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  1. #1
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    How to fit seatpost and clamp?

    My seat, post and clamp got ganked the other day and now I'm on the market for some new parts. Trouble is, I don't know what size my seat tube is and I remember last time I measured and bought one, I got the size wrong and ended up having to buy a shim . But I'm not sure what mistake I make last time as to not repeat it . So, to hopefully save myself some trouble, thought some in the community might be able to help.

    The inner diameter of the seat tube is 26.5mm and OD is 32mm. Both were measured with calipers. I'm guessing the size for the seat post should be the 26.8mm option and for the clamp, I'm thinking the 31.8mm. I haven't seen any posts that are smaller than that. What I'm concerned about is the 0.3mm extra on the seat post and the 0.2mm short on the clamp. Do seat tubes give enough to be 0.3mm larger on the post? Can I squeeze another 0.2mm out of the clamp as well? Next size up seems to be 34.9mm. Is it better to buy this one and see if it will cinch down to the 32mm? I don't know how much one can squeeze the seat tube together to a smaller size. 3mm seems like a lot to me though compared to the 10ths of mm off on the other size. Thanks for any help. It'll be nice to sit down properly again.
    Today's the day I eat bikes.

  2. #2
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    Quote Originally Posted by Dag Nabbit
    My seat, post and clamp got ganked the other day and now I'm on the market for some new parts. Trouble is, I don't know what size my seat tube is and I remember last time I measured and bought one, I got the size wrong and ended up having to buy a shim . But I'm not sure what mistake I make last time as to not repeat it . So, to hopefully save myself some trouble, thought some in the community might be able to help.

    The inner diameter of the seat tube is 26.5mm and OD is 32mm. Both were measured with calipers. I'm guessing the size for the seat post should be the 26.8mm option and for the clamp, I'm thinking the 31.8mm. I haven't seen any posts that are smaller than that. What I'm concerned about is the 0.3mm extra on the seat post and the 0.2mm short on the clamp. Do seat tubes give enough to be 0.3mm larger on the post? Can I squeeze another 0.2mm out of the clamp as well? Next size up seems to be 34.9mm. Is it better to buy this one and see if it will cinch down to the 32mm? I don't know how much one can squeeze the seat tube together to a smaller size. 3mm seems like a lot to me though compared to the 10ths of mm off on the other size. Thanks for any help. It'll be nice to sit down properly again.
    Stick a ripe banana down the seat tube, and then bring to your LBS. They should be able to measure it that way.

    What frame do you have, and year is is. Most bike makers list seat tube diameters.
    Nobody gives a s#$t you singlespeed.

  3. #3
    Break it, Fix it, Ride it
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    Quote Originally Posted by skinny-tire
    What frame do you have, and year is is. Most bike makers list seat tube diameters.
    I'm pretty sure it's a 2002 GF Marlin. It might be a 2001. Checked the archives on the GF site, but the specs don't give the measurement, just the seat post make, a Bontrager Sport. Sheldon B had some sort of seatpost database as well but lacked my entry. The later models sport a 27.2mm, a bit big I thought.

    I think I'd rather ride a watermelon to the store, rather than a banana. Its all about the sitting position for me. Already got screwed once by whoever took my seat to begin with.
    Today's the day I eat bikes.

  4. #4
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    How 'bout try the Gary fisher forum?
    I think I'm not as good as I thought.

  5. #5
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    Quote Originally Posted by ajmelin
    How 'bout try the Gary fisher forum?
    You're a genius. I forget there are other useful forums around here sometimes. Time for a x-post! Thx.
    Today's the day I eat bikes.

  6. #6
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    [QUOTE=Dag Nabbit]The inner diameter of the seat tube is 26.5mm and OD is 32mm[QUOTE]

    The most common size in use today is 31.9mm. I bet your 32mm is a 31.9. It's that easy.
    Better than most, not as good as some.

  7. #7
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    [QUOTE=surfndav][QUOTE=Dag Nabbit]The inner diameter of the seat tube is 26.5mm and OD is 32mm

    The most common size in use today is 31.9mm. I bet your 32mm is a 31.9. It's that easy.
    I think what he's asking is, when measuring the seat tube, is the measurement the actually opening size (i.e., "inner diameter) or factoring in the width of the aluminum (i.e., outer diameter)?

    I've always assumed the sizing was based on the inner diameter, but then again I could be completely wrong. I still say the only true way to find this out is stick a banana in the hole.
    Nobody gives a s#$t you singlespeed.

  8. #8
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    Quote Originally Posted by skinny-tire
    I think what he's asking is, when measuring the seat tube, is the measurement the actually opening size (i.e., "inner diameter) or factoring in the width of the aluminum (i.e., outer diameter)?

    I've always assumed the sizing was based on the inner diameter, but then again I could be completely wrong. I still say the only true way to find this out is stick a banana in the hole.
    I measured the inner diameter of the seat tube for the seatpost size and the outer diameter of the seat tube for the clamp size (I need both.) I assumed that's how they got the specs for each part (post and clamp) and haven't thought twice about that approach. The trouble seems to be that there aren't any parts available that are the sizes I measured, just close by measurements. My trouble in deciding is that I don't know how much "play" I can account for when buying the parts since the seat tube has a notch taken out of it for fitting. Will it accept a seatpost that is 0.3 or even 0.7mm larger than than my ID measurement? Will it accept a seat clamp that is 0.2mm smaller than the OD? Oh, my. This is getting confusing.... But I appreciate the attention!
    Today's the day I eat bikes.

  9. #9
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    Quote Originally Posted by Dag Nabbit
    Will it accept a seatpost that is 0.3 or even 0.7mm larger than than my ID measurement? Will it accept a seat clamp that is 0.2mm smaller than the OD? Oh, my. This is getting confusing.... But I appreciate the attention!
    I'd venture to say that if your measurements are accurate, then I doubt it will accept a larger seatpost. I went on Pricepoint, and the smallest I could find was 26.8, which is as you say .3 mm too large. I've venture to say the same is true for the collar. As you mentioned earlier, you can add shims to posts that are too small, but not sure how much --or even if there is--wiggle room there is for posts that are too big. I could be completely wrong, so maybe somebody here has experience with this...

    That said, I believe most seat posts are standard sizes, and assuming you can't easily find a 26.5mm post, perhanps your measurements are a bit off?

    Personally, when I get stumped with this kind of stuff, I break down and take to my LBS. But that's usually last resort.
    Nobody gives a s#$t you singlespeed.

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