But there’s a twist!
Here is the motor with the adapter fully seated on the rear axle:
The thing to notice here is the stepdown from 10mm to 8mm, which is “outside” of the adapter.
After seating the adapter flush with the hub and then sliding both onto the axle we are left with a ~4.8mm gap:
So the way I see it, we’re left with (3) simple options, only (1) of which is probably tenable:
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Get a 22mm bearing with a 10mm internal diameter to fit the step down (6900 bearings). These Sovereign Speed Bearings seem to fit the bill. That said, the small gap between the adapter and the axle would likely prevent the inner ring of the bearing from contacting the adapter, which would mean the outer race would seat against the adapter (not good).
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Fully seat the adapter on the motor, crank down the hub, and just let there be a nearly 5mm gap between the adapter and the hub. This will likely put too must pressure on the pins, which will likely shorten their life.
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Pull the inner bearing and spacer. The motor itself has both an internal and external bearing, if the adapter is fully seated against the hub and the outer 608 bearing is locked in place with the axle nut and speed ring(s), then the entire “assembly” should have (3) bearings distributing the force across them. The adapter pins already have a very firm seating in the hub, so the axle nut shouldn’t need to be insanely tight to make this work. Also, since the hub is Aluminum, there should be no skew in the bearing if it’s fully seated in the hub.
Here’s shot of the rear axle with the inner bearings removed and everything fully seated (without being screwed down):
As a bonus, the distance between the outer rim of the hubs on the rear axle in this configuration is ~12.3125" whereas the outer rim of the hubs on the front axle is just a fraction over 12". This means the wheel separation for the axles (in this configuration) is nearly identical.
One item to note, however, is that the threading on the axle is just barely deep enough if you use a speed ring (I may use two just in case):
I’m going to go with option 3 here, unless someone can shoot holes in this approach. Thoughts?