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Crack at bottom of seatpost

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Crack at bottom of seatpost

Old 06-26-21, 11:30 AM
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urbanknight
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Crack at bottom of seatpost

Bought my son a used road bike and did my usual inspection, which found that the bottom of the seatpost is cracked. It only goes about 1cm up and I’m inclined not to worry as there is another 220mm between it and the top of the seat tube, unless some of you see a flaw in my logic.

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Old 06-26-21, 11:37 AM
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Would you really want your son on a cracked seatpost? If it weren't a big deal you wouldn't post the thread but the fact you have posted the thread leads me to believe you are going to be a good father and get them a new post just in case. You may be able to get a carbon blade and hack off the cracked bit but you have to wonder then why it was cracked and what else is lurking in that post? If I had a son or daughter or non-gender affiliated child I wouldn't want them riding anything cracked and honestly I probably wouldn't let them ride a carbon post, I would buy them titanium because they are my precious little bundles of joy and they deserve only the best.
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Old 06-26-21, 11:57 AM
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Originally Posted by veganbikes View Post
If it weren't a big deal you wouldn't post the thread but the fact you have posted the thread leads me to believe you are going to be a good father and get them a new post just in case.
Yeah you're right. I didn't even wait for a response and am already shopping for a new seatpost. Minimal expense for peace of mind. Guess I was hoping some expert would come in and tell me it's fine, but I probably wouldn't trust them anyway.
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Old 06-26-21, 12:05 PM
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During normal use, there aren't any forces being applied to that end of the seatpost. If it was my seatpost, I'd cut it and fuggetaboutit. If it was my kid's seatpost, I'd replace it with aluminum and use the cut carbon one on my bike.
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Old 06-26-21, 12:50 PM
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I'd clean the area of the cracking, & apply resin within the tube, flowing as much resin to bring it to the outer surface. Then smooth out any excess after it's cured... or cut off the section in question & seal the exposed cut with resin.
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Old 06-26-21, 01:14 PM
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Originally Posted by urbanknight View Post
Yeah you're right. I didn't even wait for a response and am already shopping for a new seatpost. Minimal expense for peace of mind. Guess I was hoping some expert would come in and tell me it's fine, but I probably wouldn't trust them anyway.
Peace of mind is worth way more than even the most expensive seatpost and when it comes to the children Wu-Tang is for the children, we teach the children, Puffy is good but Wu-Tang is the best!
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Old 06-26-21, 01:14 PM
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I'm with @Rolla, I'd likely just trim that section off and use it. Of course cutting means the "Minimum Insertion Point" is no longer valid.

That is an odd place for a crack. Perhaps the post was used way to high, and over-tightened.
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Old 06-26-21, 01:21 PM
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Originally Posted by CliffordK View Post
I'm with @Rolla, I'd likely just trim that section off and use it. Of course cutting means the "Minimum Insertion Point" is no longer valid.

That is an odd place for a crack. Perhaps the post was used way to high, and over-tightened.
Or the tube was dropped/stepped on at some point & the installer just sent it.
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Old 06-26-21, 01:49 PM
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I got the impression from the guy’s ads that he fixes up bikes for flipping, and this bike had signs of someone too tall trying to fit on a 49cm… 90mm stem, 40mm of headset spacers, 42cm bars. I was pleased to find the spoke tension was almost perfect and the shifters didn’t need any adjustments, but a co-op mechanic could easily miss or hide things like this.

Still not terribly concerned as a 65lb kid can probably ride many compromised parts without incident, but a new seatpost for $30 is less expensive than being wrong.
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Old 06-26-21, 02:38 PM
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Originally Posted by CliffordK View Post
I'm with @Rolla, I'd likely just trim that section off and use it. Of course cutting means the "Minimum Insertion Point" is no longer valid.

That is an odd place for a crack. Perhaps the post was used way to high, and over-tightened.
That dent on the end of the post looks like it may have gotten slammed down onto a water bottle bolt/boss. Having said that I would agree with the other posters who advise replacement.
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Old 06-26-21, 02:46 PM
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As long as the crack is there, it's likely to continue to grow. If you don't want to spring for a new post, trim the current post above the end of the crack. And keep an eye on it, in case you didn't cut off enough.
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Old 06-26-21, 03:10 PM
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Originally Posted by Rolla View Post
During normal use, there aren't any forces being applied to that end of the seatpost. If it was my seatpost, I'd cut it and fuggetaboutit. If it was my kid's seatpost, I'd replace it with aluminum and use the cut carbon one on my bike.
Thats the downside to FRPs.
Steel or Aluminum? Eh, watch it for awhile. Cut off above the crack “when we get around to it.”
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Old 06-26-21, 03:14 PM
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better to use a high speed thin cut-off wheel, should you play doctor carbon & amputate it.
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Old 06-26-21, 04:15 PM
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Troul,
I have posted before on the dangers of cutting carbon-fiber. If you used a cut-off wheel there will be enough carbon dust to clog your lungs for a while. I am a Golf Equipment Professional and teach about the dangers of carbon fiber dust to golf club repair persons. The dust during cutting can be mitigated by a water bath while cutting, but the wash will dry and still be carbon fiber dust waiting for a puff of wind to get it airborne. Downdraft filtration systems work best for cutting CF but are way expensive for a once in a lifetime seat post repair. My advice is to just junk it and move on. Way too many variables and issues that can arise from cutting it, including failure at an in-opportune time. JMHO, Smiles, MH
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Old 06-26-21, 04:17 PM
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Anybody wanna buy a “lightly” used carbon seatpost? Very light, and it’s about to get a little lighter.
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Old 06-27-21, 10:54 AM
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As MH stated, cutting carbon isn't for the uninitiated. Another aspect is getting slivers into your hands.
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Old 06-27-21, 06:39 PM
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1. 10" down inside the seat tube I'd bet you wear out the rest of the fame before that crack grows an appreciable amount.
2. I would be inclined to cut the post back and possibly remove any minimum insertion marking. Not because you're going to need it, but so that some future user 5 years from now doesn't assume it to be in good condition.
3. I've done stupid things with carbon fiber and survived (and had the rash to prove it), but cutting it in open air with no mask and a short sleeve t-shirt isn't the best idea. At least my rash was on my arms instead of inside my lungs (I've met someone who did that, although with much more sanding). An easy solution is to simply tape it then cut it under a trickle of running water while wearing gloves long enough that you stay dry. Dry and lightly oil your saw when you're done and all is well. A carbide blade is nicer, but a normal blade will work. No need to push excessively hard and fray more fibers than needed.
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