I picked up a 2001 Giant OCR road bike. It came stock w/ a 27.2 diameter seatpost but also uses a stock Al spacer/shim/adapter in between the SP and inside of the seat tube. I removed the spacer, and the inside of the ST measures 31.6.
I did some research, and here's what I have so far for pros & cons to either size.
+ common size w/ lots of options
+ smaller diameter is a little more forgiving (ie: desirable flex)
- some owners complain of creaking from SP clamp area attributed to use of spacer
+ works w/o a spacer
+ less inherent creaking, since there's no spacer
- larger diameter is stiffer and arguably less comfortable
- fewer SP choices
- same model SP weighs more than in 27.2
In case you are wondering, the spacer weighs 38g. Either way, I'm unconcerned about the weight differential. My main concern is that the spacer is about 3.75" long. When you have a SP installed, there is a significant portion of the SP below the spacer that doesn't contact anything.
I don't see it as an issue w/ an Al alloy SP, but I am concerned about carbon fiber SPs. Is that void between the SP and ST inside wall a potential stress riser? Seems to me, w/ a CF SP, you would want the entire portion beneath the SP collar to be gripped around. Your thoughts?
Anecdotally, do any of you feel like you can feel the difference is stiffness and comfort between 27.2 and 31.6 diameter SPs?
Nope It's a 04'WTR, a trekking bike , Pannier racks and all, Its got an 'Airwings' spring suspension seatpost ,
shaft is aluminum, I'm about 200 pounds so never think of light weight components as practical..
had a Light AlAn Bike 25 years ago when I was younger and lighter, It had a 25.0 seat post, Campag Super record ,
migrated to a steel frame used a USE shim to put it in a 27.0 RB1, no problems there either ..
USE , UK, makes seatposts in 25.0, and 27.2, machines shims with those IDs and a number of OD's for all the other sizes , 25 to including 27.2,
27.2 ID to use on all larger ID seat tubes.