1. Scope--This Information Report presents background and rationale for SAE Recommended Practice J1106, Laboratory Testing Machine and Procedures for Measuring the Steady Force and Moment Properties of Passenger Car Tires. The purpose of SAE J1106 is to define standards for equipment design and test procedures so that data from different laboratories can be directly compared. Whereas such standardization is not a requirement for testing associated with tire development, it is necessary in the context of vehicle design and tire selection problems.

The basic approach employed in developing SAE J1106 was to consolidate and document existing technology as embodied in equipment and procedures currently employed for routine tire evaluations. Equipment and procedures whose current use is restricted to research applications were not considered. Research experience is discussed in this Information Report, however, to the extent deemed necessary to provide background and rationale for SAE J1106. Material is therefore included on speed effects, contaminants dynamic testing, traction, surface geometry, and other subjects not considered in SAE J1106. The scope was expanded in an effort to anticipate questions raised by SAE J1106.

1.1 Introduction--The motor vehicle industry is working toward a more complete understanding of the factors contributing to the motions of vehicles on the roadway. This understanding is expressed in the form of a variety of techniques for prediction of vehicle motion responses to road and operator inputs. The ability to predict vehicle motion responses is desirable if the vehicle traffic system is to be controlled and designed for optimum safety and utility.

The pneumatic tire is the primary control element in present ground vehicle systems. Forces and moments developed at the tire-road interface provide the functions of support, directional control, braking and accelerating capability. The manner in which these forces and moments are developed can be influenced through changes in tire design. Tire input-output properties must be completely identified if vehicle motion properties are to be predicted. These tire force and moment properties are analogous to airfoil properties used in the design of aircraft. The tire force and moment tests discussed in this report are, therefore, analogous to wind tunnel tests applied to various airfoil designs and have similar utility.

The report begins with a general discussion of the factors affecting tire forces and moments, and a brief historical review of associated research and testing activities. A discussion is presented of the principal requirements for tire force and moment data as they derive from considerations of vehicle dynamics and tire design. Factors affecting test machine selection and design are reviewed in a broad context ranging from overall system structure to specific subsystem requirements. Methods for testing, data processing, and presentation and interpretation of test results are discussed in detail. The report concludes with a discussion of equipment and procedures for "special test," not currently performed on a routine basis, but which provide data of significant and growing interest to the vehicle designer.

1.2 Factors Affecting Tire Forces and Moments--The steady state force and moment properties of pneumatic tires may be considered to comprise two phenomenologically distinct categories: (1) the geometric and material properties affecting shear-force potential (that is, friction) at the tire-road-interface, and (2) the elastic properties determining the shape of, and pressure distribution in, the interfacial contact area.

Measurements of frictional tire properties, which are severely confounded by synergistic interactions of tire, road surface, interface contamination (primarily water), temperature, slip and speed effects, must presently be considered to constitute research experimentation. Laboratory procedures most commonly used for routine tire evaluations are restricted to the measurement of elastic tire properties. Specifically, the object of these routine procedures is to characterize the mechanical response of the rolling tire to steady state deformations from its free inflated shape. The deformations to which the tire is subjected in the laboratory and idealizations of the deformations produced on a vehicle's tires during steady state maneuvers over a perfectly smooth road. For the measurements to be meaningful, it is necessary to establish controlled test conditions which do not differ from over-the-road operating conditions in any aspect which significantly influences the tire's elastic response. In terms of the specific factors affecting tire performance, this requirement has the following implications:

1.2.1 SPEED--The speed dependence of the steady state elastic properties of pneumatic tires is not great over the range of normal highway driving. There is probably a threshold speed, however, below which a tire's force and moment response is speed dependent. This threshold speed has not been definitely measured, but several laboratories test successfully at speeds of 1 mph.

1.2.2 TEMPERATURE--Some physical properties of rubber are known to vary with temperature so elastic properties of tires are expected to vary with temperature. The effects of temperature on elastic properties has not been carefully investigated.

1.2.3 ROAD SURFACE--The elastic response of a pneumatic tire is dictated by the compliance of its constituent parts (cords, tread, etc.) and by the relative configuration of these parts under the conditions of external loading. A major determinant of a loaded tire's configuration is the shape (macrogeometry) of the road surface. It is found experimentally that the distortions produced when a tire is loaded against a curved (either concave or, especially, convex) surface are different from those produced by a flat surface as to exert an influence on the tire's elastic response. The significance of this influence depends on the degree of curvature. Consequently, laboratory machines employed to measure tire force and moment properties for vehicle design and tire selection purposes frequently incorporate flat, rigid test surfaces.

A secondary determinant of the configuration of a loaded tire is the degree of relative motion (sliding) between tire and roadway surfaces in the interfacial contact patch. For a given loading condition, the degree of relative motion is a function of the interfacial friction coefficient, hence of the microgeometery and material of both the tire and the roadway surface. Thus, the frictional properties and elastic response of the pneumatic tire are not physically independent. While the measurement of frictional tire properties has not yet advanced beyond the realm of research, procedures for controlling the factors influencing friction in the interfacial contact patch are sufficiently well developed that the confounding influence of these factors on measurements of elastic tire response can be effectively eliminated.

1.2.4 CONTAMINANTS--The presence of contaminants (such as water, dust, or oil) at the tire-roadway interface may exert a substantial effect on the interfacial friction coefficient. Great care must accordingly be exercised to eliminate the possibility of spurious interface contamination during tire force and moment testing.

In the sections which follow, the factors mentioned above will be discussed in detail, and relevant experimental data will be presented to illustrate their implications with respect to laboratory measurement of the steady state force and moment properties of tires. First, however, a brief review will be made of some of the major historical developments relative to the characterization and measurement of tire forces and moments.

1.3 Historical Background--Although the pneumatic tire was first invented in 1845, and no less a classicist than Osborne Reynolds examined the rolling behavior of rubber wheels as early as 1876 (1), it was not until 1925 that Broulhiet(2) advanced the concept of slip angle which is the cornerstone idea behind current understanding of the mechanics of tire force and moment generation. The concept arose during Broulhiet's consideration of the automobile shimmy problem. This was also the subject of subsequent investigations in Germany which gave rise to the earliest known measurements of tire side forces in yawed rolling on a drum-type tester, by Fromm(3), in about 1931. More extensive drum measurements of tire forces and moments were performed later in the 1930's by Evans(4) and Bull(5). The latter investigator examined conditions of combined longitudinal and side slip. Early measurements of tire forces and moment generated on flat surfaces were obtained in road tests by Bradley and Allen(6) and Forster(7). Differences between tire test results obtained with flat and curved surface conditions were first documented by Bull(5).

Early theoretical analyses of the forces generated by longitudinally and side slipping tires were performed by Julien(8) and Fromm(9, 10). These analyses, which did not consider carcass deformation, nonetheless produced results which tend to agree qualitatively with experimental tire data over the whole spectrum of operating conditions from free rolling to full sliding. The basis for modern analytical treatment of the elastic response of the "running-band" theory, developed independently by von Schlippe(11) and Hadekel(12) which assumes that the tire's elastic characteristics can be determined by considering the deformation of the equatorial line; that is, the intersection of the tire surface, when undeformed, with the wheel center plane. Fiala(13) employed running-band theory, with a model accounting also for the flexibility of tread elements and the presence of sliding in the tire-road contact area, to derive expressions for steady state tire side forces and yawing moments, as functions of normal force, sideslip, and inclination angle, which agree well with experimental data over a broad range of operating condtiions(14, 15). More recently, several analysts(16, 17, 18) have adopted approaches similar to Fiala's for the case where the tire moves with combined longitudinal and sideslip, to derive general tire force and moment characterizing relationships which agree qualitatively with the limited experimental data available for the combined slip condition.

All of the so-called theoretical tire models cited above are in fact semi-empirical; that is, they involve one or more parameters whose values must be obtained by measuring tire forces and moment under prescribed and controlled test conditions. Hence, the development of these theories, rather than obviating the need for tire force and moment testing, actually adds to it.

Recent developments in tire force and moment testing methodology have proceeded along several different lines. Vehicle-towed dynamometers permitting road measurements of tire forces and moments under the broadest range of dynamic and environmental conditions have been developed both in this country(19,20) and abroad(21,22). Modern laboratory equipment for measuring tire forces and moments has evolved primarily from the low speed, "flat-plank" tester designed and built by the Dunlop Tire Company in England(23). Derivative machines have been described in the literature by Nordeen and Cortese(24), Ginn and Marlowe(25), Dugoff and Brown(20) and van Eldik Thieme(22). These flat bed machines are now employed routinely to evaluate tires for vehicle design and development purposes, and it is the technology of their design and use which constitutes the subject of the SAE Recommended Practice J1106.

Current research experimentation relative to the mechanics of tire force and moment generation includes measurements with internal drum(23) and continuous belt(26) tire testers. The influence of drum curvature on tire forces and moments is much less pronounced with the internal-track drum than with external-track machines(27). The internal-track machine also has other features (namely, variable test surface material, controllable surface water layer thickness) which are advantageous from the viewpoint of the tire mechanics researcher. The moving belt tester provides the principal advances of drum type equipment (namely, continuous, high-speed operating capability), plus a flat test surface. Its principal shortcoming for research purposes has been difficulty of water layer control for wet surface testing(28) but these problems have been solved in more recent versions.

Much of this technology has been somewhat slow to move from the research laboratory and be adopted as an integral part of the product development process. Better techniques for instrumentation, data processing, and vehicle simulation had to precede wide application of tire force and moment technology. The organization of the industry has also tended to retard this growth. The tire is an integral component of the vehicle and cannot be evaluated out of the context of the vehicle. Tire data were, therefore, of limited usefulness to the tire industry without full knowledge of vehicle properties and defined performance objectives. A more widespread understanding of vehicle dynamics and the activities of several independent research organizations with knowledge of vehicles and tires have combined to overcome many of these problems. The industry appears to be entering a time of rapid expansion in the application of tire force and moment technology.

The Ad Hoc Tire Force and Moment Subcommittee was formed with people experienced in the tire industry, vehicle industry and independent research establishment. After considering alternatives, the Committee agreed that SAE J1106 should be a concise and specific document directed at low speed laboratory testing on high friction surfaces since these tests are regularly conducted by many laboratories and produce results that are useful for design and development. The Committee anticipated that SAE J1106 would generate many questions from workers in the field who are concerned with test validation, speed effects, contaminants, surface geometry, test machine configurations, and other factors considered in Committee deliberations but not included in SAE J1106. The Information Report was drafted by the Committee to provide this additional background. Individuals on the Committee wrote particular sections which were combined with minimum editing. Most of the technical writing was done during the first half of 1973 and is an indication of technology existing prior to that time.

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