Would You Be in Favor of a Universal Seatpost Diameter Standard?

There is pretty much only 4 size that you see across all bike categories (road to DH) for round seatpost:

27.2mm / 30.9mm / 31.6mm / 34.9mm and 27.2mm is barely used in MTB. I understand the point of 34.9mm for durability on longer dropper but weight creep up fast in that size.

I think it's mostly the 30.9mm and 31.6mm that could be reduced to only 1 of the 2. 30.9mm is the better size if you need to shim a dropper in that size in a 34.9mm frame with a thicker 2mm shim compared to a 1.65mm thick shim for 31.6 dropper to 34.9mm frame. 30.9mm is lighter for the XC crowd but 31.6mm is usually better for longer travel dropper in terms of flex and durability. 

We are in a much better place than the 90s/early 2000s. Thomson offer currently their elite seat post in 18 different diameter and 34.9mm is not even one of them.

The industry can ram headset routing down our throats yet they can't get together enough to have a common seatpost diameter?!  I'd love to see survey results for how many want/don't want headset routing.

It's ridiculous to have both 30.9 and 31.6. But we should have one of them as 34.9 doesn't make sense for steel bikes but it's pretty much ideal for aluminum and carbon especially as seat tubes and post insertion lengths are getting shorter but seatposts are getting longer. I would see all hardtails and steel/ti bikes move to 31.6 and everything else move to 34.9. 

I think that making the standard 34.9 would be a good idea. Let the engineers have a little more space to make long and reliable droppers. 

A classic.

30.9 mm seatpost, then shims to fit larger.

Most of the 31.6mm and 34.9mm droppers are just a 30.9mm dropper with a thicker lower tube. The internals and the upper tube (parts that slides upy downy) are unchanged. 

The world would be just fine with only a 30.9mm dropper. Light enough for XC/gravel/road and yet still strong and stiff enough for trail/enduro/ebike.

But that is starting to change, SDG for example use a thicker stanchion in their 34.9 version. I like the idea of 34.9 for max durability, mainly in carbon and alloy frames on the enduro/DH/long dropper side of things, and a 30.9 for everything else. Steel enduro frame makers can still opt for the 34.9 version if they want, anyway weight isn't much of a consideration for them.

That is a cost savings measure by the dropper post manufacturers. If 34.9 was the standard on frames , I would expect (though I have been wrong before) seatpost manufacturers to optimize for that size.

Cost is the only thing preventing seatpost manufacturers from optimizing for every size now. 

BikeYoke already does their Max versions that use a larger structure, seals, and bushings. It doesn't weigh that much more when you make everything bigger, because wall thickness remains constant (and could even go thinner in theory). Its only the lazy companies who put their 30.9 internals into the 34.9 tube that get really heavy.

I think we should be looking at a 30mm+ stanchion diameter for long droppers. If every other tube has grown on modern bikes, why not actually use it for a performance benefit? Doesn't mean we have to get rid of the smaller sizes, but encouraging companies to make them bigger is a good idea. As lengths go up the tube should get bigger, smaller sizes should be smaller. Keep the flex profile and bushing support as close as possible between sizes. 

Meh. I just use a $10 shim. 30.9 posts are lighter anyways. 27.2 should be rigid super lightweight stuff. 

For 90% of MTBs, 31.6mm is the simple answer.  Had a conversation w/ Santa Cruz warranty dept. when they changed majority of their frames from 30.9 to 31.6mm nine years ago. They said they did it because feedback from consumers contacting them directly, from dealers, and from dropper makers was that there were *far* more complaints, shop visits, and warranty claims involving 30.9 posts due to ham-fisted consumers torquing the bejeezus out of seat collars. The thinner walls of 30.9 posts allowed for more deformation / binding / wear to occur in that scenario.  Given how many MTBers I encountered in my past life as shop wrench who had zero clue how to properly tighten a bolt, I could instantly grasp and embrace their logic.  Roadies need 27.2, clydesdales may need 34.9 to go to 240mm drop, but if I can hammer a 210mm 31.6 OneUp for 20k savagely rocky miles with zero issues, then the rest of us don't need to haul an extra-porky 34.9 dropper up and over every mountain.

I am sure we will see 270 or even 300mm dropper in the near future but I think those will only be possible in a 34.9 size.

I think the 3 sizes would be perfect, as others mentioned 30.9 and 31.6 are is a bit redudant, just pick one.

It would be very restrictive design wise IMO to limit it to 2 or 1 size, with 31.9 and 34.9 as most likely. Maybe an XC/Gravel frame wants a thin seat tube to build some compliance? Or maybe a carbon 34.9 seat tube is too thick and would create a harsh ride. As others  mentioned, a steel 34.9 tube would be way too large.

Get rid of 30.9 is the only thing id say. Otherwise seat tubes (and clamps) is one of the few bright spots in MTB standardisation id argue.

Don’t we still want metal bicycles from time to time?

The main reason seatposts are the diameters that they are are because they roughly correlate with the internal diameter of round tubes you buy from industrial suppliers.  messing too much with the seatube diameter requires machining collars you plug weld into the tube to recieve the desired diameter.  

This will be controversial, but I'd like to have a fully integrated dropper similar to ghost or syntace that is one length only (or as few as possible), where the travel can be reduced as needed and that has a standard interface that many manufacturers can use. why do we still have the stupid seatclamp for droppers and double seat tubes where dirt and water can get in between? I do not care about the diameter as long it allows to build reliable droppers with up to 220 mm travel.

More important than the question posed is that we all have an opinion about it.   Thanks for your contributions. 

Yes to a standard seatpost diameter, as long as it's not another new standard.