rgppgr

I'm looking for information as to why a weld is put on the bottom of my bottom lug shell that runs the width? Thank you.

wesmamyke

Not uncommon on entry level bikes to have seamed tubing used in the frame. Meaning it was made of flat sheet steel, rolled into a tube, and welded of course.

The bottom of the bottom bracket shell is the one place they can hide that, and make no effort to clean up the weld.

JohnDThompson 

Many bottom bracket shells, particularly on low to mid-level bikes, are bulge-formed: they start as a sheet of metal, bulges are pressed into the sheet to fit the tubes, the sheet is rolled into a cylinder and welded along the seam, the ends of the bulges are machined off to make an opening for the tubes, and then threads are cut into the cylinder for the bottom bracket cups

rgppgr

So how then are the chain stay, down and seat tube openings attached to the shell? I can see no seam that's been welded. Although it looks like the inside has been machined. So I guess the thread material was put in after the shell was machined then threaded.

Andrew R Stewart 

One way is to start with a flat sheet of steel. This sheet can be pierced through with a mandrel in specific locations and diameters. With no cutting away aspect the steel will stretch and the sockets formed. The sheet with protruding sockets is bent around another mandrel, forming the shell ready to weld. Further steps can dimension the shell, thread/face it and scallop the sockets to a pleasing shape. On some lugs (Bocama and Prugnat as example) you can see the two different lug shaping cuts, creating the shore line. (Blending these two different lug edge contours is an early thing young builders to be did back in the day). So much of basic frame building is just blacksmithy stuff. Metal can be rather plastic if dealt with properly. Think of soda cans that are formed from a sheet. Andy

dsaul

This is a case where a picture would be helpful, instead of everyone guessing at what you are describing.

rgppgr

I've done a far share of metal work so I know of what you speak. It was the sockets retaining shape as they were bent around the last mandrel I had to think about. I will post pictures on here and another thread I started with this bike to begin with. It's now bare metal. I think it's a one off by a builder back in the late 60's, early 70's. I can barely see a kerf line on both sides. Go to the other post of mine to see the whole bike before and after. I'm deciding what to do with it. Thanks again.

rgppgr

I knew that was coming dsaul . Thanks for not disappointing. Keeps me on my toes

noglider 

A piece of sheet metal, bent and welded into shape, makes a strong, reliable BB shell. It may not be pretty, but it is serviceable. In low end frames, the tubes are brazed in with brass, and in high end frames, it's the same, but the tubes are mitered before brazing. Yours looks to be brazed according to a high end practice. Yes, the threads are applied after the tubes are brazed into the shell. So I would say your frame is neither low end or top end, but it is of high quality.

Andrew R Stewart 

"Yes, the threads are applied after the tubes are brazed into the shell."

If you mean that the shell is threadless during the frame building then I will argue. If you mean that he threads are chased clean after the build, then yes (chased being the cleaning up of preexisting threads). Andy

JohnDThompson 

They're not attached separately but rather formed when the piece starts as a flat sheet. A mandrel presses bulges into the sheet for the seat tube, down tube, and chain stays, then the sheet is formed into a cylinder and welded along the cylinder seam. Then the ends of the bulges are machined off to create openings for the tubes, and threads are cut into the shell before brazing.

noglider 

Thank you for the correction.