Thank you for all the comments and the two links posted to previous threads along these lines (apparently the search function is pretty worthless since I had not seen them before).
I’m not rejecting the use of one-way needle bearing clutch assemblies but it appears that they require rather large sizes to handle the peak torque loads produced by pedaling and do not exist as ready built components built specifically and “ready to use” for bicycle applications.
Where on the other hand freewheel assemblies that can be used on the crank side are available in both right and left hand assemblies. Right hand side being the most commonly known (Sick and Cyclone parts plus others) but there are left hand side units available for Staton-Inc that would serve well on the left hand side of a double stage bottom bracket assembly for motor input:http://www.staton-inc.com/store/products/5_Hole_Flange_LEFT_HAND_thread_freewheel_sprag_clutch-967-0.htmlhttp://www.staton-inc.com/store/products/65_Tooth_25_1_4_x_1_8_LEFT_Hand_threads_Freewheel_Sprocket-695-0.htmlhttp://www.staton-inc.com/store/products/72_Tooth_35_3_8_x_3_16_LEFT_Hand_threads_Freewheel_Sprocket-756-0.htmlhttp://www.staton-inc.com/store/products/25_80_Tooth_LEFT_Hand_threads_Freewheel_Sprocket-687-0.htmlhttp://www.staton-inc.com/store/products/60_Tooth_35_LEFT_Hand_threads_Freewheel_Sprocket-679-0.html
Granted most of them use other chain sizes then common bicycle chain sizes but there is no reason not to use a different chain size on the left hand side motor chain loop. It may even be an advantage allowing greater choices of industrial type sprockets to put on the motor output shaft and also allows a simple sprocket on the motor since the freewheel is built into the left side crank sprocket. All the sprockets in question are intended for use on the left side of the rear wheel hub for use in Staton-Inc gas motor kits but they should work just as well on the left side of a double stage bottom bracket assembly especially since some of the more powerful motors used in the Staton-Inc kits are putting slightly over 2 horsepower through them so they should be more then strong enough for all but the most over-powered e-bike builds.
The information Miles
has provided with his thread (nice 3D solids drawing by the way) has been very helpful. Especially on one key point. That being that there is really no need for a second inner bearing race between the inner and outer shafts and only a tight slip fit is needed. This is due to the fact that there is only significant stress loads on the inner spindle shaft when the rider is pedaling and when they are doing so the inner and outer spindle shafts are rotating together and the bearings on the other spindle shaft carry the load. Unless of course the rider is doing light slow “fake” pedaling which isn’t putting in any power into the drive system and just letting the motor do the work while making it look like they are pedaling in which case there isn’t very much load on the spindle anyway. About the only time there would be significant load on the inner spindle while it was stationary and the outer spindle was rotating around it under load from the motor is if the rider where not pedaling and was standing up on the pedals instead of sitting down on the seat.
So one can simplify the assembly significantly by just making the inner spindle shaft a tight slip fit with a little grease in the inside of the outer spindle shaft. Yes it would wear over time more then if it had an inner race of needle bearings but that wear would be reduced according to how much the operator pedaled and standing up on the pedals while the motor was running would be a “don’t do that” item. In other words, allowing the pedals to freewheel instead of being constantly driven by the motor like the stocker-monkey set-up would be mainly a safety and convenience feature and the operator would still be encouraged to pedal to reduce the wear over time on the custom double stage bottom bracket assembly. Not a bad trade off for the simplification in the design it allows especially for the conventional smaller size bottom bracket sizes.
I did actually dig through my pile of old bike frames and components stash along with taking some measurements off of my up and running bikes and not all bottom brackets are as small as diameter as the somewhat standardized 1.37x24tpi cartridge assemblies. My bike full suspension mountain bike has a much larger bottom bracket diameter that is using a heavy 30mm diameter spindle. I didn’t pull it apart to get internal bearing race diameters but there is definitely a lot more room inside its bottom bracket and it appears to use press in industrial type sealed ball bearing race assemblies one on each side instead of screw in cartridge cups. In addition the frames designed to use the old single piece Ashtabula type cranks have bottom brackets that are larger and more roomy then the standard 1.37x24tpi cartridge bottom brackets as well so there are certainly larger size bottom brackets out there that would make things even easier.
My ultimate goal would be to put this kind of custom double stage bottom bracket assembly into a Yuba Mundo long-tail cargo bike using a sprocket drilled with six holes to match the disk brake mounting pattern on the left side of a hub motor mounted in the rear triangle to provide electric assist power through the bottom bracket just like a stoker-monkey only without having driven pedals. Now that I know it is possible I guess the next step is to save up and buy the Yuba and then pull apart its bottom bracket assembly and figure out what type it uses and take exact measurements and then go from there.
Here is an updated diagram (same color coding):