Originally Posted by Lennard Zinn
Seat-post size, on the other hand, often is dictated by the tubing manufacturer. The outside diameter of the seat tube will often be an English size, say 1.375″, and the I.D. will be dictated by the wall thickness, which the frame manufacturer selects as being appropriate for that particular rider and frame size. That’s why there are so many seat-post sizes – lots of framebuilders are trying to find the best balance between stiffness, strength and weight, while they are stuck with the English-size O.D. to make sure that a front derailleur clamp will fit on, and front derailleur clamps only come in 1-3/8″ (34.9mm, a.k.a. 35mm), 1-1/4″ (31.75mm, a.k.a 32mm), and 1-1/8″ (28.6mm).
27.2mm became the standard seat-post size because most high-end road frames in the 1970s and 1980s were lugged and were almost universally made out of Columbus SL or SLX or Reynolds 531. These seat tubes were 1-1/8” in diameter, or 28.6mm. The single-butted seat tube was 0.9mm thick at the bottom and 0.6mm thick at the top. Well, 2×0.6mm = 1.2mm, which, when subtracted from 28.6mm, yields an I.D. of 27.4mm. However, the tolerance on the wall thickness and roundness of the seat tube made it so that you rarely could fit a 27.4mm post inside, even before brazing. And then, the seat tube always got distorted during brazing, making it even less possible to fit a 27.4mm in there, but a 27.2mm fit nicely. The same goes for why a 27.0mm seat,post was often used on the bigger sizes, which were made out of Columbus SP or SPX, whose seat tubes had 1.0 X 0.7mm wall thicknesses. Because of tolerances, the predicted 27.2mm post (28.6 – 2×0.7 = 27.2mm) never fit, but a 27.0mm fit nicely.
Lennard