i have a suggestion the Rohloff' Speedhub 500/14 is a Incredible alternative to a convensional drive train sistems, yes on a full suspension MTB the mass centralization of the hub is noticiable specially do to the sprung weight that slow down the suspension reaction but on a road tandem were you are not jumping or doing extreme manuvers is not really a concern, yes it will bring a little more gyro forces to the rear wheel do to the extra mass but is not that critical



in the braking deparment they offer a disc brake compatible interface (and I think a drum set up but don't quote me on this one) on the shifting department is no comparation to anything in the market (I pretty much try it all) the 14 gears spread out at even spaces,,i think 7.3% in between each gear, you can also chose to do a little cheating and run two chainrings (i set up a few bikes this way) to multiply your options (oviuslly some overlap will become present) the smallest ring they recomend is 38T (do to the extreme torque produce by smaller gear rathios) but that is sufficient to compare with a 20T front 32T in the back, and then for the high speed stuff you can install a 44T or something and you will a have a gear exivalent to a 68T /12T(just gestimating)



another beautyful side effect of running a single rear chainring (you can NOT (in safety) run two driven and one timing ring) is that you don't need the timing chain to be on the left leaving the market wide open to any crank set in the market and not just the specific cranks made for tandems since you can run the two chains on the right side ussing the inner and outher chainring mounts of a convensional crank

personally I'm saving the money at this very moment to get one for my mountain tandem (much better ground clearance since i can run a much smaller drive chain ring plus a lot less prone the derreileur damage in the dirt)

Oh another great feature is that the speed hub is that the "Index" sistem is perform at the hub (not at the shifter) making it ideal for long and maybe complicated cable routings like on the tandem were the rate on compression and expansion on the wire can affect shifting even more

I hope this is not to complicated, my Ingles do suck plus this is a multifaceted topic, anyway hopefully it opens the door for a few more options

ricardo