It starts with a slight fore-aft vibration, which shakes the tractor. It can come and go for a while but usually gets worse. The front of the tractor moves up and down and the operator can feel the seat suspension working as his knees flex. The operator steps on the clutch and stops the tractor, but when he starts again the same sequence occurs.
He tries a little experimenting. Throttling back helps, running in a lower gear helps, raising the implement a bit stops the motion completely. The question, though, is what is happening and why? This didn’t happen yesterday — everything was fine when he quit the night before. He’s heard about this happening to other producers. Could this be “power hop?”
It’s a story Reed Turner, a longtime project engineer with the AgTech Centre in Lethbridge, Alta., has heard many times before. And the story almost always comes down to three key complaints: power hop is time consuming to deal with; it affects operations at the worst possible times; and it can be hard on equipment and uncomfortable for the operator.
The good news is that power hop is better understood than ever before and management tools are available to minimize the problem. The bad news, says Turner, is we still have no comprehensive solution to prevent it from happening in the first place.
Turner is on the case for new solutions to power hop because of a call for action by a major society of agricultural engineers. Turner, who is known across North America as an authority on tractor issues, has been selected by the American Society of Agricultural and Biological Engineers (ASABE), along with co-author Jack Wiley, to develop in-depth resource material on the subject as part of a series of ASABE lectures, which serve as teaching aids to a new generation of engineers.
“Although we have methods today that allow us to better control power hop, they tend to reduce fuel efficiency, are time consuming and do not work in every circumstance,” says Turner. “Ideally, we want a power hop fix, not just power hop control, and it should neither reduce tractor efficiency nor require operator intervention for changing field and weather conditions. This is not yet the case.”
WHAT IS POWER HOP?
Power hop is a “porpoising” type of fore/aft pitch and bounce instability that occurs on four-wheel drive (4WD) and mechanical front wheel drive (MFWD) rubber tire tractors operating under draft loads. It typically begins as a small but increasing motion that builds to uncomfortable, even damaging levels. On the Prairies, hop is most likely to occur in August when working early harvested ground. In a dry spring, it can also show up during second pass tillage or planting.
Although the two are similar in motion and sensation, it’s important to distinguish power hop from road lope, says Turner. “Road lope develops at high speeds or in road gear when the tractor is not pulling any significant load. It’s usually the result of a tire or rim not being truly round. They are two different problems with two completely different methods of correction.”
While power hop can occur on various surfaces, it becomes a real problem when it happens in a field where a farmer is trying to work, says Turner. “Soil moisture and texture are major influences on power hop,” he says. “The drier the soil, the more likely it is that power hop will occur, and the simplest cure for power hop can be a good rain.”
RESEARCH INTO CAUSES
Years of research have produced a model using three common factors that contribute to power hop: the quantity and location of vehicle mass, the combined vertical stiffness and damping of the front tires and their operating surface, and the combined vertical stiffness and damping of the rear tires and their operating surface. Turner says this model correctly predicts power hop on rigid surfaces such as concrete or pavement but does not work as well in field conditions, particularly with 4WDs.
“The effect of changing tire stiffness in the front or the rear is one area where the current model differs from field experience. The model always predicts the front as the correct end to stiffen to reduce power hop, but there are times when stiffening the rear is a better choice.”
The model also does not account for trends that have been consistently observed in the field, says Turner. “For example, we see the occurrence of power hop increase as travel speed increases. We also see power hop occur more easily on tractors with higher horsepower to weight ratios. However, neither travel speed nor power-to-weight are factors in the current model.”
HOW DO WE FIX IT?
There is no certain fix for power hop yet, says Turner. Because power hop is dependent on ground conditions, a setup that controls hop one day in one field condition may not be effective on another day in another field condition.
However, setting the tractor for optimum tractive performance is always considered the best place to start. “This means setting the correct ballast weight and weight distribution for the power, and setting minimum allowable tire inflation pressures for the weight. If power hop does not occur with this setup, life is good,” says Turner.
If power hop does occur, however, life becomes tougher. “Currently, the options producers have in this instance are to use a bigger tire or stiffen the tires on one end by raising tire pressure or adding fluid in the tires. Stiffening the tires is generally the less expensive option,” says Turner.
As explained, the current model declares the front as the end to stiffen for power hop control. Although stiffening at the front almost always controls hop on MFWD tractors, with 4WD tractors the situation is not as clear-cut, says Turner. “Particularly in softer soils, stiffening the rear tires on 4WD tractors can control power hop, sometimes with lower pressure increases than would be required on the front, resulting in smaller efficiency and soil com-