Miantaining two objects at a constant speed

Unity Engine
1

I am creating a simple bike AI. I want my AI bike to move towards player. If its ahead then slowdown and if its behind then speed up.
Heres what i did so far.

public class BikeAI : MonoBehaviour {
    Rigidbody rb;
    public GameObject playerbike;
    Vector3 newvelocity;
    // Use this for initialization
    void Start() {
        rb = GetComponent<Rigidbody>();
        playerbike = GameObject.FindGameObjectWithTag("Player");
        rb.velocity = new Vector3(10, 0, 0);
    }

    // Update is called once per frame
    void Update() {
        float zz =  playerbike.transform.position.z- transform.position.z;
        float xx = playerbike.transform.position.x - transform.position.x;
        Debug.Log(playerbike.transform.position.x);
        Debug.Log(xx);

        rb.velocity = new Vector3(xx*20,0,zz);
    }
}

Now problem is bike forward movement seems to be extremely jerky. Anyway to fix this

2

You can do something like:

Vector3 direction = bikePos - playerPos;
rb.AddForce(bikeSpeed * direction.normalized);

I think it should do!

3

Doesnt seem to work. It doesnt catchup with player