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31.8 Seat tube diameter, what size seatpost?

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31.8 Seat tube diameter, what size seatpost?

Old 04-06-11, 11:21 AM
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31.8 Seat tube diameter, what size seatpost?

I'm shopping for a Thomson Elite seatpost on eBay and I see the sizes all over the place: 26.8, 27.2, 30.9, 31.6....which is it? FWIW: The CAAD5 accepts a 31.8 clamp front derailleur.

Thanks
-Jeff
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Old 04-06-11, 11:31 AM
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Old 04-06-11, 11:37 AM
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I did use one, and it measured 29.845mm (1.175in).
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Old 04-06-11, 11:39 AM
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i googled seat post diameter for a caad5. i found 27.2.

edit: by the way, i don't have a caad5, this is just what i googled.
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Old 04-06-11, 11:51 AM
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Originally Posted by rockon
My bike has 31.8 diameter and I'm using 31.6 Thomson Master Piece. It fits perfectly
Your seat tube is 0.1 mm thick, really?
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Old 04-06-11, 11:53 AM
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Originally Posted by hao
Your seat tube is 0.1 mm thick, really?
The tubes can flare from the time it goes from the derailleur to the seatpost.
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Old 04-06-11, 11:54 AM
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Originally Posted by efiniluvr
I'm shopping for a Thomson Elite seatpost on eBay and I see the sizes all over the place: 26.8, 27.2, 30.9, 31.6....which is it? FWIW: The CAAD5 accepts a 31.8 clamp front derailleur.

Thanks
-Jeff
Have you tried calling Cannondale to ask them? It is going to be next to impossible for anyone that doesn't own (or have experience) with that frame to help you on this one. Tubes can be all different sizes so there is no sure way to know without measuring.
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Old 04-06-11, 11:58 AM
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Originally Posted by LowCel
The tubes can flare from the time it goes from the derailleur to the seatpost.
Sure, but the top is still 0.1 mm thin. That's what, the thickness of your hair?
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Old 04-06-11, 11:59 AM
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From Sheldon Brown's website

https://sheldonbrown.com/seatpost-sizes.html#c
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Old 04-06-11, 12:01 PM
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31.6mm clamp on FD and 27.2 sound reasonable, but i have a raleigh r700 from that era, where the seatpost is most definitely not 27.2, its smaller (i tried to jam a 27.2 post in there and it didn't have a hope in hell) so, i'd say go by the measurement, or stick with the post you already have, it might need a weird size like my raleigh.

and don't even get me started on my 1979 Koga Miyata, its got something like a 26.7mm or some random ass size.
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Old 04-06-11, 12:07 PM
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Originally Posted by efiniluvr
I did use one, and it measured 29.845mm (1.175in).
I meant to say I measured the diameter of the seatpost
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Old 04-06-11, 12:08 PM
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Originally Posted by rockon
My bike has 31.8 diameter and I'm using 31.6 Thomson Master Piece. It fits perfectly
Is that 31.8 seat tube diameter? or the seat post diameter?
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Old 04-06-11, 12:12 PM
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Originally Posted by hao
Sure, but the top is still 0.1 mm thin. That's what, the thickness of your hair?
But when a tube flares it can bet bigger which means that it could take a 31.8 derailleur but up near the seatpost the tube "could" be 35 or 36 mm on the outside. It all depends on how the bike is designed.
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