carbon seat post - worried about crushing it - too tight?
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  1. #1
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    carbon seat post - worried about crushing it - too tight?

    hello everyone

    I just bought my very first carbon road bike with a carbon seat post (Specialized Roubaix Comp 2005).

    I have been warned about not tightening seat post too much else I could crush it. All I have is an Allen-key to tighten it. So how tight is too tight (all jokes aside) ?

    Is it really true about "crushing carbon seat post" or should I just use same amount of force as I have on my aluminium seat post? I have not done a good job on my carbon post tightening because it still slides a few mm under my 150 lbs (67 kg) weight when I hit slightly bumpy roads.

    Are there any suggestions or comments on this topic?

    Thank you.

  2. #2

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    Tighten it until it doesn't give under your eight under use conditions. If you hear a cracking sound - you have gone too far... ;)

    FWIW, I have a carbon post and tighten it fairly tight (I haven't measured with a torque wrench), but have never had a problem. I think you would have to seriously over do it to get it to crack your tube.
    Last edited by Thorn Bait; 01-09-2005 at 06:44 PM. Reason: second thought

  3. #3
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    You know torque wrenches aren't all that expensive, and bicycle parts are. And sometimes when a part fails it can cost a lot more than just the part. Word to the wise.

  4. #4

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    Quote Originally Posted by Waxbytes
    You know torque wrenches aren't all that expensive, and bicycle parts are. And sometimes when a part fails it can cost a lot more than just the part. Word to the wise.
    I knew it was a matter of time before someone brought up the torque wrench. Damn the gold standards... that being said though, it still needs to be tight enough to not move during use, and so you could take the practical route - just make small changes in tightness, if it moves, go a little more, etc.

  5. #5
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    thanks.... re torque wrench buying.... which? how much?

    thank you both.....

    so what sort of money (approx) are we talking about when buying a torque wrench then? any particular brands? I just need one purely for bicycle purposes, nothing industrial or expensive. Simple and effective. Do I need to buy one from a bicycle shop, ideally?

  6. #6

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    Sears Craftsman Beam Torque Wrench

    Quote Originally Posted by acid_rider
    thank you both.....

    so what sort of money (approx) are we talking about when buying a torque wrench then? any particular brands? I just need one purely for bicycle purposes, nothing industrial or expensive. Simple and effective. Do I need to buy one from a bicycle shop, ideally?
    Sears has a simple one for $25. You could spend up to $300 if you got a real fancy one.

    (Word to the wise: Don't spend $300. ;) )

  7. #7

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    Index Finger Rule...

    The mech at my lbs gave me this tip when tightening the bolt on my seatpost clamp for my carbon post... tighten the bolt with the allen key until some resistance is felt... then use only ur index finger to push down the key until ur finger can no only apply anymore torque to tighten the bolt furthur... that the rough guide... i always tighten my bolts wif this rule and have never cracked any thing yet....

  8. #8
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    thank you all!

  9. #9
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    yep, i'm like that.. i'm pretty handy with a torque wrench, but dont have the allen key bits for it... wont get them either... most of these little fiddley bits dont need it. For things that require around 10-15nm, i usually go finger tight, then no more than 1/4 turn. Its pretty safe, and these things have quite a bit of margin for error. Doesn't mean you should go the full 720 degrees tho... ;)

  10. #10
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    I wouldn't worry

    IMHO, the bolt would snap before you got it tight enough tocrush a carbon seatpost.

  11. #11

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    Agreed

    Quote Originally Posted by Jim Nazium
    IMHO, the bolt would snap before you got it tight enough tocrush a carbon seatpost.
    It would take a tremendous amount of force (well, more then a seatpost clamp could exert) to actually crush a carbon post. Then main worry is tightening it to the point that it applies a point load that would make the post more likely to break from flexing under your body weight. A torque wrench isn't a bad idea, but not 100% necessary. For the really expensive carbon posts I usually use a torque wrench to cover my own @ss (if a customer breaks his/her post, I can say I torqued it properly, therefore not my fault and they are more likely to get warrantee coverage). When I say really expensive, I mean the rediculously light ones (the Alpha-Q post that weighs around 140g comes to mind). I do the same thing with clamping carbon posts in a repair stand. I'll lightly clamp my own if I'm doing minor maintenance on my bike, but will always put in a cheap Al post at work for the sake of not freaking out customers who seem to think carbon is of similar strength as glass.

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