WHAT DIAMETER SEATPOST TO USE IN A TAPERED SEAT TUBE?
January 3, 2006
I have an older Celo Europa
racing road frame from the late 1970s made from Columbus
SL tubing. I am trying to determine the correct seatpost
size for this frame. 27.2 is too big and 26.8 is too small.
I've checked the inside tubing measurement
with calipers to see if 27.0 (1.063") would fit, but
looks like it might be a little tight, as areas of the tubing
measure slightly less than 1.063". I'm having problems
finding a chart and want to make sure I’ll buy the
correct size seatpost. Can you help?
Celo
In addition to one sweet frame,
you have what experts call a “baseball bat” seat
tube (obviously, not experts in poetry), in that it tapers.
Frame makers do that to add rigidity to frames, and they
also added mild frustration when one replaces a seatpost.
A 27 mm seatpost will probably work
unless you need a rather long one—in which case you’ll
need a tapered one like the U.S.E. Aero. Alternatively,
you could use a 26.8 or even a 25.0 mm seatpost with a shim
surrounding the post at the top.
I think you can learn the needed
seatpost diameter for sure only by running that frame over
to a bike shop and trying seatposts of several sizes. (Don’t
put the 26.8 or 25.0 in without a saddle attached, else
you might lose the seatpost. No joke.)
With frames like yours, getting
a seatpost tight seems like such a huge hassle that I think
I should say few words about tightening. Ahem. First: You
can get into trouble with carbon seatposts if you overtighten
them—they’ll simply crack.
Second: Shims. Those aforementioned
experts say you should use plastic shims for carbon seatposts,
aluminum for aluminum. Tho I agree about the plastic when
using a full-circle shim (think “collar”), I believe
you can get away with an aluminum shim on a carbon seatpost
if you use just a partial shim (say, 45 to 120 degrees around).
You can easily make a custom shim
from an aluminum beer can. It cuts best wih aluminum snippers,
but I’ve done it with regular scissors. I find it helps
to drink the beer first.
Finally: If slippage persists, clean
the inside of the seat tube with isopropyl alcohol. Some
folks also have luck coating the inside of the seat tube
with resin (like Tyre-Grip,
whose maker desperately wants to break into the bike industry,
tho I don't think they had this application in mind), but
that seems extreme to me.
Mr Bike
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