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Surely this configuration is inferiour (all aspects considered), otherwise it would still be widespread today?
It's messy. And heavy.
But, it has a few other advantages compared with the standard crossover drive.A standard crossover at the stoker BB is constantly trying to twist off the stoker's BB; one side is pulled forward by the captain, the other side is pulled back by the reaction to the driving load. Stoker BBs take a lot of abuse because of this. An unconventional crossover at the captain's BB is mostly pulling the BB rearward with only a small torsional load and also brings one of the derailleurs closer to the captain who has control of it. That means they can better trim it (although unnecessary given the chainline) and shift it (more difficult on a tandem when you need to coordinate the drop in effort).
Single-side drive is the best from an engineering point of view (at least in my opinion), but it's difficult to do unless you either run a hub gear/SS/fixed or are willing to relinquish the inner position of a triple. Sheldon can direct you to this article which goes through an interesting single-side drive design.
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hamrack
Jaap
Let me rephrase; on 'modern' tandems. Surely this configuration is inferiour (all aspects considered), otherwise it would still be widespread today?
Is it really though? You need to connect the two chains instead of shortening one extra. The time saving in labour seems negligible.