Dozens of farmers and tire dealers flocked to the Purdue University Animal Science research farm last week for special demonstrations on picking tire sizes for various purposes, and on soil compaction. The event was sponsored by Michelin Tire. Purdue University Extension agronomist Tony Vyn, and Purdue Extension ag engineer Dennis Buckmaster, participated in the event. Vyn and Buckmaster pointed out the mechanics and physics of soil compaction and tire selection, respectively.
The field day involved live demonstrations, particularly a Case 330 Magnum pulling a ripper on silage ground that was just harvested. "We took a lot of silage off here in wagons that weigh 11 tons loaded," says Jeff Field, manager of cropping operations at the center. All that weight goes on four or six small tires when the wagons are pulled across the field, he notes. The point was clear- the field was somewhat compacted, particularly on the higher, reddish clay portion of the field.
"Live demonstrations don't always go like they're supposed to, and this one didn't," says Bob Rees of Michelin. They ran the tractor in the morning with tires set up on standard recommendations with standard tire pressure. Then for the afternoon, Coogle's Tires, Otterbein, put their crew to work, changing to wider-tracked tires from Michelin, designed to leave about a 50% bigger footprint and run at lower pounds per square inch of pressure.
"Six pounds per square inch is about as low as you can go," Rees says. "Any lower and you literally risk having tires come off the rim. At 6 psi, it's about the impact of a large-sized man' footstep on the soil."
Running with the standard tires, the crowd noticed some power hop, when the front tires do some bouncing off the ground. There wasn't supposed to be power hop in the afternoon. But at the first setting, 6 psi in the rear duals and 6 psi in the dual front-wheel assist tires, there was still power hop
"Make one change at a time to correct it," says Roger Lewno, a Case representative based out of Racine, Wisc. Lewno ran the tractor and ripper. "And make a big enough change that you can be sure you will solve the problem. Then once it's solved, back down in smaller steps to find the right pressures to run at."
In this case, they inflated the front tires to 10 psi, and sent Lewno back out. The power hop disappeared. "The tractor was really biting in," he says. When they tried to pull 9 shanks instead of 7 that they had been pulling, going 12 inches deep, they saw considerable wheel slippage, especially on the clay hills. That's simply because the tractor was maxing out horsepower for the job it was asked to do, officials say.
"Many people want to add more front weight to solve power hop," Lewno says. "All that does is help wear out tires. Your first step should be adding air to the front tires. Then see if that solves it."