Clamp Bike Rotation

Hello! This is about my second week of using Unity, but I've started a game for my friend. My dirt bike model is falling because it is rotating so much. (this happens only when you go on a ramp, it is a long fall and you rotate alot). Here is my steering code:

var frontWheel1: WheelCollider;
var frontWheel2: WheelCollider;
var backWheel1: WheelCollider;
var backWheel2: WheelCollider;
var backWheel: Transform;
var frontWheel: Transform;
var power: int;
var steerPower: float;
var sharpPower: float;
//var steerEquiv: float;
var brakePower: float;
var newCenter: Vector3;
function Update () {
    backWheel1.motorTorque = backWheel2.motorTorque = (-Mathf.Clamp(Input.GetAxis("Vertical"),-1,1)*power);
    backWheel1.brakeTorque = backWheel2.brakeTorque = (Input.GetAxis("Vertical") == 0 ? brakePower : 0);
    //backWheel.Rotate(0,(backWheel1.steerAngle+backWheel2.steerAngle)/(2*steerEquiv),0);
    //frontWheel.Rotate(0,(frontWheel1.steerAngle+frontWheel2.steerAngle)/(2*steerEquiv),0);
    frontWheel1.steerAngle = frontWheel2.steerAngle = (Input.GetButton("Sharp Turn")) ? sharpPower : steerPower * Input.GetAxis("Horizontal");
}
function Start(){
    rigidbody.centerOfMass = newCenter;
}

How would I clamp its Z rotation?