Originally Posted by
ColonelSanders
If one isn't looking at weird aero shaped wedge seat posts, the three most popular standard sizes for seat posts are 25.4mm(but rapidly diminishing), 27.2mm(the most popular choice) and 30.9mm.
So I want to see seat posts come in one of those 3 sizes, so I have plenty of options in getting the seat post of my choice.
Cannondale like to use 25.4mm because they say it has more compliance and that's fair enough, but what does 26.8mm give you over 27.2mm? Just less choices in seat posts.
I think you may wish to reassess how seatposts work.
The thickness of the tubing of the seat tube is the primary factor for the diameter of the seat post.
Generally speaking, stronger steel can be drawn thinner while maintaining strength, leaving a larger hole for the seat tube. Generally- the less strong the steel is, the thicker the seat tube has to be to maintain proper strength, therefore the seat post must be smaller. Traditionally- the strongest steels were Reynolds 531 and Columbus SL and SP- those were generally 27.2. Schwinn's triple butted CrMo was 26.0, Columbus Tenax was 26.6 and Miyata often used 26.8.
Additionally- if a designer wishes to have a stronger, more stout, set of tubes, they can spec the steel of the seat tube to be thicker- necessitating a smaller seat post. With the advent of aluminum, splined tubing and oversized tubing- the sizing is all over the place.
But the seatpost diameter isn't just pulled out of nowhere- it's a result of the intent of the purpose and materials used to build the frame.