Most reliable Mid drive motor kit?

[Retrorockit]

California E bike has an article on using the B rotor in the A housing with some shims and a conversion bearing size.
Perhaps tomjazz has some experience with this mod?

 

[furcifer]

" I've seen a few people breaking rotor shafts and I'm not sure why that is. I personally suspect user error but it could be a manufacturing flaw. Other than that these motors are bulletproof with very little maintenance."

The early BBSHD rotors known as A version have a couple design flaws that are all in the same place on the rotor. shaft. The B version has 4 changes made at that location.
The B version also has a bigger shaft and bigger bearing, so the end cover is different too. Usually the whole motor is replaced as a unit. My A version went 4 years of hard street riding, and finally broke testing a very heavy duty chain guide idler setup. This involved wheels up launches in low gear. It actually broke from fatigue cracking under lighter load later on and not while testing the chain guide. The A version has the pinion gear square cut all the way up to the rotor, The B version has a bigger 9mm shaft and a radiused gap between the rotor and the Pinion. The B version would be much preferred for the shunt mod. Just the shaft size =26% more cross section area. The other design changes are to prevent a crack from starting there. The keyway was also cut right up to the pinion providing a notch at the stress riser there for a crack to start.

Ahh, that makes sense. I wasn't aware of that design change or if I was I forgot. I just thought it was a change made to the controller and connectors.
I'd have to pull out my torsion labs to see if your math is correct. I recall the polar moment of inertia on a shaft varies by the diameter to the power of 4. I seem to recall a couple lectures in Design for Failure Prevention where we looked at shafts and identifying points of failure on shafts with different diameters. On shafts with gears, notches, cutouts etc. you get some force concentrations that could affect the overall "strength" more than the diameter of the shaft. For two identical solid steel round shafts of different diameters it's pretty straight forward, but I'd have to compare the two side by side and get some dimensions to confirm your math.
The sad thing is 2 years ago I could have done this easy. Now I barely remember where to begin.

 

[Retrorockit]

For 4 years I had no idea that there were 2 versions. I didn't find out until I actually managed to break mine. Full throttle takeoffs from traffic lights didn't hurt it. Just intentional abuse to see if I could provoke a chain drop caused it to fail.