1994. Effect of test method on winter traction measurements. J. Terrramech. 31 (3), 153–161.
ASTM F1805-06. Standard Test Method for Single Wheel Driving Traction in a Straight Line on Snow and Ice-Covered Surfaces..
Please cite this article in press as: Bhoopalam, AK et al., Experimental investigation of pneumatic tire performance on ice: Part 2 – Outdoor study, J Terramechanics (2015), http://dx.doi.org/10.1016/j.jterra.2015.03.001
Journal of Terramechanics
ScienceDirect Journal of Terramechanics xxx (2015) xxx–xxx
www.elsevier.com/locate/jterra
Experimental investigation of pneumatic tire performance on ice: Part 2 – Outdoor study Anudeep K. Bhoopalam a,⇑, Corina Sandu a, Saied Taheri b a
Advanced Vehicle Dynamics Laboratory, Mechanical Engineering, Virginia Tech, Blacksburg, VA 24061, USA b Center for Tire Research, Mechanical Engineering, Virginia Tech, Blacksburg, VA 24061, USA
Abstract The concept of ‘Lab to the Road’ is very challenging, especially for tire operation on ice. The changing ambient and ice conditions necessitate the need for robust engineering for the safety of vehicles on icy roads. Test method has a direct effect in measuring tire behavior on ice, with differences in the test setup and ability to maintain constant test conditions. The purpose of design for each test method also varies; certain test methods are designed to understand absolute tire performance, as compared to studying relative tire performance. This paper examines the performance of tires on ice through field tests conducted at the Keweenaw Research Center. Part-1 (Bhoopalam et al., 2015) presented a detailed indoor test program conducted on the Terramechanics Rig at the Advanced Vehicle Dynamics Laboratory, Virginia Tech, including the ice preparation procedure, test method, and test results. The two tests procedures were performed to understand how each test method influences the test results. A comparison of the laboratory (Bhoopalam et al., 2015) and field test method is also presented in this paper, with reasons for the differences in the measured values presented. Ó 2015 ISTVS. Published by Elsevier Ltd. All rights reserved.
Keywords: Tire-ice friction; Indoor tire-ice testing; Outdoor tire-ice testing
1. Introduction Laboratory evaluation of tire performance on ice offers the advantage of repeatability of ice and ambient environmental conditions; however, testing is still conducted on artificially created ice. The advantage of testing in the field comes from observing the behavior of the tire in real world conditions. The disadvantage, however, is that repeatable test conditions might not be feasible due to changing weather conditions. Wind is a major concern when testing in the field; winds often carry dust which settle on the ice surface altering surface properties. Outdoor test programs are conducted when the weather conditions stay constant DOI of original article: 10.1016/j.jterra.2015.02.006.
for a certain time period, which makes scheduling outdoor test really hard. An extensive review of literature for both indoor and outdoor tests for the performance of tires on ice has been presented in Bhoopalam and Sandu (2014). This second part presents a complete description and analysis of the outdoor test program conducted at the Keweenaw Research Center and a comparison of laboratory test results with field test results. The next section of this paper describes the outdoor test program, the test procedure and the test conditions. A comparison of the indoor and outdoor test programs and the reasons for differences in measured friction levels are discussed in the third section. Finally, conclusions and future recommendations are presented in the last.
⇑ Corresponding author.
E-mail address: [email protected] (A.K. Bhoopalam). http://dx.doi.org/10.1016/j.jterra.2015.03.001 0022-4898/Ó 2015 ISTVS. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Bhoopalam, AK et al., Experimental investigation of pneumatic tire performance on ice: Part 2 – Outdoor study, J Terramechanics (2015), http://dx.doi.org/10.1016/j.jterra.2015.03.001
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2. Field testing Outdoor tests are an actual representation of the real world conditions, to understand the performance of the SRTTs outdoors, field tests were conducted in February 2014 at the Keweenaw Research Center in Calumet, Michigan, USA by Mobility Research Inc. The outdoor testing program included studying the performance of the same two physical Standard Reference Test Tires (SRTT) (Bhoopalam et al., 2015) on ice with dry friction, at the same three levels for normal load and inflation pressure. The outdoor tests were conducted using a traction truck in the month of February 2014, with the test tire on the right-rear wheel, as seen in Fig. 1. Testing was conducted
on split-l track as seen in Fig. 2, with the left wheels on asphalt and the right wheels on the ice surface. The test program commenced in the evening to have no effect of the solar load. 2.1. Conditions for outdoor test program Outdoor testing was conducted for a reduced experiment matrix compared to the one presented in Part-1 (Bhoopalam et al., 2015) of this paper; testing with different levels for the camber angle, toe angle, and ice temperature was not a part of the outdoor test program. All outdoor tests were conducted at 0° camber and toe, at an ice temperature of 13 °C and an ambient temperature of 18 °C. The design of experiment matrix for the outdoor testing program is as shown in Table 1. 2.2. Terminology
Fig. 1. Mobility Research Inc.’s traction truck employed to evaluate tire performance on ice. Reprinted with permission from Mobility Research Inc.
The tire performance evaluated in the field at the Keweenaw Research Center is expressed in terms of a driving coefficient. The outdoor test program was carried out as per the procedure ASTM-1805 (ASTM F1805-06). ASTM-1805 defines the driving coefficient as the ratio of longitudinal force and vertical load. In Part-I (Bhoopalam et al., 2015) of this study the results are expressed are expressed in terms of the normalized drawbar pull. According to the ISTVS standards (International Society for Terrain-Vehicle Systems Standards, 1977), the drawbar pull refers to the force available for external work in a direction parallel to the horizontal surface over which the vehicle is moving, which refers to force the longitudinal force measured in the x-direction. Thus the normalized drawbar pull (Bhoopalam et al., 2015; International Society for Terrain-Vehicle Systems Standards, 1977) is analogous to the driving coefficient (ASTM F1805-06); in the interest of keeping the terminology as per ASTM1805, the results from the field testing are expressed using the expression ‘driving coefficient’. 2.3. Test method-ASTM 1805
Fig. 2. Split-l ice track at the Keweenaw Research Center.
Field testing was conducted as per ASTM-1805 (ASTM F1805-06), with the buffed and treaded tires as the candidate tires; the treaded SRTT was also used as the control tire. The control tire was run every third condition, to follow the sequence of C (control), T1, T2, C, T3, T4, C, as specified in ASTM F1805-06. Initial test procedures before
Table 1 Design of experiment matrix for outdoor evaluation of tire performance on ice. Test condition
Number of levels
Level-1
Level-2
Tires Ice temperature Inflation pressure Normal load
2 1 3 3
Treaded SRTT 10 °C Dry friction 60% Nominal pressure 145 kPa 60% Load index 4000 N
Buffed SRTT 100% Nominal Pressure 242 kPa 100% Load index 7000 N
Level-3
120% Nominal pressure 276 kPa 120% Load index 8500 N
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V tire 1 V vehicle
0.14
0.1 0.08 0.06
0.02 0 0
10
20
30
40
50
60
70
80
% Slip
Fig. 3. Variation in measured driving coefficient from 10 spin ups for the treaded SRTT. 100% inflation pressure of 242 kPa and 100% normal load of 7000 N.
ð1Þ
Vtire refers to the velocity of the tire and Vvehicle refers to the velocity of the vehicle. The velocity of the tire is measured with an encoder on the right-rear wheel. The results presented in next section have been converted in theoretical slip ratio to maintain uniformity between results, as presented in Bhoopalam et al. (2015) and in this paper.
0.12
0.04
0.16 0.14
Driving Coefficient
Practical Slip Ratio ¼
0.16
Driving Coefficient
the test program included spraying a thin layer of water (using a water truck) on the ice surface; a wide tire is then used to polish the ice surface once the water layer is frozen. Ten spin-ups were conducted for each test condition, as per Table 1, following the sequence of running the control tire every third test. Each spin-up lasted for about 3 s, during which the practical slip ratio was ramped up from 0% to 300% on the right rear wheel. The test vehicle was driven in a straight line maintaining a test speed of 8.0 ± 0.8 km/h, as specified in ASTM F1805-06. A two-axis wheel force transducer at the right-rear wheel center measures the forces in the x and z directions. The ratio of the x-force and the z-force is defined as the coefficient of friction in ASTM F1805-06. The practical slip ratio formulation as seen in Eq. (1), was used to report the findings from the outdoor test program.
3
0.12 0.1 0.08 0.06 0.04 0.02
2.4. Field test results
0 0
2.4.1. Repeatability of test results An acceptable repeatability of test data was observed for the field tests conducted at the Keweenaw Research Center. However, the variation of the measured driving coefficient was in a wider band when compared to the indoor test program (Bhoopalam et al., 2015). The curves for the 10 spin-ups can be seen in Fig. 3 for the treaded SRTT with 100% load and inflation pressure; a standard deviation of 0.006 was calculated for the measured peak values. Fig. 4 shows the measured curves for the buffed SRTT with 100% load and inflation pressure, a wider deviation was calculated for the at the peak value of 0.035 compared to the treaded SRTT. 2.4.2. Effect of normal load Outdoor tests were conducted for three levels of normal load of 60%, 100%, and 120% of the load index. At low slip ratios, the higher the normal load, the lower the friction coefficients were seen for both, the buffed and treaded
10
20
30
40
50
60
70
80
% Slip
Fig. 4. Variation in measured driving coefficient from 10 spin ups for the buffed SRTT. 100% inflation pressure of 242 kPa and 100% normal load of 7000 N.
0.16 0.14 0.12
Driving Coefficient
The following section describes the effect of the normal load, the inflation pressure, and the tread depth during operation on ice, as obtained from outdoor testing. A step jump is observed at very low slip ratios in field test results; this is due to the fact that the practical slip ratio is ramped up from 0% to 300% in a time period of 3–4 s. The range of the driving coefficient, as measured from the outdoor test program, is 0.054–0.19, as also reported by Martin and Schaefer (1996).
0.10 0.08 60% Load Index
0.06
100% Load Index
0.04 120% Load Index
0.02 0.00 0
10
20
30
40
50
60
70
80
% Slip
Fig. 5. Effect of normal load on drawbar pull for the treaded SRTT on ice with 100% inflation pressure- 242 kPa, from outdoor testing.
SRTT. The treaded SRTT exhibited this trend until a slip ratio of 30%, as seen in Fig. 5, and for the buffed tire until 17% slip ratio, as seen in Fig. 6. Above these slip ratios,
Please cite this article in press as: Bhoopalam, AK et al., Experimental investigation of pneumatic tire performance on ice: Part 2 – Outdoor study, J Terramechanics (2015), http://dx.doi.org/10.1016/j.jterra.2015.03.001
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A.K. Bhoopalam et al. / Journal of Terramechanics xxx (2015) xxx–xxx 0.16
0.16
0.14
0.14 0.12
Driving Coefficent
Driving Coefficient
0.12 0.10 0.08 60%Load Index
0.06
100%Load Index
0.04
120%Load Index
0.10 0.08 0.06
60% Pressure
0.04
100% Pressure 120% Pressure
0.02
0.02
0.00 0
10
20
30
0.00 0
10
20
30
40
50
60
70
Fig. 6. Effect of normal load on drawbar pull for the buffed SRTT on ice with 100% inflation pressure- 242 kPa, from outdoor testing.
50
60
70
80
% Slip
80
% Slip
40
Fig. 8. Effect of inflation pressure on drawbar pull for the buffed SRTT on ice with 100% normal load – 7000 N, from outdoor testing.
both, the treaded and the buffed SRTT, exhibited the same behavior and no trend was seen with variation in the normal load.
60% Load Index- Treaded SRTT
60% Load Index- Buffed SRTT
100% Load Index- Treaded SRTT
100% Load Index- Buffed SRTT
120% Load Index - Treaded SRTT
120% Load Index- Buffed SRTT
0.16
2.4.3. Effect of inflation pressure The effect of the inflation pressure on the friction coefficient was also studied through field tests. As seen from Figs. 7 and 8, a variation in the friction levels was seen at low slip ratios; however, a clear trend was observed. At higher slip ratios the effect of the inflation pressure was not noticed. For all levels of inflation pressure both, the treaded and the buffed SRTT, performed the same with friction versus slip ratio curves overlapping. 2.4.4. Effect of tread depth Comparing the buffed and the treaded SRTT at three levels of inflation pressure and three levels of normal load, the treaded SRTT provided better friction levels compared to the buffed SRTT all the time. An increase of 35% in friction levels was seen from the buffed SRTT to the treaded SRTT operating with a 100% load index and at the
Driving Coefficient
0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 0
20
40
60
80
% Slip
Fig. 9. Comparison of friction slip ratio curves for the treaded and buffed SRTT with 100% inflation pressure of 242 kPa at different normal loads, from outdoor testing.
60% Inf. Pressure-Treaded SRTT
60% Inf. Pressure-Buffed SRTT
100% Inf. Pressure-Treaded SRTT
100% Inf. Pressure-Buffed SRTT
120% Inf. Pressure-Treaded SRTT
120% Inf.Pressure-Buffed SRTT
0.16
Driving Coefficent
0.14
0.16
Driving Coefficient
0.14 0.12 0.10
0.12 0.10 0.08 0.06 0.04 0.02 0.00
0.08 60% Pressure
0
20
40
60
80
% Slip
0.06 100% Pressure
0.04
Fig. 10. Comparison of friction slip ratio curves for the treaded and buffed SRTT with 100% normal load of 7000 N with different inflation pressures, from outdoor testing.
120% Pressure
0.02 0.00 0
20
40
60
80
% Slip
Fig. 7. Effect of inflation pressure on drawbar pull for the treaded SRTT on ice with 100% normal load – 7000 N, from outdoor testing.
nominal inflation pressure, as seen from Figs. 9 and 10. The effect of inflation pressure and normal load on the tire was not predominant at high slip ratios, all the friction
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levels were seen to overlap after a slip ratio of 65%. Hence, at high slip ratios, the SRTT performs similarly, irrespective of the tread depth, inflation pressure, and the load on the tire during operation on ice.
This section compares the test results from the indoor and outdoor test programs. The effects of operational parameters, namely normal load and inflation pressure, as captured by the indoor and the outdoor programs, are explained in following sub-sections. The reasons for the differences in the performance measurements between the two test programs are explained in the next section.
120%Load Outdoor 120% Load Indoor
0.25 0.20 0.15 0.10 0.05 0.00 0
10
20
30
40
50
60
70
80
Slip Ratio %
Fig. 12. Comparison of drawbar pull/friction slip ratio curves for the buffed SRTT with 100% inflation pressure of 242 kPa at different normal loads, from outdoor and indoor test programs.
3.1. Effect of normal load The effect of normal load during the operation of the treaded SRTT on ice as measured in the laboratory was shown in Bhoopalam et al. (2015). From the indoor experiments, the higher the normal load, the lower the drawbar pull was measured. Whereas with higher normal loads, higher friction levels were measured in the field; this trend was observed until a slip ratio of 20%. Above 20% slip ratio, no clear trend was observed from the field tests and the curves for the three normal load cases were seen overlapping, as in Fig. 11. A comparison of the performance of the buffed SRTT on ice as measured on the Terramechanics Rig and on the outdoor ice track is shown in Fig. 12. The buffed SRTT tire behaved the same as the treaded SRTT until a slip ratio of 15% when tested in the laboratory. At slip ratios of 15% and above no clear trend was observed with variation of normal load. From the field testing at the Keweenaw Research Center, at low slip ratios until 12% higher friction levels were measured with higher normal load, in the mid slip ratio range no clear trend was observed. High slip ratios above 40% the 60% load case yielded the maximum friction.
60%Load-Outdoor
100%Load-Outdoor
60% Load-Indoor
3.2. Effect of inflation pressure Comparing the effect of inflation pressure as measured from the indoor and outdoor test program, from indoor studies for the treaded SRTT, after a slip ratio of 20%, with lower inflation pressure, a higher drawbar pull was measured, as seen in Fig. 13. After a slip ratio of 50%, a reverse trend is seen due to effect of heat generation in the contact patch. The results from the field tests show a lower friction level with a lower inflation pressure up to a slip ratio of 9%, after which the effect of inflation pressure is not captured by the outdoor test procedure and curves for the three inflation pressure cases are seen overlapping. The buffed tire exhibited no clear trend until a slip ratio of 15%, after which the tire with nominal inflation pressure showed the highest drawbar pull, as seen in Fig. 14. Both 60% and 120% inflation pressure cases measured a lower drawbar pull compared to the 100% inflation pressure, from indoor testing. The field tests for the buffed tire did not capture any effect of the inflation pressure; from Fig. 14 all the three curves are seen overlapping.
120%Load-Outdoor
100% Load-Indoor
120% Load-Outdoor
0.35
60% Pressure-Outdoor
100% Pressure-Outdoor
60% Pressure-Indoor 120% Pressure-Outdoor
100% Pressure-Indoor 120% Pressure-Indoor
0.25
0.30
Driving Coefficient / Normalised Drawbar Pull
Driving Coefficient / Normalised Drawbar Pull
100%Load Outdoor 100% Load Indoor
0.30
Driving Coefficient / Normalised Drawbar Pull
3. Comparison of indoor and outdoor test programs
60%Load Outdoor 60% Load Indoor
5
0.25 0.20 0.15 0.10 0.05
0.20 0.15 0.10 0.05 0.00
0.00 0
10
20
30
40
50
60
70
80
Slip Ratio %
Fig. 11. Comparison of drawbar pull/friction slip ratio curves for the treaded SRTT with 100% inflation pressure of 242 kPa at different normal loads, from outdoor and indoor test programs.
0
10
20
30
40
50
60
70
80
90
Slip Ratio %
Fig. 13. Comparison of drawbar pull/friction slip ratio curves for the treaded SRTT with 100% normal load of 7000 N with different inflation pressures, from outdoor and indoor test programs.
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A.K. Bhoopalam et al. / Journal of Terramechanics xxx (2015) xxx–xxx 60% Pressure Outdoor 60% Pressure-Indoor 120% Pressure Outdoor
90
100% Pressure Outdoor 100% Pressure-Indoor 120% Pressure-Indoor
80
Shore-A Hardness
Driving Coefficient / Normalised Drawbar Pull
0.25
0.20
0.15
0.10
70 60
Treaded-SRTT 50
Buffed-SRTT 40
0.05 30 -20
-15
-10
0.00 0
10
20
30
40
50
60
70
0
5
10
15
20
25
Temperature (¶C)
80
Slip Ratio %
-5
Fig. 15. Variation of tread hardness with temperature for the SRTTs.
Fig. 14. Comparison of drawbar pull/friction slip ratio curves for the buffed SRTT with 100% normal load of 7000 N with different inflation pressures, from outdoor and indoor test programs.
4. Reasons for difference in friction measurements – indoor versus outdoor test programs The previous section compares the friction measurements/tire performance from the indoor and the outdoor test programs. As one might have observed, the results from the two test programs are not similar. This section describes the investigations carried out to understand the reasons for the differences noticed in the friction measurements. Table 2, lists the differences in the calculation procedures and the test conditions. A difference by 3–5 °C in the ice temperature and by 30 °C in the ambient temperature existed; such large differences in temperature alter the mechanical properties of the rubber compound of the SRTT and thus the performance of the SRTT (a later subsection describes the variation of tread hardness of the SRTT with temperature). A difference by 2 times in the ice thickness existed between the two test programs; however, we consider that a 3 in. ice thickness in the laboratory would not likely cause a performance variation as compared to the tire performance on ice thickness of 6 in. Ice in the laboratory was created using water from the Town of Blacksburg, which is fit for drinking. Whereas at the Keweenaw Research Center ice was created using water from an underground well; one can expect a high iron or copper content in this water due the old mines present in the town of Calumet, Michigan. The chemical content of the water sample used in ice creation also alters Table 2 Comparison of test conditions and calculations for the indoor and outdoor test program. Parameter
AVDL
MRI
Slip ratio calculation Water source Ice thickness Ice temperature Ambient temperature Longitudinal velocity
Theoretical slip ratio Town of Blacksburg 2.5–3 in. 8 to 10 °C 11–12 °C 0.218 km/h
Practical slip ratio Underground well 6 in. 13 °C 18 °C 8 km/h
the performance levels of the tire; the review paper (Bhoopalam and Sandu, 2014) contains additional information about previous studies in this subject. There existed a difference of 40 times in the longitudinal velocity of the tire in the field versus the laboratory; an increased longitudinal velocity means increased heat generation in the contact patch, which definitely reduces the friction levels. 4.1. Ice resurfacing procedure The ice preparation and resurfacing procedures are explained in Bhoopalam et al. (2015) for the indoor and Section 2.2 for the outdoor test program, respectively. In the indoor investigations the tire runs on fresh ice for every test condition with resurfacing procedures followed between every test run. Whereas in outdoor tests, an initial water layer is sprayed on the ice after the water layer is frozen, a wide tire is used to polish the ice surface. After which the testing was conducted, with no resurfacing between the runs. The initial polishing and no resurfacing the ice between the runs leads to smoothing of ice surface and there by a reduction in the friction level. 4.2. Slip ratio control Part-1 (Bhoopalam et al., 2015) of this study and Section 2.2 describe the slip ratio control procedure followed while testing indoors and outdoors, respectively. For indoor tests the slip ratio is maintained at a steady-state slip ratio of time period of 20 s; during field testing the slip ratio is ramped up from 0% to maximum in the 3 s to 4 s. This difference in slip ratio control leads to change in heat generation in the contact phenomenon. The ramp slip ratio control and 8 km/h longitudinal velocity during filed testing lead to shorter adhesion times and thus a reduced friction level is seen when compared to outdoor testing. 4.3. Tread hardness versus temperature The hardness of the SRTT tread was measured using a durometer at different temperatures as in Fig. 15, a 20% increase in tread hardness was observed from the indoor
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8000
7000
7000
6950
6000
Indoor
5000
Outdoor
6900 6850 6800
4000 6750 3000
6700
2000
Outdoor (N)
Indoor (N)
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6650
1000
6600
0
6550 0
0.5
1
1.5
2
2.5
3
Time (s)
Fig. 16. Comparison of time histories of normal load, indoor testing versus outdoor testing.
laboratory temperature to the outdoor test temperature. This measurement also confirms the findings in Bhoopalam et al. (2015), a reduced drawbar pull is seen with a reduction in ambient temperature. The increase in thread hardness is also a reason for the reduction in friction levels from indoor to outdoor testing. 4.4. Normal load time histories The time histories of the normal load of the two test procedures were compared as seen in Fig. 16. The Terramechanics Rig is equipped with an active normal load control system as described in Bhoopalam et al. (2015), which maintains a contents level of load throughout the entire run. On the other hand, a variation of 150–200 N was observed during the 3 s of slip ratio ramp-up during outdoor tests. A change in normal load means a change in effective rolling radius and a change in the slip ratio. Maintaining a constant normal load during filed testing is a challenge with the traction truck experiencing a certain amount of bounce. We estimate that the variation in normal load is not a major, but a minor contributor to the difference friction levels measured in the laboratory and in the field. 5. Discussion: comparison with prior research The effect of test method studied by Shoop et al. (1994) measured the same peak friction coefficient with two test vehicles on the field. The measured peak friction value of 0.18 is close to tests conducted as a part of this study at the Keweenaw Research Center. However, Shoop et al. (1994) also report a different peak friction value with a third friction tester as a result of using a different test method, similar to obtaining different values in the laboratory versus field tests in this study. Shoop et al. (1994) and Coutermarsh and Shoop (2009) also report the ice condition affecting the magnitude of the traction coefficient and different shaped traction curves. Our study reports a similar observation based on the two test procedures, indoor and outdoor. Martin and Schaefer (1996) and
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Hunter, 1993 classify winter surfaces into different types and report different friction coefficients for each of them. Differences in the ice surfaces between the lab and in the field will definitely lead to different friction coefficients. Roberts, 1981) observed an increase in the glass transition temperature and resilience of the tread compound with decrease in temperature, which further led to an increase in the friction coefficient value. The increase of hardness of the tread rubber, as seen in Fig. 15, together with changes in other mechanical properties of the tread (Roberts, 1981), explain the higher friction values from laboratory tests versus those obtained in field conditions. 6. Conclusions The outdoor testing program led to the understanding of the effect of inflation pressure, normal load, and tread depth during the operation of the SRTT on ice. The variations of friction levels with normal load were seen in an opposite trend compared to indoor testing at low slip ratios and at high slip ratios the effect of the normal load was not noticed. No increasing or decreasing trend of friction levels was observed with variation in the inflation pressure when tested as per ASTM-1805. However, the effect of the tread in clearly improving traction on icy surfaces was captured even with the outdoor testing program. Outdoor testing as per ASTM-1805 is mainly used by the industry for relative performance analysis of different tires, rather than a method to study the effect of different operational parameters, which should be kept in mind. The reasons for differences in the recorded friction levels were investigated, with the temperature, change in tire properties and test setup being identified as major factors causing this discrepancy. The design purpose of the test method should also be noted. The in-house developed indoor AVDL test method was successful in capturing the effect of different operational parameters, where ASTM-1805 was successful in capturing the effect normal load and inflation pressure. Certain test methods are widely used to study the relative tire performance with respect to the SRTT and minimal procedures exist to study the absolute tire performance. Hence, the demand of the day is the establishment of standard test setups and standard test methods for uniform evaluation of tire performance on ice. 7. Future recommendations Current regulations require the tire manufacturer to rate the braking performance of tires, thus experimental investigations to understand braking performance of tires on ice are necessary. Future directions could also include evaluation of cornering performance of tire on ice by varying operational parameters. A tire-ice model capable of simulating tire performance of ice with consideration of all the ambient, ice, and tire conditions would help to reduce the cost involved for experimental studies.
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Acknowledgments This study has been partially supported by the Center for Tire Research (CenTiRe), an NSF-I/UCRC (Industry/University Cooperative Research) at Virginia Tech and by the Advanced Vehicle Dynamics Laboratory (AVDL). The authors hereby wish to thank the project mentors and the members of the industrial advisory board of CenTiRe for their support and guidance. The authors also would like to extend their thanks to Mr. Paul Schultz, President, Mobility Research Inc., for conducting the outdoor testing program at the Keweenaw Research Center and to Michelin for providing the SRTTs for this study. References
Bhoopalam, A.K., Sandu, C., 2014. Review of the state of the art in experimental studies and mathematical modeling of tire performance on ice. J. Terrramech. 53, 19–35. Bhoopalam, A.K., Sandu, C., Taheri, S., 2015. Experimental investigation of pneumatic tire performance on ice: Part-1 Indoor Study. J. Terrramech. http://dx.doi.org/10.1016/j.jterra.2015.02.006. Coutermarsh, B., Shoop, S., 2009. Tire slip-angle force measurements on winter surfaces. J. Terrramech. 46 (4), 157–163. International society for terrain-vehicle systems standards, 1977. J. Terrramech. 14 (3), 153–182. Hunter, J., 1993. Reconstructing Collisions Involving Ice and Slippery Surfaces. SAE Technical Paper 930896. Martin, D., Schaefer, G., 1996. Tire-road Friction in Winter Conditions for Accident Reconstruction. SAE Technical Paper 960657. Roberts, A.D., 1981. Rubber–ice adhesion and friction. J Adhes 13 (1), 77–86. Shoop, S., Young, B., Alger, R., Davis, J., 1994. Effect of test method on winter traction measurements. J. Terrramech. 31 (3), 153–161.
ASTM F1805-06. Standard Test Method for Single Wheel Driving Traction in a Straight Line on Snow and Ice-Covered Surfaces.
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