27.0mm seatpost for 27.2mm seattube?

[RS]

Somebody gave me a nice Thomson seatpost, its 250mm x 27.0 but my
bikes all use a 27.2 seattube. They seem to clamp down ok but I'm
cautious. Any problems or issues with this? I'm more comfortable using
a 27.2 post in a 27.2 tube.

thanks, Rick

[* * Chas]

"RS" <r_sch...@comcast.net> wrote in message
news:_OKdnVL3DNHP5BbY...@comcast.com...

What is your frame made of? If your frame is made of CF or aluminum I'd be
very cautious.

How much of the seatpost is going to be inserted into the seat tube?

Does a 27.2mm seat post fit tight in your seat tube?

The seat tubes in a lot of steel frames come from the factory out of round
or they have high spots that haven't been reamed or honed round so smaller
diameter seat posts sometimes provide an easier fit.

The seat tube clamps on the circumference of the seatpost.

A 27mm seat post is only .2mm smaller in diameter but it's over half a
millimeter smaller in circumference - .63mm or .025".

So it's a definite maybe.

Chas.

[John Forrest Tomlinson]

I don't think you'll hurt the frame, but you may be more likely to
snap the binder bolt. If you have any reason to untighten/tighten if
often, a soda can shim is called for.
--
JT
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Remove "remove" to reply
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[Ernie Willson]

I wouldn't worry about it at all.
EJ in NJ

[RS]

In article <_OKdnVL3DNHP5BbY...@comcast.com>,
r_sch...@comcast.net says...

Frames I would be using this on are aluminum, older Cannondales. It slips
in easily and has a little play but seems to tighten down fine. Its a nice
seat post and the price was right. ;)

[Mark Hickey]

RS <r_sch...@comcast.net> wrote:

Others have suggested that this won't be a problem. I'll disagree
with them. Although the difference seems awfully small, the clamping
force required to adequately clamp the seat post will be MUCH higher
than it would be with a 27.2mm post. There's no good reason to risk
damaging your frame because you have a nice seat post that doesn't
fit. I'd suggest either shimming it, or selling it on Ebay so you can
buy a similar one in the right size.

Mark Hickey
Habanero Cycles
http://www.habcycles.com
Home of the $795 ti frame

[dusto...@mac.com]

Yes, lots higher, IME: I once bent a steel lug trying to use a 27.0
post in a 27.2 frame. Didn't really pay attention (not unusual in those
days) until after the damage was done. It even closed the vertical slot
at the top of the seat tube. Luckily, and I still have that frame, ca.
1980-81, nothing broke when too tight or when things were carefully
returned close-as-possible to orig. config.

Aluminum doesn't take to being deformed as well as steel.

Shim? OK. Better, sell it and get something you want that fits, if for
no other reason than ease of adjustment and transplant (you said
"bikes"). That Thompson might be a good ebay or Craigslist item due to
someone needing a 27.0 post for their bike.

Isn't there a cheap, good post made by Kalloy that is inexpensive
enough to have more than one, when using multiple bikes? (of course,
then you'd want more saddles...) --D-y

[Ozark Bicycle]

John Forrest Tomlinson wrote:
> On Thu, 21 Dec 2006 22:21:06 -0800, RS <r_sch...@comcast.net> wrote:
>
> >Somebody gave me a nice Thomson seatpost, its 250mm x 27.0 but my
> >bikes all use a 27.2 seattube. They seem to clamp down ok but I'm
> >cautious. Any problems or issues with this? I'm more comfortable using
> >a 27.2 post in a 27.2 tube.
>
> I don't think you'll hurt the frame, but you may be more likely to
> snap the binder bolt. If you have any reason to untighten/tighten if
> often, a soda can shim is called for.
>

Be sure to drill out the shim to save 0.0345gr!

[Gary Young]

I've sometimes seen it suggested that you can use a knurling tool to
increase the diameter of the seatpost. Is that safe and effective?

[RS]

In article <vl6oo25thv2m01j8g...@4ax.com>,
ma...@habcycles.com says...

I think you're right, I'm going to sell it and get the right size. Anyone got a
27.2x250mm Thomson at a fair price? thanks

[Donga]

Why not try a shim? Easily made by cutting up a coke can with some
strong scissors. I've been using the same set-up you describe, with a
shim, with no dramas.

Donga

[b...@mambo.ucolick.org]

Seatposts and seat tubes vary slightly from nominal
sizes. AFAIK, knurling is for slightly increasing the
diameter of a marginally undersized seatpost, not for
bringing it up a full size.

Soda can shims work. Given that, using an undersized
seatpost without shimming it is a bad idea. You are insuring
that the tube clamps the post only along a very small length,
which increases stress on both post and tube (though I'd
be more worried about damaging the tube or binder than
the post).