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Singlespeed & Fixed Gear "I still feel that variable gears are only for people over forty-five. Isn't it better to triumph by the strength of your muscles than by the artifice of a derailer? We are getting soft...As for me, give me a fixed gear!"-- Henri Desgrange (31 January 1865 - 16 August 1940)

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Old 09-10-07, 10:15 AM   #1
jdms mvp
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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.
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Old 09-10-07, 10:17 AM   #2
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No.
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Old 09-10-07, 10:21 AM   #3
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no to both questions?
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Old 09-10-07, 10:22 AM   #4
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get it right. i've ruined a frame and cracked a seatpost over a .2mm difference.
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Old 09-10-07, 10:24 AM   #5
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get it right. i've ruined a frame and cracked a seatpost over a .2mm difference.
ahh thanks
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Old 09-10-07, 10:24 AM   #6
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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.
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Old 09-10-07, 10:25 AM   #7
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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.
will do. thanks for the quick responses.
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Old 09-10-07, 12:06 PM   #8
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There are shims, that will allow one to use a 27mm seat post in a 27.2 seat tube.
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Old 09-25-07, 08:14 PM   #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
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Old 09-25-07, 08:36 PM   #10
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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.
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Old 09-25-07, 09:40 PM   #11
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There are shims, that will allow one to use a 27mm seat post in a 27.2 seat tube.
They're called beer cans.
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Old 09-26-07, 09:58 AM   #12
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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?
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Old 09-26-07, 10:00 AM   #13
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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.
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Old 09-26-07, 10:05 AM   #14
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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...
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Old 09-26-07, 10:08 AM   #15
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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?
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Old 09-26-07, 10:17 AM   #16
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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?
yea maybe i'm incorrect? i'm just asking someone to educate me

starting to make more sense to me
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Old 09-26-07, 10:20 AM   #17
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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.
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Old 09-26-07, 01:59 PM   #18
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thanks for the link
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