History/ logic to seat tube and seatpost diameters?

[Veloo]

Just wondering if there is any (historical) reasoning behind the different seat tube and resulting seatpost diameters?

Of course 27.2 being the most common, but Chain Reaction has a range of posts from 26 to 34.9 diameters.

I've owned 27.2, 31.6 and even 26.8 posts myself.

Were those diameters some standard from another industry that just found its way over to bikes?

[C40_guy]

Historical quirk.

The Royal Shim Industry lobby was very strong in the 1970s, and advocated for a krazy mix of seat tube diameters so that the shim industry would grow and prosper.

Even today, those of us riding carbon Colnagos continue to curse that lobby as we search, usually in vain, for 27.2 to 28.0 shims, or the unobtanium 28.0 seat post.

[unterhausen]

they came from tubing dimensions. Not sure how they came up with .6 wall 1 1/8" tubing, but that's 27.2

[colker]

26.4 was common. 27.2 is already oversize. Some bikes even had 25.0.
Tubing got thinner so in order to keep stiffness they widenned the diameters. I guess that´s how it went.

[colker]

Historical quirk.

The Royal Shim Industry lobby was very strong in the 1970s, and advocated for a krazy mix of seat tube diameters so that the shim industry would grow and prosper.

Even today, those of us riding carbon Colnagos continue to curse that lobby as we search, usually in vain, for 27.2 to 28.0 shims, or the unobtanium 28.0 seat post.

How do you sell a seatpost labelled Colnago for 3x the cost of others? Make it 28.0

[Mark McM]

Like Unterhausen said, seatpost diameters can from commonly used tube diameters. In the days of lugged frames, tube diameters were limited to the availability of lugs.

Still, there were a small variety of different tube inside diameters, so there are a small variety of different seatpost diameters. Fortunately, there are more restrictions on steerer tube diameter, so there is smaller number of headset and stem sizes to worry about.

[Hindmost]

Eddy rode 27.2. So, there's really not a lot to say.

[Veloo]

I believe it was on the (old) Vitus frames that used the 25.0 seatposts.

So how does a builder decide on seat tube diameter?

My Schwinn S96.2 MTB is a 26.8 seatpost.

I know the Lynskeys used 31.6 at one time but mine is a 27.2.