I agree with Sheldon, and would like to share an anecdote.
My '84 Trek 610 was supplied when new by the shop (incredibly they are still in business!!!) with a too-small post. I never knew it, and was too naïve to think that tapering slot was a problem. But I started after a while to have binder bolt breakage and finally the little ears Trek used to capture binder bolt cracked off. Trek guaranteed and repaired the frame, but I still have problems getting a good-fitting post. Now I use either a Thomsen with a .002" brass shim 360 degrees, or an American Classic which is a bit meatier, at least a better fit. But what a set of hassles!!
My '80 Woodrup might have had some similar problems. Here the very heavy seatstay attachments have made the front half of the seat lug very rigid, and only the rear half moves to grab the seatpost when I secure it. No slippage yet, but ... Again not a sense of security.
Listen to Sheldon -- make it fit right!!!
Ken Freeman Ann Arbor, MI
-----Original Message----- From: classicrendezvous-bounces@bikelist.org [mailto:classicrendezvous-bounces@bikelist.org] On Behalf Of Sheldon Brown Sent: Friday, July 20, 2007 6:43 PM To: John Hurley; classicrendezvous@bikelist.org Subject: Re: [CR]Still pondering seat posts
Quoth John Hurley:
>Is my math right? Seat post diameters come in 0.2 mm increments. 0.2
>mm is about 1/128 inch or about the thickness of two sheets of paper
>(at 1-7/8" per ream). This means the next bigger size seat post is
>like wrapping one thickness of paper around the previous size post.
>The clamping difference would of course be a matter of circumference.
>Changing to the next smaller size post would mean drawing the clamp
>tighter by about 1/64 inch, about the thickness of a business card.
>
>I didn't realize the tolerances were so close, and I'm surprised by it.
>Sounds like you could use the next smaller post and never notice the
>difference.
>
>The explanation might be that in practice, the lug opening has to be
>significantly larger than the post if the post is to slide in easily,
>so the clamp has quite a bit more to do than just closing a gap the
>thickness of a business card. Apparently it doesn't take much before
>the post becomes too small to be effectively clamped.
.2 difference in diameter means .63 mm difference in circumference, so the gap in the back of the lug would have to close that much farther.
There's also a funny thing that happens when you try to do this, at least with a lugged seat cluster. Only the back part of the seat tube/lug actually bends, and as the "ears" of the lug bend inward, they tend to punch forward where they come together. This will often deform the seatpost, putting a dent in the back of it and giving it a slightly cardioid cross-section.
You really need the correct size.
Sheldon "Easy As Pi" Brown
--
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