Hey everyone, lining up to build a super light cx bike. The main tubes are 0.7-0.4-0.7 wall thickness. I’ve tig welded an equally thin 4130 tube before, and while very annoying, didn’t have issues once built.
However, ive been seeing some things said here and there about how tubing that thin should only be brazed due to lower temp etc. Does anyone have experience and opinions building super light tig bikes? Anyone have scientific research on this?
Thanks!
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You can perfectly well TIG weld it. I’ve used 0.7 wall tubes and even 0.6mm seat-stays.
Make sure you get an externally butted seat-tube. Note that almost all the joints on the frame are thick-to-thin (the only exception is the small region where the seat-tube intersects the down-tube, and bridge-tubes). So always start the arc on the thicker tube and never forget which is which. Then dip the rod, and pulse the pedal up a bit as you wash the puddle onto the thinner one. This works for me anyway. Any small error and things will go south very quickly so be careful and if in doubt lift off the pedal at once.
I like to TIG braze bridge tubes as they are non-structural anyway and there is much less risk of weakening the tube you care about-- the stay-- but you can also weld those. Just be extra careful because in these cases both tubes are thin.
If you use Reynolds 853 the heat of welding should actually make it a bit stronger! But it’s fine to TIG any of these tubes (and the stays I use are 525 which is just regular 4130 cromoly).
I have a document from Reynolds which discusses welding. It certainly doesn’t say that you shouldn’t use it on the thinner tubes. They suggest using ER-70S-2 , ER70S-6 or ER80S-2, which are all very normal mild-steel rods, on everything except the stainless tubes. I use one of the ER70 ones. Some people like to TIG all frames with a stainless filler. They don’t mention this in that document.
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I only made a small number of TIG frames with .7 — Col. EL or Prestige mostly but also some tandem keel tubes in .028" 4130. None of them broke, that I know of, but some people don’t tell you when their frame cracks if it was years later. I am not a great TIG welder, so there’s a good chance you can weld 'em as well as I did and likely better!
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I would maybe consider back purging the frame while welding to give a bit more “safety factor” to both the process and the result. I feel that things don’t go south as quickly when there’s less oxygen present around the work area…
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Interesting. I’ve never set up back-purging as I don’t do stainless or Ti. It’s quite a bit more hassle. However there are a few places where I use a bit of aluminium foil to keep the gas where I need it.
Thanks great info. I used a mix of er70s-2 and er312 on the thin tubes I’ve welded. Stainless seems to flow better at lower temp which makes it easier to do a pretty weld, but should probably be backpurged.
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Been thinking about this too. Just did some stainless vessels at work and having a purge really makes the whole process better.
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Yes the thin steel frames I welded (a few dozen tops) had an internal purge, because we were set up to make Ti frames (and Metax stainless, and Aermet 100 alloy), so all the stuff to purge was right there. Why not? I wasn’t the guy paying the argon bill, and on the rare occasion where I blew a hole through the tube, it made fixing that a lot easier than if the hole was made of oxidized steel. Yes I know, “just don’t make holes in the tube while welding”, great advice, not easy to live by 100%.
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