I have attached an Xtracycle FreeRadical to a Giant LaFree Lite electric-assist bike. (Total payload of ~500lbs before i got off to take pic!) . It worked with the stock Nexus 4-speed transmission, but loaded climbing and top speed under power were annoyingly limited, so I swapped in an SRAM Dual Drive system (had a machinist friend mill down the chainring and drive sprocket to accept the necessary 9-speed chain).

I can now climb the steepest San Francisco hills with wife and child aboard, and reach 25mph under power on my daily 18-mile+ commute with ~25% charge to spare. It's charged from our large rooftop photovoltaic array. - Todd Fahrner