Originally Posted by commander_taco
I still have one unaswered question: How come lugs are stronger? How can a bond between steel and bronze be stronger than the bond between steel and steel? It just does not work that way owing to the metallurgical differences (also note that steel and bronze do not react). For the record, solder is weaker than a weld. Lugs may be strong enough for bicycle application (altough anything is debatable in the absence of hard data), but I like to hear an explanation as to *why* lugs are stronger than welds. It is easy to make an assertion without proof...I guess you see my point.
thanx.
When you use lugs the 2 surfaces of the steel are bonded with brass or silver. it the case of a welded frame the two points of the steel become one via a bead of steel.
the major difference between the two is the size of the area where the 2 pieces of the joint.
In the case of a lug, the tube and the lug have a large area where they overlap, so stresses on the joint are spread through this area.
In the case of a weld, the the width of the joint, and the small overlap of the 2 tubes (basically 2 thin edges) make it more susceptible to stress fractures. Also, the act of welding itself with its high heat melts part of the tubes and changes the molecular structure of the steel (some time can be fixed with heat treatment).
so strength-per square inch of a weld is probably much stronger then a brazed lug, but in a bike application the lugs are stringer..
However, when you use lugs, especially for a custom bike you will be limited by the geometry of the lugs. Lugs are made in different angles and it gives you enough flexibility to build almost any bike. But if you are anal about the geometry you can use welds to achieve any angle you want just by cutting the tubes accordingly.
that said, I have ordered a custom bike made with lug. It is more expensive, maybe a few grams heavier, and maybe half a second off the "perfect" geometry, but IT LOOKS SO PRETTY..
