PURPOSE: To prevent generation of difference between respective moments at engine brake and foot brake to improve stability by coinciding with each other a king pin axis and the axis standing with same angle as a tire chamber angle from a king pin offset point.

CONSTITUTION: A tire 19 is fixedly secured on a hub 20 and rotated with a drive shaft 21. The hub 20 is pivotally fitted to a knuckle 22, and a joint 17 is connected to the under side of the knuckle 22. A cylinder 15 is connected to the upper side of the knuckle 22 and a joint 16 is connected to the upper side of piston 7. In this case, the point of intersection between the axis connecting to each other both the upper and lower joints 16, 17 and a tire grounding face is instituted as a point Q. An aerial center point P is instituted on the inclined line standing from the point Q with the same angle as a tire chamber angle. A line segment PQ is instituted as a king pin axis 25, and steering of the tire 19 is operated along this.