there's something strange about the car

hello everyone, i'm new to unity, when i try to drive the car forward, it runs fine, but when the other way around, the car seems to want to flip over, i just want the car to go backwards, you can see this picture:

 if (Input.GetKey(KeyCode.UpArrow))
         {
             rigidbody.AddForce(new Vector3(transform.forward.x, 0, transform.forward.z) * 5, ForceMode.VelocityChange);
         }
         if (Input.GetKey(KeyCode.DownArrow))
         {
             rigidbody.AddForce(new Vector3(transform.forward.x, 0, transform.forward.z) * -5, ForceMode.VelocityChange);
         }

when i press down arrow, the car does not reverse, but wants to overturn, please help thank you