UM-HSRI -78-31 Final Report THE STATE OF KNOWLEDGE RELATING TIRE DESIGN TO THOSE TRACTION PROPERTIES WHICH MAY INFLUENCE VEHICLE SAFETY P. 0. No. 6640 Robert 0. Ervin Highway Safety Research Ins ti tute The University of Michigan Prepared for: Bolt, Beranek and Newman, Inc. Tochnical Report Docu.l.ntocik~Page 1. RW Mc 2. C..mnmaw Accmssiaa 3. Rmeirmf'r Cda4.e Ho. UM-HSRI -78-31 4. Titlm adSubntl* 5. Report Oetm STATE OF KNOWLEDGE RELATING TIRE DESIGN TO July 1978 THOSE TRACTION PROPERTIES WHICH MY INFLUENCE 6. Pu'-$*0v9-*a*~-~~ VEHICLE SAFETY. , 8. Pnlwrtag Otglliu#iuR- No. 7. w3 R. D. Ervin UM-HSRI-78-31 I 9. Pubdaq Oqmirdian MI. ad Ubmsa 10. Wad Unct No. Highway Safety Research Institute The University of Plichigan 11. camwadw (irantnu. Ann Arbor, Michigan 48109 P.O. 6640-1 13. of RW ad Pwid bmd 12 hndy +mq Mmad Mhss Final Bolt, Beranek, and Newman, Inc. 50 Moul ton Street Cambridge, Massachusetts I'd Abam The various mechanical properties of pneumatic tires are reviewed in light of the influence of tires on vehicle maneuvering behavior. The report serves as a primer in the phenomenology of tire force and moment productior+providi ng a broad overview of exis ti ng ti re measurements and interpreting ti re behavior in terms of elementary vehicle dynamics considerations. Example data are presented i1 lustrati ng the basic responses of the ti re to braking and cornering slip and the sensitivities of those responses to operating and design variables. The state of knowledge is summarized covering the condi tions of braki ng , cornering, combined s 1 ip, and mobi 1i ty on soil or snow. Sensitivities of tire performance to inflation pressure, load, velocity, surface texture, water depth, carcass design, tread design, and tread rubber compounding are discussed. I 17. Kvlads tire mechanics, vehicle 18. Discribtior, Stat-at maneuveri ng, traffic safety, braki ng , cornering , combi ned s 1 ip , mobi 1i ty UNLIMITED I I 19. kd+yClosaif. (of this m) ] f). kari+yC1osmif. (of ?his pqm) I 21. No. of Pages I 22. Pricm I NONE I NONE TABLE OF CONTENTS INTRODUCTION .......................... 1 1. BRAKING PROPERTIES ..................... 5 1 . 1 Longitudinal Stiffness. Cs .............. 7 1.1.1 Sensitivity of Cs to Inflation Pressure .................... 7 1.1.2 Sensitivity of Cs to Vertical Load ....... 8 1 . 1 .4 Sensi ti vity of Cs to Surface Texture .................... 8 1.1.5 Sensitivity of Cs to Water Depth ........ 8 1.1.6 Sensitivityof Cs toCarcassDesign ...... 11 1.1.7 Sensitivity of Cs to Tread Design ....... 11 1.1.8 Sensitivity of Cs to Tread Compound ...... 11 1.1.9 Relevance of CS to Maneuvering Behavior of Vehicles .................. 14 1.2 Peak and Slide Braking Limits andus . 14 , p~ 1.2.1 Sensitivity of p and ps to Inflation Pressure ....................P 14 1.2.2 Sensitivity of p andp, tovertical Load ... 16 P 1.2.3 Sensitivity of p, and pS to Velocity ...... 19 1.2.4 Sensitivi:ty of p and ps to Surface Texture ....................P 21 1.2.5 Sensitivity of pp and p . to Water Depth 24 5 .... 1.2.6 Sensitivity of p and ps to Carcass Design...................... P 27 1.2.7 Sensitivity of p and ps to Tread Design .... 30 P 1.2.8 Sensi ti vi ty of p and pS to Tread Compound ....................P 36 1.2.9 Relevance of p and u to Vehicle Maneuveri ng prBperti e$ ............. 40 2 . CORNERING PROPERTIES (Discussion of Aligning. Overturning. and Rolling Resistance Moments) ........ 44 2.1 Cornering Stiffness, C 56 a ................ 2.1 .I Sensitivity of C to Inflation Pressure. .................,.ci 56 2.1.2 Sensitivity of C to Vertical Load. 58 a ...... 2.1.3 Sensitivity of C to Vel oci ty ......... 62 C1 2.1.4 Sensi ti vity of c to Surface Texture. 62 C1 ..... 2.1.5 Sensitivity of C to Water Depth. 62 a ....... 2.1.6 Sensitivity of C to Carcass Construction. .................ci 62 2.1.7 Sensitivity of C toTread Design 64 a ....... 2.1.8 Sensitivity of C to Tread Compound 69 a ...... 2.1.9 The Involvement of C in Determining a Vehicle Maneuvering Properties. ........ 71 Camber Stiffness, C 74 Y ................. 2.2.1 Sensitivity of C to Inflation Pressure.................... Y 74 2.2.2 Sensitivity of C to Vertical Load. 76 Y ...... 2.2.3 Sensitivity of C' to Velocity. 76 Y ........ 2.2.4 Sensitivity of C' to SurfaceTexture. 80 V ..... 2.2.5 Sensitivity of C' towater Depth. 80 Y ....... 2.2.6 Sensitivity of C' to Carcass Construction. ............... 80 2.2.7 Sensitivity of C to Tread Design ....... 80 Y 2.2.8 Sensitivity of C toTread Compound. ..... 82 Y 2.2.9 Significance of c to Vehicle Maneuvering Properties. ..,...............Y 82 2.3 Peak and Lateral Traction Coefficient , ....... 85 2.3.1 Sensitivity of u to Inflation Pressure .... 85 Y 2.3.2 Sensitivity of p to Vertical Load. ...... 85 Y 2.3.3 Sensi ti vi ty of p to Velocity ......... 89 Y 2.3.4 Sensitivityof P toSurfaceTexture. ..... 93 Y 2.3.5 Sensitivity of u to Water Depth. ....... 93 Y 2.3.6 Sensitivity of u to Carcass Design ...... 98 Y 2.3.7 Sensitivity of u to Tread Design ....... 98 Y 2.3.8 Sensitivity of u, to Tread Ccmpound ...... 102 J 2.3.9 Significance of P to Vehicle Maneuvering Properties. ..................Y 102 3. COMBINED BRAKING AND CORNERING PROPERTIES. 106 4. TRACTION OR MOBILITY ON DEFORMABLE SURFACES. 115 5. REFERENCES . 124 INTRODUCTION This report is intended as a general discussion of the state of knowledge which may connect tire design to vehicle safety. It is primarily intended as a primer in ti re mechanics, identifying the various mechanisms by which ti res generate steering and braking forces and the sensitivities which those mechanisms have to design and operating variables. It is pertinent, then, before considering ti re properties per se to reflect on the manner in which a connection might be made between tires and vehicle pre-cras h safety. Motor vehicle safety, or conversely, the occurrence of motor vehicle traffic accidents, is known to depend upon the interaction of a broad set of factors. These have been broken down in various investigations of vehicle safety into the factors constituting a com- plete description of the traffic system itself. Such diverse elements as roadway geometry, surface condition, road sys tem signing , climatic variables, driver physiology, psychology, and driving history, vehicle braking and hand1 ing properties, and various mechanisms by which these properties interact with one another are be1 ieved to be all relevant to accident potential. The linking of any single factor to the accident record is, of course, a difficult exercise since the interaction mechanisms are so strong and since the factors of potential importance are so numerous. Nevertheless, many researchers have studied the correlations between individual factors describing the highway, drivery vehicle, traffic environment and accident occurrence. Regarding the specific area of the relationship between vehicle dynamic performance and accident involvement, certain studies have been made to establish the extent of correlations which may be hypothesized as evidenci ng causal relationships. To a much larger degree, however, research which is intended to examine the vehicle's role in pre-crash safety is simply based upon notions of suspected relationships, such as the fol 1owi ng : 1) High'levels of braking capability should render a vehicle more capable of avoiding collisions or of minimi zing the velocity prevailing upon impact. 2) High levels of cornering or lateral acceleration performance should render a vehicle more capable of obstacle avoidance. 3) The retention of yaw stability up to the cornering limits imposed by the saturation of tir'e shear forces should render the vehicle more likely to be success- fully controlled throughout any obstacle avoidance maneuver. Such hypotheses have underpinned most studies which have sought to expand vehicle dynamics techno1 ogy in a direction benefiting vehicle safety. Thus it is largely on the basis of the link provided by intuitive hypothesis that we can presume to examine the state of knowledge relating tire design characteristics to vehicle safety. That is, we have a safety interest in tire dejign (putting aside structural integrity issues such as puncture and blowout resistance) only insofar as the design of a tire determines its traction qualities which, in turn, influence the braking and steering properties of the vehicle which, in turn, influence the accident avoidance potential of the driver/ vehicle/hi ghway system. Insofar as improper inflation pressures, im- proper application of a given tire size or type, excessive tread wear, or excessive ti re 1oadi ng may comprani se the as-designed traction quality of a tire, the relationship between tire design and vehicle safety wi 11 be obscured. Insofar as other non-tire-re1 ated properties inherent to the vehicle tend to reduce accident avoidance potential (such as the low rollover immunity of certain heavy trucks), the safety importance of the tire's design characteristics is reduced. Neverthe- less, there seems ample reason to believe that the relationship of tire design to traction properties and thence to vehicle dynamic behavior constitutes , to a significant degree, a connection between tire design and vehicle safety. The discussion in this report is broken down into sections addressing different aspects of ti re performance, as follows: 1 ) Braking properties 2) Cornering properties 3) Cmbi ned braki ng and cornering properties 4) Mobil i ty (deformable surfaces) properties Of these four areas, only the first three are seriously suggested to affect the safety quality of vehicles.