E-Tagalong for Antique Tandem

This is a project to make a trailer to both assist and brake. While this trailer is theoretically flexible to any bike, my primary use for it will be connected to my antique tandem to make that an ebike without messing with the original drivetrain which is cool in its own right but challenging both up and down hills with only 1 gear and a lone coaster brake providing slowing power (I choose not to describe it as stopping power intentionally).

A little history: I found this tandem for sale for cheap, needing some help, but it had a cool linkage steering system meaning you could solo the bike from either seat and that brand of weird is right up my alley, so I bought it. As I did more investigation I found, to my best estimation, this bike is ~80-90yrs old. As was typical at the time, this bike has a skip-tooth chain and coaster brake hub. Somewhat less typical is the front hub which has a very early through-axle. The threaded axle must be completely removed before the wheel comes out of the fork and the fork legs have holes, not drop-outs.

Riding such a heavy bike with a seating position that was built for a time when people were on average shorter, with a single gear, I quickly found myself wishing for a little help. At the same time, descending the west hills of Portland had me concerned for my safety and the health of that little coaster brake which was making a valiant effort, but coming up short on the necessary braking demands.

I already had experience with hub motors capable of regen on my cargo bike so I decided to apply that to the tandem, but there was a problem. I couldn't change the front wheel, the through-axle situation doesn't match any hub motors (even an all-axle would have had to have custom adapters because the through axle is not a modern standard size, and even if it could I wasn't sure I trusted the old fork to handle the forces of a hub motor. The rear was also out of the question, the bike has no provisions to mount any other form of brakes beyond the coaster on either wheel so losing the coaster would have meant having no mechanical brakes at all, only regen, which I did not want to do. Changing the rear would have also meant losing the skip-tooth chain and stylized chainring that goes with it, or having a skip tooth sprocket custom machined to fit a cassette body.

I was left that the easiest option to electrify the tandem would be to build it a trailer. As a side benefit, since I sometimes take the tandem to events where event laps may not require the assist, I will be able to ride to the event assisted, unhook, ride the event unassisted, then reattach and ride home assisted. This should allow for a smaller (and thus lighter and cheaper) battery to be adequate for my rides.

Now that you understand why I would do such a bizarre thing, the build specs are:
Motor: SX1 modded to do regen laced in a 20in wheel
Controller: V5 Baserunner
Battery: 52V

Stay tuned for part 2 when I have acquired a battery and whatever UI winds up going on this bike and am actually able to take it for a ride.