I'd say get a heavy duty chain, horse lead rope, or a wire rope. They all will hold much more force then your dog can dish out. We once had a Lab/Golden Retriever mix that liked to chase cars. One time she ran all the way up the highway because she would always break free of her collar/leash so I kinda know your situation. But then we got the right kind that would hold her. We had to give her up before our move because where we were moving doesn't allow dogs (although you come here and everybody owns one) and the new owner (who was my mom's friend) stupidly let her loose in their yard even though my mom told her that she chased cars and not to let her loose, and she got hit by one and died. Just thinking about it makes me want to cry. Princess was the best dog anyone could have. So please, don't let this happen.