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Old 09-24-11, 03:47 PM   #1
shmily_dana
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Seat Post Failure

Took a fall because my seat post failed where the seat connects. Fortunately, I walked away from the crash. Needless to say, I don't want to do that again. Would periodic inspections have revealed a problem?
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Old 09-24-11, 03:55 PM   #2
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really short on details , dude...
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Old 09-24-11, 04:22 PM   #3
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Did it crack or did the clamp just come loose? What type of post, what type of clamp, etc? Need more info.
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Old 09-24-11, 05:10 PM   #4
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Routine inspections, or simply being very aware of how your bike rides, sounds or feels may have given you warning, but then again the bracket could have simply cracked without any prior warning signs.
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Old 09-24-11, 05:12 PM   #5
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I have a colleague downstairs who when on a sprint against somebody else, and in the process of rising and spinning hard off the seat just prior to standing up, his carbon fibre post fractured dramatically. No splinters in the privates and he can still live to reproduce someday, but whoa... nasty. It was sudden and unexpected.
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Old 09-24-11, 05:28 PM   #6
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"his carbon fibre post fractured dramatically...It was sudden and unexpected. " This is what scares me about carbon composites.
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Old 09-25-11, 12:36 AM   #7
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I don't know make and model of the seat post. I know the post is carbon. Don't know what the clamp is made of. I was riding on flat road with some minor pavement cracks at about 20mph. I heard and felt a loud snap/crack noise. My first thought was a blow out. Then I was wondering why I was on the ground. I am a big guy. I've been riding on this bicycle for just over 2 years (14k miles). For my size, should I replace periodically?
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Old 09-25-11, 06:38 AM   #8
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Quote:
Originally Posted by shmily_dana View Post


I don't know make and model of the seat post. I know the post is carbon. Don't know what the clamp is made of. I was riding on flat road with some minor pavement cracks at about 20mph. I heard and felt a loud snap/crack noise. My first thought was a blow out. Then I was wondering why I was on the ground. I am a big guy. I've been riding on this bicycle for just over 2 years (14k miles). For my size, should I replace periodically?
It looks like the saddle was mounted as far rearward as possible, which may've contributed to the failure. You may want to replace with an aluminum seat post that has a little more set back.

Brad
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Old 09-25-11, 07:36 AM   #9
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Quote:
Originally Posted by shmily_dana View Post


I don't know make and model of the seat post. I know the post is carbon. Don't know what the clamp is made of. I was riding on flat road with some minor pavement cracks at about 20mph. I heard and felt a loud snap/crack noise. My first thought was a blow out. Then I was wondering why I was on the ground. I am a big guy. I've been riding on this bicycle for just over 2 years (14k miles). For my size, should I replace periodically?
If you're worried, just use a metal post.
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Old 09-25-11, 07:39 AM   #10
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I'm 200lbs and had the oem seatpost snap on me. I just bought a Thomson Elite and never looked back.
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Old 09-25-11, 08:55 AM   #11
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Interesting that with all the concern about CF, the part of this CF seat post that failed was the alloy clamp.
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Old 09-25-11, 09:46 AM   #12
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Quote:
Originally Posted by bradtx View Post
It looks like the saddle was mounted as far rearward as possible, which may've contributed to the failure. You may want to replace with an aluminum seat post that has a little more set back.

Brad
^This.

If you have to slam the seat that far on the rails to be comfortable, you may want to look into getting a professional fit. If that's not an option, choose a seatpost with enough offset that the clamp is roughly in the middle of the seat rails. Slammed one way or the other you risk broken seatpost or rails as well as less comfort since the rails can't flex the way they're designed to.
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Old 09-25-11, 10:13 AM   #13
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Interesting that with all the concern about CF, the part of this CF seat post that failed was the alloy clamp.
^This.
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Old 09-25-11, 10:42 AM   #14
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My Guess, From picture, is the bolt that clamps the saddle rails,
and holds the angle adjustment is what broke,
the seatpost itself, apparently, did not.

go back to the bike shop that you bought the bike from.
they should warrantee the whole seatpost...

Well the enlargements, offered below,

certainly helped the tiny attachment that were not so clear
my eyes are about to be 64, also, next month.


FWIW,
I have not had a problem in 25 years with my old 2 bolt Campag seatposts,
nor the more recent American Classic ones , also a 2 bolt, or 1 bolt and one setscrew.

also got a Race Face one , for it's low profile head,
they took a whole new look at how you fit a saddle on ,
and have a bolt that holds the rails in solidly from the sides,
and a linkage that adjusts the angle , held in place with a band clamp
on the seatpost itself, with steel pivot pieces joining the moving parts.

Next purchase you know what may be a weak point.

Paul's Has a New Seat post, with a bit more setback , of aluminum,
made on their CNC machines in Chico California..

And Rivendale sells a lugged Nitto made
Chrome moly tubed seat post with even more setback..

and a Cane Creek ThudBuster, is a good solid seatpost, their ST
has a road bike application, and does take the bumps out
of the surprise holes that sneak up on you, before you can rise up
off the saddle to use the leg suspension.

I like the LT one I have on my Bike Friday..

But it needs better than 8" between the frame and the saddle height.

Last edited by fietsbob; 09-25-11 at 02:30 PM.
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Old 09-25-11, 10:51 AM   #15
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Quote:
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My Guess, From picture, is the bolt that clamps the saddle rails,
and holds the angle adjustment is what broke,
the seatpost itself, apparently, did not.

go back to the bike shop that you bought the bike from.
they should warrantee the whole seatpost...


I'd say that it's pretty clearly the head of the post that broke Bob. Not the carbon post, not just the bolt.

If the bike is less than a year old and purchased through a shop, I agree, they should cover it through warranty.
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Old 09-25-11, 12:30 PM   #16
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This is a 2 year old photo of the bicycle. I don't have a close-up of the seat post. Please confirm your comments about the seat being to far rearwards. If the seat was too far rearwards, would I have fallen forwards when it broke? I actually fell backwards. I don't have a side view of me on the bicycle, so I can't see where my center of gravity is over the seat.
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Old 09-25-11, 01:37 PM   #17
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Quote:
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This is a 2 year old photo of the bicycle. I don't have a close-up of the seat post. Please confirm your comments about the seat being to far rearwards. If the seat was too far rearwards, would I have fallen forwards when it broke? I actually fell backwards. I don't have a side view of me on the bicycle, so I can't see where my center of gravity is over the seat.
Well schmily, in this picture that you provided earlier a portion of the post head is still attached to the very front of the clamping section of the seat rails:



Looking at where the head snapped, it's entirely logical that you fell back. While I haven't seen a post snap where yours did, I have seen lightweight titanium and carbon seat rails bend or fail when clamped to one extreme or the other of the clamping zone. It's just a matter of leverage. While one might think if a manufacturer provided that much adjustment, it would be safe to clamp it anywhere, that doesn't always work out in real life usage, especially when parts are changed out or combined in ways that were never planned for.

I don't mean to offend, but when I see riders sliding seats to one extreme, installing overly long or short stems or over/under extend the seatpost, it indicates a fit issue. It could be that the particular manufacturer's specs for top tube to seat height proportions don't match up with your particular physiology well, or that you simply need another size.

If the seat clamp merely slid forward in the incident, you can disregard my comments.
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Old 09-25-11, 01:59 PM   #18
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I like these. I have two on my bikes. The two bolts makes it easy to adjust the seat angle for my Brooks saddles. http://ctxtv.wmppt.servertrust.com/product-p/sp2.htm
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Old 09-25-11, 02:42 PM   #19
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Obviously from the #16 picture.. it's a big rider.. not light,

I do see the bolt hole, adds a design flaw, it took a lot of metal away ,
and the rider weight used that weak point like a fulcrum with every bump.

Ill go back to my Cane Creek Thudbuster ST suggestion,
it will take the harshness out of the ride ,
and the bumps will be absorbed by the suspension block ,
rather than seeking a weakness in the saddle clamp-head to flex.

The Elastomers are offered in different rider weight ranges..

Last edited by fietsbob; 09-25-11 at 02:45 PM.
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Old 09-25-11, 05:13 PM   #20
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Originally Posted by Wordbiker View Post


.
From the picture of the bike, OP is a large rider. From the picture of the seat with clamp still on the rails, the seat was slammed all the way back. From this picture of the seat clamp the failure was a classic fatigue failure in the aluminum clamp. The crack started at the valley between two locating ribs (stress concentration) and grew downward due to the cyclic load from cycling. Large rider means a large load, and having the seat all the way back increased the load due to the lever arm to the sit area on the seat.

Back to the OP question, I doubt that you would have seen the crack during inspection even if you disassembled the seat post clamp since there would have been no load on it at the time. It would take something like a dye penetrant test to show up.
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