Ok sorry been super busy this weekend, @ccostel thanks for the video and your test to break it.
I think you should screw it to a mount and then try to pull it off.
@chinzw I dont know about thermal stress since the motor never gets hot. But view the close up below.
New updates are that I have broken a third motor with a similar design from the same manufacturer.
Im getting a better idea of what causes this since i was able to brake this motor in just a few rides.
First common issue with these 3 motors out of the box, they make a weird screeching (that isnt the normal one) while braking very hard. sounds like magnets rubbing the stator for a short time till i lay off the brakes. So i decided to keep doing this and short stop more often. The sound was constantly only during strong braking. Then the next day i realized that the place where its usually breaks is becoming uneven/bent a little.
Next day I had to short stop on some cobblestone. (which i rarely ever ride on since i’ve memorized all the newly paved streets) Then I checked the motor to see if it survived. Sure enough the piece was completely broken now.
My theory is that under hard braking with low gearing and strong vibrations and tight enough belt is enough to bend the motor “can” so that its magnets rub the stator and make a weird noise. This constant bending and pulling on the motor as well as the weight of this heavy motor being pulled towards the ground during vibrations will eventually fatigue the weakest point being the motor cap that hold the motor to the mount. It need to be stronger since this part is the only thing attaching the motor to the mount and must withstand all these forces.