1. Of the total power used in agriculture, about one-half is used for field work and one-fifth for hauling.

2. Of the total primary power used in Iowa (1930) in farm operations, tractors comprise 39.4 percent and trucks 20 percent.

3. The maximum tractive efficiency of tractors tested under various conditions varied from 40 percent for soft field conditions to 84 percent for smooth hard sod.

CONCERNING STEEL TRACTOR WHEELS

4. The rolling resistance of tractors over the tractive surfaces was the principal cause for low efficiency.

5. Lugs or grousers of excessive length used to increase adhesion on a firm surface or turf may cause considerable loss in efficiency. With a wheel tractor weighing 5,620 pounds, the power required to overcome rolling resistance at a speed of 3 miles per hour varied from 2.45 horse power with drive wheels without lugs to 6.3 horse power with drive wheels equipped with 4-inch spade lugs. Because of the lugs, rolling resistance on oat stubble did not differ greatly from that on freshly plowed land.

6. On a loose soil of uniform texture, an increase in length of spade lugs from 4 to 7 inches increasingly lowered tractive efficiency.

7. On a loose soil of uniform texture, an increase in the width of the tire by use of an extension rim gave higher tractive efficiency.

8. On soil with a loose surface, but firm subsurface, a spade lug 9 inches long reaching firm soil resulted in a slightly increased efficiency over 6 and 7-inch lugs but was less than for 4 and 5-inch lugs.

9. Five-inch angle lugs mounted on a wheel 42 inches in diameter with a rim 12-inches wide gave higher tractive efficiency than spade lugs on freshly prepared loose soil.

10. Extension angle iron lugs increased tractive efficiency on loose soil materially, about one-fifth to one-fourth.

11. Angle iron lugs extending over wheel rims were advantageous on sticky soil, because the soil did not pack in between the lugs.

12. Increasing the weight from 1,750 to 2,250 pounds on a 12 x 42-inch traction wheel equipped with spade lugs increased the drawbar pull 75 to 100 pounds at maximum efficiency. The drawbar pull was increased approximately 200 pounds when the wheel was equipped with extension rims and angle lugs.

13. Angle iron lugs gave slightly better results with a 6-inch rim extension than without on freshly prepared loose soil.

14. Open type traction wheels performed practically the same as 12-inch rim wheels with lugs on firm traction surfaces of cinders or sod. The rim did not function, as the weight was carried entirely on the lugs.

15. On loose freshly prepared soil where the space between the lugs did not fill with soil, the rim wheel gave slightly higher tractive efficiency than open wheels.

16. The tractive efficiency of steel drive wheels was progressively raised by increasing the diameter from 38 to 58 inches by 4-inch increments.

17. The effect of wheel diameter is more marked on less firm traction surfaces.

CONCERNING LOW PRESSURE PNEUMATIC TIRES

18. The rolling resistance of a wheel tractor. defined herewith as drawbar pull or its equivalent required to move the tractor over a given surface, was materially reduced by low pressure pneumatic tires for all conditions observed.

19. On a smooth hard surface the maximum tractive efficiency of a tractor equipped with pneumatic tires was 84 percent.

20. The maximum drawbar pull of a tractor equipped with low pressure pneumatic tires was materially reduced on stubble and loose soil.

21. The maximum drawbar pull of tractors equipped with low pressure pneumatic tires can be increased by additional weight, chains or lugs.

22. The maximum tractive efficiency was increased progressively with a decrease of inflation pressure from 20 to 16, 12 and 8 pounds per square inch.

CONCERNING TRACKS

23. The tractive efficiency of a track tractor as observed is not materially influenced by normal variations of traction surfaces.

24. On freshly prepared loose soil maximum tractive efficiency of a track was lowered by increasing the height of hitch.