A California bicycle shop posted the photo below of a tandem made by Specialized S-Works — the part of the company which produces high-end, innovative bicycles.
This is a show bicycle and concept bicycle, “must be better because it’s got a carbon-fiber frame and has (wow), shaft drive.” However, drive from the rear crankset to the rear wheel reverts to chain drive, which allows of multiple drive ratios.
A reader where the photo was posted on Facebook remarked that “formula one racing motorcycles all use a chain drive. With that shaft drive the power has to change 90° twice. Minimum 5% power loss compared to chain drive.” As only the power from the captain (front rider) passes along the shaft, power loss on this tandem might be somewhat less.
The frame has to be heavier to resist the torque produced by the shaft. A tubular shaft could probably be as light as a chain.
With no seat tubes, the frame may provide a greater suspension effect, but at the cost of greater weight due to lack of triangulation. In a tandem, there is a major requirement for torsional stiffness around a longitudinal (front-to-rear) axis, and computer analysis may have shown that the structure necessary to achieve this also optimizes strength and vertical stiffness/suspension without seat tubes.
The saddles and seatposts are integrated and non-adjustable. Saddle height can only be changed by replacing a saddle. Handlebars can only be adjusted by replacing the entire front end assembly. This bicycle, then, is effectively limited to use by the same pair or riders. If not for the inefficiency of the shaft drive, I’d say that it might be suitable for pro racers, for whom cost is no object. But is there any tandem racing at that level?
Useful new ideas sometimes spin off the design an impractical concept bicycle — but in this case, in my opinion, they would relate the ride quality and durability of the unusual frame, rather than to the shaft drive.