# Seat post clamp sizing



## Vitamin G (Oct 11, 2004)

What??? There's no dedicated seatpost forum? :lol:

Anyway.... I wanted to replace the Bontrager quick release clamp on my 2006 X-Cal with a Salsa lip-lock clamp. So, I took the Bontrager clamp off, and noticed that it said 32.0 on it.

OK, must be 32.0 mm. I will order that size. Big mistake. The Salsa doesn't fit. Furthermore, it's nowhere near 32.0 mm ID, coming closer to 28 mm.

My bike has a 27.2 mm diameter seatpost. 

How do I go about measuring for a replacement clamp?


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## boomn (Jul 6, 2007)

This calls for some double-checking. I've had a couple Salsa clamps and they're all within a fraction of a millimeter of the stated size. 

They have a lip that sticks in on the top side that stops it from sliding down the frame and therefore has a smaller ID. Do you have it right side up?


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## theMeat (Jan 5, 2010)

Hmm, something sounds fishy. With a 27.2 seatpost, anything from like 31.8 - 32 should be perfect.


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## dru (Sep 4, 2006)

The 32 you saw on the original clamp should refer to the O.D. of the seat tube. The clamp you need has zilch to do with the seat post diameter. 

If you are not sure what size you need measure the O.D. of your seat tube, as it is entirely possible that someone put the wrong size clamp on your frame in the first place. You'll need a fairly accurate set of calipers preferably better that +/- 0.1mm error.

BTW, what size was stamped on the Salsa clamp? I have both 28.6 and a 30 on my two MTB's. The #s are quite clearly marked. I wonder if they shipped you the wrong size by mistake. 

Drew


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## theMeat (Jan 5, 2010)

Zilch? Well anyways, Google comes throu again. Found that a 2007, which is made from the same exact ZR9000 aluminum material as 2006 and also has a 27.2 seat post, has a 31.8 front derailluer clamp, and found a guy with a 2010, which is also made from aluminum but 6066 aluminum, with a 27.2 seatpost, paired with a 32.0 seat clamp so I'd say it's a safe bet that you're right that your old clamp said 32.0, and I'm right that 31.8 - 32.0 should fit perfect. 
Somethings up with your new clamp.

BTW, far as i can remember, any bike I have or have had with a 27.2 seatpost, whether ti steel or aluminum, had either a 31.8 or 32 mm clamp.


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## dru (Sep 4, 2006)

theMeat said:


> Zilch?


Yes, zilch.

The post clamp size is entirely based on what seat tube the builder uses. I built a frame with a 28.6 O.D. seat tube. After welding/brazing the tube is supposed to be reamed to take a 27.2 mm seat post. I didn't bother since I had an old 26.8 post that fit perfectly. Obviously what size post I ended up using is irrelevant to the clamp I.D.

My other MTB, a Salsa has a seat tube with much thicker walls at the clamping area. The Salsa takes a 27.2 mm seat tube yet uses a 30.0 mm clamp since the seat tube O.D. is also 30 mm.

The clamp I.D. must match the seat tube O.D., that is all.



> BTW, any bike I have or have had with a 27.2 seat post, whether ti steel or aluminum, had either a 31.8 or 32 mm clamp.


I think you've experienced nothing more than a very strange coincidence here or you are mistaken.

I say that simply because if your clamps were correct for your frames' tubing O.D. the wall thickness on all of them (Al, Steel, Ti) would be 2.4 mm. That thickness would be applicable to an aluminum frame but certainly not steel or ti.

Steel and Ti seat tubes typically have wall thicknesses of roughly a millimeter in the seat post/seat stay/top tube junction, unless the seat post clamp area is sleeved, which isn't a very common construction technique any more, and one I've never heard of for titanium construction.

Drew


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## theMeat (Jan 5, 2010)

Yep, see your point dru and it is of coarse true, but if you know the seatpost is 27.2 then you atleast know it's not smaller than that, or even the same size as that, and of coarse would have to be a bit bigger so not zilch. But it is a cool word. 
Think the OPs question is answered anyways so...No need to beat "theMeat"


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## Cormac (Aug 6, 2011)

where can you find a set of decent calipers for not much dinero?


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## dru (Sep 4, 2006)

*auto parts stores*

Home depot, etc. should have digital or mechanical calipers. Cheap ones can be found for 10 bucks. Most bike tubing is very size specific so you really shouldn't have too much difficulty figuring out what size clamp to buy if you are fairly accurate in your measuring even if you used a ruler. Common sizes in steel or ti are 28.6 and 31.8. External butted (in the seat clamp area of the tubing) 28.6 will usually take a 30 mm clamp. If you look at what size clamps Salsa offers, it will match up pretty good with what tubing choices are out there.

Drew


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## Vitamin G (Oct 11, 2004)

I have a cheap plastic calipers, and I did some measuring tonight.

OD of seat tube was 32 mm

ID of Bontrager clamp was 32 mm

ID of Salsa clamp was 31 mm

maybe I just need to open up the Salsa clamp as much as possible and work it on the seat tube? 

I was cautious and didn't want to damage anything when I tried the Salsa clamp the first time.


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## dru (Sep 4, 2006)

I'd get a screw driver and open up the gap a bit on that Salsa clamp. You won't hurt it, but only open it up enough to get it on the frame.

Drew


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