Fillet Braze Crack Diagnosis and Repair- Mtbr.com
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  1. #1
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    Fillet Braze Crack Diagnosis and Repair

    It might be time for me to learn how frame builders fix broken bikes. The task always seems more daunting than building in the first place. Paint stripping cutting, grinding old joints smooth, Installing new tubes. I don't really know how repairs work at all.

    So this is what I have.
    * My first brazed frame. I built this mtb frame for myself. My previous 5 frames were tig welded.
    * I wanted to learn fillet brazing for aesthetics, working with lightweight tubes etc. This frame is Nova 8-5-8. Nothing too swanky.
    * My first DIY paint job. 2 stage base, clear. With lots of primer underneath sanded to smooth some of my rough spots. I filed and sanded the bronze fillets, but they weren't perfect. So I touched up before paint just for the hell of it. Maybe my flaw here.
    * I rode the bike 3 times so far. It rides great. No creaking or anything that feels odd or broken. I got after it pretty good one day on some ledgy rocky trails. It was so fun, I couldn't resist letting go of the brakes dropping some knee high rocks and drops.

    Here is a photo of the crack that appeared this week. It kinda freaked me out and I haven't ridden it since. I haven't had a problem with any of my bikes I have built so far, though I'm always paranoid and convinced that I'll F*&( one up someday. Also, this is the first brazed frame. And first one that wasn't powdercoated. So all new process here.
    I am hoping this might just be the paint cracking. But. I don't know if I believe that. So, what to do.
    Fillet Braze Crack Diagnosis and Repair-crack.jpg

    Any thoughts on how I go about diagnosing and fixing this?
    Should I strip the paint and get a look at the metal right away?
    Can I reflow the brass and add some more filler to remedy the bad joint?

  2. #2
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    Try your best to be zen about it and use it as a learning experience.

    The likelihood that's just a paint crack is near zero. My best guess is that either the joint wasn't properly tinned or there's a void in there somewhere that cracked through the fillet. I'd want to know exactly what caused the crack in the first place and personally, I'd cut most of the DT out so I could peek inside that joint to see what the penetration looked like and then replace the entire DT.
    Sean Chaney :: Owner/Builder :: Vertigo Cycles

  3. #3
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    Is that the way repairs usually go? Cut the tube out, And grind/file the leftover nubs smooth at the head tube and bottom bracket so that you can cleanly fit a new tube in there?

  4. #4
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    Yep, as lengthy a process as it sounds, you gotta do the hard yards with this one.
    Treat it as an adventure to see the actual fault and go on and do the rebuild correctively.
    You won't move forward if you don't with your growing skills.
    Even experienced builders have an off day with the torch.

    Carry on.

    Eric
    If I don't make an attempt, how will I know if it will work?

  5. #5
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    Thanks guys for the encouragement. I'll resist my temptation to keep riding it. I'm not afraid of getting in there and repairing. But to this point I've only built from scratch with clean tubes. I haven't had a reason learn how repairs work. I can see how this is a necessary skill. Frames crack, and you fix em, or rebuild from ground zero I suppose.

    Alrighty then off we go. I'll post a picture or two when I get in there. Hopefully I can learn what I did wrong and make some technique adjustments.

  6. #6
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    I'd first get the paint off and see if it really is a crack. Quite an odd place to crack right in the middle of the fillet. Also a bit puzzled by the white colour unless that is just the camera flash.*

    Brazing is pretty tolerant of beginners. Pretty much all the framebuild courses teach brazing for that reason rather than TIG. If you can TIG, then you should have a reasonable ability and understanding of heat control.

    My first frame was pretty poor teach-yourself fillets with way too much heat - that managed a few years rough rigid mtb then using as regular work commuter 12 years later and it is still in one piece. And it was elevated chainstay and the damn thing still hasn't cracked where I expected it would die. So if that thing has lasted I really can't imagine yours has failed so quickly.

    I presume the mitre was a reasonable fit? Which brazing rod and flux did you use? Was there still a decent sized fillet after filing?

    * day job is principal engineer in an automotive fatigue test lab so I look at LOADS of cracked things.

  7. #7
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    Quote Originally Posted by mickuk View Post
    I'd first get the paint off and see if it really is a crack. Quite an odd place to crack right in the middle of the fillet. Also a bit puzzled by the white colour unless that is just the camera flash.*

    Brazing is pretty tolerant of beginners. Pretty much all the framebuild courses teach brazing for that reason rather than TIG. If you can TIG, then you should have a reasonable ability and understanding of heat control.

    My first frame was pretty poor teach-yourself fillets with way too much heat - that managed a few years rough rigid mtb then using as regular work commuter 12 years later and it is still in one piece. And it was elevated chainstay and the damn thing still hasn't cracked where I expected it would die. So if that thing has lasted I really can't imagine yours has failed so quickly.

    I presume the mitre was a reasonable fit? Which brazing rod and flux did you use? Was there still a decent sized fillet after filing?

    * day job is principal engineer in an automotive fatigue test lab so I look at LOADS of cracked things.
    The white color is the lite colored primer that is under the paint and the bright flash from my phone camera. It doesn't stand out that much normally.

    I still haven't had a chance to get in there and remove paint to diagnose. But I will soon. I have to learn something from this eventually.

    Miters were pretty tight. I have become reasonably good at those on my first 4 tig welded frames. My fillets were ugly and bumpy, but I filed them smooth and still had a couple mm of material left in the joints everywhere. I have had the same experience with my welded frames. Been beating on some for years with some pretty hard mountain biking and they keep on going... Surprisingly.

    Thought I'd drop a photo of the frame before paint. Not really a close-up of the joint and crack. It's all I have though.

    Fillet Braze Crack Diagnosis and Repair-20200324_010759.jpg

  8. #8
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    It seems logical to me that since that angle is obtuse, you accidentally filed that fillet too thin. That weld takes on a lot of load under heavy braking.
    "Things that are complex are not useful, Things that are useful are simple."
    Mikhail Kalashnikov

  9. #9
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    In the unpainted photo (from what you can see) the fillet looks maybe a touch small, but hard to say? Studying the painted close up, I think thereís just not enough filler. Get some paint stripped and letís see whatís going on.

  10. #10
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    bare naked.

    I had to use the super zoom on my camera to even see the crack. Mostly invisible with my 50 year old vision. But with the aid of technology there it is... Cracked for sure.

    Fillet Braze Crack Diagnosis and Repair-20200618_174316_hdr.jpg

    If I get this thing stripped clean. Can I file it down a bit closer, and flux it up and reflow it and add a better fatter fillet?

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