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  1. #1
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    Seatpost clamp size for a 29.8 seatpost

    Hi !

    Could you pls let me know the seatpost clamp size, that I should buy for a 29.8 Seatpost. Unfortunately the availability in india is nil, I would have to go online for a purchase. I have already tried a 31.8 clamp and been unsuccessful.

    If possible could I have a table for all the compatibility like : SeatPost size---- Seat post clamp. This would certainly help !

    Pls advise

    rgds

  2. #2
    Senior Member FlatSix911's Avatar
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    I would suggest using a 31.8 clamp with a 2mm shim to fit a 29.8 seatpost
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  3. #3
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    Quote Originally Posted by FlatSix911 View Post
    I would suggest using a 31.8 clamp with a 2mm shim to fit a 29.8 seatpost
    This keeps coming up.

    The OP said a 29.8 post, not frame and we need to take him at his word.

    Clamps are sized for the seat tube the post fits in not the post itself. So if the post is 29.8, then assuredly the seat tube is bigger.

    Until fairly recently, life was easy and seat tubes commonly were one of three diameters (imperial sizes are standard) 1-1/8" (28.6mm), 1-1/4" (31.8) and 1/3/8" (35mm). Note that the metric sizes are rounded off and may be expressed as 31.75 for example. What made things nice is the seat post collar was almost always the same size as the front derailleur band which fit the same tube.

    However that was too good to last and now there all sorts of new clamp sizes, and they no longer automatically match the FD band.

    In this case I'd expect that a 29.8mm post would almost surely need a 1-3/8" collar, aka 34.9, or 35mm. That allows 3mm wall thickness for the seat tube which is about par for an aluminum or carbon frame. If the frame happens to be steel then it might take a 31.8, but that's already been ruled out.

    BTW- since there aren't that many possible sizes (yet) you might measure it by wrapping a strip of paper around the top of the seat tube, marking the overlap, then laying it out and dividing the circumference by 3.14 to get the diameter. Then going to the nearest size stock clamp
    Last edited by FBinNY; 02-13-12 at 10:14 PM.
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    Thomson has a clamp in that size. Ebay.

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    Quote Originally Posted by ultraman6970 View Post
    Thomson has a clamp in that size. Ebay.
    Yes, but before buying it the OP has to decide whether he has a 29.8 seat post (marked on the post itself) or a smaller post in a frame that takes a 29.8 collar.

    The issue is the difference between the ID and OD of the seat tube.
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  6. #6
    coprolite fietsbob's Avatar
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    Bike shop.. they can measure your bike, 1st hand,
    if not in stock, there is the order book ..
    and It will be handed over the counter when it comes in ..

    I only used a Clamp directly on a Seat post on my Folding bike when it slipped down.
    and the QR seat tube/ post clamp got it's edge lip removed to do that.

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    If the seatpost is 29.8 or 30 mm pretty much the seat tube has to be at least 31.6. Isnt better for him just go to home depot and get a cheappo caliper, learn to use it and keep it?? a caliper is something used almost all the time.

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    coprolite fietsbob's Avatar
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    the digital micrometer calipers really are not that dear,
    China produces some of those too,

  9. #9
    Senior Member Bill Kapaun's Avatar
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    Quote Originally Posted by ultraman6970 View Post
    If the seatpost is 29.8 or 30 mm pretty much the seat tube has to be at least 31.6. Isnt better for him just go to home depot and get a cheappo caliper, learn to use it and keep it?? a caliper is something used almost all the time.
    Apparently you missed the part about- "...Unfortunately the availability in india is nil...."

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    Quote Originally Posted by ultraman6970 View Post
    If the seatpost is 29.8 or 30 mm pretty much the seat tube has to be at least 31.6. Isnt better for him just go to home depot and get a cheappo caliper, learn to use it and keep it?? a caliper is something used almost all the time.
    On steel frames the tube walls are about .7-.9mm thick, so the OD (clamp size) is 1.4-1.8mm larger than the ID (seat post size). Carbon and aluminum frames would have OD's more like 4-6mm larger than the seatpost size.

    In any case the OP already tried a 31.8mm (1-1/4") clamp and it wasn't a go. Unfortunately he didn't say whether the clamp was too small or too big, leaving us guessing. my best guess, based on thr assumption that it's not likely to be a steel frame is that he needs a 1-3/8" or 34.9mm (aka 35mm) clamp. But that's just an educated guess.
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    Thank you so much for your replies, yes the 31.8 clamp was too small for the particular seattube. And yes it is an alloy frame, I have bike with a 34.9 quick release collar and tried it on, the fit was marginally on the larger side( ie the collar is slightly loose on the seattube). Is there a DIY shim that I could make for this little gap. Over tightening the quick allen after turning the key does solve it , but I am not in the comfort zone.

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    Quote Originally Posted by atlantis View Post
    I have bike with a 34.9 quick release collar and tried it on, the fit was marginally on the larger side( ie the collar is slightly loose on the seattube). Is there a DIY shim that I could make for this little gap. Over tightening the quick allen after turning the key does solve it , but I am not in the comfort zone.
    I suspected from the beginning that a 34.9 collar is correct, and this seems to confirm that. It shouldn't need to be shimmed. The test is how close the ears in back are when it's closed tight. If there's still 2-3mm between the ears, than all is good, and you haven't overtightened it, just adjusted it to clamp correctly. (there's variation (tolerance) in tube and post diameters, which is why the QR's are adjustable in the first place)

    If you're still concerned, and feel you want to shim it slightly, you can do so with a few turns of scotch tape on the part of the tube under the collar. There's no movement there and the tape will last as long as your bike. If the shim thickness is a bit more than a turn or two of scotch tape, then try cloth surgical tape where you'll probably need only one turn.
    Last edited by FBinNY; 02-21-12 at 09:25 AM.
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    WARNING, I'm from New York. Thin skinned people should maintain safe distance.

  13. #13
    Senior Member Jed19's Avatar
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    [QUOTE=FBinNY;13849195]BTW- since there aren't that many possible sizes (yet) you might measure it by wrapping a strip of paper around the top of the seat tube, marking the overlap, then laying it out and dividing the circumference by 3.14 to get the diameter. Then going to the nearest size stock clamp[/QUOTE]

    Measuring clamp size is no brain surgery.

    And why guess?

    FB nailed the answer.
    Regards,

    Jed

  14. #14
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    India??? thought they did not ride bicycles but darn scooters and 500 dollars family cars.

    Quote Originally Posted by Bill Kapaun View Post
    Apparently you missed the part about- "...Unfortunately the availability in india is nil...."

  15. #15
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    Quote Originally Posted by ultraman6970 View Post
    India??? thought they did not ride bicycles but darn scooters and 500 dollars family cars.
    http://www.youtube.com/watch?v=nEpbkhUMDbM

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