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  1. #1
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    Thomson Seatpost Broke today

    I'm OK, which is the main thing. Seriously, if I had not been riding on smooth concrete, this would have been a compete disaster. Added help was that I have a small Cannondale seatbag for a spare tube under my seat with small velcro loop that goes around the post in addition to the seat rails. That's the only thing that saved me from an aluminun colonoscopy. I have 5 Thomson seatposts and never had an issue with any of them till today. Across those posts I have over 100,000 care free miles. Owned this particular post 10 months and about 2000 miles. It's a Thomson Setback 31.6 410mm I have been using in Cannondale Caffeine framesets. The post has never been dropped, wrecked, knock over or even put in a repair stand.

    Frankly I did not think this was possible because I read that the bolts are supposed to break before the seatpost. I have not even heard of a Thomson failure before.






  2. #2
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    That really sucks. Call them and they'll take care of you.

  3. #3
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    Wow, that sucks. Although, not everyone can brag about breaking a Thomson, so at least you have that much going for you.

  4. #4
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    I'm guessing they will be very concerned, send you a new one, and ask for the broken one for analysis

  5. #5
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    Ever hit a garage?

  6. #6
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    Quote Originally Posted by Jerk_Chicken
    Ever hit a garage?
    'Zakly what I was thinking .

  7. #7
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    Quote Originally Posted by Jerk_Chicken
    Ever hit a garage?
    Nope. Don't even own a roof rack. Don't even own a garage.

    Had to ride 18 miles home standing. That gets a little old after awhile. But not too bad. The weather was nice and I had the wind at my back.

  8. #8
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    Quote Originally Posted by apacherider
    Nope. Don't even own a roof rack. Don't even own a garage.

    Had to ride 18 miles home standing. That gets a little old after awhile. But not too bad. The weather was nice and I had the wind at my back.
    Wow, that is just nuts...looks like something absolutely catastrophic happened there. 18mi...no cell phone? I called the wife today to come and rescue me with the truck because I flatted on the road bike a 2nd time 15 miles from home, grrr.

  9. #9
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    Is this even common for any kind of kind of seat post, including non name brands? Just thinking about the potential injury from taking that up the rear is disturbing.

  10. #10
    Now with 10% more!
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    Now I've seen it all.

  11. #11
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    Maybe it was a manufacturing defect that took 2k miles to finally break?

  12. #12
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    Any chance of getting a macro close up of the cross section on both ends? I'd love to see a close up of the damage.

    I had a post fail on me on the trainer. But the funny part was that I was on the trainer b/c I had a broken collar bone so I couldn't use my arm/ride outside.

    When the post broke, I collapsed to the floor banging my arm/shoulder into pretty much everything, that really hurt. But kinda funny now....

  13. #13
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    Quote Originally Posted by civil
    Any chance of getting a macro close up of the cross section on both ends? I'd love to see a close up of the damage.



    That's about as good a photo as I can get with my crummy camera. I know nothing about metal or fatigue. The front side is the clean break side, the rear is the jagged side. From what I understand, the seatpost has an elliptical build to it with more material on the front/back rather than the sides. Does it look like the rear end lacks some material or is that just a result of the deforming when it broke?

    In the photo below, see all the little teeny tiny fractures? They are on the rear facing side of the seat post. I don't know if they occured over time and I did not notice or if they occured as a result of the post at the moment it broke.


  14. #14
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    Well, I'll make some generalizations here, but here's a quick guess..

    Judging by the pic (I can't really tell b/c I can't see enough detail), it looks like a crack may have initiated here (arrow) and slowly propagated outward towards the line. Every loading cycle would cause the crack to propagate further. (front side of post in Tension and back side in Compression, cracks propagated through tension side due to opening and closing of crack from loading cycles)

    The surface looks smooth up until the line, indicating the area that the fatigue crack propagated through.

    To the right of the line, the remaining material wasn't strong enough to hold the stress and a shear failure (rough surface indicates this) occured, which would have deformed the rear end of the surface. The little wrinkles on the back of the post could be some straining due to the final shearing.

    Do I'll chalk it up to a fatigue failure due to some stress riser/imperfection.

    *edit* Glad you were OK
    Attached Images Attached Images

  15. #15
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    Geez that scares the crap out of me, and its a 31.6mm post too!

    I guess I will be examining my 31.6mm straight post tonight real careful. From the lack of shiny rub points in the fracture zone it looks like it happened more or less all at once, sure looks thin on the relieved side.

    Thanks for sharing with us, you may have saved someone elses rectum if they have a similar post issue. Glad you weren't hurt, it could have been real nasty.

    Kinda makes me rethink the weight savings from these seat posts, for the small amount of weight a straight gauge tube sure sounds safer.

  16. #16
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    Can't say I've seen a report of anything like the OP experienced FWIW. Everyone's gonna have a problem here and there. I think you can usually rely on Thomson's products more than many others in any case...
    "...the people get the government they deserve..."
    suum quique

  17. #17
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    OP, how much do you weigh?




    /hoping it's twice as much as I weigh

  18. #18
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    Quote Originally Posted by RookieBeotch
    Is this even common for any kind of kind of seat post, including non name brands? Just thinking about the potential injury from taking that up the rear is disturbing.
    I've broken a post, and I don't think there is a huge danger of getting sodomized by the post. When it breaks you have the seat between your a$$ and the top of the post. Now, if you forget about it as you ride on, that's a different story.

  19. #19
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    Thompson will want to see this.

    I'd send it to them for science alone. Now whether you want to see if they will cover it, that is another issue but I don't sense that you feel they are obliged

  20. #20
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    Also looks to me like a MFR issue.
    Just goes to show ALL parts need a regular check.

    Oh, I would not have expected it to fail at that point. Should have been more flex further down the shaft. I would be very sure this was a fault from production.

  21. #21
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    I ride nothing but Thomson, that blows my mind I thought they were indestructable. Glad to hear your ok, but 18 miles standing up...Your an ANIMAL!!!!

  22. #22
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    was anything sharp on your saddle bag rubbing the seatpost?

  23. #23
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    Caution;  Merge;  Workers Ahead! Thanks Apacherider

    Apacherider,

    I noted a similar incident on a Thomson Elite seatpost on my blog. Read it here.

    Seems like a good way to connect...

    As you will see in my writeup and the attached pictures, I did the best I could to collect all relevant details from the rider as to how he used it. Given his information (and trusting it), I don't see how the bolting ears could have just snapped off like that.

    In your case, again...I can't see how 2000 miles of riding will cause the post to crack at that spot.

    What's your weight? And did you follow all installation instructions in the manual, like torque settings on the clamps? Did the post also take a fall anytime you remember, and like someone asked before me, do you remember having put anything sharp in your saddlebag that may have marked up the post? (I do not see how that can happen when something sharp is inside your bag but for the sake of information collection, let's have it all from you...)


    Thanks...

    Ron
    Cozy Beehive Bike Blog

  24. #24
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    I noted a similar incident on a Thomson Elite seatpost on my blog. Read it here.
    This failure is not similar.

  25. #25
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    Anodized Finish ?

    Quote Originally Posted by Jerk_Chicken
    This failure is not similar.
    No, no one said its a similar failure. I said the incident is similar as in it involves a Thomson post.

    Failures with Thomson are pretty rare given their reputation among cyclists for making strong components. But these two incidents occurred not very far apart from each other (both this year). Marketing materials from Thomson claim "over 40% on ultimate strength test than the strongest production seat posts on the market" and the presence of a unique 'bending fuse' to prevent catastrophic failure. In my writeup, we determined that this fuse could be nothing more but a fancy term for the cushion of safety achieved by the elliptical bore, specifically in the lateral sections of the post where it is thinnest, as opposed to the bolt ear locations which are thickest. Due to this, the sides of the post folds before the fore and aft sections having the bolt ears do, offering some degree of protection to the cyclist from a fall.

    What could be interesting about the pictures provided by apacherider is that the ideas behind the "bend, not break" philosophy and the elliptical bore may be insignificant in reality...because as Civil pointed out here before, the crack originated on the thicker section (pointed by red arrow). But that's needs confirming through a fracture analysis.

    Just one final thought. The black finish is beautiful. It is chemically polished and then anodized. But my mind keeps asking whether this treatment makes the post any more susceptible for failure. Jobst Brandt, mechanical engineer and author of "The Bicyle Wheel", didn't favor anodized rims, advising that when rims are anodized, a thin layer of porus, Al oxide is the result and the problem is that this oxide material is extremely hard but brittle. This makes the rim more susceptible to cracking and fatigue failure. This is actually a well known issue in materials circles. As Al bends elastically, the anodized surface cracks and the crack grows into the body of the Al. Anodized aluminum only worsens the fatigue limitations of Al. You can't bend anodized Al significantly without cracking it. The cracks that develop on the coating are stress risers and potential sources for fatigue failure in the substrate metal.

    Could the same be said for this seatpost, which also has an anodized finish? My feeling is that anodizing is a bad choice for a part made to take bending.
    Last edited by Cozy.Beehive; 04-12-2009 at 01:00 AM.

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