are you able to share more on the gear ratios from the pedals to the generator and pedals to the rear hub?
Yes, as you surmised, the diversion of human power from the rear wheel to the generator is accoplished by a simple freewheeel. Once the vehicle is over about 12 kph the human energy goes only to the generator. What is cool about th esystme is that it is dead simple, there are no levers or buttons to push, it just happens.
The generator does not start to produce power or offer significant resistance until the pedal RPM is at about 80, so the rider's energy can all go to getting the vehicle up to speed until the transition.
The generator unit used is the 48 Volt version of this one.
https://www.aliexpress.com/item/32878945779.html?spm=a2g0s.9042311.0.0.2b8e4c4dllVRN5 It needs to be spinning at about 800 RPM to produce 72 volts. The resistance at the pedals changes as the battery voltage drops, because the generator produces power at a lower RPM when it's looking at a lower voltage in the battery. This actually produces a nice feedback situation, if one is demanding hight speed and power, the battery is drawn down and the rider feels a slight increase in resistance at the pedals. But, with no gears to mess around with, it functions like an automatic transmission for a bicycle.
The Gear Rations I'm using:
Cranks are driving a 38 tooth ring to a 13 tooth on the Transfer Hub.
From the Transfer Hub top the back wheel there is another 13 tooth ring driving a 24 tooth cog on the back wheel.
Also from the Transfer Hub, on the other side, bolted to the disk brake mounts, is a 54 tooth #25 chain ring driving an 18 tooth #25 cog threaded onto the generator.
One of the benefits of this set-up is that the free hub on the Transfer Hub allows the rider to stop pedaling without any backlash from the generator that is spinning at 800 RPM. Although the generator quickly slows down to below 800 due to electrical resistance, it still spins for quite some time once the generator output is below battery voltage.
This leads me to wonder whether there is a system (either manual or automatic) to gradually increase the rider watts absorbed by the generator during this transition (and in general?).
I was looking to keep the system as simple and intuitive as possible so there is no adjustment, however as the power generated (and therefore the resistant at the pedals) goes up as a function of RPM, one is able to pedal gently and produce just a 50 or so Watts, or if one is looking for more exercise one just has to pedal a little faster and the wattage/resistance goes up very quickly. This effect also makes for a seamless transition betweeen human-power-to-the-back-wheel and human-power-to-the-generator.