Anti-roll bar doesn't work

I'm making a racing game that has trucks as vehicles. For the truck movement, I'm using WheelColliders. After some testing, I found out that after a certain speed (~60km/h), turning made the truck flip to the side and hit the ground.

I found an Anti-Roll script and applied it to the truck, but the truck still flips but after a higher speed (~90km/h) and its physics seem a bit unstable.

Hhow can I prevent the truck from flipping like that? Here are the scripts I'm using:

Truck movement script:

using UnityEngine; using UnityEngine.UI; using System.Collections;

public class WheelTest : MonoBehaviour {

 public WheelCollider WheelFL;
 public WheelCollider WheelFR;
 public WheelCollider WheelRL;
 public WheelCollider WheelRR;
 public Text txt;
 public float speed;
 public AudioSource EngineSound;
 
 public GameObject COM;
 
 public float steerMax = 8f;
 public float motorMax = 30f;
 public float brakeMax = 60f;
 
 public float steer = 0f;
 public float motor = 0f;
 public float brake = 0f;

 // Use this for initialization
 void Start () 
 {
     COM.transform.position = rigidbody.centerOfMass;
 }
 
 void Update () {
 int speedDisp = Mathf.RoundToInt(speed);
 txt.text = speedDisp + "km/h";
 
 EngineSound.volume = 0.5f + speed/50;
 EngineSound.pitch = 0.75f + speed/75;
 }
 
 // Update is called once per frame
 void FixedUpdate () 
 {
     steer = Mathf.Clamp(Input.GetAxis("Horizontal"), -1, 1);
     motor = Mathf.Clamp(Input.GetAxis("Vertical"), 0, 1);
     brake = Mathf.Clamp(Input.GetAxis("Vertical"), -1, 0) * -1;
     
     speed = transform.InverseTransformDirection(rigidbody.velocity).z * 3.6f;
     WheelRL.motorTorque = motorMax * motor;
     WheelRR.motorTorque = motorMax * motor;
     WheelRL.brakeTorque = brakeMax * brake;
     WheelRR.brakeTorque = brakeMax * brake;
     WheelFL.steerAngle = steerMax * steer;
     WheelFR.steerAngle = steerMax * steer;
     
 }

}

Anti-Roll Bar:

#pragma strict

var WheelL : WheelCollider; var WheelR : WheelCollider; var AntiRoll = 5000.0;

function FixedUpdate () { var hit : WheelHit; var travelL = 1.0; var travelR = 1.0;

 var groundedL = WheelL.GetGroundHit(hit);
 if (groundedL)
     travelL = (-WheelL.transform.InverseTransformPoint(hit.point).y - WheelL.radius) / WheelL.suspensionDistance;
 
 var groundedR = WheelR.GetGroundHit(hit);
 if (groundedR)
     travelR = (-WheelR.transform.InverseTransformPoint(hit.point).y - WheelR.radius) / WheelR.suspensionDistance;
 
 var antiRollForce = (travelL - travelR) * AntiRoll;
 
 if (groundedL)
     rigidbody.AddForceAtPosition(WheelL.transform.up * -antiRollForce,
            WheelL.transform.position); 
 if (groundedR)
     rigidbody.AddForceAtPosition(WheelR.transform.up * antiRollForce,
            WheelR.transform.position); 
 }