Originally Posted by
FBinNY
The downtube and lower head lug are in tension. When they fail, the front end drops down and pushes the front wheel forward. What happens exactly depends on the strength and ductility of the top tube, and strength of the top/head joint.
From first hand experience, I've seen one where the top tube held and bent upward in a way that the rider could control the bike and avert a crash. Note that front wheel braking in this instance would improve the conditions.
I had an old Murray Eliminator that had that happen while my brother was riding it. The front end lowered and the front wheel went forward, and he skidded to a stop when the pedals started hitting the ground. Didn't crash, thankfully.
My dad had a friend of his with a welder bend the headtube back into position and re-weld the bottom tube in place. It held for the remainder of the bike's life when a few years later one side of the handlebar snapped off. That one didn't end as pretty.