Andrew R Stewart 

[QUOTE=shahidbashir92;22143146]In the case of bike frame lugs, the brazing is under sheer forces only - as the tubes are inserted into the lug. The joint will be as strong as the metal of which the lug is made. If you used the same brazing material in place of a weld, it would fail - because its now subject to variety of forces (pulling apart and sheer).[/QUO

Not quite right. This statement speaks to tensile strength of fillers with little understanding of what a well joined joint consists of. A well made joint has the filler join the two tube's contact surface, the miter face. Whether it's a lug, a fillet or a weld if the two tubes don't touch (or have the filler preferred gap) the joint is compromised. Now in addition to proper tube miter set up is how much surface area of filler is needed to be "strong enough" (and any greater strength is excess, although "enough" strength is a debatable aspect). Silver (as in the common 56% filler usually referenced) is real strong with the right gap but looses that level quickly as the gap increases (pretty much scant fractions of an inch from the crotch of a tube to tube contact). So greater surface at a close gap is needed to attain enough strength. Brass/bronze has a lower ultimate tensile strength but retains that lever far further from the crotch of a joint. (The rule I learned was the root thickness of a fillet should be 3 or 4 times the tube wall thickness). Welds have the highest tensile strength but have greater base material strength changes happen right at the crotch (and where the stresses are focused).For the quoted statement to apply the lugged joint would have to have no tube to tube contact or pretty poor penetration of the filler, never getting deep into the joint enough to flow between the tube contact area.

Any type of joining can be designed to be stronger or weaker. Gaps, joint prep, filler amount, filler surface area contacting the tubes, penetration can all be jiggled to suit the fillers and the skills of the builders. Tube surface prep, purging, second heating cycles (like a fillet pass on welds) further affect stuff.

I tend to dismiss this type of discussion because well done joints of any method are fine and last a long time AND the discussion is instead one of preference and aesthetics, rarely one of actual joint engineering.

Not yet mentioned is how joints fail (well one mention of welds cracking was made). In the early/mid 1980s Bicycling mag had an article about this stuff. They had an experienced builder fab samples of three joining methods. Silvered lugs, brass fillets and steel welding. They had a lab test various aspects of the resulting structure (remember that? We are talking about riding bikes and not make believe models). Hardness of the tube at different distances from the crotch (which was claimed to be a pretty good reference for after joined strength). They applied bending forces enough to cause each joint method to fail and noted the progression of the failure, where along the tube/proximity to the crotch the plastic deformation happened. Base material annealing and HAZ was discussed. The conclusions were that all methods fail but with different manors and all can be more then strong enough.

In real life of making frames (from steel) these joining methods chosen are much more about cost, builder's skills, marketing aesthetics then theories. Like has been said each type of joining can be done really poorly or well and each can be more then enough. Still a rider's ability to stress a bike can exceed the strength of a bike they actually want to buy and ride.

BTW we haven't brought up bonded steel frames like the Technium Raleighs or the die cast junctions of the Kabukis. Andy