PURPOSE: To carry out the self steering action exceedingly smoothly by connecting a knuckle arm and the push rod of a lock chamber, and the cylinder of the lock chamber and an axle housing through each ball joint.

CONSTITUTION: The left and right knuckles 3 on which the rear shaft tire of a rear two-shaft vehicle is installed are pivotally installed on an axle housing 5 in turnable manner around a king pin 4, and a knuckle arm 6 is connected with both the edges of a tie rod 8. In this case, the fixing device 30 of a self steering shaft is constituted by fitting a piston 31 which constitutes a part of a lock chamber 37 into a cylinder 35, and the edge part of a push rod 33 is connected with the second knuckle arm 23 through a ball joint 34. The pushing rod 33 is fitted in an intermediate sleeve 32, ant applies a resistance to the shift in self steering, and the spring holder 35c of the cylinder 35 is connected with the axle housing 5 side through a ball joint 42.