Constrain local X movement

Howdy, I'm attempting to make a little skate game. And all I want to do is make it so that when the player has jumped, they continue along the same vector, but can rotate around local Y ( e.g to do a 360 ollie or whatever ). Here's what I have so far:

 using UnityEngine;
 
 public class PlayerController : MonoBehaviour
 {
     Rigidbody playerRigidbody;
 
     float acceleration = 5, deceleration = 2;
     float currentSpeed, minSpeed = 0, maxSpeed = 20;
     float turn, turnSpeed = 2;
     float jumpForce, jumpForceModifier = 4, minJumpForce = 2, maxJumpForce = 5;
 
     Vector3 preJumpVector;
 
     void Awake()
     {
         playerRigidbody = GetComponent<Rigidbody>();
     }
 
     void Start()
     {
         jumpForce = minJumpForce;
     }
 
     void Update()
     {
         Impetus();
         Turn();
         Jump();
     }
 
     bool OnGround()
     {
         return (Physics.Raycast(transform.position, Vector3.down, .1f));
     }
 
     void Impetus()
     {
         if (OnGround())
         {
             if (Input.GetKeyDown(KeyCode.W))
                 currentSpeed += acceleration;
             else if (Input.GetKeyDown(KeyCode.S))
                 currentSpeed -= acceleration;
 
             transform.Translate(Vector3.forward * Time.deltaTime * currentSpeed);
         }
 
         else
         {            
             transform.Translate(preJumpVector * Time.deltaTime * currentSpeed);     //here's where I think it needs to change
         }
         
         currentSpeed -= deceleration * Time.deltaTime;
         currentSpeed = Mathf.Clamp(currentSpeed, minSpeed, maxSpeed);
     }
 
     void Turn()
     {
         turn = Input.GetAxis("Horizontal");
         transform.Rotate(Vector3.up * turn * turnSpeed);
     }
 
     void Jump()
     {
         if (Input.GetButton("Jump") && jumpForce <= maxJumpForce)
         {
             jumpForce += jumpForceModifier * Time.deltaTime;
         }
 
         if (Input.GetButtonUp("Jump") && OnGround())
         {
             playerRigidbody.AddForce(Vector3.up * jumpForce, ForceMode.Impulse);
             jumpForce = minJumpForce;
             preJumpVector = Vector3.forward;   //think this should give the pre jump vector, then I want to constrain movement to this whilst not OnGround(), but still be able to rotate
         }
     }
 }