PhilThee

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

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.

PhilThee

Yea I knew that but I didn't word that correctly

Svr

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.

Svr

So the seat tube on your frame is only .2mm thick? I think you need to measure again.

Urbanbiketech

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)

PhilThee

But on the bright side...

I'll bet that thing is HELLA LIGHT!!!

Flanderflop

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.

Flanderflop

Wait, I'm thinking deraileur clamps ignore me.

Namenda

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.

FarHorizon

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!

hueyhoolihan

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.

fietsbob

why do you have to seek crowd support to measure something Yourself .

What has become of the Younger generation?

bikeman715

They want everything done for them .

FarHorizon

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...

bellhydro

Seat Post Clamp Diameter: The internal diameter of the seat clamp must correspond with the external diameter of the seat tube – too big, and the seat post may move in the frame, with the risk of cracking; too small and the collar can’t fit on the top of the tube (and you might damage your frame in trying to force it on). There are five standard seat tube diameters on the market: 28.6mm, 30.0mm, 31.8mm, 34.9mm, and 36.4mm, so once you know the size of your seat tube (measure with a calipers or check your manufacturer’s specifications) make sure to buy a seatclamp that matches.

fietsbob

It has happened that a lot of standards, for bicycles seem weird to newbies,
like steel tube diameters

out side are actually imperial fractions, but expressed in metric equivalents..

1 inch = 25.4mm ..

wesmamyke

It's worth mentioning that seat tube size and front derailleur clamp size don't always equal the post clamp size. I've had a couple frames that had thicker sleeved sections at the top and took one size larger clamps.

greeniearts

Good info--thanks. The OD of my seat tube is 35.05mm (with a post diameter of 31.24mm). Is the 36.4mm clamp referencing the diameter of the clamp when fully closed? -- if so, I might need to procure a 34.9mm version and wiggle it onto my bike...