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  1. #1
    qqy
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    How bad is this seatpost crack and what can be done? (Columbus Thermachrom steel)



    Overtightend the seatpost collar and did this to my favorite bike in the world. Since it's not load bearing, I was hoping there's a reasonable fix to this. It's been surprisingly difficult to get a local store or framebuilder to give me a quote or suggestion on what to do. I rode it for a bit after doing this unknowingly and it hasn't gotten much worse.

    So far, the range of options is: braze in a lug (most extreme) to bead weld it to hold the crack. Looks like the seatpost is a bit too small, so maybe weld in a shim?

    Any help is greatly appreciated...

  2. #2
    Randomhead
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    find someone with a internal hole gauge to try to figure out what the right seatpost size is and get one that size. It does make sense that the undersized seat post caused the problem. Unfortunately, with a crack that big it might not stop growing. In fact, my thought is that it ends at the white dot that is almost all the way to the weld. I think you could braze half of a tube as an overlay on top of this, but it would be hard to make it look right.

  3. #3
    qqy
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    Thanks for the reply. Sounds like the 'braze in a lug' may be the best option.. ugh..

  4. #4
    Decrepit Member Scooper's Avatar
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    If it were mine, I'd just braze on a 1" wide 1mm thick reinforcing strip of 4130 formed around the rear half of the seat tube centered vertically over the crack, then redrill the hole and cut the slot in the reinforcement.
    - Stan

  5. #5
    qqy
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    That's actually exactly what I want. I've seen this before with this very frame and the solution was to braze a diamond-shaped patch over the back of the tube and recut the notch.

    Very good solution. I think this is what I will ask for, and if it fails, then resort to a lug. Thanks to both suggestions above - this gives me a bit more confidence that this can be resolved reasonably.

  6. #6
    Randomhead
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    actually, scooper just expressed himself more clearly -- that's what I meant. However, you do need to get a seatpost that is the right size or this will fail

  7. #7
    Senior Member
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    I would think that drilling a hole at the end of the crack would help keep it from spreading. Then possibly brazing the crack and filling the hole with silver or brass to finish it off.

  8. #8
    Randomhead
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    do you have any experience with repairs like that?

  9. #9
    Senior Member GrayJay's Avatar
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    I have a MTB made from TT OX2 tubing that had a similar crack develop shortly after I purchased the frame. I brazed a 2" long sleeve into the top of the seattube and used a smaller seatpost to fit the sleeve ID. Except for the seatpost diameter, the repair is nearly undetectable and I have been riding it that way for nearly 20 years.

  10. #10
    Senior Member
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    No; but I my understanding is that the hole at the bottom of the seatpost cut was there to prevent stress risers from occurring. Of course, the crack still occurred, so maybe I'm missing something. The brazing of the crack and filling the hole would be purely cosmetic.

  11. #11
    Framebuilder
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    Quote Originally Posted by randomgear View Post
    No; but I my understanding is that the hole at the bottom of the seatpost cut was there to prevent stress risers from occurring. Of course, the crack still occurred, so maybe I'm missing something. The brazing of the crack and filling the hole would be purely cosmetic.
    I might get flamed for this, but I see this kind of thing so often I'm gonna say it anyway- If you have no experience with this type of repair, why would you reply in a thread about making said repair?

  12. #12
    Senior Member
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    School learning verses actual experience in steel crack repair. Drilling a hole to stop a crack was discussed in a materials class in college, roughly 20 years ago.
    Some googleing brought up a couple results on the subject:
    in airplanes - http://www.mechanicsupport.com/metal_fatigue_crack.html
    in highway signs - http://www.ce.udel.edu/cibe/news%20a...hop/Dexter.pdf
    about halfway through there is a formula for determining the size for a stop-hole diameter - rule of thumb is listed as diameter should be 1/3 the length of the crack - not good news for bike repairs.

  13. #13
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    I do have a technical question, why the cut notch is so long??

  14. #14
    Randomhead
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    Quote Originally Posted by randomgear View Post
    School learning verses actual experience in steel crack repair. Drilling a hole to stop a crack was discussed in a materials class in college, roughly 20 years ago.
    the thing is, you need to keep the crack tip from opening. If the crack tip doesn't open, there is no advantage to drilling a hole, the crack still isn't going to move. Stop drilling alone will not stop a crack. It removes the crack tip and should delay the formations of new cracks for some short period of time. I went through extensive training in the subject of fixing damaged aircraft and I don't remember the subject of stop drilling ever coming up at all. That's because it does nearly nothing. The crack in the OP is possibly somewhat different because it is apparently mode III crack. But it is long enough that it might propagate in mode I. The bottom line is that it needs reinforcement back at the slot.

  15. #15
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    I had something similar to this decades ago and it was fixed with a TIG welder in just a few minutes. Ugly as hell, but never failed.

  16. #16
    Senior Member pyeyo's Avatar
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    Is there enough seat tube above the top tube to slot the front after putting the diamond shape patch on?
    I repaired a mountiain bike with a similar problem but sleeving the entire seat tube mitered to tuck into the top tube with the diamond extending down between the seat stays. after painting it looked to be part of the frame.
    You will have to ream the seat tube after you repair it.

  17. #17
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    Engineering 101 thats a propagating crack and is only going to get bigger if you dont do anything about it, id suggest a lug but im sure there are some other acceptable less extensive ways to fix it

  18. #18
    qqy
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    Quote Originally Posted by randomgear View Post
    I would think that drilling a hole at the end of the crack would help keep it from spreading. Then possibly brazing the crack and filling the hole with silver or brass to finish it off.
    Forgot about this thread. This is basically what was done and it seems to have worked! 800+ miles since and the repair has held up perfectly.

  19. #19
    ex frame builder
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    First drill a small hole (Say 1/16 in') at the end of the crack to stop it from spreading further. Later if you want to fix perminantly, braze a piece of wire in the hole and a little over the crack. File the surplus wire sticking through inside the seat tube.
    History, photos and tech articles on my website. Also check "Dave's Bike Blog."

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