Why do some frame manufacturers make their frames for 27.2 seatposts, while others seem to favor 31.6? What are the advantages and disadvantages of each?

Thanks.

27.2 is an old standard giving you more options which is good if you need a lot of setback or are just particular. For example, Campagnolo C-Record and SP10-RE posts hold their adjustment without a lot of clamping force and have a classic look. I don't want anything else after breaking a cradle, breaking two clamp bolts, and needing 16 NM of torque to keep a different single bolt level (yeah, "carbon assembly" friction paste would be a better option).

31.6 allows a lighter and stiffer seat post

Traditionally 27.2 would be for road bikes which have skinnier tubing, 31.8 for mountain bikes which had fatter (stronger, stiffer) tubing. Of course these days that's blurred but I think it's still more common to find the skinny seat posts on road bikes (both my road bikes take 27.2).

Manufacturers seem to be slowly going back to 27.2 for a less harsh ride. I think Cannondale even went down to 25.4 for all their '16 bikes.

Yes to Lazyass' post. Even with a 31.6 seattube you can use a 27.2 post with a cylindrical shim. More comfy. A lot of Ti and steel bikes that have oversize seattubes call for the 31.6 post.

One advantage of 31.6 - you can use either post. The shims are easy to make and get. My framemaker uses oversized ti seatposts and shims them down to 27.2 as a matter of course.

Ben

Cannondale went 25.4 for the Synapse a few years ago. Supposed to ride better.

Yeah. I didn't care for the 4ZA 31.8 post that came with my Ridley so I got a shim to use with the 27.2 Specialized post I already had.

Funny stuff with Cannondale going down to 25.4mm. I hope the trend continues and 25mm seatposts like my Look KG386 uses make a comeback. It would feel modern again.

What's the story with the 30.9. That's what I wanna know!

Some engineer born straight out of hell. For that matter, same for 25.4. They just need to LEAVE 27.2 alone!! It's the perfect happy medium. These mfgrs are endlessly pursue a solution in search of a problem.

25.4 was pretty much the standard for lower-end bikes going back at least to the 60s, nicer bikes with same OD seattube had thinner walls, thus bigger ID.

If you're really into gaspipe you get even smaller seatposts. I have an olde 1959 cruiser with 22.2.

Also have a tandem with 25.4s, Free Spirit with 26.0, Mongoose and the ParkPre have Tange seattube with 26.8 post, Scapin is Aelle so it has the 27.0, standard 27.2 in the Reynolds 853 on the Lemond, KHS has True Temper OX3 which takes a 28.6...

You can fit a BB motor in a 31.6, but not in a 27.2.

Well, not yet anyway...

Hell yes !! Done and done !

They don't make frames to fit seat posts, they use the appropriate sized seat post to fit the frame.

Yeah they do. Cannondale specifically designed their new frames for a 25.4 for a smoother ride.

In the 80s, I had a Campy Super Record equipped Gianni Motta with Columbus SL tubing with a 27.0 set post. Some SL frames were 27.2 and some were 27.0.

Just a couple months ago, I had a friend ask me if I still had any 27.0 seat posts. He was rehabbing an early 90s tandem from a famous maker that needs a 27.0 stoker seat post.

That was the truth in the past. Not so much now. When all was steel, the inside butt diameter determined the seatpost diameter. So seatposts were all over the place. For example the post needed for Columbus SL would have likely been larger than the one needed for SLX. Nowadays the top of the seat tube, even on steel, is usually either molded to 27.2 or 31.6, is actually one of those diameters or is normalized with an integral (fixed in place) shim to standardize the seat post to either one of those depending upon the material and design. There are certainly still exceptions, but they are much less common than in the past.