Traction Aids

A number of techniques are available for increasing the tractive performance of a tractor. One of the earliest techniques was to use dual or triple tires on each rear axle of a 2WD tractor. Careful studies have shown that such use of duals or triples provides little increase in C_ni or in η_t. However, the duals or triples increase the load-carrying capacity as discussed in Section 7.3.1, thus permitting much higher dynamic loads on the drive axles and, consequently, higher drawbar pull. In soft ground, duals or triples also reduce the sinkage of the tires into the soil. Whether single, dual, or triple tires are used, use of increased axle loads leads to increased soil compaction. Soil compaction can be avoided and axle stress can be reduced by use of travel speeds of 7.2 km/h or higher. As Equation 7.15 shows, use of higher travel speeds reduces the amount of pull needed to achieve a given drawbar power level.

Only the rear wheels of early 2WD tractors were powered; the front wheels produced motion resistance and thus only a negative contribution to tractor pull. Two methods have been used to power both front and rear wheels. Tractors designated as 4WD have all wheels powered. All of the wheels are of equal size and the rear wheels run in the tracks of the front wheels. Thus, in addition to the pulling contribution of the front wheels, the soil strengthening they provide (see Equation 7.14) increases the pulling capacity of the rear wheels. More recently, FWA (Front Wheel Assist, an alternate name is MFWD for Mechanical Front Wheel Drive) tractors have been designed as 2WD tractors with a front wheel assist option. The peripheral speeds of the front and rear wheels are closely matched with a slightly higher peripheral speed for the front wheels. The front wheels of FWA tractors are intermediate in size between conventional, unpowered front wheels and powered rear wheels; the front wheels on FWA tractors are also equipped with lugs to aid pulling. Since the rear tires of 2WD and FWA tractors are always wider than the front tires, the rear tires must form part of their own rut and Equation 7.14 somewhat overestimates the effective cone index for the rear tires.

Optimum weight distribution varies according to type of tractor. For 2WD tractors, only enough dynamic weight is needed on the front axle to provide reliable steering. With typical weight transfer, reliable steering is achieved if approximately 25% to 30% of the static weight is carried on the front axle. For 4WD tractors, approximately 55% to 60% of the static weight is carried on the front axle; weight transfer then generates approximately equal dynamic loads on the front and rear axles. Ballasting of FWA tractors depends upon their use. When the optional FWA is disengaged, the tractor should be ballasted the same as a 2WD tractor. Conversely, when the FWA is engaged, ballasting should be that of a 4WD tractor.

To allow use of more ballast and to reduce sinkage in soft soils, some FWA tractors are equipped with dual or triple wheels on the rear axles, but steering considerations generally require use of a single tire on each front axle. Most 4WD tractors have articulated steering, i.e., the tractor has a vertical hinge between the front and rear axles that allows those axles to move out of their parallel orientation for a turn. Such tractors may be equipped with dual or triple tires on both the front and rear axles, i.e., there may be 8 or 12 tires on the tractor. As before, the benefit of the dual or triple tires is to reduce sinkage in soft soils and to allow the use of more ballast to increase traction.

lug height, lug angle, and number of lugs on a tire, but few consistently clear trends have been observed from such studies. Bias-ply construction was used on all early tires, but tires with radial plies are now gaining wider use on farm equipment. Some studies have shown that radial tires provide a significant improvement in net coefficient of traction. Note that, in traction mechanics theory, traction parameters k₁, k₂, and k₃ for radial tires differ from those for bias ply tires. The reader can thus use the traction mechanics equations to assess performance differences between radial and bias ply tires under various conditions of tractor loading and soil strength.

Where rice is grown in flooded paddies, effective soil cone indexes will typically be less than 0.5 N/mm² . The performance of conventional tires becomes unacceptable in such conditions because of high wheel slippage and adhesion of sticky soil to the tires. One solution has been the use of tires with very high lugs. Another solution is to attach an auxiliary wheel with steel lugs to the side of each tire. The steel lugs improve the traction coefficient and, for road transport, may be folded to avoid contact with the road surface.