Shock absorber

1. A shock absorber comprising:

a shock absorber main body having

a cylinder housing

a fluid,

a piston that is inserted into the cylinder housing to be free to slide and divides an interior of the cylinder housing into a contraction side chamber and an expansion side chamber, and

a piston rod inserted into the cylinder and coupled to the piston; and

a damping force adjustment mechanism having

a flow passage that allows the fluid from the cylinder housing to pass during only one of an expansion operation and a contraction operation of the shock absorber main body,

a valve seat provided midway in the flow passage, a valve body capable of advancing and retreating relative to the valve seat, and

a stepper motor that is configured to adjust a flow passage area by driving the valve body to advance and retreat relative to the valve seat, wherein

a valve body position in which step-out may occur in the stepper motor if the valve body is moved further toward the valve seat therefrom is set as a step-out boundary position, and

a zone between a seated position in which the valve body is seated on the valve seat and the step-out boundary position is set as a step-out zone, and

when the valve body is to be moved to the seated position through the step-out zone, the stepper motor is energized to move the valve body by a distance that is at least twice a distance to the seated position.

2. The shock absorber as defined in claim 1 , wherein a zone between a maximum separation position in which the valve body is maximally separated from the valve seat and the step-out boundary position is set as a non-step-out zone, and

when the valve body is to be moved through the non-step-out zone, the stepper motor is energized to move the valve body by an equal distance to a desired movement distance.

3. The shock absorber as defined in claim 1 , wherein a plurality of valve body stop positions are set in the non-step-out zone and a valve body stop position closest to the step-out boundary position, from among the plurality of valve body stop positions, is set as a boundary stop position, and

when the valve body is to be moved to the seated position, the stepper motor is energized to move the valve body by a distance that is at least twice a distance from the boundary stop position to the seated position.

4. The shock absorber as defined in claim 1 , wherein the valve seat is provided on a tip end of the piston rod, and

the flow passage penetrates the piston rod and connects either the contraction side chamber or the expansion side chamber in the shock absorber main body to a reservoir provided on an exterior of the cylinder.

5. The shock absorber as defined in claim 4 , further comprising:

a vehicle body side tube coupled to a vehicle body of a saddle-ridden vehicle;

an axle side tube coupled to a vehicle wheel of the saddle-ridden vehicle; and

a valve housing that houses the valve body,

wherein the piston rod is coupled to the vehicle body side tube via the valve housing,

the cylinder housing is coupled to the axle side tube, and

the stepper motor is fixed to the valve housing and disposed to project from an upper end opening portion of the vehicle body side tube.

6. The shock absorber as defined in claim 1 , wherein the stepper motor is energized by pulse signals received at a specific period.

7. The shock absorber as defined in claim 1 , wherein the stepper motor is energized for a duration greater than or equal to a duration required to drive the valve body from the step-out zone to the seated position.

8. The shock absorber as defined in claim 1 , wherein the valve body is a needle valve or a poppet valve.