Originally Posted by
Kuromori
Of course it is possible to have an entirely inadequate lug for pure lugged construction without any butt brazing, and with a well mitered joint, it doesn't matter, as the lug has largely been made redundant, which is good practice, and I'm not advocating anything contrary to that. As I noted, you don't need miters with the right lugs, and clearly a half mm bikini lug is of questionable strength and not the right lug for that.
I don't want to speculate too much on inferior methods, but the wall thickness of the lug tubes are thick enough to be a butt on a cycle tube, presumably the miters are tight on the lugs and have good strength, and there's a large brass fillet to help reduce stress risers at the miter. The amount of steel-to-steel contact seems to be in the range of sufficient at any point along the joint. The concern of a stress riser where the laminated structure effectively thins due to a poor miter is conveniently reinforced by a large fillet. I hesitate to draw any conclusions, since of course, there is no good reason to risk trying to make an intentionally inferior joint.
I just wanted to point out that, in my opinion, the "truism" is more about making the lugs largely redundant and irrelevant when it comes to joint integrity (yet still useful as a reinforcement like a gusset and for holding tubes in place), but isn't really true as a general rule for lugged joints, otherwise we'd all have serious problems with our lugged BB shells and chainstays which have exactly zero tube-to-tube brazing.
So true. I'm sure most anyone who's done frame repairs has seen horrible mitering and gaps/voids in even some high-end hero brands.
Without a good miter and thorough brazing, the lug becomes the only structure there at the joint, and they're often made of **** steel and methods. I have seen cracks in lugs that could only happen if the underlying miter and/or brazing were substandard. But other lugs are up to the task and don't require the tubes to be joined inside.
I guy who worked at Bianchi's Reparto Corse told me this story, about when they were making Gianni Bugno's new frame. This was when he was reigning World Pro Road Champion, so 1991 or '92. The guy who sets up the tacking jig has no CAD drawing, not even an excel spreadsheet, he just sort "knows" what angle the DT miter needs to be for this frame size. The mitering guy takes a stab at making a DT, oops a little too long, cut it shorter. Second try and the DT is now a little too short. No problem, let's not waste a valuable steel tube, I'll just adjust the jig to fit. The miter angle is off, leaving a gap. For the reigning World Champion. They didn't know what the BB height would be, or whether the TT would be precisely level, but don't worry, he'll ride whatever we give him.
Perfect miters and thorough brazing with an internal fillet all the way around are selling points for artisan builders, but they most add to the warm feeling the owner gets when he thinks about them. Not anything a rider can detect by riding. Except in those rare cases where the joint breaks -- you can feel that!
