Rigidbody with multiple colliders

adding colliders will change the center of mass and rotation behavior of a rigidbody. To set those manually to the center of the object use following code. This is a must setup rule for unity 5.x+ project with mass drag or vehicles game play. This may not a bug for unity 3.x / 4.x project.

void Start()
{
    rigidbody = GetComponent<Rigidbody>();
    rigidbody.centerOfMass = Vector3.zero;
    rigidbody.inertiaTensorRotation = new Quaternion(0, 0, 0, 1);
}

void Start()
{
    rigidbody = GetComponent<Rigidbody>();
    rigidbody.centerOfMass = Vector3.zero;
    rigidbody.inertiaTensorRotation = new Quaternion(0, 0, 0, 1);
}

Ref: https://forum.unity.com/threads/rigidbody-with-multiple-colliders.524352/