The tractor can run by the power of the internal combustion engine through the transmission system, so that the driving wheel to obtain the driving torque Mk, the driving wheel to obtain the driving torque through the tire pattern and the tire surface to the ground small, backward horizontal force (tangential force), and the ground to the same driving force, the opposite direction of the horizontal reaction force Pk, This PK reaction force is the driving force that pushes the tractor forward (also known as the feeding force). When the driving force PK is enough to overcome the rolling resistance of the front and rear wheels and the traction resistance of the farm implements, the tractor will move forward.
If the driving wheel is fragmented to the ground, that is, the driving force PK is equal to zero, the driving wheel can only idle in situ, and the tractor can not run; If the sum of rolling resistance and traction resistance is greater than the driving force PK, the tractor can not run. Thus it can be seen that the wheel tractor running is realized by the interaction between the driving torque driving wheel and the ground, and the driving force should be greater than the sum of rolling resistance and traction resistance.