Broken Wheel Stud Our Newell That Hurt

[Twins]

This kind of goes hand in hand with what I was told about changing studs with aluminum wheels. I switched to aluminum wheels, but didn't change the studs because there was a couple of treads showing with the nuts tight. I can't understand how an extra 1/4" of thead sticking out is going add strength to the stud. That said, I broke a front stud this morning. I did get stuck in a lane with raised manholes, and hit a couple hard. Can someone enlighten me as to why the studs are weakend if not enough treads are showing?What is the difference if two threads are showing or a 1/4" of them? The threads go all the way to the hub, there is no shoulder. Do the longer studs have one? I learned a valuable lesson and it will never happen again.