The mechanics of unsprung wheel tractors has received extensive study in the last 40 years. The quantitative approach to the problem essentially began with the work of McKibben (7) in the 1920s. Twenty years later, Worthington (12) analyzed the effect of pneumatic tires on tractor stabiIity. Later, Buchele (3) drew on land-locomotion theory to introduce soil variables into the equations for tractor stability. Differential equations were avoided in these analyses by assuming that the tractor moved with zero or constant acceleration. Thus, vibration and actual tipping of the tractor were beyond the scope of the analyses.