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  1. #1
    Randomhead
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    anyone ever replace a seat tube on a lugged bike?

    I have this obsessive need to get my first frame on the road after 30 years, and somewhere along the way the seat tube got badly dented/creased so I'd like to replace it. I'm assuming the frame will spring when I take the old seat tube out. I thought about holding the top tube in position, but I really don't have anything that will do that. Any thoughts?

  2. #2
    tuz
    tuz is offline
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    I'm not sure what you mean about holding the TT. I've never done a repair, but Dave Moulton has a nice article about it.

    http://davesbikeblog.squarespace.com...or-repair.html
    homebuilt commuter, mixte, road and track (+ Ryffranck road)
    bla bla blog

  3. #3
    Randomhead
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    thanks for the link. His objection to the seat cluster is that it is hard to take apart with heat. I think I will grind the tube out of the lug to avoid that problem.

    My concern is that there are stresses in the frame that will be released, changing the geometry

  4. #4
    Framebuilder
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    You think it would move enough to matter? I'd imagine the seatstays would hold the tt in place vertically.
    I've never replaced a st, but I have done a few tt's, dt's and chainstays. If the frames moved at all when they were cut (all the mass produced lugged ones did) it seemed like is was due to a built-in tension from the headtube angle...maybe because they were forcing the lug angles to fit the jig? On some frames the kerf would close up and on others it would spread apart. If you're not cutting the dt or tt, that shouldn't be a problem.
    I'm thinking there's a good chance your seat lug won't move at all, of course, there's only one way to find out !

  5. #5
    Randomhead
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    this was my first frame. I'm almost afraid to see how the brazing was, I have no recollection of making it at all after all these years.

    If you look at how much movement you can get from witch-wanding, it's not surprising that frames move when one of their tubes is removed. I built an occasional Trek where, for whatever reason, I would have to put some strain to move the top tube to the top of the seat tube. We brazed the top tube, head tube, and down tube together, and then put the seat tube on so the feedback on this was pretty immediate.

    I guess I can always move the top tube back into place with a car jack or something.

  6. #6
    Senior Member GrayJay's Avatar
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    Before resorting to cutting out the ST, you might see how much repair you can first accomplish by pounding a seatpost size mandral back-n-forth over the dented area from inside the tube. Even if it is creased, having a solid object inside the tube gives you something to push the crease back outward. Once the mandral has popped the dent out (easy part), you can use an auto-body hammer to tap all around the crease, thus pushing it back out. Even if you cannot remove the crease 100%, it should be possible to get it pushed out 99%, enought that it only takes a tiny bit of filler to smooth over.
    Difficulties would be if it were a double butted seattube instead of single butted or if the dent were below a H2O bottle fitting that might prevent passage of the mandral down to the dented section.

  7. #7
    Randomhead
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    tried that, didn't like the results. It was really mangled, now it's mangled in a different way. I don't really think it will ever be round, and the mangling is up near the seat lug where it would be annoying to deal with. Frame has never been painted so I see no reason not to just replace the tube.

  8. #8
    coprolite fietsbob's Avatar
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    Rosebud torch tip ? and OOdles of heat ..
    to remelt the brass in the BB shell-seat tube joint.

    need to cut out a section .. and pull whats left out when brass is melted.
    Last edited by fietsbob; 03-22-12 at 05:40 PM.

  9. #9
    Randomhead
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    if you read the op, I have specific question about holding the geometry of the bike. But since you mentioned rosebuds, I'll mention that when I looked into them, the only ones available for my torch draw more acetylene than my tank can deliver and wouldn't be safe.

    Enough suspense, I'm gonna go cut out a seat tube

  10. #10
    Andrew R Stewart Andrew R Stewart's Avatar
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    Eric- If you cut the seat tube at it's mid point the frame's shift will be shown by the cut ends of the ST misaligning WRT to each other. Kind of like drop out aligning tool cups. Any twist or lengthwise shift (if minor) could be corrected for at this point using the cut ST as both the levers and alignment indicators. As an option to replacing the entire tube you might consider sleeving a shorter replacement in instead. The BB sleeve could be internal. The one just below the seat lug might be external. Artistic styling might make the upper one look less wrong. If you choose to replace the whole tube I'd grind out the tube from the lug and shell. Hopefully you can maintain close enough a fit to be able to use Silver. I braze my seat stays to the lug with Silver, I'd hate to see Silver flowing in an area I didn't intend for...

    Revisiting an old frame is an interesting learning chance. i have done stay replacement on a many year old frame of mine before as well as better aligned a few miles after "completion". Keep us informed to your efforts. Andy.

  11. #11
    Framebuilder
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    Quote Originally Posted by unterhausen View Post
    If you look at how much movement you can get from witch-wanding, it's not surprising that frames move when one of their tubes is removed. .
    Good point, I'll bet those old Eye-talian frames saw some serious (and probably) uneven heating during construction. I guess the amount of movement when cut would be a testament to the care and skill of the builder.
    I cut a ruined tt out of a Steelman frame once...zero movement of the other tubes.

    So what are we gonna learn about the Unterhausen of the 70's?

  12. #12
    Randomhead
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    Quote Originally Posted by Live Wire View Post
    So what are we gonna learn about the Unterhausen of the 70's?
    actually, the funniest thing about this frame was that it has a fastback style seat cluster, and I didn't leave enough room between the seat stays to allow the seat post to be clamped. That's funny, but not "haha" funny.

    I'm reconsidering the whole idea of replacing the tube because I worked it over with a wooden block as GrayJay suggested and it looks like it might be acceptable.

  13. #13
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    Wow, Unterhausen, I'd find a new obsession. I mean, this is the first one you built, as you moved on you set it aside and it's now become a museum piece (of sorts). I wouldn't mess with the first frame I made. Hang it up as is, and live the memories it made for you. Just make another. I don't think you really want to ride that one anyway.
    Craig

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