Seat post to seat post tube measurement

[teachndad]

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.

[john_childs]

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

[teachndad]

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.

[teachndad]

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.

Gene wrote:

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.