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  1. #1
    Senior Member TimJ's Avatar
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    Seatpost clamp size

    Probably a dumb question, but is the diameter listed for a seatpost clamp the diameter of the seatpost it's supposed to clamp of the diameter of the clamp itself?

    I'm guessing the clamp itself since tube thickness would make measuring the other way pointless, but follow-up question: is sizing pretty standard? I'm away from my bike right now but I want to place an order for some stuff. My bike takes a 30.8 seatpost, and itís aluminum, so 34.9, which seems to be a standard, sounds about right.

    Think I can assume itís 34.9?

  2. #2
    Perineal Pressurized dobber's Avatar
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    Clamp diameters are spec'd on the OD of the tubing.

    Best thing to do is measure with a pair of calipers. A 4mm differential is about 0.160". Does your tubing walls look like they are 0.08" (~5/64") thick?. That strikes me as quite a beefy frame.
    This is Africa, 1943. War spits out its violence overhead and the sandy graveyard swallows it up. Her name is King Nine, B-25, medium bomber, Twelfth Air Force. On a hot, still morning she took off from Tunisia to bomb the southern tip of Italy. An errant piece of flak tore a hole in a wing tank and, like a wounded bird, this is where she landed, not to return on this day, or any other day.

  3. #3
    me have long head tube TallRider's Avatar
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    Yeah, clamp diameter has to be wider than the seatpost, otherwise there'd be no room for the frame between the clamp and the seatpost. Also, the diff sounds about right. 2mm thick tubing at the seatpost-insertion end of the seattube sounds thinner than what I've got on my aluminum frame, actually. No reason to convert mm to inches for somethign like this, btw.

  4. #4
    Perineal Pressurized dobber's Avatar
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    Quote Originally Posted by timcupery
    No reason to convert mm to inches for somethign like this, btw.
    Unless you're trying to give somebody a sense of scale. Not everybody can convert mm to inches off the top of thier head.
    This is Africa, 1943. War spits out its violence overhead and the sandy graveyard swallows it up. Her name is King Nine, B-25, medium bomber, Twelfth Air Force. On a hot, still morning she took off from Tunisia to bomb the southern tip of Italy. An errant piece of flak tore a hole in a wing tank and, like a wounded bird, this is where she landed, not to return on this day, or any other day.

  5. #5
    me have long head tube TallRider's Avatar
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    Quote Originally Posted by dobber
    Unless you're trying to give somebody a sense of scale. Not everybody can convert mm to inches off the top of thier head.
    True. I've just been working with bike stuff for so long that metric is just as intuitive as inches.

    Btw, my frame, which has a 27.2mm seatpost, uses a 31.0mm or 31.2mm seatclamp; I can't remember what the exact dimension is. The frame is aluminum.

  6. #6
    Senior Member TimJ's Avatar
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    I'll bet it's correct. I'm just going to order it rather than be prudent and pragmatic. It's an older rockhopper frame and the tubing is pretty beefy.

    It was sort of a dumb question, but sometimes a part will be sold by the measurement of what it fits - like quill stems that fit a 1 1/8 headset seem to be called 1 1/8 stems.

    Thanks.

  7. #7
    Perineal Pressurized dobber's Avatar
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    Quote Originally Posted by timcupery
    True. I've just been working with bike stuff for so long that metric is just as intuitive as inches.

    Btw, my frame, which has a 27.2mm seatpost, uses a 31.0mm or 31.2mm seatclamp; I can't remember what the exact dimension is. The frame is aluminum.
    Hey, you're right, I just went an looked up an order I placed last year for a seatpost clamp. 31.8mm, it's an aluminum frame with a 27.2mm seatpost. That tubing must be thicker than I thought, it is a tank though.
    This is Africa, 1943. War spits out its violence overhead and the sandy graveyard swallows it up. Her name is King Nine, B-25, medium bomber, Twelfth Air Force. On a hot, still morning she took off from Tunisia to bomb the southern tip of Italy. An errant piece of flak tore a hole in a wing tank and, like a wounded bird, this is where she landed, not to return on this day, or any other day.

  8. #8
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    Seat tubes pretty much come in three standard outside diameters of 1-1/8" (28.6 mm), 1-1/4" (31.8 mm) and 1-3/8" (34.9 mm) and those are the three sizes the external clamps come in. Sometimes you will see the size numbers rounded off in the ads (34.9 is called 35 mm for example) but those are the choices. A ruler should be able to tell you what size you need.

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