Prediction of the force distribution between the soil and a pneumatic wheel

The tractive performance parameters (rolling resistance, drawbar pull and tractive efficiency) of agricultural vehicles in soft soil conditions are determined by the normal and shear stresses between the wheel or track and the soil. Satisfactory analysis of these parameters depends on the accurate prediction of the forces between the soil and the wheels or tracks of the vehicle. In order to predict these forces accurately, the basic phenomena of wheel or track and soil interaction must be well understood. The path of each point at the interface and the soil deformation at that point also needs to be known. For a rigid wheel or track, the path of a point through the interface is purely dependent on the slip and sinkage of the wheel or track. If, however, the wheel is fitted with a pneumatic tyre or the track is flexible, the path is also dependent on the resilience of the wheel or track. The interaction between pneumatic wheel and soil is dealt with in this paper. The method applied to this problem takes account of the fact that the path at the tyre or track-to-soil interface depends not only on slip and sinkage but also on the resilience of the tyre or track. The forces generated at each point on the interface are calculated by conventional soil-vehicle mechanics techniques. These are then integrated to provide the overall force at the interface. Results obtained by this method are validated by comparing them with those calculated from semi-empirical equations obtained from experimental data. In all cases, the general trend of the data was correctly predicted although the absolute values had significant errors of up to ±20%. The most likely source of error was due to the inaccuracy of measurement of soil properties.