Rigidbody.centerOfMass  

var centerOfMass : Vector3

Description

The center of mass relative to the transform's origin.

If you don't set the center of mass from a script it will be calculated automatically from all colliders attached to the rigidbody. Setting the center of mass is often useful when simulating cars to make them more stable. A car with a lower center of mass is less likely to topple over.

Note: centerOfMass is relative to the transform's position and rotation, but will not reflect the transform's scale!

rigidbody.centerOfMass = Vector3 (0, -2, 0);

using UnityEngine;
using System.Collections;

public class example : MonoBehaviour {
    void Example() {
        rigidbody.centerOfMass = new Vector3(0, -2, 0);
    }
}

import UnityEngine
import System.Collections

class example(MonoBehaviour):

    def Example():
        rigidbody.centerOfMass = Vector3(0, -2, 0)