Thanks for your comments Mike. I may not have made that clear. The point was that the margin of the heat affected zone, which represents a discontinuity in the strength of the tubing (stronger on the farside than the nearside), when it coincides with the lug point, multiplies the chance of fatigue failure at that point. That discontinuity, by itself, will not likely cause a fatigue failure, but when you put it right where the point of the beer can opener is, it can. Putting it out beyond the lug point (but still in the butted part of the tube) would seem to be a good thing. There is no way to avoid having a discontinuity or you would have a cold joint (no bonding of the materials). And remember, in the test, the strongest joint, requiring the most force to buckle, was the one done with the rosebud torch to evenly preheat the material, and brazed quickly with brass. Silver and brass brazed only with a small torch and no preheating required about the same force to buckle.
And the point of shining the lugs was not to clean them. They are pretty darn clean right out of the box, and are degreased in solvent prior to use. It was to break the surface layer which may be weak. Remember, Bill observed a thin layer of metal attached to the brazing material, meaning it is not a dirty braze that is coming apart but a shear layer in the lug material. Could be only a few molecules thick. And that observation was not a result of our test but of seeing bikes that had been crashed and trying to figure out why the lug sometimes separated. Some lugs have a tight fit out of the box. Too tight in some cases to allow full penetration with even silver. The amount of abrasion needed to shine them will not increase the gap enough to make any kind of brazing a problem. I am sure every good framebuilder shines up his tubing and lugs before assembly. I just looked at a Henry James seat lug that we have already shined and it is still so tight you can barely get it on the tube. That tube will be dressed a bit more to provide the proper gap.
I will bow out of this discussion now as I don't have much more to add. We don't want to bore these poor guys silly. They want to talk about nice vintage bikes (Wait til you see pics of the Paris I am restoring!). But thanks for the opportunity to share our experiences. Anyone is welcome to ask questions off-list.
Bob Freeman Elliott Bay Bicycles 2116 Western Ave Seattle, WA 98121 206-441-8144 Home of Davidson Handbuilt Bicycles
In a message dated 9/27/2005 1:57:44 P.M. Pacific Standard Time, firstname.lastname@example.org writes:
Ouch....I think there be a logic problem here...
The claim that the stress riser being lower down improves fatique life does not follow. Where the weakest point of the joint is may well become irrelevant if all parts of the joint are now much less prone to fatique failure than they would be with brass brazing.
Again, it comes down what others have said - it is not what methods or materials are used, but rather how appropriately the material of choice is used in the application. And the mention that after testing it was decided to "shine" the lugs seems to confirm that the lugs may not have been really "clean" on the failed joints. Of course, too much "shining" (by use of a reamer or grinding) and the tolerances of the lugs could be shot with respect to use of silver braze.
Mike Kone in Boulder CO