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  1. #1
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    Warped Seat Tube at Top Tube Joint

    What am I doing wrong here? Every time I braze a non-sleeved seat tube/top tube joint I end up with a warped seat tube. I've tried going hot and fast, slow and cold, big fillet and small fillet. The image attached is the frame I'm currently building. This time I brazed the joint in four segments and let the tube cool between each round. I was hoping to keep more strength in the seat tube to avoid warping by only have a 1/4 of the joint hot at any time...obviously this didn't work. Any thoughts?

    Warped Seat Tube at Top Tube Joint-img_2888.jpg

  2. #2
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    It's just the nature of steel. It expands when heated and retracts past the initial position when cooled. Thicker material will help.

    -Joel
    NOTHING WORKS LIKE CLOCKWORK

    www.clockworkbikes.com

  3. #3
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    What seat tube did you use? This is another great reason to use a sleeve at the joint, it will almost totally prevent this problem.

    -Walt
    Waltworks Custom Bicycles
    Park City, UT USA
    www.waltworks.com
    waltworks.blogspot.com

  4. #4
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    When you heat a spot in a tube, it wants to expand but the cooler metal around it won't let it. So it expands in by getting slightly fatter. But when it cools down it wants to shrink back in all directions so it both wants to get thinner and smaller. This pulls the are around the heated spot back in, which pulls the tube into an arc.

    The trick is to heat it equally at least on both sides if not all away around the tube.

  5. #5
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    Hey Walt, the seat tube is from Nova-31.7mm diameter x 0.8/0.6mm wall.

    I'm hoping once I braze the stays on it will pull a bit of the bend back then when I cut the tube to length deformation should be minimal. I'm using an aluminum shim to fit a 27.2 post so there shouldn't be an issue with seat post insertion.

    Dr. Welby, thanks for the tip. I'll try heating 360 degrees of the seat tube next time and see if that eliminated the warping issue.

  6. #6
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    That's too thin anyway. It's not intended to be fillet brazed, it's for an OS lugged frame, I bet. You are just going to have it crack eventually, even if you can ream it enough to get a post in.

    Sleeve the next one, or use Nova's 31.8 external butt seat tube (can't remember the part number off the top of my head). Generally speaking the mininum wall thickness, un-sleeved, for fillet or TIG is about 1.2-1.3mm though if you're a heat-control stud and the rider isn't too big/aggro, 1.1mm is ok.

    Edit: Yes, unfortunately the gist of this is that you have to throw away the frame you're working on now and start over.

    -Walt
    Last edited by Walt; 04-01-2013 at 01:20 PM.
    Waltworks Custom Bicycles
    Park City, UT USA
    www.waltworks.com
    waltworks.blogspot.com

  7. #7
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    Quote Originally Posted by Walt View Post
    You are just going to have it crack eventually, even if you can ream it enough to get a post in.
    Yeah, that's a goner. Just for fun you can heat up the other side of the tube and see if you can "witch-wand" back to straight.

  8. #8
    650b me
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    Heat as evenly as possible all the way around the tube. Where your top tube intersects, could it be that the seat tube wall is only .6mm? That would make it worse, and would be a bad idea in the first place. And as the others have said, you need more beef at the top of the tube. If you don't want to go the sleeved route, you can buy externally butted seat tubes from Nova that are 1.2 at the top.

  9. #9
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    I'm beginning to understand why most people don't buy tube sets. The seat tube came with a tube set that was obviously intended for braze/tig construction (38mm bent down tube). I should have stopped to consider the seat tube wall thickness before moving forward with the build, live and learn I guess.

    I'm not ready to toss the frame yet, I see no harm in attempting to straighten the seat tube with a bit of heat and attempting an internal sleeve. It looks like a 1 1/4 tube with .095" wall thickness could be made to work. If nothing else I will learn a lot from this one.

  10. #10
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    If you can get a really tight fitting plug in there you might be ok, but it's going to be really hard to do that as the seat tube isn't at all round anymore (most likely). Still, if it's for you, just mess with it and figure out a fix of some kind, then ride it. If the seat tube cracks, c'est la vie, the frame won't come apart at speed or anything.

    I might call Nova and complain if I were you. WTF is that seat tube doing in that tubeset?

    -Walt
    Waltworks Custom Bicycles
    Park City, UT USA
    www.waltworks.com
    waltworks.blogspot.com

  11. #11
    WIGGLER
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    I would do a little Coconino'ish sleeve on it!!
    PAYASO 36er.....Live the Circus

  12. #12
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    Quote Originally Posted by dr.welby View Post
    The trick is to heat it equally at least on both sides if not all away around the tube.
    This is true and goes for other tubes like the head tube and BB shell, too. One prominent builder suggested to me that you flux the other side of the tube you are working on, you'll have an indication of when it's up to temperature.

  13. #13
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    Quote Originally Posted by whydomylegshurt? View Post
    I'm beginning to understand why most people don't buy tube sets. The seat tube came with a tube set that was obviously intended for braze/tig construction (38mm bent down tube). I should have stopped to consider the seat tube wall thickness before moving forward with the build, live and learn I guess.
    Did they send you the wrong tube? All of the Nova tube sets for mtb show the 28.6 1.2/.6/.9 ST as the one it should have.

  14. #14
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    I tried calling Nova to complain about the B.S. 46mm head tube that was included in that same tube set. That didn't go anywhere so now I'm doing my best to avoid them. I already tossed one front triangle due to the headtube, too bad I didn't use this seat tube in that same frame.

    In the end I'm just having fun with the learning process, I guess this is why you don't go pro until you build yourself a whole load of frames.

  15. #15
    Nemophilist
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    Hey;

    In your potential wanding experiments, you might do well to include the TT as well. It will be subject to same same movements, and heating it in the right spot in conjunction with the same on the ST might walk it substantially around.
    Most people ply the Well Trodden Path. A few seek a different way, and leave a Trail behind.
    - John Hajny, a.k.a. TrailMaker

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