Here is an article that goes into some detail about this type of fabrication per the Schwinn high end bikes.

Fillet-Brazed Schwinn Bicycles 1938-1978

I personally build this way as I do custom angles and such but for traditional the luged build is good. mostly for the motored bike we need the tubing not to be too thin. The top end bikes usually use tubing that is very thin to save weight. You can get away with this when you have a weak motor like a human being. An engine has at lest 3 times the power and many times the vibration of a person. This translates to a falling apart bike if it is not made stronger than a light weight racer or a department store junko bike. To be safe a 1960 to 1990 vintage touring or mountain bike will usually have strong enough tubing especially if it is Chromium Molybdenum, or cro moly steel tubing Aluminum and carbon fiber fatigue and break easily compared to cro moly steel which is a spring steel. Now if you have some bucks get a titanium fabricator to make a titanium frame to motorized bike standards and you will maybe have the most indestructible frame going.

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LennyHarp of Lenny's Bikes & Things
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