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  1. #1
    RetroGrouchWrench Rural Roadie's Avatar
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    Seat tube to seat post clearance

    Got around to measureing several seatposts and they don't measure what is stamped into them so this designation must be for the bore of the seat tube the post is to fit.
    What is the usual clearance between the two, I will post my measurements later but would like to know what the standard is.
    Inch or metric is fine by me.
    TIA
    Kevin

  2. #2
    GT enthusiast midwestmntnbkr's Avatar
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    Quote Originally Posted by Rural Roadie
    Got around to measureing several seatposts and they don't measure what is stamped into them so this designation must be for the bore of the seat tube the post is to fit.
    What is the usual clearance between the two, I will post my measurements later but would like to know what the standard is.
    Inch or metric is fine by me.
    TIA
    Kevin

    The inside of the seat tube on your frame should be the same as the outside of your seatpost. their is no clearance to speak of. The stamped measurement on the post should be the exact size of the post. You really should use a micrometer or caliper to measure them. The measurements come in so many sizes and many of them are only .2mm different or less.

    Someone correct me if I am wrong.
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  3. #3
    Junior Member LukeH's Avatar
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    midwestmtnbkr is correct!

    The seat post should measure exactly the same as the seat tube. You risk stressing the frame if they do not match!

    It is possible to use shims to make a smaller seat post fit a larger seat tube, however I would avoid this as I have experienced movement in the seat post during high stress i.e. racing!

    Hope this helps.

    Luke

  4. #4
    Can't ride enough! Da Tinker's Avatar
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    So can anyone explain my problem: I have an old frame I am building up as a fixie. My caliper tells me seatpost ID of 26.2 mm and yet the 26.0 mm post from my bride's bike fits great. Her post mics 26.0. Don't you have to have a bit of clearance?
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  5. #5
    Senior Member demoncyclist's Avatar
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    It needs to be tight. The clamp should be able to secure it without being tight enough to stress the seatpost. The seat tube does most of the work just by friction.
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    Quote Originally Posted by Da Tinker
    So can anyone explain my problem: I have an old frame I am building up as a fixie. My caliper tells me seatpost ID of 26.2 mm and yet the 26.0 mm post from my bride's bike fits great. Her post mics 26.0. Don't you have to have a bit of clearance?
    Yes there has to be a slight difference.

  7. #7
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    Quote Originally Posted by demoncyclist
    The seat tube does most of the work just by friction.
    That's just NOT right. Repeat, NOT right.

  8. #8
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    Quote Originally Posted by Rural Roadie
    Got around to measureing several seatposts and they don't measure what is stamped into them so this designation must be for the bore of the seat tube the post is to fit.
    What is the usual clearance between the two, I will post my measurements later but would like to know what the standard is.
    Inch or metric is fine by me.
    TIA
    Kevin
    The seat post should measure what is stamped it.There are slight manufacturing tolerances.Sometimes one might get the wrong stamp.

  9. #9
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    Quote Originally Posted by LukeH
    midwestmtnbkr is correct!

    The seat post should measure exactly the same as the seat tube. You risk stressing the frame if they do not match!
    If it measured exactly the same, it would not fit. This is not rocket science. There has to be a difference.There are manufacturers tolerances in both seatpost and seattubes, that have to be accomodated.

  10. #10
    GT enthusiast midwestmntnbkr's Avatar
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    Quote Originally Posted by tommy2pants
    If it measured exactly the same, it would not fit. This is not rocket science. There has to be a difference.There are manufacturers tolerances in both seatpost and seattubes, that have to be accomodated.
    Their is no difference...the tiny bit of clearance is not measureable. we're talking tenths of milllimeters here. less than a human hair.

    If you have a 26.2mm seatpost it will not fit a 26.0 frame. IT is that close.
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  11. #11
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    Quote Originally Posted by midwestmntnbkr
    Their is no difference...the tiny bit of clearance is not measureable. we're talking tenths of milllimeters here. less than a human hair.

    If you have a 26.2mm seatpost it will not fit a 26.0 frame. IT is that close.
    Go back and read it again.He said the id of the seatpost,(meaning seattube) measures 26.2 and the post measures 26.0. Like I said it isn't rocket science, 10ths of milimeters certainly IS measureable and certailnly a 26.2 post does not go in a 26.0 seattube,assuming the post is marked and or measured correctly.

  12. #12
    GT enthusiast midwestmntnbkr's Avatar
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    Quote Originally Posted by tommy2pants
    Go back and read it again.He said the id of the seatpost,(meaning seattube) measures 26.2 and the post measures 26.0. Like I said it isn't rocket science, 10ths of milimeters certainly IS measureable and certailnly a 26.2 post does not go in a 26.0 seattube,assuming the post is marked and or measured correctly.
    You're the expert here. whatever
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    you're taking up too much space"


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  13. #13
    The Red Lantern Rev.Chuck's Avatar
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    Friction is what keeps a seatpost from moving in the seattube. From Webster:friction, the force that resists relative motion between two bodies in contact.
    You are using a clamp to increase the level of friction to the point the seatpost will not move, much like you press down on the brake pedal hard enough to push the pads into the rotor to completely stop it so you don't roll back into the car behind you. The condition is static but it is still friction.

    There can be some very fine tolerances. I used to rebuild rotary swash plate piston pumps for hydraulic drives and the piston clearance is so tight if you warm one in your hand it will not fit in the pump.

    A inside/outside caliper is(typically) not accurate enough to measure a seat post and esp. not the seattube. You need an outside mic and a snap guage.
    Last edited by Rev.Chuck; 01-12-04 at 08:35 PM.
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  14. #14
    RetroGrouchWrench Rural Roadie's Avatar
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    When I'm talking clearence I mean .003", as a trained machinest this is a lot, the thickness of a sheet of paper.
    Okay here is the first example, Sakae post marked 26.6mm and it measures 26.49mm for a clearence of .11mm or about .0043".
    Another post was just .05mm under listing.
    So what is the industry standard or better yet what do the good framebuilders allow? Maybe a link to a website with this kinda techinal info?

    BTW the only time I had trouble with a post staying put under my mass was a post undersized becuse of a lump in the seat tube, trued up the bore and using the correct post fixed it.

    FYI, this was measured with a carbide faced Mitutoyo 25 to 50 mm micrometer rated accurate to .001mm.
    Less than $20 bucks lightly used from ebay.

  15. #15
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    Quote Originally Posted by Rev.Chuck
    Friction is what keeps a seatpost from moving in the seattube. From Webster:friction, the force that resists relative motion between two bodies in contact.
    You are using a clamp to increase the level of friction to the point the seatpost will not move, much like you press down on the brake pedal hard enough to push the pads into the rotor to completely stop it so you don't roll back into the car behind you. The condition is static but it is still friction.

    There can be some very fine tolerances. I used to rebuild rotary swash plate piston pumps for hydraulic drives and the piston clearance is so tight if you warm one in your hand it will not fit in the pump.

    A inside/outside caliper is(typically) not accurate enough to measure a seat post and esp. not the seattube. You need an outside mic and a snap guage.
    Several of my 27.2 post measured 27.2 by my crude measurements.All the holes they fit into measure slightly bigger. Some fits are sloppier than others. The clamp provides the friction to keep them in place.

  16. #16
    Senior Member gazedrop's Avatar
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    Are you guys calibrating your micrometers against a known accurate standard at 68 degrees F on the day of measurement? Many smileys to show that I am JUST KIDDING!!!

    But seriously, I don't know if there actually are any published tolerances on seatpost clearance (I know... I'm a whole lotta help!) But I would be willing to postulate that the intended clearance should be less than 0.1 mm. I am basing this on the availability of seatposts rated at 26.0, 26.2. 26.8, 27.0, 27.2, etc.

    I would also say that 0.1 mm or less is definately a ballpark because a seat tube is finished off by a person handholding an electric drill driving a 3 bladed reamer, followed by a flex hone... So, it depends upon his mood and the relative tension of his anus...

    (A hand-lapped seatpost? Geez, THAT would be anal!)

    Although, I have read (on www.roadbikereview.com) that Moots waranteed a chronically slipping post because it measured "On the small side" of their tolerances...

    Check with the bicycle manufacturer to be sure of the intended seat tube size if you're unsure.

    And then don't get all dyslexic when ordering that 26.8 post for your 28.6 bike!

    -Erik

  17. #17
    RetroGrouchWrench Rural Roadie's Avatar
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    And some bikes have too much clearence and the good ansure is to turn down the next size seatpost for a custom fit, sure glad I have a metal lathe handy.
    And yes one of my clunkers is shimmed with a soda pop can shim.


    "Are you guys calibrating your micrometers against a known accurate standard at 68 degrees F on the day of measurement? "

    Yes, but the alloy seatpost will expand and contract more the the mic will from temp change.

  18. #18
    opinionated SOB cycletourist's Avatar
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    I have 3 bikes that all claim to use a 27.2mm seatpost. But the same post fits differently in all three. There is definitely some variance in seat tube inside diameters. Maybe as much as 3 tenths of millimeter. My guess is seatposts are the same.

  19. #19
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    Quote Originally Posted by cycletourist
    I have 3 bikes that all claim to use a 27.2mm seatpost. But the same post fits differently in all three. There is definitely some variance in seat tube inside diameters. Maybe as much as 3 tenths of millimeter. My guess is seatposts are the same.
    You are just not AR enough for this thread. One should buy very expensive slightly oversized posts, and have them professionally milled to +/- .ooo1mm to fit into professionally reamed and honed seattubes to the same tolerances. The seatpost material should match the seattube material so that temperature changes wil not cause differential rates of expansion and contraction rates.

  20. #20
    DEADBEEF khuon's Avatar
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    Quote Originally Posted by tommy2pants
    You are just not AR enough for this thread. One should buy very expensive slightly oversized posts, and have them professionally milled to +/- .ooo1mm to fit into professionally reamed and honed seattubes to the same tolerances. The seatpost material should match the seattube material so that temperature changes wil not cause differential rates of expansion and contraction rates.
    And then they'll gaul up. Good luck trying to get the post back out.
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    Quote Originally Posted by khuon
    And then they'll gaul up. Good luck trying to get the post back out.
    Gaul: A celt of ancient Gaul....Webster.

  22. #22
    hello roadfix's Avatar
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    If it's tight, I ram it in...
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  23. #23
    DEADBEEF khuon's Avatar
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    Quote Originally Posted by tommy2pants
    Gaul: A celt of ancient Gaul....Webster.
    Sorry...

    s/gaul/gall/
    1999 K2 OzM 2001 Aegis Aro Svelte OCP Club Member
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  24. #24
    Senior Member gazedrop's Avatar
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    Weren't the ancient Gauls known for their machining abilities? Coulda' swore that I saw some exquisite copper seatposts in a museum somewhere...

    Sorry all! Couldn't resist. And I accept partial blame for degeneration of this thread!

    But does this indicate that there is not a standard clearance (Rural Roadie's original question)?

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