Well, technically they have to flex and do flex. They may not flex much though.
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Yes, aluminium components do flex without failing. If you want an extreme, but everyday, example just get a window seat on a passenger jet and watch that wing on takeoff. Very flexy, but stresses remain below the fatigue limit. It’s the same for any material.
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Fair enough, but the amount of deflection of the wing per 300mm of length is not much. Yeah aluminum flexes but not enough to make a complaint seatpost that lasts.
The Cyclingabout website has done vibration testing of various seat posts, standard and designed for compliance.
I recently switched from a Paul seatpost to Cane Creek eeSilk+ on my steel all-road bike (max 38mm tires). Similar weight, definitely smoother ride, while not affecting my pedaling or position in any noticeable way. Plus Cane Creek just lowered the price, and they participate in the EC member program.
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I know you said “stem”, but I will add handlebars to the discussion.
My 2006 Litespeed Terramo was very buzzy at the front end with AL handlebars. The Zipp SL-70 aero carbon handlebars transformed the bike.
- They do not flex longitudinally, whereas the old ones flexed when I accelerated. Result: bike now feels snappier when putting power on the pedals.
- They do flex vertically, dampening the road buzz considerably, to where 120mile rides are far more comfortable.
Thompson stuff are unbelievably stiff, extremely uncomfortable, in my experience. I would use them for an alleycat bike, a fixie, but not on a gravel bike.
Finally, a cheap set of carbon rims can add considerable vertical compliance to Ti frame (again, off-topic potentially, but worth mentioning?). I tuned the ride by increasing the rear wheel’s spoke tension for a more efficient rear power transfer and kept the “flexy” front spoke tension to further tame the buzz.
Thanks very much for this detailed response. Really appreciated. Seems like I can now rule out Thomson. I was going to stray from carbon bars as I figured if the bike was dropped this would be the first contact point. As per your advice I might need to rethink this. Instead I was trying to stick to carbon seat post and potentially stem. Appreciate the Zipp recommendation as was looking at their components as an option. Again thanks.
I used to have a Titanium Litespeed Ultimate G gravel bike which I purchased with Litespeed’s own Ti seatpost and also separately bought an ENVE Carbon seatpost on the assumption it would be more compliant. My personal assessment was that the Ti seatpost was noticeably more compliant than the carbon one but appreciate it may just the ENVE post being particularly rigid.
I always appreciated the stiffness of carbon.
To pile on top of all of the above, a flexy carbon seatpost is definitely noticeable. Some notes from what I’ve used:
The Canyon/Ergon VCLS S15 is the most comfortable I’ve used, but I would second that it can get a bit of a bob in choppy/high cadence situations (wasn’t a big deal for my needs, but if you’ve got a lot of seatpost out it might be).
Ritchey WCS Carbon FlexLink looks like it is not currently in the brand’s range, but is another that gives tangible comfort and flex. Would absolutely get again – maybe my favourite ‘normal’-looking but comfy post.
Syntace P6 HiFlex is pretty excellent, if you can find one. They’re an older model but you can find them floating about sometimes.
I also really like the Canyon VCLS S13 (now discontinued, I think?) and have that on a bike. Really neat (albeit slightly fiddly) clamp design that allows you to adjust the setback from 15mm to 35mm, which is really handy while you’re dialling in your fit.
And Roval Terra (which I have on another bike) has a bit of flex but relatively subtle IMO. Least flexy of the five discussed here, in my experience, although I haven’t got a huge amount of extension on that bike. I’d put that as the last of the carbon seatposts I’ve experienced that I’d recommend the combo of comfort/clamp design/etc. before we get into carbon posts that are just … there. Which is most of the others I’ve used (Easton, Zipp, etc)
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Thanks for the detailed reply mate. And importantly the recommendation of a ‘normal’ looking seat post. Considering its titanium frame (and I am fussy) I like the idea of just having one that doesn’t look too futuristic… This said the Canyon doesn’t look too bad. Adding to that is being choosy about the brand so it will too suit the frame, ideally sticking with the one brand across the post, stem and bars. Again really appreciate the insight.
On carbon posts (apologies for the digression), a really flexy one is this Litepro one. A lot of casual riding folks I know love this for its light-ish weight and cheap price, but boy is it all kinds of wrong.
I bought one for a burner folding bike that I use for cafe runs, grabbing takeouts, etc where I would actually feel sorry for the thief, if it was stolen. Given the amount of exposed seatpost, circa 400mm, cos folding bike with tiny, 406mm wheels, the flex is so unnerving, you’d think it was about to give, rocking back and forth like it does.
You can still get the S13, which looks traditional: Canyon S13 VCLS 27.2mm CF Seatpost | CANYON BE
Litepro, don’t they also make fishing rods? 
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Ah nice! Couldn’t see it on the Aussie version of the site. It’s really good - I think it’s less compliant than the S15 or Ritchey I mentioned, but I’m a bit smitten with the adjustability.
Hardly spend time angling, so I honestly wouldn’t know 
Regarding these seatposts in general: the Canyon versions are still remarkably less expensive than the Ergon versions. If one can live with the ‘canyon’ name/logo on it…
I’ve got many titanium bikes - road race, gravel, all-road/endurance, adventure… Among them is a Litespeed Ultimate G2 that I have set up for winter biking on frozen and rutted bike paths. I use 42mm winter studded tires and a cane creek suspension seatpost. The post definitely helps with comfort in those conditions. Most of my other Ti bikes I use either a Thompson Elite or Masterpiece post, or a Ti post (Thompson, Moots, Bingham, TiCycle, etc…). I find either to be plenty comfortable for gravel and road use. Why not carbon seatposts? Only because these are metal bikes and I just happen to think they look better with a metal post. I guess I get plenty of compliance from the frame, wheels, and tire choice.
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My titanium gravel bike came with a nice looking oversize 31,6mm titanium seatpost. It was so stiff that the bike was uncomfortable to ride. I changed to a Canyon/Ergon semi-suspension post and it made a huge difference. I don’t think there is anything to gain with a carbon stem, but a carbon bar can be more comfortable.
Just FYI, the spring bladed version of the Canyon seatpost comes in 2 versions, with different setback (S14 and S15). The S14 looks less futuristic.
The ‘old’ S13 looks even more normal, but supposedly has less flex.