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Numerous tests with tractors and other farm machines equipped with rubber tires reveal the relative advantages and disadvantages.

Advantages of rubber-tired tractors are (1) higher operating speeds, (2) less power required for the same load, (3) less fuel consumption, (4) decreased rolling resistance, (5) less vibration, (6) easier handling qualities, and (7) greater comfort for the operator.¹

Disadvantages are (1) difficulty of holding on listed ground, (2) greater slippage on wet soil, (3) greater initial cost, and (4) possibility of punctures.

When used on other farm machines, such as combines, sprayers, and potato planters and diggers, rubber tires reduce the drawbar pull, the fuel consumed by the tractor, vibration, and dust, as well as make transportation easier from field to field and along the highways.

Kinds of Rubber Tires. In general, rubber tires used on farm equipment can be divided into two classes, namely, traction and free-rolling.

Traction Tires. Traction tires have specially designed treads which grip the soil with sufficient bite to give a high degree of traction for pulling heavy loads with a minimum of slippage. Figure 6–17 shows several special-purpose traction treads. Traction tires are used on tractors and self-propelled equipment. They are designed to carry a load as well as give traction.

FIG. 6–17. Types of rear-wheel tractor tires: A, heavy-duty deep tread; B, standard-type farm tread; C, cane-field special tread; D, general-purpose and industrial tread. (Goodyear Tire and Rubber Company.)

Free-rolling Tires. Tires of this type are also called implement tires. They are designed primarily to carry loads. The rib tread is constructed to aid the wheel in rolling straight forward and to prevent as much sideslip as possible (Fig. 6–18). Where moderate traction is required in an implement tire, the tread has a lug design of medium height (Fig. 6–18). Such tires are used on manure spreaders and other machines that require ground traction for their operation.

Agricultural Tire Code. The Tire and Rim Association and Rubber Manufacturers Association have developed and approved a code numbering system for tires, as shown in Table 6–6. The F, R, and G series are traction-type tires while the I series are implement tires. This code designation is stamped on the side wall of all tires just under the size and ply rating.

TABLE 6–6. AGRICULTURAL TIRE CODE DESIGNATIONS

Ply Rating of Tires. The Tire and Rim Association and the Rubber Manufacturers Association have defined ply rating as follows: “The term ‘ply rating’ is used to identify a given type of tire with its maximum recommended load, when used in a special service. It is an index of tire strength and does not necessarily represent the number of cord plies in the tire.” The ply rating for agricultural tires ranges from two to ten depending upon the type of service. Small, lightweight tractors require only a two-ply tire while a large, heavy tractor carrying mounted equipment requires eight to ten-ply tires.

FIG. 6–18. Free-rolling-implement rib tires and tire with tread for moderate traction. (Goodyear Tire and Rubber Company.)

Tire Sizes. Tractor tire sizes are designated by the tire cross-sectional diameter and the rim diameter. The tire size for front tractor tires is given as 4.00-15, 5.00-16, 5.25-21, 6.50-16, 7.00-16, and many others. The SAE Technical Committee recommends that the sizes of rear tractor tires be as follows: sectional diameters are to range in consecutive numbers from 6 to 15 with no fractional sizes. The rim diameters recommended are 24, 26, 28, 30, 34, 38, and 42. Typical stampings on the tire for size are shown as 9–24, 11–28, 12–26, 12–38, and 15–30.

Inflation Pressures. The use of proper inflation pressures is an important factor in the satisfactory performance and maintenance of tractor and implement tires. The recommended pressures vary with the tire size, the number of plies, and the service load. The maximum recommended pressure in pounds for the front wheels of tractors are: two-ply, 20 to 28; four-ply, 24 to 56; six-ply, 32 to 60; and eight-ply, 40 to 80. The minimum inflation pressure for the rear tires on tractors is 12 pounds. When a tractor tire runs in a furrow while plowing, the pressure for the rear tire in the furrow should be increased by 4 pounds. The maximum recommended pressures in pounds for implement tires are two-ply, 20 to 24; four-ply, 16 to 52; six-ply, 24 to 44; eight-ply, 24 to 48; and ten-ply, 28 to 48.

FIG. 6–19. Types of wide-base rims for tires: A, wide-base drop-center with shallow well; B, wide-base drop-center with deep well; C, wide-base semidrop-center with split side ring.

Agricultural tractor and implement tires are designed for a maximum speed of 20 m.p.h. Tires used on farm trailers should be of the automotive or truck type which are designed for high speeds.

Rims for Tractor and Implement Tires. Rims now in use are usually of the drop-center type with a shallow or deep well. The trend is toward the use of wide-base rims with a shallow drop center (Fig. 6–19). The wide-base rim on a trailing implement allows for better lateral stability and provides a better-shaped tire section to carry loads. A cross section of a wide-base semi-drop-center rim is shown in Fig. 6–19. There is a removable split side ring, which makes it easier to remove and mount heavy tires. The effect of rim width on tire performance is shown in Fig. 6–20.

FIG. 6–20. The effect of rim width on tire performance is shown by the travel reduction, torque input, drawbar pull, and work efficiency for 14 to 26-inch, six-ply, bartype (R-1) tires mounted on 12, 14, 16, and 18-inch rims and operated on dry sand at 12 pounds per square inch inflation and 3,120 pounds static load. Dynamic load was equal to static load plus 0.2 of 1 per cent of drawbar pull. Broken-line curves for button-type (R-3). (U.S. Department of Agriculture, Tillage Machinery Laboratory.)

Life of Agricultural Pneumatic Tires. There are many factors that affect the life of tires used on tractors and implements, such as (1) type of use, (2) type of farming and crop, (3) abrasive wear, (4) cuts and chipping, (5) punctures and blowouts, (6) exposure to weather, (7) improper inflation, (8) annual use, and (9) general care.

The bar graph in Fig. 6–21 shows the average life of front and rear tractor tires for various sections of the United States. The difference in the life of tractor tires in the different areas is greatly influenced by many of the factors enumerated above. High speeds, overloads, and under-inflation will increase tire temperatures, damage the tire, and shorten its service life.

Traction of Tires. In the foregoing discussion it was shown that traction tires had treads designed to bite into the soil to give traction. Numerous tests have been conducted to determine the effect of various surface conditions on tire traction. The traction efficiency for a tractor equipped with pneumatic tires is expressed as the ratio of the drawbar horsepower to the engine horsepower. This is true regardless of type of wheel equipment or tread.

FIG. 6–21. Bar graph showing life of tractor tires in the various sections of the United States. (Farm Implement News.)

Traction of the rear wheels on tractors is increased by adding weights to them. This is done either by attaching weights to the outside of the wheels or by filling the inner tube with liquid. If water is used in the tube, an antifreeze solution should be used where freezing conditions occur. The type of tread also is important in giving traction under different conditions (Fig. 6–22).

Effect of Weighting Tires. Reed et al. state that “the effectiveness of tractor tires is markedly affected by the effective pressure at the bottom of the tire while under load. Higher pressures, whether caused by air or liquid, reduce the tire’s effectiveness in loose sandy soils, but increase effectiveness on concrete and extremely firm soils.” The performance of tires with three types of weights is shown in Table 6–7 and in Figs. 6–23 and 6–24.

FIG. 6–22. Influence of type of tire tread on traction. (U.S. Department of Agriculture, Tillage Machinery Laboratory.)

FIG. 6–23. Chart showing the increase in inflation pressure for tires filled with air, 81 per cent water, and 100 per cent water when the static load was varied from zero to 3,700 pounds. The initial inflation was 12 pounds per square inch with the tire on concrete and a static load of 2,325 pounds. (U.S. Department of Agriculture, Tillage Machinery Laboratory.)

FIG. 6–24. Chart showing the effects of three kinds of ballast on the performance of tractor tires in packed Lakeland sand and Hiwassee sandy-loam soils. The inflation pressure was measured at the tire valve. (U.S. Department of Agriculture, Tillage Machinery Laboratory.)

TABLE 6–7. PERFORMANCE OF THREE TIRES IN TWO SOILS AT SELECTED LOADS