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7.1 Introduction Traction may be defined as the force derived from the interaction between a device and a medium that can be used to facilitate a desired motion over the medium. The usual traction device converts rotary motion derived from an engine into useful linear motion. Anchor devices such as winch sprags are exceptions to the usual concept, but they are traction devices since they provide traction by interacting with a medium-usually soil. Although the basic soil reactions are similar, the continuous rolling action of a wheel or track requires a different analysis than the stationary action of a sprag. The traction and transport devices considered here operate off the road and are construed to be wheels and tracks-that is, parts of vehicles rather than complete vehicles such as tractors. The number of off -the-road vehicles is rapidly increasing for agriculture, military, and construction purposes. The total engine power available for conversion into useful pull is generally in excess of the traction capacity that can be developed between the traction device and the soil. In other words, the limitations of the vehicle in respect to off-the road movement are usually the limitations of the traction device. Furthermore, the efficiency with which a traction device converts energy into pull is usually extremely poor when the device is operating on soil. Work at the National Tillage Machinery Laboratory shows that a pneumatic tire operating on a concrete surface has an average power efficiency of approximately 75 percent. The same tire operating on various soils has an average efficiency of less than 50 percent. Nebraska Tractor Test results indicate that pneumatic tired tractors operating on concrete lose approximately 5 percent more in thermal efficiency when the useful work output is expended through the drawbar than when it is expended through the power takeoff. Obviously, loss in thermal efficiency on soil will be even greater. Based on this minimum loss in thermal efficiency, some 152 million gallons of gasoline valued at $42 million are lost anually because of the inefficiency of the pneumatic tire. There are soil conditions where adequate traction and satisfactory efficiency can be obtained; but, because of economic, social, and political Pressures, such tasks as pest control, crop harvests, and military, mining, and construction operations are performed on extremely poor soil conditions where adequate traction cannot be attained. With the exception of loose dry soils, volcanic ash, or sand, most of the adverse conditions are associated with wet soils. Some of the land conditions on which operations must be conducted are so extreme that entirely new principles of vehicle design may be required.