Empirical Analysis of Pneumatic Tire Friction on Ice Troy Nigel Holley Thesis submitted to the Faculty of the Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree of Master of Science in Mechanical Engineering Corina Sandu Mehdi Ahmadian Saied Taheri September 14, 2010 Blacksburg, VA Keywords: Friction, Ice, Terramechanics, Tires Empirical Analysis of Pneumatic Tire Friction on Ice Troy Nigel Holley Abstract Pneumatic tire friction on ice is an under-researched area of tire mechanics. This study covers the design and analysis of a series of pneumatic tire tests on a flat-level ice road surface. The terramechanics rig of the Advanced Vehicle Dynamics Lab (AVDL) is a single-wheel test rig that allows for the experimental analysis of the forces and moments on a tire, providing directly the data for the drawbar pull of said tire, thus supporting the calculation of friction based on this data. This indoor testing apparatus allows for some degree of replication by helping to maintain test conditions and by imposing a desired tire slip; the normal load, camber angle, toe angle, and other testing configurations can also be pre-set, as required. Methods of and issues related to controlling the production of ice and maintaining the conditions of numerous factors for each trial run were also documented. The AVDL terramechanics rig allowed for the collection of data from tests that varied the tire tread, tire inflation pressure, normal load on the wheel, and the slip ratio of the moving tire. This empirical data was then analyzed through the statistical analysis program JMP 8 in order to determine which factors (or combination of factors) significantly influence pneumatic tire friction on ice. The analysis verified that the slip ratio had a significant effect on the observed coefficient of friction, which decreased as the slip ratio increased. The combinations of the slip ratio and inflation pressure and the slip ratio and tire setup also had a significant effect on the observed coefficient of friction. The tests appear to have validated the theory that the drawbar pull and the traction was higher for the tire with tread. Acknowledgements I’d like to thank several people that helped me over the course of this research and in completing my degree: Dr. Sandu for the chance to work at CVeSS and for the incredible opportunity to obtain a Master’s Degree in Mechanical Engineering from Virginia Tech. Adam Woodward for his work on obtaining the enclosure and alleviating the friction problems plaguing the rig, along with Colin, Heather, and Vivake from NASA. Ben Taylor for his continued support with the Terramechanics Rig and the software and responding to minor questions I had. Michael Craft for his help on modifying the crossbar and moving around literally tons of equipment and sand and assisting with some of the cleanup. Clemente Negode for his steel angle idea for the camber. Thanks to my good friend [and future doctor] Ozma for giving me moral support over the summer. Special thanks to the National Science Foundation for their support through the NSF Grant Opportunities for Academic Liaison with Industry (GOALI) grant and the Graduate School Recruiting Office for their support through the Powell Fellowship. And of course thanks, Ma, for constant checking to see how I was doing and making sure I was alright… iii Table of Contents 1 Introduction .......................................................................................................................... 1 1.1 Motivation of the Study.................................................................................................... 1 1.2 Research Objective ........................................................................................................... 2 1.3 Research Approach .......................................................................................................... 2 1.4 Thesis Outline .................................................................................................................. 3 2 Review of Literature ............................................................................................................ 4 2.1 Review of Literature: Test Methodology ......................................................................... 4 2.2 Combined Testing Parameters ....................................................................................... 11 2.3 Review of Literature: Data From Previous Tire on Ice Tests ........................................ 11 2.4 Review of Literature: Coefficient of Friction Models ................................................... 16 2.5 Review of Literature: Slip Ratio of Preceding Models .................................................. 28 3 Background of Pneumatic Tires: Tire Design & Mechanical Properties .......................... 30 3.1 Tire Nomenclature.......................................................................................................... 32 3.1.1 Tire Nomenclature: Tire Section Width ................................................................. 33 3.1.2 Tire Nomenclature: Aspect Ratio ........................................................................... 33 3.1.3 Tire Nomenclature: Maximum Load ...................................................................... 34 3.1.4 Tire Nomenclature: Construction Code Indicator ................................................... 34 3.1.5 Tire Nomenclature: Wheel Diameter ...................................................................... 35 3.1.6 Tire Nomenclature: Load Index .............................................................................. 35 iv 3.1.7 Tire Nomenclature: Speed Rating ........................................................................... 37 3.2 Tire Nomenclature.......................................................................................................... 39 3.3 Tire Construction............................................................................................................ 41 3.3.1 Tire Construction: Tread ......................................................................................... 43 3.3.2 Tire Construction: Belt ........................................................................................... 45 3.3.3 Tire Construction: Breaker & Carcass .................................................................... 45 3.3.4 Tire Construction: Bead Wire ................................................................................. 46 3.3.5 Tire Construction: Sidewall .................................................................................... 46 3.3.6 Tire Construction: Tire Casing, Shoulder, and Crown ........................................... 47 3.4 Tread Pattern Construction............................................................................................. 47 3.5 Chemical Composition of the Tire ................................................................................. 50 3.6 Effective Rolling Radius ................................................................................................ 51 3.7 Tire Friction on Various Surfaces .................................................................................. 52 4 Characteristics of Ice and Tire Friction on Ice ................................................................... 53 4.1 Water Layer Formation .................................................................................................. 54 4.2 Differences in the Formation of Ice ............................................................................... 55 4.3 Main Factors that Affect Tire Friction on Ice ................................................................ 55 5 Test Equipment .................................................................................................................. 57 5.1 Test Equipment: Terramechanics Rig ............................................................................ 57 5.1.1 Test Equipment: Terramechanics Rig: Enclosure Modifications ........................... 60 v 5.1.2 Test Equipment: Terramechanics Rig: Air Springs and Modifications .................. 61 5.2 Test Equipment: Kistler RoaDyn P650 Wheel Force Sensor ........................................ 67 5.3 Test Equipment: Terramechanics Rig Computers ......................................................... 70 5.4 Test Equipment: Motion Control Software and Hardware ............................................ 70 5.5 Test Equipment: Data Acquisition ................................................................................. 71 5.6 Test Equipment: Pressure Pad ........................................................................................ 73 5.7 Test Equipment: Ice-Making Equipment ....................................................................... 75 5.7.1 Test Equipment: Ice-Making Equipment: Creation of Ice Test Surface................. 78 6 Design of Experiment ........................................................................................................ 82 6.1 Design of Experiment: Main Experiment ...................................................................... 82 6.2 Design of Experiment: Test Procedure and Slip ............................................................ 91 7 Results and Discussion of Results ..................................................................................... 96 7.1 Results and Discussion of Results: Filtering the Data ..................................................