Depends on your team weight and how hard your stoker cranks on their handlebars.
We've used Easton EC-70s for several years and recently changed over to FSA SL-Ks. No problems with any of them. I've ranged in weight from 155 - 169lbs during their use and Debbie is sub 120lbs and does not use a lot of upper body strength when she rides. Your results may vary.
We're only about 10lbs heavier. Wouldn't an alloy stoker stem cause cracks in a carbon seatpost?
Only if you ignore the torque specs. Carbon is tough stuff.... Seriously.
All I can offer is our empirical evidence: 5 years of problem-free use of the Easton EC-70 seatposts on both of our road tandems. The only reason that we recently switched brands and models is because I needed to switch Debbie over to a no set-back seatpost after a recent fitting update.
Like any seatpost, you need to guard against putting any stress risers into the posts and must be mindful of the torque specifications. But, aside from that, I don't see them being any more prone to failure than any of the aluminum SL posts that many teams / manufacturers have been using. Now, I would most likely steer clear of any super-lightweight carbon seatposts for the captain's posts, and I might not recommend using one for a captain who has a lot of seatpost mast extended where their stoker's bars are mounted up high.
I replaced the POC seatpost that came on my Santana with a Uno Seraph which is relatively lightweight, low cost and easy to adjust (order from Precision Tandems). Another more expensive option is Thomson Elite which is lighter, a work of art and I have on my half bike.
Was it a cleat adjustment which prompted the seatpost change?
No. It was a just a riding position make-over coupled with an equipment correction that was long overdue.
Debbie wasn't a serious cyclist prior to the tandem and didn't have a well-refined riding position. Like many developing cyclists and stokers in particular, as she's gotten stronger and as we've switched bikes her position has had to evolve as has our equipment. Our original seatposts back in the 90's were Ritchey (aka, Kalloy) models with mild set-back. Along the way I had an opportunity to pick up some Easton carbon posts for "beta testing" on the tandems in '02 and, while they have some really nice features, the one feature that has always been problematic for Debbie's riding position was the excessive set-back on their standard posts. As it was, her saddle had to be pushed almost all the way forward on the Easton's clamp to achieve the correct set-back.
During a fitting that was intended to "fix" some issues that had developed over the last year one of the recommended changes from our objective fitting pro was to further reduce her set-back which wasn't going to happen on the Easton posts. So, I opted to pick up a set of four attractively priced FSA SL-K posts on Ebay: two zero set-back models for Debbie and two normal set-back models for me. They're not as robust as the Eastons, but so far so good.
ah, makes sense. I ended up doing the same after getting a Record kit (with post) and then swapping out the setback post for a regular Thompson.
I was thinking of an arch cleat position for my fiancee who is the stoker on the tandem (she had foot surgery as a kid and it affects her pedaling). This would necessitate a lower, more forward position. Still only in the thought stage, no experimenting or anything.
Good question. I had a think about this; if a post is designed to take 90kg of single-bike rider hitting it hard, 90kg of tandem captain plus leverage from a stem will be a larger load. For carbon seat posts, the designer will naturally have considered a clamping load as the seat post needs to be held in the frame. However I'm concerned about potential for large bending and torsional loads from the stoker bars as probably the designer won't have expected this when designing the post. His job is to lay out fibres to cover the loads he expects to make best use of the material's anisotropic (stronger in one direction than another) nature. This is a bit less of an issue for Al products as the material is isotropic (as strong in one direction as another). All this considered, the loads are normal use are unlikely to be sufficient to break anything, so if you're 55kg and your stoker is 50kg, there is probably a large safety factor.
My own decision was not to go with anything too light as injuring both of us by causing a crash would negate any weight saving / cool factor. I would thus avoid the sorts of seatposts discussed on weighweenies that weigh about 100g. My recommendation is to use Thomson posts if you can use zero setback models (the pre-bent ones are ugly IMO). Why Thomson? a) they are strong enough for aggressive mountain biking b) their failure mode is to bend rather than snap - read their website for their views on this c) when I emailed their technical dept to ask about tandem approval, they stated that although they had not tested their stems and posts on a tandem, many tandem pairs had used them satisfactorily. They did not say 'do not use... we do not endorse...', which I is as positive as you are likely to get given the non mass market nature of the hobby.