I recently purchased a fat bike and started upgrading different components that I thought were lacking immediately, as I always do. It came with a 1500 watt BBSHD motor and an Sram X5 9 speed drivetrain. The brakes are Tektro E740 and actually perform as well as my Magura MT7 so they get to stay. It was a hard decision deciding what derailleur to use. Because my hub is the old fashioned type (HG not XD) I was unable to use any of the 12 speed cassettes with the 10 tooth gear, so that meant if I wanted to go Sram my only option for cassette is the NX. It has an 11 tooth as the smallest cog, which really is fine for me, I don't even use that small of a cog or even the two next larger ones no longer get any use. So I decided to go with the Di2 XT derailleur, XTR shifter, XT 11-42(46) cassette, and I needed two chains (crazy long chainstay) so I used one XT and an XTR. It is a really, really nice system which it had better be for the price. It shifts so smoothly and precisely, by far the best I have ever had the opportunity to use. At this point I am now running a BBSHD at 1500 watts with the stock Bafang controller. I used the Bafang programming cable to edit a few motor settings but found that as usual I still wanted more torque. The high end was around 35 mph which to me is more than I need so I saw no reason to upgrade from a 52 volt battery I just needed more amps. I love amps.
My next upgrade was the controller. Typically I would have wanted a solution that was internal to the motor, like the original Bafang controller but combine the difficulty in obtaining one with the fact that my motor is built into the bike frame and I wound up deciding to go with the BAC 855. I was lucky that the motor was built in and had protection from the bike frame so I didn't even wind up needing to use the 3D printed cover for the Bafang motor. If you want to hear more about this just send me a DM. A 2" piece of double sided sticky mounting tape and two zip ties secured the controller perfectly to an unobtrusive area on my frame. The BAC 855 has a feature that will send a small amount (<100) of wattage to the motor even when the throttle is not engaged and there is no peddling. It does this to keep the ride smoother, so when the throttle is engaged the chain is not moving from 0 - quickly immediately which can cause breakage issues. This way it provides a smoother ramped up feeling to acceleration as opposed to the jerky on and off feeling it has stock.
The Di2 system has barely needed adjustment at all, and has managed the pesky chain line problem for me with no alterations to the drivetrain except for a spacer next to the Lekkie Bling Ring up front. Also, the small amount of wattage being sent to the motor when in "neutral" is just enough for the Di2 system to grab and shift gears if need be even when I shift 2 or 3 at once (which is also made possible by di2). It is seamless and as smooth as it gets. I have had no problems with the chain coming off or breaking and have not needed to rework the cassette at all and am able to make use of the entire 11-42 tooth range. Initially I installed the 46 tooth cassette but realized it was unneeded and when I needed to replace it I did so with its 42 tooth counterpart. I was having all sorts of issues prior to moving to Di2, but since its implementation have had none. I just adjusted the derailleur using the Di2 head unit and moved it 3 clicks to the left and that's it!
My next upgrade was the controller. Typically I would have wanted a solution that was internal to the motor, like the original Bafang controller but combine the difficulty in obtaining one with the fact that my motor is built into the bike frame and I wound up deciding to go with the BAC 855. I was lucky that the motor was built in and had protection from the bike frame so I didn't even wind up needing to use the 3D printed cover for the Bafang motor. If you want to hear more about this just send me a DM. A 2" piece of double sided sticky mounting tape and two zip ties secured the controller perfectly to an unobtrusive area on my frame. The BAC 855 has a feature that will send a small amount (<100) of wattage to the motor even when the throttle is not engaged and there is no peddling. It does this to keep the ride smoother, so when the throttle is engaged the chain is not moving from 0 - quickly immediately which can cause breakage issues. This way it provides a smoother ramped up feeling to acceleration as opposed to the jerky on and off feeling it has stock.
The Di2 system has barely needed adjustment at all, and has managed the pesky chain line problem for me with no alterations to the drivetrain except for a spacer next to the Lekkie Bling Ring up front. Also, the small amount of wattage being sent to the motor when in "neutral" is just enough for the Di2 system to grab and shift gears if need be even when I shift 2 or 3 at once (which is also made possible by di2). It is seamless and as smooth as it gets. I have had no problems with the chain coming off or breaking and have not needed to rework the cassette at all and am able to make use of the entire 11-42 tooth range. Initially I installed the 46 tooth cassette but realized it was unneeded and when I needed to replace it I did so with its 42 tooth counterpart. I was having all sorts of issues prior to moving to Di2, but since its implementation have had none. I just adjusted the derailleur using the Di2 head unit and moved it 3 clicks to the left and that's it!