Optimizing tractive performance

Canadian researchers correlate analytical predictions and experimental data to determine that the proper matching of front-and rear-tire sizes, as well as careful control of inflation pressure and normal load of the tires, is necessary to achieve optimum tractive performance of four-wheel-drive tractors.

Analytical studies have shown that for a four-wheel-drive vehicle to achieve its optimum tractive performance under a given operating condition, the thrust (or driving torque) distribution between the front and rear drive axles should be such that the slips of the front and rear tires are equal. For most agricultural and industrial four-wheel-drive tractors, including those with front-wheel assist, the front-and rear-drive axles are rigidly coupled so that the ratio of the angular speed of the front tires to that of the rear tires is fixed. For this type of four-wheel-drive tractor, the optimum thrust distribution, hence the optimum tractive performance, is achieved when the theoretical speed of the front tires is equal to that of the rear. The ratio of the theoretical speed of the front tires to that of the rear is referred to as “the theoretical speed ratio.” Theoretical speed is the product of the angular speed and rolling radius of the tire under free-rolling conditions in the field, referred to as the free-rolling radius. The free-rolling radius of a tire usually varies with inflation pressure and normal load.

In plowing or similar field operations, tire slip is significant. Consequently, the efficiency of slip requires special attention. For a four-wheel-drive tractor, the slip efficiency can be determined from the ratio of power losses due to slip at the front-and rear-driven tires, to the sum of the power at the driven tires. Researchers from the Department of Mechanical and Aerospace Engineering at Carleton University and Canada's Department of Agriculture and Agri-Food conducted field tests to prove that to achieve the maximum slip efficiency, the ratio of the free-rolling tires must be carefully controlled so that in operation, the theoretical speed ration is equal to one. Only under those circumstances will the slip of the front tires be equal to that of the rear tires, the thrust distribution between the drive axles at an optimum, and the efficiency of slip a maximum.