Join Date: Dec 2005
Location: London, UK
Bikes: Trek T200 plus enough others to fill a large shed
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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.