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Old 08-31-06, 04:07 PM   #1
PhilThee
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Aftermarket Clamp-How do you get the size of the seat post clamp correct?

How do you get the size of the seat post clamp correct?

My seatpost is 27.2
Do I simply get a 27.2 seatpost clamp?
Do they compensate for the amount of material between the post and the clamp?

Do I have to take a pair of calipers to it?

It's a 2006 Cannondale R1000
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Old 08-31-06, 04:24 PM   #2
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I don't know about an industry standard tube thickness, but I can tell you that your seatpost diameter is not the seattube diameter. My Rocky Mountain Flow uses a 26.8mm seatpost and a 27.2mm clamp.

I had to use a vernier caliper to measure mine.
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Old 08-31-06, 04:59 PM   #3
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Quote:
Originally Posted by Urbanbiketech
I can tell you that your seatpost diameter is not the seattube diameter.
Yea I knew that but I didn't word that correctly
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Old 08-31-06, 05:24 PM   #4
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Common seatpost clamp sizes are 28.6mm (1 1/8"), 30mm, 31.8mm (1 1/4"), and 34.9mm (1 3/8").

Measure the diameter of your seat tube where the clamp sits. Seatpost diameter often has little to do with seatpost clamp size, while frame materials often do.
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Old 08-31-06, 05:30 PM   #5
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Quote:
Originally Posted by Urbanbiketech
I don't know about an industry standard tube thickness, but I can tell you that your seatpost diameter is not the seattube diameter. My Rocky Mountain Flow uses a 26.8mm seatpost and a 27.2mm clamp.

I had to use a vernier caliper to measure mine.
So the seat tube on your frame is only .2mm thick? I think you need to measure again.
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Old 08-31-06, 06:28 PM   #6
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Oops. My bad. It is 28.6mm. I was doing it from memory, but I had a look at the clamp (on which the size is stamped)
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Old 08-31-06, 08:04 PM   #7
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Originally Posted by Svr
So the seat tube on your frame is only .2mm thick? I think you need to measure again.
But on the bright side...

I'll bet that thing is HELLA LIGHT!!!
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Old 08-31-06, 09:39 PM   #8
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If you have a 27.2 seat tube, you probably need a 31.8 clamp. Don't throw stuff at me if I'm wrong, I would measure it if you can, but if I had to guess, that is what I would say.
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Old 08-31-06, 09:42 PM   #9
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Wait, I'm thinking deraileur clamps ignore me.
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Old 08-31-06, 09:49 PM   #10
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My frame takes a 27.2 seatpost, and a 31.8 FD clamp. I use a 32 seat clamp, it works perfectly.

Phil--are you using a clamp-on or braze-on FD? If clamp-on, you will have an idea what seat clamp to use.
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Old 03-22-15, 05:19 AM   #11
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If your frame is steel or thin wall aluminum and your seat post diameter is 27.2mm, then a 31.8 clamp will fit. If your frame is aluminum and the seat tube walls are thicker, then a 31.8 clamp will NOT fit. You need the 34.9mm size. The only ways to tell for sure are to either measure the seat tube at the clamp location OR to see what size the factory clamp was (some, but not all, have the size stamped on them).

Do NOT try to pry open a 31.8 clamp to fit a 34.9 seat tube. The pinch-bolt either won't be long enough and/or the holes will no longer be parallel.

I speak from experience after ordering not one but two 31.8mm clamps that won't fit my aluminum frame!

I reopen this 2006 thread because a Google search directed me here.

PS: For my bike, a 34.9mm clamp won't fit either! My seat tube diameter is 33.6mm! How ******** is that? I guess I'll need to use a 34.9 with shims? The only clamp offered by the manufacturer is a QR type, and I want a bolt-on to prevent snatch-and-run theft of my expensive Brooks saddle. If I can find a hemispherical metal washer (to fill the QR slot on the factory clamp), then I can modify the thing into a bolt-on... Hardware store, here I come!
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Old 03-22-15, 10:40 AM   #12
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this is one of those times when i can measure if i want to, order it, wait, and hope for the best when it shows up or... take the bike to the LBS, where i'm pretty sure they will have whatever size i need, and make sure. the price difference is immaterial and i immediatly see if it's the right size or not, know it will work, and i'll have it right now.
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Old 03-22-15, 01:03 PM   #13
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Do I have to take a pair of calipers to it?
why do you have to seek crowd support to measure something Yourself .

What has become of the Younger generation?
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Old 03-22-15, 01:45 PM   #14
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why do you have to seek crowd support to measure something Yourself .

What has become of the Younger generation?
They want everything done for them .
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Old 03-22-15, 01:58 PM   #15
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Well, stick that seatpost where the sun don't shine! LOL. The LBS not only didn't have the right seatpost clamp (despite being the dealer for the brand), but also had not realized that such a size existed! Something new every day...
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