How to access wheel collider in a child object in Unity

Question

In Unity, my player gameObject has 4 wheels and all of them contain wheel colliders. I want to get the rpm of these.

I write this code, but at line 33 it is throwing index/argument out of bound exception, which means the index it's trying to access with 'GetChild(i)' is not accessible. line 33 is

 wColliders[i] = gameObject.transform.GetChild(i).gameObject.GetComponent<WheelCollider>();

My player has 5 child objects and 4 of them have a wheel collider. how to solve it?

```
public class PlayerController : MonoBehaviour
 {

    //[SerializeField] private float speed = 10f;
    [SerializeField] private float speed;
    [SerializeField] private float rpm;
    private float totalRpm;
    [SerializeField] private float horsePower;
    [SerializeField] private float turnSpeed;
     public float horizontalInput;
     public float forwardInput;

    [SerializeField] private GameObject centerOfMass;
    [SerializeField] TextMeshProUGUI speedometerText;
    [SerializeField] TextMeshProUGUI rpmText;
    
    List<WheelCollider> wColliders;
    private Rigidbody playerRb;
    // Start is called before the first frame update
    void Start()
    {
        playerRb = GetComponent<Rigidbody>();

        playerRb.centerOfMass = centerOfMass.transform.position;

        wColliders = new List<WheelCollider>();
        for(int i = 0; i < 4; i++)
        {
            wColliders[i] = gameObject.transform.GetChild(i).gameObject.GetComponent<WheelCollider>();
        }
    }

    // Update is called once per frame
    void FixedUpdate()
    {
        horizontalInput = Input.GetAxis("Horizontal");
        forwardInput = Input.GetAxis("Vertical");

        playerRb.AddRelativeForce(Vector3.forward * horsePower * forwardInput);
       
        transform.Rotate(Vector3.up, Time.deltaTime * turnSpeed * horizontalInput);

        speed = playerRb.velocity.magnitude;
        speed = Mathf.RoundToInt(speed);
        speedometerText.SetText("Speed: " + speed);

        for(int i = 0; i < wColliders.Count; i++)
        {
            totalRpm += wColliders[i].rpm;
        }

        rpm = Mathf.Round(totalRpm / wColliders.Count);

        rpmText.SetText("RPM: " + rpm);
    }
}

Answer 1

You get the IndexOutOfBoundsException not because Unity doesn't find the child but rather because you try to access/set

wColliders[i]

of an empty list. There are no items added yet so you can not access them via the index.

---

You would rather use List<T>.Add

for(var i = 0; i < 4; i++)
{
    wColliders.Add(transform.GetChild(i).GetComponent<WheelCollider>());
}

---

Or actually rather simply use GetComponentsInChildren

wColliders = GetComponentsInChildren<WheelCollider>(true).ToList();