Kurt Sperry wrote:
> As steel is generally considered essentially immune from
> fatigue within its elastic limit in real world conditions, I
> would be surprised to see a properly built and used (only
> deformations within its elastic limit) 753 frame last any
> less time than an equivalent 531 frame of thicker gauge pipe.
Kurt, that limit that stresses need to stay below, for the steel part to last forever, is called the endurance limit. This is substantially lower than the elastic limit. No doubt some engineer here will know the real-world percentage for steels of this type but I think it's less than half - probably much less. Stresses near the elastic limit will cause fatigue in a VERY short time. Luckily no normal rider would be able to induce stresses in that range by normal riding.
Due to the pressure to build racing frames as light as possible, the endurance limit is quite often surpassed, especially by heavy/strong risers, and especially at stress risers. As you may know, stress risers are geometric discontinuities, such as an abrupt change in section thickness, sharp-edged holes and edges etc. Stresses there can be many times the average stress on the part. All bike frames have 'em to some degree, no matter how much care is put into the build. Also, all 753 frames are weakened where the tubing was tempered by the heat cycle of silver brazing - no matter how good the brazer. (753 should be left stronger than 531 of the same thickness even after brazing, but 531 is usually thicker)
Even top-notch skill and care on the part of the framebuilder cannot ensure that a 753 frame will last forever, and in fact many of them have broken. This is not to say 753 was bad tubing or that it failed in its purpose. I believe that was to allow a lighter frame for light riders and/or those who don't mind that it might need to be thrown away after a while, if ridden long/hard enough to cause fatigue cracks. Remember, many frames with heavier tubing have cracked too - even when carefully built by skilled builders - so this is not just a problem with 753. Some riders can break almost anything!
Still, for SOME of us who don't pedal too hard or ride such long distances, 753 can turn out to last a lifetime. It may even have lasted a long, long time for Andy Hampsten -- but for him to have a fatigue crack while racing would have been disastrous publicity, so Merckx couldn't risk it. Much cheaper to keep Andy in fresh frames.
I know two people who were able to buy Andy's cast-offs, and knowing how cautious Eddy was, these are probably a good investment -- very unlikely to break for a typical non-racer. And when they do, the crack typically grows slowly and gives lot of warning, at least to someone who cleans the frame now and then and looks for cracks.
Seattle WA USA