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A report from the AgTech Centre

Tractor road lope means something is out of round

Farm tractors can develop a hopping or porpoising motion when operated at certain speeds under certain conditions, but don't confuse that phenomenon known as power hop with road lope, says an engineer with Alberta Agriculture, Food and Rural Development (AAFRD).

"They are two different problems with two completely different methods of correction," says Reed Turner, a mechanical engineer with AAFRD's AgTech Centre in Lethbridge.

Power hop is a critical frequency problem that can develop when four-wheel drive and mechanical-front wheel drive (MFWD) tractors are operated under load, at certain speeds, and in certain field conditions. Some tractors will develop a rhythmic increasing for-aft, up-down, porpoising motion that makes it difficult for the operator to control the tractor. Power hop can usually be corrected by adjusting tire air pressure, tractor weight distribution, operating speed, or a combination of these.

Road lope, on the other hand, is usually a similar motion that develops at higher speeds or in road gear when the tractor is not pulling any significant load. It usually results because a tire or rim is not truly round.

"Tractors have no suspension," explains Turner. "And if the wheel is a bit out of round, and you run it down the road at any speed, you get to a frequency where it will start to bounce. That's not a power hop situation."

A common scenario that leads to road lope involves radial tires that sit overnight, get cold and develop flat spots on the bottom. "There's not much you can do but drive it for a few miles, preferably at low speed, until the tire warms up and the flat spot disappears," says Turner.

If the bounce persists after several miles when tires are warm, the lope may be caused by a structural problem, he says. "There's good probability one of the tires is not concentric, which means you may want to take it back to the dealer, or there could be damage to a rim. In either case, the remedy is to fix whatever part is out of round."

Road lope can develop on two-wheel drive or four-wheel drive tractors, while power hop is only associated with four-wheel drive or MFWD tractors.

The AgTech Centre is part of AAFRD's Technical Services Division and has a mandate to support all aspects of agricultural sustainability.

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