I went ahead and bought the "upgrade" motor for my RadMini. It wasn't a need, and I bought it partly out of curiosity, but it provided a nice opportunity for testing and comparison.
If you spend any time on e-bike websites and forums, I'm sure you've heard the claim that Rad motors are not "true" 750 watt motors, and you need to buy an upgrade. This idea started in an online forum, then became popular through YouTube. Oddly, most opinions on this are based on looks, and while there are many things you can determine by looks, electrical specifications are not one of them. That being said, I do like the motor and will be keeping it. It's a high quality unit and worth the money, depending on your needs.
Side by side comparison
The Rad and upgrade motors do have some differences in design. Most notable is the magnet shell. This is narrower on the Rad motor, and wider on the upgrade. What does this mean? It means the shell is wider. What seems to throw people off is that the Rad motor has this empty space between the stator windings and the mounting plate. The upgrade motor does not have this space... or does it? It turns out the upgrade motor also has an empty space. It's just in a different location.
Although it seems the upgrade motor has bigger windings, in reality, it does not. The stator coils are the same size, as are the magnets.
The installation of the new motor itself went fine, with only one hitch: I was using a cast bit to remove the T-20 Torx head screws with anti-tamper pins, and the bit shattered. To avoid that, I purchased a forged Craftsman folding driver set, and used a butane mini-torch to soften the Locktite for the screws. The only settings changes required to the controller display was changing the speed pulse sensor (P2) from five to six.
Doing the math
I was a bit surprised when I could not get the motor specifications, so I had to do some testing on my own. Below are the results.
Stock Rad motor
Top speed: 23 mph
22 inch wheel RPM at 23 mph: 351.592
Motor RPM at 23 mph: 1757.96 (351.592 x 5)
Motor rating: 32.55 kV (approximate - electronically limited)
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Bafang upgrade motor
Top speed: 27.4 mph
22 inch wheel RPM at 27.4 mph: 418.85
Motor RPM at 27.4 mph: 2094.26 (418.85 x 5)
Motor rating: 38.78 kV
The Rad kV rating is an estimate because it has a built-in limiter that starts reducing power electronically, so the motor is not free to reach it's natural RPM. The Bafang upgrade motor does not have this limiter, and this was one of the reasons I wanted to try it out. You can see the difference on the wattage being used at top speed: The Rad motor is only pulling a couple hundred watts at top speed, even though over a thousand watts is available, while the upgrade motor actually is pulling over a thousand watts at top speed.
Acceleration and hill climb tests
Tests with both motors were done on a RadMini 4 with dual batteries and a 35 amp controller, battery voltage at 54.3 volts, and the tires at 12 PSI. Performance would be better with more air in the tires, but since the first tests were done at low pressure, I kept them the same.
Original Rad motor 0 to 20 mph: 4.9 seconds
Bafang upgrade motor 0 to 20 mph: 4.9 seconds
Wait, what? You read that right, and I ran the tests multiple times. The motors had the same acceleration times. They are both a blast to ride with the 35 amp controller, with the Rad motor pulling ahead off the line and the upgrade motor catching it.
The hill climb test was a bit more subjective, but basically I drove up an absurd 28% grade berm for a freeway overpass. The Rad motor seemed to have a bit more low RPM torque, getting up without me touching the pedals. The upgrade motor had a little harder time, and although it made it, it was not as confidence inspiring.
In top speed, there was no comparison. The Rad motor can hit 23 mph if you change your settings, but the power starts being electronically reduced above 20. If you are already riding at 20 mph and go full throttle, the response is like "Well, ok, give me a minute" as it gradually gathers momentum on the restricted power. With the upgrade motor, it's prime time, and giving the throttle a full twist at 20 mph responds with the attitude of "Hold my beer!". Yes, it does top out at 27/28 mph because kV x voltage = RPM, but it gets you there decisively.
In conclusion, if you have off road or private areas where you can safely use the extra speed, the Bafang upgrade motor is worth every cent. If you need to keep it locked to 20 mph though, the Rad motor will do the same job and can handle the 1,600 watt power levels fine.