It happens right in the middle of important fieldwork. The tractor starts lurching, or “porpoising” in a fore/aft pitch and bounce oscillation. It rattles the seat, the cab and especially the operator and can take the precision out of even the most high-tech tractors.

This phenomenon is “power hop.” And it can force halt to operation and adjustments to speed, load, tire pressures and/or ballast, says Reed Turner, long-time farm machinery consulting engineer for the AgTech Centre in Lethridge, Alberta, Canada, and major tractor manufacturers.

“The worst problem with power hop is that Murphy’s Law says it happens at the worst time,” he says. “Suddenly you just can’t go in the gear you want. Adjustments have to be made in order to continue the operation on that field.”

Turner explains that “tractors have a natural frequency. When they experience resonance at that frequency, you get power hop.”

Since soil moisture and texture heavily influence whether or not power hop occurs, widespread drought conditions the past two years could create more power-hop problems. “The drier the soil, the more likely it is that power hop will occur,” Turner says. “Power hop is extremely rare when operating on soils that are very moist most of the time.”

Preventing power hop begins with making sure the tractor has proper tire and ballast settings. “That means setting the correct ballast weight and weight distribution for the power needed and setting minimum allowable tire inflation pressures for that weight,” Turner says.

Proper ballasting. This is critical because balance and slip level are the most important factors in power hop control. “Power hop incidence and power hop control on 4WD tractors are particularly sensitive to weight split,” Turner says. “Ballasting the tractor influences both total tractor weight and the fore-aft balance.

For 4WD tractors used with standard towed tillage and seeding implements, no more than 55% of the static ballasted weight should be on the front axle. “Tractors used to tow scrapers that generate high vertical drawbar loads, as well as tractors used for deep ripping with an integral ripper should have 65-70% of the weight on the front axle,” Turner says. “Power hop does not typically occur in these applications.”

Tuning of ballast primarily involves adjusting the total weight and front/rear weight split for the tractor horsepower, desired travel speed and implement load. This enables tires to operate on the soil at an optimum slip level, typically around 10%, at the desired travel speed, Turner says.

Higher travel speeds require less ballast because drawbar pulls are lower. However, if all of a tractor’s available horsepower is to be transferred to the ground at slower speeds, ballast should be increased, Turner explains. “Ballasting a tractor enough to pull at full power below about 4 to 4.5 mph is usually not a great idea,” he says. “Power hop will be more likely, and even if it doesn’t occur, drive-train life will be shortened.”

Tire pressure. It’s important to set the correct minimum tire pressures for the tractor’s ballasted weight. This involves using settings recommended by the tire’s manufacturer, in what are called “load-inflation tables.” Always start with these pressures, “but, even with correct settings, sometimes a tractor that operates perfectly in one field, might start power hopping when moved to another,” Turner says.

“If power hop occurs and everything else is set correctly, the quick fix is to raise tire pressures on one end of the tractor,” he says. “That stiffens the tires on that end, and the tractor is less likely to hop. Current tractor modeling suggests the front as the end to stiffen for power hop control.”

Stiffening at the front almost always controls hop on MFWD tractors, but with 4WD tractors the situation is not as clear-cut. In softer soils, stiffening their rear tires on can sometimes control power hop with lower pressure increases than would be required on the front. “This can result in smaller efficiency and soil compaction penalties,” Turner says.

He encourages growers with hopping 4WDs “to experiment with each end separately to see which one will control the hop with the least amount of change away from the optimum.”

The more you have to raise pressures to control power hop, the more fuel efficiency decreases. So operators may want to experiment enough to be able to control hop with the lowest possible increase in tire pressure.

Don’t just run with the hop control settings all the time, Turner concludes. “As soon as you can, return the tires to the minimum allowable pressure. That will significantly increase tractor productivity and fuel economy, reduce soil compaction, improve ride quality and reduce tire wear,” he says.

For more detailed information power hop, go to