mlwarriner

so the specs i can find on my incoming frame don't list seatpost diameter. but the front derailleur is listed as having a 31.8mm clamp.

i need a seatpost, as i'm sure the one from my cadex won't fit. the options i find regularly are 30.9mm and 31.6mm diameter.

is it fairly safe to assume the 30.9mm post will fit my frame?

dsbrantjr

I'd recommend against assuming. Seatposts come in 0.2mm increments, too big won't fit in and too small won't clamp properly and depending on clamp construction could risk damaging the seat tube.

FBinNY 

Never assume anything, especially seatpost diameters. For example there are 30.9 and 30.8 seatposts, a difference of only 0.1mm in diameter. Either look up the spec on the makers site, or caliper the existing post.

If you don't have the original post, you can measure the ID of the tube, but be forewarned that there's often distortion at the top, so measure as follows for an accurate measurement:

Tighten the seat clamp carefully until the slot is the same width top and bottom. This will restore the tube to it's true size before slotting. But it may be oval, so measure twice at right angles, and average the diameters, to get the right answer. If you worked very carefully you should have gotten something 0.02- 0.05mm over a standard seatpost diameter, ie 30.92 - 30.95mm.

commo_soulja

sheldon brown has a good chart of seatpost sizes by brand/madel

fietsbob

I agree with the others, assume nothing..

the tube wall thickness is not uniformly the same, so the ID varies.

Some seat post manufacturers make a couple sizes, then machine shims
for larger seat tube ID sizes.

measure , accurate to the nearest 0.2mm ..

HillRider

A 31.8 mm (1-1/4") outside diameter seat tube is likely to require a seatpost significantly smaller in diameter. If you are lucky it will take a 27.2 mm which is very common. Assume nothing until you have the frame in hand and then measure carefully or go to an LBS and have them gauge the frame.