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
>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
Harris Cyclery, West Newton, Massachusetts
Phone 617-244-9772 FAX 617-244-1041
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