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Simple Seatpost Shim Question

  • 11-11-2010 10:14am
    #1
    petethedrummer
    Registered Users, Registered Users 2 Posts: 7,604 ✭✭✭


    I ruineded my carbon seatpost by over tightening a few months ago. It has held up well but I think I should replace it before catastrophic failure.

    The seat tube diameter is 31.8mm, the seatpost diameter is 27.2mm and it sits inside a shim. Is there any reason why I can't use a 31.8mm carbon seatpost and do away with the shim?

    This would increase my options while shopping for a cut price seat post.


Comments

  • #2
    blorg
    Registered Users, Registered Users 2 Posts: 15,995 ✭✭✭✭blorg


    You can use a 31.8mm post, yes, and it will be stiffer than a 27.2mm too. It is not absolutely necessary but using carbon assembly compound is a good idea; it both reduces the risk of over-tightening (less torque is required to hold the post in place) and the seatpost getting stuck.


  • #3
    petethedrummer
    Registered Users, Registered Users 2 Posts: 7,604 ✭✭✭petethedrummer


    sound Blorg. I thought it would be fine but just wanted to be sure.

    I put my 31.8 Alu seatpost from my MTB in last night and didn't notice any difference in ride comfort so I might just stick with it.


  • #4
    el tel
    Registered Users, Registered Users 2 Posts: 2,991 ✭✭✭el tel


    How thick are the walls of carbon seatposts in general? I'm guessing they must not be thick (strong) enough if they fail due to overtightening.


  • #5
    SurferDude41
    Closed Accounts Posts: 339 ✭✭SurferDude41


    Are you sure thar the seatpost you require is 31.8mm?
    Most are 31.6 or 27.2 nowadays.
    What make of frame are you riding?


  • #6
    petethedrummer
    Registered Users, Registered Users 2 Posts: 7,604 ✭✭✭petethedrummer


    Could be, probably is. Easton EA30 is what I have in it at the minute. I just measured roughly. Gonna get a calipers to verify all my various seatpost requirements.


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  • #7
    blorg
    Registered Users, Registered Users 2 Posts: 15,995 ✭✭✭✭blorg


    @el tel- carbon is very strong, stronger than alloy for a given weight, but it is sensitive to crushing in a way that alloy is not.


  • #8
    el tel
    Registered Users, Registered Users 2 Posts: 2,991 ✭✭✭el tel


    Given its limitations in that regard you,d think they'd use more resin at the lower end of the seat post shank. Butting or tapering would be handy to do. Anyone know the wall thickness dimensions of a seatpost?


  • #9
    blorg
    Registered Users, Registered Users 2 Posts: 15,995 ✭✭✭✭blorg


    That would make the post heavier and is entirely unnecessary if you just don't overtighten. As different people will place their saddle at different heights you would need to do it on basically the whole length of the post. I'm also not entirely sure it would fix the susceptibility to crushing.

    The posts are strong enough in use; overtightening to the point of crushing a post is mis-use.

    FWIW I have lots of carbon seatposts and the lighter/more expensive ones do have thinner walls than the heavier ones- although most of the weight tends to be saved in clamp design. I have never broken one. Carbon posts do tend to have thicker walls than alloy ones.

    I have thinner and thicker alloy posts too for that matter.


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