D - see John Tetz's idea at IHPVA.
I wanted a very light power assist only. Tetz pioneered that.
Other sites say you want something so big and heavy that you then NEED it to accelerate quickly or to go uphill. Basically a motor that can deliver 100W to match the typical rider's 100W.
Today I was looking into the idea of rewiring (or findig ) a hub motor to 12V that
1) has a direct drive and no freewheel, and
2) a planetary gear for better torque.
These ARE quite light compared to the typical alternative.
I like the idea of a constant-draw generator that
1) can be used via battery as a motor
2) charges that battery
3) powers lights at night or
4) powers a car-style cigarette power adapter for various appliances such as
i) music player
ii) cell phone
iii) GPS unit.
On a road trip (or cross country tour), I'm always gonna be using it for those 4 uses anyway, so don't mind the constant draw. So long as it is not too much.
However, not cheap and much hassle to install a hub motor for all that.
I found this decade-old obsolete ZAP Zeta III rim motor.
There IS 1 on ebay this month. POS with dead battery, but I can do that externally. Plus the battery tek has come a long ways since then. I'd like brushless too but meh.
Clearly this won't deliver much torque. But it lets me explore some ideas.
For brief and strong assist (at 2x the motor capacity, for a short period before the overheating causes permanent damage), the former 12V hub motor is better.
Based on my results with the ZAP, maybe I'll be encouraged to continue this project.
With so little juice, the structural components could be trimmed down to save weight.
Again, I'd note that the steady RPM at the pedal makes the crank and bottom bracket ultimately a much more sensible place to place a dynamo or generator.
I wonder about my 'nitro boost' idea for a brief power surge when you need it using a capacitor array.
When a big dog is chasing you, there is no such thing as too much power!