Abstract: A roll control structure adapted to automatically control an earth working machine, such as a rotary cultivator so that the machine becomes horizontal in the lateral direction irrespective of the inclination of the ground surface, i.e., in such a manner that the machine assumes a posture preset by the operator. The roll control structure is characterized in that it is provided with left and right stroke sensors for detecting strokes, i.e. a distance between arbitrary points on the left and right lift arms in an earth working implement connecting link mechanism and those on the left and right lower links in the same mechanism to determine an angle of inclination in the lateral direction of the implement with respect to a tractor on the basis of a difference between the detected strokes.

Abstract: A speed-changing transmission device for vehicles includes a speed-changing shaft connected to rear wheels and disposed at the rear side of an input shaft driven by an engine co-axially with the input shaft, a counter shaft mounted in parallel with the speed-changing shaft and driven by the input shaft, and forward and backward speed-changing gear systems through which the speed-changing gear is connected to the counter shaft to constitute a speed-changing transmission mechanism. A forward one-way clutch is disposed between the input shaft and the speed-changing shaft of the speed-changing mechanism. In the power transmission path formed by the backward speed-changing gear system, provided is a backward clutch and a backward one-way clutch parallel with the backward clutch. The connection between the engine and the speed-changing the speed is turned into "OFF" state during parking of the vehicle, so that an engine braking effect is obtained to prevent the vehicle from starting naturally.