Quite a fiasco with the rear brake getting it not to rub. As I have mentioned before I got it to work but it was barely adequate and not something I want to do often. FWIW, I have worked on /built bikes since 1994. Also played with cars at the track(turbo miata) so Im no stranger to wrenching. Im no ace mechanic but I get things done.
I have 5 other disc equipped bikes and none have this issue.
The problem seemed to be when tightening the rear caliber, the caliber would shift inwards..it would actually 'pop' into place. I finally got it to barely work with a feeler gauge in between a pad and the rotor while also applying the brake lever while tightening the caliber. I was worried, it would pop while out riding so I wanted to fix the issue.
I 'speculated' that shimming the rotor out 'might' solve the problem so I bought some 0.2mm disk rotor shims.
Got the shims yesterday so decided to try them.
Unbolting the rotor revealed something horrendous ...several of the bolts were actually bent. As such removing them pretty much ruined the bolt holes and you could see big aluminum slivers on the removed bolts. In addition, the bolts were shorter than typical m5x10 rotor bolts at about 8.5mm to account for the shallow bolt holes in the hub motor (holes were about 9.6mm). The rotor is about 2mm thick. Not much room for thread engagement in soft aluminum.
I had some existing m5x10mm rotor bolts so I paired them with 2 0.2mm rotor shims for a total thickness of 0.4mm. I was able to bolt all this up but 3 of the 6 bolts (with blue loctite) were basically stripped. That being said, I bolted it all up and adjusted the calibers and BINGO, it worked well. Easiest caliber alignment ever for this bike.
I decided to check the threads on the removed bolts by running a die over them and the stock threads were just garbage, the term 'horrendous' comes to mind.
This is of course, not safe so I decided to fix the stripped holes with helicoils.
Initially tried to use the existing hole depth and it was just too shallow to get a helicoil to work. Hard to go into all the details, but after trying 5 different things(and 10 inserts) I decided I needed to disassemble the motor and drill the holes thru the hub to have enough hole depth.
Disassembling the hub motor is pretty easy but you need torx security bits (basically a torx bit with a hole in the middle). Luckily I had these so I disassembled the motor. While seperating the motor/a shim(15mmx19mm.0.2mm) used to preload a ball bearing was destroyed (see video by grin technologies when they disassemble a bafang).
This video gives a good descriotion
Once I seperated the motor, drilling the holes thru/tapping and inserting the helicoils was pretty easy although I did have to modify the plastic insertion tool with a dremel for clearance. as there was a step that wouldnt allow the tool to sit flush.
After all that, helicoils were inserted super easy. Had to be super careful to make sure no leftover metal fragments to get caught in the gears.
So the only thing left at this point is to get yet more rotor bolts (many were ruined while doing all of this). get some lithium grease to relube the gears and find a replacement for the destroyed shim. Luckily there appears to a shim that will work used in the RC car industry (15mmx18mmx0.2mm).
So, basically waiting for lithium grease and shims, will update once I get those.