Thanks for all of the great replies to Joules so far. To answer some of your questions:
--- The build was motivated by a couple of things. One, I wanted to see if I could regain a few shop skills. The last time I had touched a mill was during Nixon's first term. Second, I was frustrated because I meet very few kids who get excited by actually building something. I wanted to show local kids that having ideas and building them is FUN! We need lots of kids to believe that, I think.
--- Joules plus the Trek tandem plus the battery/electronics box weigh about 200 pounds. Had I found you guys sooner, I could have saved quite a bit of that with Matt's (Recumpence's) drive. That concept is genius!
--- Most of Joules' body is 6061 aluminum, with load-bearing bushings for bearings and such made from 316 stainless. Bearings, shafts, urethane, screws, sprockets, chain, etc. mostly came from McMaster-Carr (a treasure for builders), with the Gates timing pulleys and belts from Grainger.
--- All of Joules was built in the basement over the course of about 4 months, plus lots of semi-idle time on other things. I used a Grizzly mill-drill and an old Jet lathe. Body shapes in 0.25 in. aluminum used a jigsaw, router plus templates (scary!) and a lot of filing.
--- Here are a couple more photos, of the electronics box innards and some of the speed-reducer. Joules uses two Ping 48V, 15AH packs with an Alltrax controller. Given the available torque, the current limiter on the controller is set to 60A to keep the bike (and me) intact.