31.6 vs 27.2 seatpost sizing

[SadieKate]

This has probably been discussed to death and then some in the forum, but I can't find it. Other than needing a beefier post to deal with the length of post exposed due to sloping top tubes is there any reason why a 31.6 post is needed on a road bike? Just matching size with mountain bikes?

Are all custom ti builders using 31.6 these days? If I have a horizontal (or durn near horizontal) top tube why does a builder need to use 31.6? Are the appropriate ti tubes no longer available for 27.2?

Am I going to feel a difference in the ride? I'm fairly lightweight.

Feeling a bit cranky about needing to buy a new seatpost for what I see as simple marketing hype, so tell me all the "rationale" for why a 31.6 seatpost is necessary for a small rider with a horizontal top tube.

[Peter B]

You can always use one of these (http://www.speedgoat.com/product.asp?part=42718&cat=270&brand=192).

[ss-jimbo]

The inside diameter of standard ti tubes is 31.6mm. Almost any ti bike with a 27.2 seatpost has an insert welded into the frame. This can cause problems. Trust me, I know. I've had two ti frames, from the same builder break at the seat tube/top tube junction because of a problem with the seatpost insert. By leaving the insert out, and specifying a 31.6 post, this problem is eliminated.

[davids]

Feeling a bit cranky about needing to buy a new seatpost for what I see as simple marketing hype, so tell me all the "rationale" for why a 31.6 seatpost is necessary for a small rider with a horizontal top tube.
ss-jimbo is right - It's the diameter of the Ti tubing.

It could be worse - The Nove asks for a 30.9 diameter post. I went through a long search to find an '06 Ritchey WCS in that diameter. I'm glad I did, though - It looks much better than the shimmed 27.2 I used at first.

[SadieKate]

The inside diameter of standard ti tubes is 31.6mm. Almost any ti bike with a 27.2 seatpost has an insert welded into the frame.Maybe the new standard? We have 3 Litespeeds from 1999-2001 and all take 27.2 posts, including 2 mtbikes. Absolutely no problems with several thousand miles. The outer diameter of the seat tube on the road bike is only 31.7 below the seat collar so obviously it must be a new standard.

[Peter P.]

Another reason is if the bike is for particularly tall riders, and if the post has a commensurately long extension. See Lennard Zinn's Project Big and Tall titanium frames.

For a given diameter, titanium will be noodlier than steel (the standard), so upping the diameter will return the frame rigidity to an acceptable level. Increasing the diameters of the top and down tubes alone may not be enough, and I've read on some framebuilder's site that this is a common complaint of tall riders when they choose titanium.

You already said you're a light rider, and perhaps you're shorter too, but the frame manufacturer may have wanted to keep their costs down by stocking only one diameter seat tube.

Of course, it's also less expensive to stock a tube with commercially available dimensions than to custom order tubes with I.D.s to fit standard posts.

[malcolm]

SadieKate, how long a post do you need? I've got a brand new salsa that has been laying around for about 2 years still in original package. I bought it for a frame I sold before I built up. I'll let it go cheap, real cheap. It's silver.

[SadieKate]

Peter B - it's a custom bike, and yes, I'm short, and yes, I'm going through a builder with access to lots of tubes, and yes, I can keep writing sentences with horrible comma-splicing. That's my point: I'm short, lightweight, level top tube (so very minimal extension) and it's a custom builder with access to lots of tubing. I can't even find seatpost diameters on Serotta's site. Why can't builders put seatpost, BB measurements, etc., on their site for those of us smart enough to build up our bikes? Maybe Serotta has it, but I can't find it. I know. I'm getting testy.

Malcom - is that one of the old silver Shaft (http://www.salsacycles.com/seatposts.html) in 31.6? I was going to use my already-owned Chorus carbon for "regular" riding and swap out with an AC alloy for when I need to clamp on a rack. I also already own a Specialized Pave which I use reversed when setting up the bike for summer time trials. This is ridiculous. Your Salsa could fit the ticket. I'll PM you.

[malcolm]

Let me know it is just taking up space.

http://forums.thepaceline.net/showthread.php?t=13226&highlight=salsa+shaft

ignore price

[Jeff N.]

Seven Cycles (Bill Holland too, if you ask him to; maybe some others) uses a carbon insert to size down to 27.2. It's bonded in place and creates no problems at all. Jeff N.