Especially on hardtails?

Just curius

Narrower (26.8, 27.2mm) seatposts tend to flex more, which helps seated comfort, especially at longer extension, so whats the point in so many bikes coming with 30.9, 31.6, 34.9mm or other sizes? Especially on usually stiffer aluminium frames?
And why did 25.0, 25.4 etc get ditched, surely they'd be more comfortable still?

Does the extra stiffness make a better trade off in pedalling efficiency over the loss in seatpost flex?

Or is it simply as a result of speccing larger tubes on the bike elsewhere mean that a fatter seattube is needed to weld up against and shimming the seatpost down (on new complete bikes) just isn't 'acceptable'?

The properties of aluminium mean that alu bike frame designs typically use larger tube diameters than steel frames. I think that's the only reason seatposts are specced with a bigger diameter. If you really want to you can use a shim to reduce the seatpost size.

I do sometimes wonder if a lot of the perceived comfort of steel frames is simply because they tend to have narrower seatposts?! This only holds for bum related jarring of course.

I believe the larger alu seatposts also use less material and so are ever so slightly lighter – if that makes sense…Could be totally wrong though!!!

Jay

25.4 etc died out with shite thick tubing.

Otherwise it's abut stiffness I guess.

Yeah Jay, maybe the wall thickness of larger seatposts is less

I heard a while ago, from an engineer, that the larger diameter means you can use less material for the same level of stiffness and therefore the weight/stiffness ratio is improved.

I might have got this wrong though.

With this in mind, surely if a smaller diameter seatpost needs more material the it will be just as stiff as a larger diameter?

Jay

Given a wide diameter tube and a narrow diameter tube that use the same amount of material, the wide diameter one will be stiffer (second moment of area, innit?).

If you take this too far you will end up with stupidly thin tube walls where the theory goes kaput because the walls will crumple instead of flexing.

Weight weenies suggests that narrower seatposts, if anything, tend to be slightly lighter than wider ones (looking at Easton seatposts) – this means they will be more flexy too.