A big improvement!

 

The new reverse-transfer gear as shown here has been installed, and we test rode it this evening around Fremont. it worked like a charm. Alicia and I were both able to start or stop pedaling independently, yet still contribute to driving the wheel. There were no chain derailments caused by any uneven tension between us. So the BMX freewheel was a great solution to the problem, and also helped straighten the chain line a bit, due to it's smaller diameter.

 

This is the old part that it replaced. Just a crankset (or what's left of it) chopped up from a triple. So long, sucka!

 

Here's a complete view of the stoker drivetrain. You can see the crossed chain pulling on the bottom of the BMX freewheel (remember it is flipped around to drive in the proper direction through the bottom bracket axle. The two derailleur pulleys work together to manage the non-tensioned side of the chain and allow it to cross itself. The lower pulley is on a spring tensioner to take up slack in the chain. This previously was the cause of my problems, allowing uneven tension to pull the chain loose and flop off the wheels. A rigid pulley system would prevent this from happening...up to a point, then something in the system would eventually bend. So that's why i had a spring tensioner. Both non-ideal for two novice tandem riders. So now that I have the freewheel solution, the spring tensioner is perfect for just keeping the chain taut.

 

Another view of the drivetrain. Here you can see both transfer gears. captain side and stoker side. If I have time I'd like to replace the captain's transfer gear, with one of my alternate versions to save weight and also get an even better chain line. But we'll see how the rest of the week unfolds. I may have run out of time for niceities, but for now, the Double Take is rideable, and I am so relieved.