RE: [CR]Front end impact question


Content-class: urn:content-classes:message
Subject: RE: [CR]Front end impact question
Date: Sun, 22 Jan 2006 23:11:40 -0800
Thread-Topic: [CR]Front end impact question
Thread-Index: AcYfzQr+Yi1M4sT2R1+qbCxx8lKB9AAHAlWQ
From: "Mark Bulgier" <Mark@bulgier.net>
To: "David Bilenkey" <dbilenkey@sympatico.ca>, <classicrendezvous@bikelist.org>


David Bilenkey wrote:
>
> I have a 1972 Bevilacqua racing frame that I was given a few years ago.
> I got around to examining it closely today. I noticed the slightest
> bulge under the TT and DT just behind the head lugs and
> paint cracks up top.
> [snip] Anyone have any different
> thoughts about how safe this frame would be to ride?

I have personally broken three frames that came to me with crash-bulges in them. For commuting and training (in my racing days) I was not picky and was a bit of a dumpster-diver, so I rode frames that others had given up on. I liked the steeper head angle and shorter front-center better than the original geometry better anyway.

All three fatigue cracked right at the bulges, no coincidence I'm sure. But they all took a lot of miles to break. I kept riding them after I found the cracks. I'm not THAT cheap, really, but I just wanted to learn how they would eventually break and how long it would take. It took a really long time! Maybe a thousand miles from when I first saw the crack at a few millimeters, until I was finally able to part the downtube in two by bunny-hopping and nose-wheelies. You'd be impressed at how difficult it was to get that last cm or so of uncracked tube to part! (This is a composite memory of three frames but I seem to recall that their deaths were all remarkably similar.)

I'm not saying I recommend riding cracked frames. Just pointing out that the failure mode of steel frames that have been buckled is usually very mellow and slow - tons of warning. But they* will eventually break - their fatigue life has been severely shortened in the vicinity of the bulge. (*Speaking of lightweights here - Varsities definitely will, and UO-8s probably will run forever with bulged tubes.)

Just conjecture here, but I don't think it is the bulge per se, or even the fact that the crash exceeded the steel's yield strength. I think it is the stresses that are locked into the steel after the event. I'm betting that if the frame could be stress-relieved, the chances of fatigue cracking there would go way down. I don't know anyone who can do that in a controlled manner though. 'Twould be more expensive than a crashed frame is worth anyway.

Mark Bulgier
Seattle WA USA