Lateral force from the butt sat on the saddle through the dropper shaft is the killer of dropper bushings, not so much the flex in the dropper shaft.
30.9 mm has serious physics based challenges for anything but the shortest posts and the lightest riders. 31.6 mm is just a 30.9 mm with a beefed up side wall so the seat clamp force problem associated with 30.9 mm is less of an issue (use a torque wrench and it is less of an issue anyway).
31.6 mm does a decent enough job on posts up to 210 mm.
Other than BikeYoke using 34.9 mm to give room for bigger bushings and better seals, every other brands' 34.9 mm is just a 30.9 mm with even more weight but no engineering advantages (for the post - the bigger seat tube can give shock mounting advantages) that the 31.6 mm format doesn't already provide.
With carbon frames (and most aluminium frames) the strength of the seat post doesn't add any value to the frame strength, the frame/ seat post already has to be strong enough to cope with the lever effect of the planned length of seat post and the probable weight of the rider that needs a frame of that size.
The technology has existed for a long time to make strong, light 35 mm aluminium tubes (with coatings if the expense justifies the reward). The ability to make a really good 35 mm tube in large quantities with high levels of quality control is well established.
The move to metric shocks was mainly to allow longer bushing and seal overlap for shocks.
Why doesn't the industry just combine the two advances (and they are engineering based advances as opposed to marketing department 'advances') and develop a light weight 35 mm dropper (with the appropriate seat tube and seat post clamp size) and get one with strong, light, easily serviceable and long lasting.
Provide support for legacy post sizes for 10-15 years (about how long the longest lasting version will probably last) but move forward with a format that supports; the lengths we expect on a modern bike (150 - 240; short-ies through to tall-ies), the ease and longevity of use between services and a high value for money ratio.
Also why buy a non serviceable cartridge based post when there are equally good fully serviceable (at home - not "send it back to the factory" posts available?
April 7, 2022, 1:17 p.m. - AndrewR
Lateral force from the butt sat on the saddle through the dropper shaft is the killer of dropper bushings, not so much the flex in the dropper shaft. 30.9 mm has serious physics based challenges for anything but the shortest posts and the lightest riders. 31.6 mm is just a 30.9 mm with a beefed up side wall so the seat clamp force problem associated with 30.9 mm is less of an issue (use a torque wrench and it is less of an issue anyway). 31.6 mm does a decent enough job on posts up to 210 mm. Other than BikeYoke using 34.9 mm to give room for bigger bushings and better seals, every other brands' 34.9 mm is just a 30.9 mm with even more weight but no engineering advantages (for the post - the bigger seat tube can give shock mounting advantages) that the 31.6 mm format doesn't already provide. With carbon frames (and most aluminium frames) the strength of the seat post doesn't add any value to the frame strength, the frame/ seat post already has to be strong enough to cope with the lever effect of the planned length of seat post and the probable weight of the rider that needs a frame of that size. The technology has existed for a long time to make strong, light 35 mm aluminium tubes (with coatings if the expense justifies the reward). The ability to make a really good 35 mm tube in large quantities with high levels of quality control is well established. The move to metric shocks was mainly to allow longer bushing and seal overlap for shocks. Why doesn't the industry just combine the two advances (and they are engineering based advances as opposed to marketing department 'advances') and develop a light weight 35 mm dropper (with the appropriate seat tube and seat post clamp size) and get one with strong, light, easily serviceable and long lasting. Provide support for legacy post sizes for 10-15 years (about how long the longest lasting version will probably last) but move forward with a format that supports; the lengths we expect on a modern bike (150 - 240; short-ies through to tall-ies), the ease and longevity of use between services and a high value for money ratio. Also why buy a non serviceable cartridge based post when there are equally good fully serviceable (at home - not "send it back to the factory" posts available?