How to measure diameter of seat tube?
 

Hi,
I am building what appears to be a mid to late 80's Bianchi, I have yet to determine the exact model.
While shopping for for used campy seat tubes I noticed they come in a variety of diameters such as 26.2, 26.8. 27.2, and so on.
How do I measure the diameter of my own seat tube to ensure that I purchase the correct size seat post.

I've check at sheldon brown's site and the information available there does not cover this.

Can anyone help a mathematically challenged newb?

Thanks

CW

Do you have a vernier caliper? Those numbers are the diameter of the post in mm. I doubt you will be able to get an accurate measurement without one.

You need to measure the opening size in the top of the seat tube (where the seat post goes).

If you can figure out the year and model, this link might help.
http://sheldonbrown.com/seatpost-sizes.html#b

http://www.youtube.com/watch?v=SmXfGan_NXQ

This is the best way, but may not be practical.

You can measure the top of the tube with calipers, but this section may not be perfectly round. I assume there is not an old seat post with the frame that you can measure.

Welcome to the world of bicycle mechanics. If you are planning to do a majority the work yourself, you'll most likely going to need a Caliper. A 'necessity' in your tool box...! General Tools 6" Carbon Fiber Digital .

as much as it pains me to say it, this is one situation where i would bring my bike to an LBS :(

I'll just have to take her down to an LBS. I'm not entirely certain of the model of the bike. It looks like a Stelvio but has different tubing than the US versions and has campy dropouts.
Based on Sheldon Brown its mostly likely 27.2 but no way to be 100% certain...

W.T.F.!!!

carbon fiber digital???

Real Men ride and use Steel!!!
My old man is over 80 and still has his steel Mauser from early 50's
I have a Mitutoyo over 20 years- which is among the best.
Honestly I have had some cheap pairs and they just dont last a lifetime.
Digital?! will eventually break.
this is a tool which will last your lifetime, thru all your hobbys to come.
Model: 530-312

http://nano-machinery.com/catalog/im...yo_530-312.jpg

I finally bought a tool that I had been dreaming of for years: a turned aluminum rod with all the sizes in steps over the length. Drop it in the frame, the one that fits is the size. I always found it hard to be accurate with calipers, so unless you have a bunch of seatposts around, its hard to judge sometimes.

Sorry, that sounds like someone in serious need of an upgrade.

Do you know how much "better" carbon fibre is? Really, it's all the rage these days...

Well...I was more or less thinking on an economical scale ~pun ~ !

But as whole, I can agree. That's why I ride steel...:p OTOH: 2 steely's have broke on me. I'm ecstatic that my daughter gave one for Xmas that reads fractions, decimal, & metric. Wixey :)

Also: my eyes have soured through the years. Reading your eg is a an 'exercise in futility' for moire...

I am pretty sure that will take a 27.2 post. you don't have another post lying arouns to try?

http://www.bikeforums.net/showthread...59cm?highlight How is the build coming? How did you make out with a BB?


Dbakl, Those are great tools I bought a set a few years ago. I had thought the set we had at one shop was chromed steel but it is likely they were change to alloy to save money.

One problem to be aware of is the deformed seat post cavity. Always take several measurements and average them out, when using a Vernier caliper. Also...

You can get bar, dial and digital calipers. I gave up trying to use the little gradations on the bar, opting for the dial model...

http://bikeforums.net/attachment.php...hmentid=256666 http://bikeforums.net/attachment.php...hmentid=256667

Not sure how to advise using the seat post gauge, since I don't know if it will take deformity into consideration.

Ditto. Pretty difficult to accurately measure the inside of a tube with vernier calipers. I go around and measure several places.

I keep seat post's in most sizes, this is how I check a seat post size.

Here you go

http://www.gagesgalore.com/Mitutoyo/..._Series511.htm

even my best digital calipers with multiple readings only gets me in the ballpark. I have a graduated sizer, but it's a jeweler's gauge intended for ring sizing, so doesn't go big enough for 27.2 and beyond...and you're right that it won't account for a deformed or ovalized opening. The T-gauges combined with an accurate caliper is really the way to get down into the tube and get an accurate measurement. My set is nothing fancy but they work for me.

Hijacking: What do folks use (preferably a common hand tool) to make a deformed seat tube opening round again?

^ screwdriver.

Response to Hijacking:

Prying and squeezing a seat lug to make it round amounts to cold-setting. Applied judiciously, no damage. But if it's overdone, it can damage the lug. If the setting seems to take less effort (less overbend) as you continue to try to make it round, the lug/tube metal is seriously weakening at the locations with the thinnest cross-section.

I think the thing to do is to get a post in there that you think might work and torque it to near-spec. If the slot in the seat tube is narrowing, go to the next larger post. Clean the inside of the seat tube and lube it to insert/manipulate the post in the tube. Having the saddle attached to the seat post is a great help with leverage.

With this the lug may become more round, but at least the stresses are in accord with the shape of the metal.

Don't go any bigger than what fits with the slot edges parallel what near-tight. The bolt holes on both sides need to remain parallel and co-axial. Otherwise dirt in the threads will seize the bolt and you'll have bolt breakage, once a year or so.

Calipers are not the right tool for measuring seat tube i.d., but they're very handy for a lot of other things.

I use a bore gauge myself (with a micrometer, obviously), to get down below the slit of the seat tube clamp. As many have mentioned above, the area of the slit may well have been distorted due to prior abuse, and in that case will not yield a correct reading of the seattube ID.

Before measuring with a bore gauge, you want to make sure you're measuring the ID of the tube and not crud/or corrosion that has built up inside.

If that requires cleaning out the seat tube (e.g. with solvent & rags, fine sandpaper, or a brake cylinder hone), you probably want to do that with a bare frame and not let the removed detritus descend into and contaminate the assembled bottom bracket.

Nuff said?