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Head feeding combine harvesters are requested high turning performance, because they must turn again and again during harvesting in slippage paddy of about 0.3ha in area. Thus they are equipped with rubber crawler and they cannot adapt Clutch and Brake system that is low turning performance. If 2 HST for a traveling and a turning are combined with planetary gear units, engine power can be transmitted to both of left and right crawlers during even turning. Output of HST for traveling is transmitted to each sun-gear of the left and right units. Output of HST for turning is transmitted to each ring-gear of the units provided the rotational direction of the left ring-gear is different from the right one. Each carrier of the units is connected with half-axles for crawlers. This system is called Forced Differential Steering System. If HST for turning does not rotate, both of the ring-gears are locked. Thus the left and right carriers are rotated at same speed, so that vehicle runs straight. The system, however, has a defect in turning that an operator is requested different steering operation at forward and reverse, respectively. This problem has been solved by invention of conical-linkage-mechanism. The mechanism consists of only mechanical linkage without electric device. Vehicle speed is changed by an inclination of the bottom circle of the cone by a speed selector lever. Turning angle is changed by rotation of the bottom circumference by a steering wheel. The idea depends on the distance between top of the cone and any point of the bottom circumference is always equal. Therefore vehicle can be turned smoothly in proportion to the steering wheel angle in spite of vehicle speed. Yanmar Co. names the system Full-time Drive System (FDS). The system can be applied to all vehicles with crawler.