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Tire trends: The low-pressure look

If you're like many farmers, you haven't quite adjusted to the look of “cheeky” tires. The slightly bulging profile of your tractor's rear radials still makes you instinctively reach for the air compressor hose. But it's a look worth embracing, say agricultural engineers and tire experts, especially for no-till and reduced-tillage fields.

Those droopy-looking sidewalls — the result of reduced inflation pressure — are part of today's radial tire design and can translate to a longer footprint, increased traction, reduced soil compaction and even a smoother ride.

“You can reduce compaction by between 20 and 40% just by reducing inflation pressure,” says Georges Mokbel, business marketing manager for Michelin. “Reducing the pressure from 14 to 8 pounds per square inch can make a measurable difference in compaction.”

No-tillers say that the difference can be magnified in firmer no-till soils. And recent research conducted by agricultural engineer Tom Way, National Soil Dynamics Lab, USDA Agricultural Research Service, Auburn, AL, supports that claim.

Testing tillage impact

In a project Way conducted with Tadashi Kishimoto of the Obishiro University of Agriculture and Veterinary Medicine in Japan, the engineers used a radial-ply tractor tire equipped with six sensors on its tread to study tire pressure on tilled and no-tilled soils. Three of the sensors were mounted on the face of the long lug, and three were placed on the undertread area.

The two researchers found that pressure exerted by the tire on the soil was least uniform on untilled clay soils and more uniform on tilled soils. “This wasn't necessarily a surprise to us,” Way explains. “On no-till soils, the structure is firmer, so the tire lug takes most of the load, whereas on tilled soils, the undertread takes more of the load.”

What was somewhat surprising, Way says, was that the study showed no significant difference in tractive efficiency between soil and tillage types. “We expected more power going to the drawbar on firmer, less-tilled soils, but tractive efficiency was about 76% on both tilled and no-till soils,” he says. “In turn, we expected better fuel efficiency on no-till, but that advantage wasn't there, in this case.”

Lowering the pressure

What was evident in the study, as in several previous studies, was that no matter what the soil or tillage type, using the correct tire inflation pressure was critical in getting the most tractive efficiency and the least soil compaction. “My guess is that the majority of farmers overinflate their tractor tires, which reduces traction,” Way says.

His hunch is supported by a Kansas State University study in which the majority of radial tractor tires checked were overinflated by 5 to 8 psi. Mokbel thinks those numbers are even higher. “Too many farmers are still in a bias tire frame of mind when it comes to inflation pressures,” he says.

Notes Way, “If a farmer stays toward the low end of manufacturer's recommendations for tire inflation, based on axle weight, he'll get the most benefit from the lower inflation pressure, without voiding a warranty.” And for most manufacturers, those low-end pressures are in the single digits. “You definitely want to stay below 14 pounds per square inch,” says Dave Weed, Goodyear farm tire applications engineer. “Depending on the application, you can even go down to 6 psi.”

Checking pressure regularly

Way says, in an ideal situation, a tractor operator would check and change inflation pressures in all tires when switching from road to field conditions. “I realize most farmers don't take the time to do this, but it would be the best way to reduce compaction and maintain maximum tractive efficiency, while prolonging the life of the tires,” he states.

Weed adds that it's most crucial to check tire pressures and make adjustments when changing operations and/or the weight of the tractor and implement. “Total gross load is the key. When that changes, your tire inflation pressure should change,” he says.

Inflating on the fly

Being able to change the inflation pressure in your tractor tires with the flip of a switch would be the ideal way to make adjustments from road to field, and between different soil types and textures. According to one agricultural engineer, that system is not only possible but would pay for itself in reduced tire wear, less compaction, better traction and a more comfortable ride.

Central Tire Inflation

Brian Adams has spent years pondering such a system, known in the tire industry as a Central Tire Inflation (CTI) system, first as an engineer for Case IH, and then as a doctoral student when he wrote his dissertation on it. “Not only is this type of system entirely feasible for farm tractors, but the economics are there, too,” he says. “Such a system already exists as an aftermarket add-on for tractors in Europe.”

The military and the forestry industry in this country have used CTI systems for years to improve the mobility of their vehicles. Adams says a CTI system for tractors would be very similar, comprised of an air system that compresses, cools, dries and stores the air in a tank; air lines that run from the tank to a rotary seal on each hub; and special wheel valves on each tire's rim.


In his research, Adams built and installed such a system on a Case IH 8950, measuring changes in ride quality, traction and soil compaction. He found that reducing tire pressure with the CTI system improved ride nearly 100%, on average, while increasing tractive efficiency by 5%. He says that although less time was spent on measuring soil compaction, tire ruts were 10% more shallow with reduced tire inflation pressures set with the CTI system.

“A farmer could certainly make these tire inflation adjustments manually, but few would ever take the time,” Adams says. “This system simplifies and speeds the process.”

He claims that if this type of system could eliminate the need for using duals or triples, savings in additional tires would be in the thousands of dollars. He says, “Add in the improved traction and tractor performance, the benefits of reduced soil compaction, and the significant improvement in operator comfort, and this system could more than pay for itself.”

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