There is usually a turnbuckle of some kind which allows some fine tuning of the spring force on the door.

I agree that these springs have a tendency to fail every so often. I've already found one of mine snapped in two. If they are anchored on both ends then there is not much chance of the spring going into orbit. However, if the spring is just hooked around the turnbuckle, a broken section could go for quite a ride.