Simply put...
(A) Is the caliper (part that grabs the disk/rotor to brake) behind the fork & disk/rotor with the mouth of the calipers facing towards the front (like "typical" bike)?
Or...
(B) Is the caliper in front of the fork & disk/rotor with the mouth of the calipers towards the rear (i.e. reverse of "typical")?
Picture in your mind the tire spinning normally & moving forward... Then a caliper behind the fork & disk/rotor (i.e. A) grabs the rotor to brake... The combined forces make it want to rotate & move downward, foward & outwards!
Now picture the same wheel spinning & moving forward... But the caliper is in front of the fork & disk/rotor (i.e. B) & grabbing... Now the combined forces are upwards, rearward & inward into the strongest parts of the mounting system...
It's actually just common sense & physics! It's no surprise that hub motors (being far more spun mass, inertia, torque & additional sources of force to continue moving [motor] then pedal bikes) would not react well to this!
E-bikers try to fight this situation by using torque arms (I assume what you called "moment arms" or momentum arms [is what you're saying] are actually torque arms)...
But... It's a bandaid on a gushing major wound! Torque arms are not a bad things in general & given the lack of decent support within the mount, should be used (best are like doc bass torque arms)! But it does not fix that we are ignoring the serious catastrophic design flaw within the system.
As I understand from your post, you actually had some torque arms in place at the time... So clearly it won't entirely solve or prevent the underlying flawed design!
The real answer is a 2 part one.
(A) However it's done, use beefier, heavy duty forks & axle mounts either from the factory or with add on torque arms that are integrated into the fork/mount as single piece (like the doc bass arms)....
& (not or)
(B) Relocating the caliper in front of the fork & disk/rotor!
This problem isn't unique to hub motors & e-bikes. It 1st was widely noticed with quick release wheels. People discovered that if damaged or not properly used, it readily climbed out of the fork & axle mount when the disk brake was used. Some then realized, if the caliper are put in front of the fork & disk/rotor, then even if the quick mount was damaged or improperly used, it would never result in the wheel climbing out of the fork & axle mounts...
However (as far as I know) no one connected the dots between hub motors & quick release wheels until I did. Since then, just about all the experts I asked agreed it was the same underlying design flaw & solution!
Technically, the design flaw is present in all disk brake bikes with calipers behind the fork & disk/rotor... It's just that the majority of the time, the forces involved aren't strong enough & the fork/axle mount is not weak enough for that problem to show itself. Unfortunately, that's not the case with e-bikes/hub motors & weakened forks/axle mounts!
Adam
