Isn't the key distinction here the magnitude of the bulge? The original post states the "slightest of bulges". My guess is that frames with a very slight bulge might never fail, while others with more extreme bulges are doomed to die if ridden hard. I've seen a number of frames with bulges that the owners never saw, that have been ridden for years and years without incident.
Now when a crack starts (not just in the paint) that is the kiss of death. But the scarry thing is that if we all took a look at some of the bikes we owned really carefully, we would find things that we would rather not think about. And if you don't find distortion on many old frames from a crash, you may find distortion from overheating. That doesn't give much confidence either.
Now the other thing to factor in is how the frame was built. A frame brazed at high temperature looses ductility and becomes more brittle - such a frame I suspect is much more likely to fail down the road. And a frame that has been brazed with silver and retains its ductility? Perhaps much less of an issue.
Mike Kone in Boulder CO
>
\r?\n> David Bilenkey wrote:
\r?\n> >
\r?\n> > I have a 1972 Bevilacqua racing frame that I was given a few years
\r?\n> ago.
\r?\n> > I got around to examining it closely today. I noticed the slightest
\r?\n> > bulge under the TT and DT just behind the head lugs and
\r?\n> > paint cracks up top.
\r?\n> > [snip] Anyone have any different
\r?\n> > thoughts about how safe this frame would be to ride?
\r?\n>
\r?\n> I have personally broken three frames that came to me with crash-bulges
\r?\n> in them. For commuting and training (in my racing days) I was not picky
\r?\n> and was a bit of a dumpster-diver, so I rode frames that others had
\r?\n> given up on. I liked the steeper head angle and shorter front-center
\r?\n> better than the original geometry better anyway.
\r?\n>
\r?\n> All three fatigue cracked right at the bulges, no coincidence I'm sure.
\r?\n> But they all took a lot of miles to break. I kept riding them after I
\r?\n> found the cracks. I'm not THAT cheap, really, but I just wanted to
\r?\n> learn how they would eventually break and how long it would take. It
\r?\n> took a really long time! Maybe a thousand miles from when I first saw
\r?\n> the crack at a few millimeters, until I was finally able to part the
\r?\n> downtube in two by bunny-hopping and nose-wheelies. You'd be impressed
\r?\n> at how difficult it was to get that last cm or so of uncracked tube to
\r?\n> part! (This is a composite memory of three frames but I seem to recall
\r?\n> that their deaths were all remarkably similar.)
\r?\n>
\r?\n> I'm not saying I recommend riding cracked frames. Just pointing out
\r?\n> that the failure mode of steel frames that have been buckled is usually
\r?\n> very mellow and slow - tons of warning. But they* will eventually break
\r?\n> - their fatigue life has been severely shortened in the vicinity of the
\r?\n> bulge. (*Speaking of lightweights here - Varsities definitely will, and
\r?\n> UO-8s probably will run forever with bulged tubes.)
\r?\n>
\r?\n> Just conjecture here, but I don't think it is the bulge per se, or even
\r?\n> the fact that the crash exceeded the steel's yield strength. I think it
\r?\n> is the stresses that are locked into the steel after the event. I'm
\r?\n> betting that if the frame could be stress-relieved, the chances of
\r?\n> fatigue cracking there would go way down. I don't know anyone who can
\r?\n> do that in a controlled manner though. 'Twould be more expensive than a
\r?\n> crashed frame is worth anyway.
\r?\n>
\r?\n> Mark Bulgier
\r?\n> Seattle WA USA