Keep rotation when changing parent in Unity 3D

Questions & Answers
1

Hello, I’m trying to implement arrows. Every frame the arrow changes its rotation depending on its velocity, but when it hits something and I try to attach the arrow to the object rotation goes wrong. I’m sure that the problem occurs when changing parent and I tried everything without result. This is the script:

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Arrow : MonoBehaviour
{
    private Rigidbody rb;
    private bool hit;
    private Quaternion startingRot;

    void Start()
    {
        rb = GetComponent<Rigidbody>();
        hit = false;
        startingRot = transform.rotation;
    }


    void FixedUpdate()
    {
        if (!hit && rb.velocity.magnitude>0)
        {
            transform.localRotation = Quaternion.LookRotation(rb.velocity);
        }
    }

    private void OnTriggerEnter(Collider other)
    {
        hit = true;
        rb.constraints = RigidbodyConstraints.FreezeRotation;
        transform.SetParent(other.transform, true);
        rb.isKinematic = true;
        rb.velocity = Vector3.zero;
    }

}
2

This was an interesting problem (the video-clip was a very good demonstration). It seems like you can’t re-parent a rigid body without getting some weird behavior. I tried several things, like disable the rigid body and re-parent first in the next frame, and so on, but it still didn’t work.

The only solution that works for me is to remove the rigid body before re-parenting.

        Destroy(rb);
        transform.SetParent(other.transform, true);

If you don’t like the idea of removing behaviors in run-time you could have a second copy of the arrow, without the rigid body and simply replace the original arrow with the copy.

Good hunting!