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Ideally, the theoretical drawbar performance characteristics should be identical to the one experimentally obtained. In reality, there are some differences between the two characteristics. One difference lies in that the actual speed of the tractor, which is analytically obtained, is not equal to the actual speed of the tractor that is experimentally obtained. We assume that the actual speed that is obtained on a theoretical basis depends on engine speed, transmission gear ratio and tractor slip. In reality this speed also depends on the variable value of the tractor wheel radius: for instance, it depends on the tractor variation of weight distribution on the two axles during operation.

The paper presents a mathematical model that determines the theoretical drawbar performance characteristics of the wheel tractor taking into account the modification of the wheel radius during operation. The paper also presents applications of the mathematical model developed on tractor MAT 81, constructed in Craiova, Romania. We further examine the calculus systematic errors that may occur when we assume the tractor wheel radius is constant.

tractor; drawbar performance; wheel radius