Originally Posted by
CliffordK
I think someone said that Peugeot used some kind of internal lug on some of their bikes. It is difficult to visualize from the outside, but likely could be seen if one removes the headset, or bottom bracket.
The bike above does have strengthening rings brazed onto the head tube.
Peugeot used a lugless, internal brazing process, whereby preformed doughnuts of brazing material were placed inside the frames, at a tube joint. Whem heated, the doughnut melted and capillary action drew some of the brazing material through the mitred joint. A large fillet formed on the inside, with a smaller fillet on the outside. The advantage of this system was that the joint quality could be dtermined by visually examining the parameters of the external fillet. With lugs, either external or internal, there's no convenient way to determine joint quality. Peugeot also cliamed that it wasd stronger and lighter than using lugs. Motobecane/MBK used a simlar process which they called Inexternal Brazing. I believe that Gitane also used a similar process.