Seat post to seat post tube measurement

Seat post to seat post tube measurement

Post by teachnda » Fri, 02 Aug 2002 13:16:38


If the seat post tube on my muni is 26.5mm ID, then do I need a seat
post with that diameter, or do should I have  a seat post sized 26.4mm.
I can't find seat post with a 26.5mm diameter, only 26.4mm.

My assumption is that, then, seat posts are usually 1mm smaller than
their corresponding seat post tube.

Is this correct?

Thanks.

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teachndad - The Munieer

Rod Wylie
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Seat post to seat post tube measurement

Post by john_child » Mon, 05 Aug 2002 15:28:27

The seatpost diameter should be the same as the ID of the seat tube.  A
0.1 mm difference is a big difference as far as seatposts go.  Are you
sure your seat tube ID is 26.5 mm?  That seems like a non-standard
diameter for a seatpost.  I don't know if you'll be able to find a
seatpost in that size.  Thomson makes a seatpost with a 26.6 mm
diameter, but they don't make a 26.5 mm or 26.4 mm diameter post. You
might want to have a bike shop double check the ID of the seat tube with
an accurate set of callipers.  If your seat tube is really 26.5 you
might have to get creative with fashoning a shim.

When putting in a seatpost do not force it in the seat tube.  If you
have to force it, it is not the right size and will possibly get stuck.
It should be a snug fit, but you shouldn't have to force it.  You should
also grease the seatpost and/or the inside of the seat tube so the post
doesn't get stuck in the seat tube due to oxidation (see
<http://www.sheldonbrown.com/stuck_seatposts.html> for the reasons why
you should grease a seatpost).

john_childs

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Seat post to seat post tube measurement

Post by teachnda » Mon, 05 Aug 2002 16:24:34

John,

The frame builder told me by mistake that it was 26.5mm.  He works with
two size tubes and he forgot which one he used. You're right, that is a
weird size.  I went to two bike shops to get it checked.  Niether one
could tell me the exact measurement, but one did say that a 29.4mm
should fit, but there might be some play.  Some bike shops really stink.
It's really a big mess right now.

The outer diameter of the seat post is 31.8 mm.  That I know, for sure,
because the seat collar of that size fits nice an snug.

The builder did the math and came up with an inner diameter of
29.972mm, but you have to assume some shrinkage during welding.  So I
have no clear answer.

Currently, I will keep a shimmed 25.4mm seat post in there for now.  It
works, but I would like something bigger and closer to the seat post
tube.

Work the  maze.

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teachndad - The Munieer

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Seat post to seat post tube measurement

Post by teachnda » Tue, 06 Aug 2002 01:26:19

I got some clarity from the frame builder today and I will use his
words to help clarify why I didn't  get a clear answer from him.

It certainly made things clear for me.

The shim is stronger than the shimless.  I never used that
thin of a wall tube (.035mm)without a shim inside or a sleeve outside.
I would  NOT try to use a post without a shim.  The wall is so thin that
without the shim it will be more stress on the tube than if used
without the shim.....if you do not have a perfect seat tube fit.  That
is the reason I did not know the exact ID since I would not go without
some inner or outer reinforcement. I do not know if 29.4 is too
small...it sounds like it by the
measurement....but I do not have the tube here to measure the heat
shrink and paint level. When I did it I just measured the ID before
heated and painted and got the closest shim to accept a 25.4 seat
post....I even had to put  it in a lathe to get a good fit.  .035 is to
thin to expect a perfect post fit and for the seat collar to clamp on
and not deform the seat tube if the seat post is not a very very perfect
fit.  The thicker shim and wide slot in the seat tube allow some
flexibility for clamping.

I'd be interested to know if anyone else out there has shims in their
munis.

Work the maze.

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teachndad - The Munieer

Rod Wylie
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