Just like fifteen million Model T's, metal gear-converted Bafangs: some will run like coffee mills and some will run rather quieter than that.
TIP: do as I did with my new eZee (plastic geared). FILL the housing partly with light motor oil (I used Mobil One SAE 20,
and ran for hours and hours, slowly enough to not churn the motor windings HARD with oil (which acts pretty ruff on windings and wire ties
at thousands of rpm).
Now, the eZee just happens to have a half dozen threaded hole, disk rotor mounts: perfect place to inject the temporary oil fill.
Presuming the Bafang has no such holes: you'd need to, while it's open, drill and tap the cover for a 6/32 (or other) small screw, to be your oil
fill-and-drain place. Make two such holes: one for fill and one for air-venting to allow drainage. Tip: one oil hol on the side, so you can drain whilst
the bike is laid on its side, and the other hole might be elsewhere.
Scenario: your newly metal geared Bafang. Install several ounces of light oil. Run unloaded for an hour.
DUMP the oil out into a clean, white plate. Look for metal sparklies (wear products of the new, soft, planet gears).
Refill and repeat. The noisy motor will run quieter, right off the bat (oil cushion).
With each run and drain, more sparkles (metal flake) will come out with the drainage.
Eventually, when the new planets are "worn in", metal generation will nearly cease.
The uneven "rrrr...rrrr...rrrrr" will eventually become more like a steady "rrrrrrrrrr" and softer in volume of sound.
Do not run the motor at super speeds with a full case of oil! That would be so hard on the windings, ties, and hall sensor mounts
...liquids at high speeds act almost like solids.
Instead, leave your Bafang nearly empty of oil when it is in actual service: an ounce or two at most,
or "none" at all (the teeth of the outer, ring gear will stay "wet",
and so, wet the planets.
And at intervals, to clean out all wear products, re-fill the motor with fresh oil, run slow, and then instantly dump the contents.
Look for the sparkles. They will generate for an indefinitely long time, I predict, but, as the involutes gradually self-machine (polish),
the metal generation rate will diminish greatly.
Do not run metal to metal gearing with mere "grease". It won't work well long term: high speeds, and the grease gets "pushed out of the way":
dry metal gears will gall and make a super mess of themselves, etc.
THIS IS AN EXPERIMENTAL TECHNIQUE. I CAN'T BE RESPONSIBLE FOR DISASTERS.
I HAVE NO PRACTICAL EXPERIENCE OTHER THAN WITH MY YET-UNUSED eZEE MOTOR.
YOUR RESULTS WILL VARY.
OIL IS THE FIRST CHOICE FOR GEAR LUBRICATION.
THINK: AUTO transmissions. AUTOMATIC TRANSMISSIONS: Other than for their need for special additives in ATF, due to their helical/sliding action teeth,
ATF -could work- for our need, EXCEPT that ATF may have a detergent, dissolution effect on any silicone-based Hall sensor adhesive.
THERE IS A RISK, that even plain, synthetic motor oil (or any motor oil) may, in time, de-anchor any glued-in Hall sensors. We don't know, not yet.
Oil oil oil. I know oil and break-in/wear in. Rrrrrrrr can become ummmmmmmm. That's our aim.
Be careful, take a risk, but don't blame me if you f anything in the doing of this new "new" trick: as old as gearing;] but new
to this new kind of hub motor.
Good luck must be made, not found like some lost penny.