Wobbling position after setting a new Vector3

Hi all! I made a script for a game object to copy other objects position only on one axis.

public GameObject wheel;

void Update ()
    transform.position = new Vector3(

It works, but the object is not still on y and z axis, wobbles for a tiny bit, and on collision it jumps out of place by even further distance.

Is there any other way to copy just one axis?

Thanks in advance!