But it seems that niether is true: 2) the lug opening is not significantly larger than the post diameter and should be snug but not binding on a properly sized seatpost. And 1) using an .2mm undersized post (without proper shim) will slide down eventually under body weight no matter how tight you make the seat lug bolt. For example I had a 27 in a 27.2 bike (N.R. post didn't notice the missing ".2" on the label at first) that fit snugly in the seat lug and tightened fine, but would start sliding down after a few miles on the road no matter what I did. Finally realized my mistake and got the proper size, then no problem.
Mitch Harris Little Rock Canyon, Utah
On 7/20/07, John Hurley <JHurley@jdabrams.com> wrote:
>
> List,
>
> 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.
>
> John Hurley,
> Austin, Texas USA