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  1. #1
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    Broken Seat Tube

    So I was out riding today and the seat tube on my carbon hardtail 29er broke. Is it common for seat tubes to break? Anything that can be done to prevent them from breaking once I get a new frame? Which parts of the bike get stressed the most during riding?
    Thanks for your help everyone

  2. #2
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    9 out of 10 times (maybe more like 99 out of 100) when a seat tube breaks, it's because the seat post wasn't inserted deep enough. This is true regardless of the frame material.

    Many people think that inserting a post up to the minimum insertion line is enough, but that isn't always the case. The min-insert line is related to the post flexing (camming) out of the frame, but doesn't address the needs of modern frame design where the seat tube extends above the top tube. The second consideration which is key to seat tube survival is that the seat post must extend deep enough to end below the bottom of the top tube, preferably an inch or so below that.

    Seat tubes break when the post ends above the top tube joint, so all the post's stress is placed on the unsupported section of seat tube above the joint. To protect your next frame , get a longer post, make sure it fits well, and that way the post will buttress the seat tube and take it's fair share of the stress of the rider's shifting weight.
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  3. #3
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    I forgot to say that this seat tube broke below the top tube by about 3 inches.+

  4. #4
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    Quote Originally Posted by dabee1106 View Post
    I forgot to say that this seat tube broke below the top tube by about 3 inches.+
    I did leave room for the 1 out of 10 (or 100) that failed for another reason.

    There's no reason for a failure there except possibly if a post that ended there was carrying a very heavy rider, or had such a sloppy fit that the end scored the ID of the seat tube. It could also be a function of the details of construction, like if the seat tube had an aluminum liner which ended there, creating a stress riser.

    Otherwise it's a fluke, possibly a structural defect, and you shouldn't worry about a repeat in a decent quality frame, unless you weigh upwards of 250#s and maybe shouldn't be riding a carbon frame in the first place.
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    “One accurate measurement is worth a thousand expert opinions” - Adm Grace Murray Hopper - USN

    WARNING, I'm from New York. Thin skinned people should maintain safe distance.

  5. #5
    Senior Member catonec's Avatar
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    Quote Originally Posted by dabee1106 View Post
    I forgot to say that this seat tube broke below the top tube by about 3 inches.+
    whaaat?? never heard of that before, not from just riding, must have been a nasty crash caused a crack in the carbon and eventually it broke open.

    I would say normally the seat tube between the top tube and BB take very little stress compared to the down tube and seatstays. If it was due to a crack then the frame did not fail, you did.
    2010 Kestrel RT900SL, 800k carbon, chorus/record, speedplay, zonda
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  6. #6
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    And then there is a the problem facing us almost everyday in this modern world- Quality sucks! It may have broken just because it wasnt made right.Now that you have broken one, the chances of it happening again are next to nothing right? ( obscure refernce to: World according to Garp)-SP

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