Wheel script

using UnityEngine;
using System.Collections;

public class Wheel : MonoBehaviour {

    float spinAngle = 0;

    // Update is called once per frame
    void Update () {

        // assume the wheel art (the hub) is the only child
        Transform hub = transform.GetChild(0);

        // get wheel collider reference as correct type:
        WheelCollider wheel = collider as WheelCollider;

        // *6 converts RPM to Degrees per second (i.e. *360 and /60 )
        spinAngle += wheel.rpm * 6 * Time.deltaTime;

        // rotate hub according to spin and steering
        hub.localRotation = Quaternion.Euler( spinAngle, wheel.steerAngle, 0 );

        // use a raycast to detect current spring position
        RaycastHit hit;
        if ( Physics.Raycast ( transform.position, -transform.up, out hit,  wheel.suspensionDistance + wheel.radius ) ) {
            // wheel is touching the ground
            float springPos = -(hit.distance-wheel.radius);
            hub.localPosition =  Vector3.up * springPos;

        } else {
            // wheel is off the ground, use full spring distance
            hub.localPosition =  -Vector3.up * wheel.suspensionDistance;

        }

    }
}