Originally Posted by
OLDYELLR
But most motorcycle frames are made of what is known as gaspipe in bicycle circles. That stuff could be arc welded with no problems, just like the cheapest bicycles. (Well, okay, there were some racing frames made of chrome moly, but it was still welded and I never heard of 0.5mm thick double butted tubing being used to build motorcycle frames.) Okay, today there is lightweight high alloy steel that can be TIG welded, but those joints look like crap compared to lugs or fillet brazing. So today you have KOF (Keeper of the Flame) bikes that are lugged or fillet brazed because that's the traditional way and looks cool, not like a frame TIG welded on a robot production line in China.
In 1971 I was at the local motorcycle dealer, and he had the latest Penton Motocross & trials machines with Reynolds 531 transfers on the frame and swing arm!
Its a question of money.
An interesting aside, in the aircraft industry, 4130 needs to be welded, not brazed. but also required post weld heat treatment for engine cradles and mounts, usually done mounted onto huge cast iron fixtures to keep from heat distortion, that after a number of cycles need to be replaced. Brazing with brass is not allowed due to the possible creation of micro cracks where the brass infuses in the steel grain structure and cools at a different rate, creating a surface layer of micro cracks that can grow.
Why fillet brazed bike frames don't fall out of the sky (pun intended) all the time I do not know. But, Reynolds 531 is a bit different than Columbus of the day, columbus being closer to 4130.
I do agree with the other previous poster that a tigged steel frame is probably heavier than a lugged one, or can easily be. The weight of thicker butted ends & head tube wall thickness coupled with seat tube sleeves (I forgot to count the seatpost binder collar)