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  1. #1
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    QUESTION regarding seatpost diameter

    followup question:
    so do i buy a stated "27.0mm" seat post for a 27.mm seat tube? intuition would tell me that it wouldn't work because the inner diameter of the tube and outer diamter of the post are the same?

    can someone educate me?

    -Nathan







    for a 27mm seat post diameter tube is possible to use a 27.2mm tube. the measurement "appears" to be miniscule, but would this work?

    on the flip side....will a 27.0mm post work in a 27.2mm tube and properly clamp or tighten?

    thanks in advanced
    Last edited by jdms mvp; 09-25-07 at 08:26 PM.

  2. #2
    jerk store mathletics's Avatar
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    No.

  3. #3
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    no to both questions?

  4. #4
    antisocialite dirtyphotons's Avatar
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    get it right. i've ruined a frame and cracked a seatpost over a .2mm difference.
    Quote Originally Posted by Sheldon Brown
    Because when fashion conflicts with function, I vote for function.

  5. #5
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    Quote Originally Posted by dirtyphotons View Post
    get it right. i've ruined a frame and cracked a seatpost over a .2mm difference.
    ahh thanks

  6. #6
    jerk store mathletics's Avatar
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    Yes, no to both questions. Seatposts come in 2mm increments from 25mm to 30whatever for a reason. Get the right one.

  7. #7
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    Quote Originally Posted by mathletics View Post
    Yes, no to both questions. Seatposts come in 2mm increments from 25mm to 30whatever for a reason. Get the right one.
    will do. thanks for the quick responses.

  8. #8
    Should be out Riding
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    There are shims, that will allow one to use a 27mm seat post in a 27.2 seat tube.

  9. #9
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    so do i buy a stated 27.0mm seat post for a 27.mm seat tube? intuition would tell me that it wouldn't work because the inner diameter of the tube and outer diamter of the post are the same?

    can someone educate me?

    -Natha

  10. #10
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    Quote Originally Posted by jdms mvp View Post
    so do i buy a stated 27.0mm seat post for a 27.mm seat tube? intuition would tell me that it wouldn't work because the inner diameter of the tube and outer diamter of the post are the same?

    can someone educate me?

    -Natha
    Nope it'll be fine. It's supposed to fit snugly. Any other way is bad.

    Remember to grease the seatpost before installing it.

  11. #11
    Hello. crushkilldstroy's Avatar
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    Quote Originally Posted by lvleph View Post
    There are shims, that will allow one to use a 27mm seat post in a 27.2 seat tube.
    They're called beer cans.
    Quote Originally Posted by Jacquie Phelan
    Until mountain biking came along, the bike scene was ruled by a small elite cadre of people who seemed allergic to enthusiasm.

  12. #12
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    Quote Originally Posted by mander View Post
    Nope it'll be fine. It's supposed to fit snugly. Any other way is bad.

    Remember to grease the seatpost before installing it.
    so when a bike manufacturer states 27.0mm on their post it'll be a tiny bit smaller than 27.0mm to fit in a 27.0mm tube?

  13. #13
    antisocialite dirtyphotons's Avatar
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    Quote Originally Posted by jdms mvp View Post
    so when a bike manufacturer states 27.0mm on their post it'll be a tiny bit smaller than 27.0mm to fit in a 27.0mm tube?
    when manufacturers list the "size" of the seatpost they're referring to the outer diameter of the tube. when they list the "size" of a seattube, they're referring to the inner diameter of the tube. they should be exactly the same.
    Quote Originally Posted by Sheldon Brown
    Because when fashion conflicts with function, I vote for function.

  14. #14
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    Quote Originally Posted by dirtyphotons View Post
    when manufacturers list the "size" of the seatpost they're referring to the outer diameter of the tube. when they list the "size" of a seattube, they're referring to the inner diameter of the tube. they should be exactly the same.
    well the questions i'm asking is if a Inner diameter and out dimeter are the same, then in theory* one pipe should NOT be able to fit in the other...

  15. #15
    antisocialite dirtyphotons's Avatar
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    in theory, if one pipe has the exact inner diameter as the outer of another. they will fit inside each other with exactly no play. what theory are you using?
    Quote Originally Posted by Sheldon Brown
    Because when fashion conflicts with function, I vote for function.

  16. #16
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    Quote Originally Posted by dirtyphotons View Post
    in theory, if one pipe has the exact inner diameter as the outer of another. they will fit inside each other with exactly no play. what theory are you using?
    yea maybe i'm incorrect? i'm just asking someone to educate me

    starting to make more sense to me

  17. #17
    jack of one or two trades Aeroplane's Avatar
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    Quote Originally Posted by jdms mvp View Post
    well the questions i'm asking is if a Inner diameter and out dimeter are the same, then in theory* one pipe should NOT be able to fit in the other...
    In theory, yes. But those sizes that are printed aren't exact. In fact, there will be deviation in thickness between seatposts of the same "size" even by the same manufacturer. These are called "tolerances", and everybody takes them into account for you.

    Surly has a little essay on their site that covers this in detail. Read and be educated.

    So just match the numbers.
    Quote Originally Posted by Dr Irwin Goldstein
    Men should never ride bicycles. Riding should be banned and outlawed. It is
    the most irrational form of exercise I could ever bring to discussion.

  18. #18
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    thanks for the link

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