Vertical Velocity for fps

I am making an fps roller skating game where your character moves with momentum. The current code work fine when on the ground, but it has no effect on vertical velocity other than when jumping. The goal is to allow the character to gain speed on the ground and use that momentum to catch more air while ramping. thoughts?

 using UnityEngine;
 using System.Collections;
 
 [RequireComponent (typeof(CharacterController))]
 public class FirstPersonController : MonoBehaviour {
 
     public float movementSpeed = 0.0f;
     public float airRate = 0.0f;
     public float minSpeed = 0.0f;
     public float maxSpeed = 0.0f;
     public float moveRate = 0.0f;
     public float jumpSpeed = 0.0f;
 
     public float mouseSens = 0.0f;
     float verRotate = 0;
     public float upDownRange = 0.0f;
 
     float verVelocity = 0;
     public CharacterController cc;
 
 
     // Use this for initialization
     void Start () {
         Cursor.lockState = CursorLockMode.Locked;
         Cursor.visible = false;
         cc = GetComponent<CharacterController>();
         movementSpeed = minSpeed;
     }
 
     // Update is called once per frame
     void FixedUpdate() {
 
 
         // Rotation
         float rotLeftRight = Input.GetAxis("Mouse X") * mouseSens;
         transform.Rotate(0, rotLeftRight, 0);
 
         verRotate -= Input.GetAxis("Mouse Y") * mouseSens;
         verRotate = Mathf.Clamp(verRotate, -upDownRange, upDownRange);
         Camera.main.transform.localRotation = Quaternion.Euler(verRotate, 0, 0);
 
         // Movement
 
 
         float forwardSpeed;
 
         // speed decreasing while in air
         if(movementSpeed > minSpeed && !cc.isGrounded)
         {
                 movementSpeed -= airRate;
         }
         
         //braking button
         else if (Input.GetButton("Fire3")) {
             if (movementSpeed > minSpeed) {
                 movementSpeed -= moveRate * 3;
             }
 
         }
         // Gaining speed when pressing w
         else if (Input.GetButton("Vertical")) {
             if (movementSpeed < maxSpeed) {
                 movementSpeed += moveRate;
             }
 
         }
 
 
         //decreasing speed when not pressing w 
         else
         {
             if (movementSpeed > minSpeed)
             {
                 movementSpeed -= moveRate;
             }
         }
         forwardSpeed = movementSpeed;
 
         float sideSpeed = 0.0f;
 
         //side-to-side movement
         if (cc.isGrounded)
         { 
         sideSpeed = Input.GetAxis("Horizontal") * movementSpeed;
         }
 
         //gravity
         verVelocity += (Physics.gravity.y) * Time.deltaTime;
 
         //Jump
         if (cc.isGrounded && Input.GetButton("Jump"))
         {
             verVelocity = jumpSpeed;
         }
 
 
 
         Vector3 speed = new Vector3(sideSpeed, verVelocity, forwardSpeed);
         speed = transform.rotation * speed;
         cc.Move(speed* Time.fixedDeltaTime);
 
     }
 }