Tandem Cycling - Two different seat post diameters on same frame?

OK, I did warn everybody that the newbie questions would be starting.......................

I have been messing around with various saddles, seat posts, and trailer brackets etc. In doing so it appears there are two different diameter seat posts on my Trek T900.

One says 27.2mm on it, and was installed in the stoker position, and the other says 29.something mm. I can't read what is after the decimal point. This larger diameter post was in the captain's position.

From what I have seen here and in other places, there are numerous different seat post diameters. I can understand why perhaps different manufacturers would have different sizes etc. on their single frames but does my bike really have two different sized tubes on the same frame? There is probably a reasonable answer to this so if anybody knows why I'd like to hear it.

The smaller stoker post is a suspension post and thus won't quite go down as low as I need it to. Thus I was going to swap it with the non-suspension captain's post to get the extra adjustability I need. Since the captain's post is larger it won't fit in the narrower stoker position and vise versa. I gather I shouldn't just be reefing down the larger tube to grip the smaller seat post tightly which is why the shims are available!

I'm will most likely end up buying a new, non-suspension post in the correct diameter but just curious why it is setup this way with the two diameters.

Thanks.

Did you look for a shim inside the stokers seattube?

27.2 is a common size, especially for suspension posts. These are then shimmed to fit the frame.

Then again, maybe it's that way by design.

BTW, we run a 29.8 post front and a 27.2 titanium post in the back, with a shim.

PK

+1 - daVinci is set up with a shim in the Stoker position. Both seat post tubes are 29.8 mm.

This was a used bike, wasn't it?

we recently bought a used tandem with a 29.8 captains post but a different stoker (without a noticable shim). I take it that the previous owner or the bikestore from which he bought put a post on there that he had lying around. Same goes for the captains stem. It must have come from another bike, since it was severely scratched on the underside of the stem.

I guess that comes with buying a used bike. Sometimes you will find unexpected part combos

As other posters have stated, it is an issue that can be resolved, by either a shim or a new post
Duppie

T900's came with two different seat tube diameters.
other T900 owners can confirm this.

i have seen this in 2009 C'Dales seat tubes as well.
captain's is 31.6 while stoker's is 27.2.

Well as crazy as two different diameter tubes may seem, at least I can now blame TREK and not my own mental acuity ;) Now I know what the issue is, you are correct, it's easily solved with various combo's of posts and shims.

Thanks again for all the excellent input, this really is a great forum.