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Framebuilder Question: Never seen a weld on this area before

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Framebuilder Question: Never seen a weld on this area before

Old 11-13-21, 09:48 AM
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Framebuilder Question: Never seen a weld on this area before

cheers everybody,

recently I stumbled upon quite a nice steel frame on the net. however I realised that the seat tube was welded in an interesting way that I have never seen before.

maybe someone can explain to me why they chose to weld the seat tube like they did.

thanks in advance


Last edited by vxchtx; 11-13-21 at 10:20 AM.
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Old 11-13-21, 10:40 AM
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That's there because many sizes of tubes are not available in convenient sizes to fit easily available seat posts. Or more likely, the tubes that fit the easily available seat posts are too heavy. So builders weld in a collar.

You can shop all the seat tube collars that paragon machine makes on this page https://www.paragonmachineworks.com/...t+tube+collars
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Old 11-13-21, 02:57 PM
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Not an uncommon way to get the top of the seat tube to a spec ID, as Eric mentions. Seen a lot of TI frames and some steel ones done this way. Andy
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Old 11-14-21, 06:17 AM
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I use seat collars like that to make some of my MTB seat tubes to fit dropper posts or to make a seat tube out of a tube intended for a different purpose. I sometimes use a tube intended to be a bent downtube for a seat tube and that leaves me with a 34.9mm tube with a .6mm wall where I cut the tube to the proper length for a seat tube. In order to get the tube to fit a 30.9 or 31.6 seat post and have enough strength to weld the top tube and seat stays to it, I attach a 4" section of a much thicker walled tube to the top of it.

This is a 4" section of 4130 tube that I turned with a step to fit the inside of the 34.9 x .6mm wall tube and the interior dimension to fit a 31.6 seat tube.

That tube section fusion welded into the seat tube. If the fit is tight enough, it can be fusion welded and the weld just about disappears under powder coat.

This is the finished frame with that seat tube.
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