Power Hop Instability of Tractors

Power hop is a “porpoising” type of pitch/bounce oscillation superimposed on the forward motion of tractors equipped with pneumatic tires. It is most often observed with tractors having two powered axles pulling towed implements in moderate to high draft field operations on dry soils. As power hop begins, the tractor exhibits gradually increasing vibration amplitudes until the operator, of necessity, has to take action to maintain control of the vehicle. In this paper, the phenomenon is shown to be a self-excited vibration. This result follows from a stability analysis of a mathematical model of the tractor-implement system. The model results were confirmed with extensive tests conducted on both concrete test tracks and in numerous different soil conditions. Adjustments of fore-aft weight distribution and of front and rear vertical stiffnesses as influenced by tire inflation pressures are shown to be required for power hop control. During the course of research conducted in collaboration with several tractor tire manufacturers, it was found necessary to extend the tire load-inflation pressure tables to lower pressure limits in order to reduce stiffnesses enough to control power hop. The new tables were introduced early in 1992 and are now being used worldwide. In addition to controlling power hop, use of these lower inflation pressures has been shown to significantly increase tractor productivity and fuel economy, reduce soil compaction, improve ride quality, and reduce tire wear.