Mash / Cinelli Vigorelli

Guessing stiffness increase will be proportional to R squared? Where's tester when you need him?

For a right circular cylinder, bending stiffness is proportional to r⁴. For a tube that becomes r₁⁴-r₂⁴ where r₁ is the outside radius and r₂ is the bore radius. That means that if you simply scale up a tube from 27.2 to 31.6, keeping the ratio of diameter to wall thickness constant, it gets to be about 80% stiffer. Of course, there would be no point in doing this, the whole point of oversized tubes is to increase stiffness without using more material, so you cut the wall thickness if you can. In general, nobody was complaining about a lack of stiffness from 27.2mm seat posts for the past 80 or so years, so the move to a new 'standard' of 31.6mm has been driven by the need to fit it into a large diameter seat tube, not any actual need to stiffen the post itself. Oversize seat posts are generally heavier than their 27.2mm brethren because it turns out that if you try to make the same amount of material stretch to the larger diameter the walls get thin enough to be a problem, especially where the bending load peaks at the top of the seat tube. As a result, the seat post gets to be very stiff, and people start blaming it for discomfort, when what they should be looking at, in approximately this order, is positioning, tyre selection/pressure and saddle choice. Oh, and in a great many cases, rider avoirdupois. I was never in the least bit bothered about riding 18mm tubulars at 140psi with an unpadded saddle back when I was not much over 10st. :-)