PURPOSE: To aim at securing the safety of superconfidence spot turning in such a way as enlarging the radius of gyration with more throttling action in proportion to the increase of speed, by coupling a variable throttle valve of a torque variable clutch to a side clutch, while connecting a throttle governing member of the throttle valve to a gearshift lever.

CONSTITUTION: When a side clutch lever is pulled for turning, a support plate 35 pushes a spool 37 of a variable throttle valve 36 inward whereby the transmission torque of a torque variable clutch 18 goes up, stopping a reversing clutch 22a making normal rotation with a brake applied sofar and thus becoming confidence spot turning but, after this period of time, it slowly starts in reverse rotation, turning to superconfidence spot turning. However, since a cam 44 rotates as interlocked with a stepless gearshift lever 55, if this lever 55 is located at the highest speed position D, the cam 44 takes the most erected attitude so that the spool 37 is permitted of shift along only a slight disface; inasmuch as the transmission torque comes little, a machine body turns with a large radius of gyration and therefore superconfidence spot turning is attainable at the lowest speed position E of the lever 55.