Analytical studies reveal that for a four-wheel-drive tractor with rigidly coupled drive axle to achieve the optimum tractive performance under a given operating condition, the theoretical speed (the product of angular speed and free rolling radius) of the front tires must be equal to that of the rear tires. Previous field test data obtained using an instrumented tractor, equipped with tires 16.9R26 on the front axle and 20.8R38 on the rear axle, substantiates the analytical findings. This paper presents the results of further field tests using another set of tires, 13.6R28 and 18.4R38. Latest test data further confirm the analytical findings. It is concluded that to achieve optimum tractive performance in the field, proper matching of the sizes of the front and rear tires and careful control of the inflation pressure and normal load of the tires are of importance.
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