Selection of the proper tire size, shape, and construction for a given off-road operation must remain far more an art than a science until a proven parametric description of the tire-soil system is available. The most reasonable way to obtain this description involves two stages of study: first in the laboratory and then in the field. The U. S. Array Engineer Waterways Experiment Station (WES) has analyzed data from extensive tests of a large number of single wheels in dynamometer carriage soil-bin arrangements to determine the most important independent parameters of the soil-tire system and to consolidate functions of these parameters to single dimensionless prediction terms. Tire performance is described by towed force/load, and by pull/load, sinkage/tire diameter, and torque/ load X active tire radius, the latter three terms measured at near-maximum pull. For the most part, prediction terms were developed by determining the interrelations that tire performance tests indicate exist among the dimensionless, independent terms that describe the soil-single tire system. Data were analyzed from laboratory tests that included a very broad range of tire-soil conditions. Soil strengths for the full range reasonably anticipated in the field for sand and for clay were studied; tire sizes ranged from 36 to 75 cm in diameter d, and from 4 to 41 cm in width b; tire shapes of d/b values from 1.1 to 18.0 and tire flexibilities (i.e. hard-surface tire deflection/ tire section height) from 0.08 to 0.45 were included; conventional pneumatic tires of both near-circular and near-rectangular cross-sectional areas were studied; and pneumatic tire hard-surface contact pressures ranged from 14.5 to 633 kN/m~2. One basic dimensionless tire performance prediction term was developed for sand and another for clay. These two terms can be used to predict with good accuracy single-tire performance in environments described by the full ranges of most of the independent parameters described above. Furthermore, the model-prototype relations inherent in a dimensional analysis approach indicate that the relations of this study likely can be extrapolated over a much wider range of soil-tire conditions. Equations also were developed that describe relations between the laboratory-developed basic prediction terms and performance data obtained in field tests of full-scale wheeled vehicles. These equations help greatly to quantify the soil-tire system for field operations, and thus allow rational selection or design of tires to attain a given level of performance for a particular off-road situation.
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