how will this frame fail?
I've recently found a bubble/ripple in my head tube, just below the upper lug. It's accompanied by a slight bend in the headtube. Picture below. It's 531 from 1979 or so. There are no other bubbles/ripples/bends/cracks or any other sign of damage to any other tube.
I took this to two different shops. The mechanic at one shop told me that the tube would eventually crack, ruining the frame, but the failure would not be catastrophic in a crash-causing way. The mechanic at the second shop took one look at the bike and told me that the bubble has "nullified the strength of the lug" so that the top tube will completely separate from the head tube at any moment, pitching me onto my face. He told me I should not even ride the bike home. I asked him for some clarification, but couldn't find out anything more what he meant (the shop was near closing, and I think he didn't want to have a longer conversation). I don't really understand how the bend in the head tube would have "nullified" the lug - what would that mean?
I have no doubt that this tube will continue to fail, but my question is, what should I expect as it does? A sudden catastrophic failure in which the tubes separate, or a gradual bending and cracking?
Thanks for any insight you can give!
There is really no upside for a mechanic to tell you it is safe to ride when there is a visible problem.
I don't see the doomsday scenerio depicted by the second mechanic happening. As long as the ring around the top headset cup remains intact, that can't happen. Even if it is a pressed and welded lug, the head tube is brazed into it, so I can't see the lug coming apart.
If the head tube breaks around where it is bulged, the steerer and headset will hold it somewhat togethor, and the top tube will not seperate from the head tube. That may cause enough binding in the steering and general squirrelyness that you ultimatly crash, especially if you are riding near a curb or similar tight situation where you don't have a lot of room to sort things out, but there is a also a fair chance that you just notice some mild changes in handling and finally realize the head tube has fractured.
The steerer is going to keep the two parts of the HT aligned enough that I also can't see this failure domino-ing to to other joints quickly.
I'm an engineer and a gearhead, but not a framebuilder, BTW. Also I put several thousand miles on my old Nishiki with worse issues than that after hitting a curb.
ETA: The bulge is most likely the result of overheating/annealing when it was brazed. The bad news is that it can make the steel soft and weak. The good news is that softer steel is less likely to crack suddenly.
I got the exact same type of bends in my Bianchi Volpe and kept riding it. I lasted a few more weeks 'til I had to make a hard stop. The frame and front forks flexed enough to let the front tire hit the down tube and instantly over the bars I went. If you do not want to debraze all the lugs, replace the bad rubes and rebraze it all stop riding it before you get hurt.
That's a very strange deformation. It almost looks like the lower portion of the headtube is bent back a little, but then I would think there would be problems with the downtube as well. What does does the downtube look like? Does the fork look normal?
Thanks for the feedback and advice.
It is a strange deformation, and I'm not sure what the cause is. When looking at the entire bike, it looks more like the top of the head tube is bent back, rather than the bottom, but of course there's no way to distinguish (I suppose if I knew the exact original ht angle, and had a protractor . . .) The top and down tubes are absolutely fine as far as I or anybody else can see or feel. No rippling or bending. The fork is also fine. The bend seems to have developed very slowly. Perhaps a weakness developed in that spot on the head tube.
I've shown it to a few more mechanics, all of whom agree with the first mechanic - it will continue to fail, but the headset and lug will hold it in place. I'm looking at having it replaced and rebrazed, or buying a new frame. But I'm still hoping to understand how this happened.
It looks like the lug is coming loose also.