1-3/8" x .035 Seat Tube Junction Too Thin? Cracking?

So I’ve pretty much wrapped up my first TIG frame and realized may have messed up.

I used a 1-3/8" to 31.6mm seat collar from Paragon, but in my design I welded the TT/ST/CS Junction all down low below the seat collar, so they are welded to the .035 wall section of tube. Am I massively screwed? Reading on thos forum it sounds like that wall thickness is not strong enough and will crack at some point.

The collar has a pretty large brace going down to the top tube but I feel like still having the welds on the thin section was a big mistake.

Anything to be done now? Could I do some sort of crazy external collar or side gussets from seat stays to top tube? Or do I just ride it until it breaks.

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My hardtail is like that and I have had no problems. My vote is ride it, watch it and start planning the next one.

p.s. Happy new frame! Where are the pictures?



Nice looking bike! I recon ride it and if it breaks down the line then fix it.


That bike looks great, especially considering it’s your first frame!

I think there is a high likelihood that it should be fine. Wall thickness is only half the equation, the tube diameter dominates the strength equation, and 35x.09mm is beefy. I think most SS-ST failures you see are on 29mm tubes with thin .7 butts.

Total guess: I think half of the frame failures are engineering mistakes, and the other half are fabrication errors. If you try reinforcing and gusseting the area, you are probably more likely to introduce fabrication errors (heat, contamination, etc…)

I also recommend using a pre-butted seat tube instead of the topper. IMO seat toppers are an outdated process: 35MM Ø SEAT TUBE DEDICCIAI BUTTED FOR 31.6 SEAT POST 1.65/.9 X 600MM — BICYCLE FABRICATION SUPPLY

The first time is never perfect, I have to constantly remind myself of that. You already know what to do next time (place the stress on the thick butts), so if it breaks, just build it better!


That’s a really nice looking bike! Ride the hell out of it.

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Thanks y’all, glad to hear it won’t explode off the first jump :crazy_face:. I think I was mostly fear mongering after reading through a couple other threads.

Bike went off to powdercoat yesterday with Good Weather Bicycle in Seattle. They have a been great to work with so far. I’ll report back once the frame is done.


Great looking bike! Nice work!