That's really interesting. You are pushing the boundaries of the BBS02 system! Why do you run a 20" wheel though?
Are you actually pedaling at 118RPM cadence?
I barely hit 30mph only with a long runway (like 2 city blocks) and lots of pedaling
Thats with 42t on the front,
12t on the back, knobby tires (not efficient!) and a fresh 12s battery.
It should be pretty easy to avoid the issues I had. One set of crank arms got ruined when I tried to use some crappy old pedals
and frocked up the threading. After a couple of hundred miles the pedals worked themselves off the cranks. On the current left crank, I didn't torque it in sufficiently and now the taper part has widened so it wiggles a little with every stroke. Just torque the crank retaining screw in really tight and it'll be fine
The chain ring I got was actually second hand and the seller was getting rid of it due to chain suck on his bike. Now there are more options for a 42t offset chain ring... I really should get a new one. I'm also eyeing a rear derailleur with a clutch.
Incidentally, if you get the BBS02, I'd suggest you get a spider adaptor and a 44t chain ring from bafang in additional to the stock 48t. You might need smaller gears up front because of the 29" in the rear, and the spider adaptors lets you choose anything you like. The 44t is pretty cheap and gives you another gear to try.