Tractor and implement configuration for reducing power hop

Agricultural tractors come to jump and bounce severely in specific field conditions. This phenomenon of instability, commonly called "power hop", is more likely to happen when the soil is dry, has been worked recently, and when the tractor is pulling disks, or a chisel. Many efforts have already been made to control the phenomenon, and some empirical techniques proved somewhat efficient. Yet, these techniques sometimes prevent the farmers from using their tractors in optimal conditions in terms of tire inflation pressures, tractor ballast… So far, no radical solution has been found regarding tire conception, tire carcass construction, materials, and other tire characteristics… Thus, the objective of the work presented here is double: first to understand analytically the origin of the instability, and secondly, to find configuration parameters that can improve system stability. A simplified analytical model is presented, which demonstrates the possibly instable nature of the system, using the Laplace transform approach. The influence of various parameters (tractor pitch inertia, wheel base, drawbar height) and operating conditions (drawbar force) can then be known. Experimental tests were conducted both on a hard soil (concrete) and a soft soil (wheat stubble) to verify some of the predictions. These tests confirm the predicted interest of lowering the drawbar height close to ground level.