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Thread: Seatstay Seat Lug Joint Cracked after First Ride. Help! How Should I Fix?

  1. #1
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    Sep 2019
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    Default Seatstay Seat Lug Joint Cracked after First Ride. Help! How Should I Fix?

    Hello Everyone,

    I just finished building my first bike. I took it out for its first shakedown ride before paint and noticed afterward that the brazed junction between the seatstay and the seat lug has a big crack. I'm starting to think about how best to do the repair and what went wrong the first time, I wanted to get some advise before I dive in.

    To join the seatstay and the lug I made a fitting that closely matched the profile of the binder bolt boss and the chainstay ID (see pictures below). I then bronze brazed both stays into place using the fittings.

    It looks like it cracked right at the joint between the seat lug and the fitting.

    What is the likely cause of the failure? Did I not use enough braze or get good enough penetration? My guess is yes, in hindsight I should have got more fillet in there but that was my first try at fillet brazing anything.

    As far as the repair goes, should I grind out all of the braze before trying again or just reflow and lay more on top?

    My other thought was to pin the joints before I attempt to redo it. I'm hoping this will retain the alignment. Any thoughts on this method?

    Finally, even if I beef up the joint, is this just going to happen again? I'm wondering if it is due to the stress applied when the binder bolt is tightened?

    Thanks all,

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  2. #2
    Join Date
    May 2008
    Better to be ruined than to be silent atmo.
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    Default Re: Seatstay Seat Lug Joint Cracked after First Ride. Help! How Should I Fix?

    Without having witnessed you braze this, it’s all speculation regarding why it happened. But if the mating surfaces aren’t completely bonded, no amount of filler on the periphery will carry the load.

    I would replace the stays, not patch it up trying a second or third braze.

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