1. This study shows a theoretical prediction method on the upper limit angle of tracor use, facing forward to the contour line of slope field.
2. Tractor and its running surface are assumed to be ideal systems, that is they each are rigid bodies with smooth surfaces. Then traction force is developed only by the friction force between them. The power factor on the traction is also assumed in constant number.
3. The resistance force on the driven wheel for side slip is in proportion to the friction force. But the one on the drive wheel is effected by both the friction force and the traction force, which are in accordance with the princlple of friction circle.
4. The upper limit angle of 2 wheel tractor becomes larger with the friction force, and on the contrary smaller with the traction force.
5. The one of 4 wheel tractor is related to the position of center of gravity, hitch height and hitch eccentricity, and they work in adverse effect on front and rear wheel, respectively.