Picking the right chainring for your BBSHD, BBS02, or TSDZ2 is important but can be tricky. There are a lot of factors to consider in getting the one that will be the best all around. Let's take a look at some of the more important ones.
That is a total of 27 combinations or speeds. With the mid-drive motor you typically have just one chainring which means your 27 speed is now a 9 speed. This is not as bad as it sounds in fact in some respects it is desirable. Riders find that they need fewer gears with electric assist and it is nice having fewer gears to shift through.
If your bike originally had 3 gears up front and they were 52 tooth, 42 tooth, and 38 tooth, you might want to pick the middle or gear or 42 tooth chainring as the best all around. If you still want a wide range of gears, especially low gears then you can trade out your rear cassette for a wider range cassette and have the full range you need. You can read more about that here.
As it turns out most newer mountain bikes and now many city and road bikes have only a single chainring up front and a wide range cassette in the back. This is known as a 1x drive-train. So adding a mid-drive doesn't require any of the trade offs mentioned above. In fact if you have an older bike you can look at going to a mid-drive to be a 1x drive-train upgrade as well.
If you really want more than one chainring it is possible. It is actually common with the TSDZ2 motor in fact we sell dual chainrings for that motor. It is less common with Bafang but is still possible. The problem is many front derailleurs won't go out far enough to reach the outer chainring. Since the TSDZ2 doesn't stick out quite as far as the Bafang motors more front derailleurs will work with the TSDZ2. If you just want to have it on one or the other based on city or mountain riding then you can move it by hand before you start out. The TSDZ2 comes with a 110 BCD spider. You simply mount a different chainring on either side of the spider. After-market spiders are available for the BBS02 and BBSHD as well and dual chainrings can be used with them as well. However, this is rarely done because of the cross-chain problems that arise from not using a single offset chainring.
Why are some mid-drive motor chainrings offset? Each mid-drive motor comes with a chainring, usually steel and between 42 to 46 teeth. This is usually a good size for all around riding and also 42 teeth is the typically the smallest you can go and have the chainring offset to return your chainline as close as possible to your original chainring(s). This simply means that the motor pushes your chainring mount point to the outside and by dishing or offsetting the chainring they can move it back inboard closer to where your original chainrings were. Chainrings smaller than 42T can't be dished because they would hit the motor housing so they are further outboard and the chainline will be off. This is not a problem on many bikes but you want to try to keep the chain as straight as possible. 42T is simply too big for many mountain bikers so they get a smaller chainring for overall lower gearing and the chainline is still fine through much of the range but is more cross chained in the lowest (large) gears on the rear wheel. Recently Lekkie introduced a 40T chainring for both the BBSHD and BBS02 with great offsets achieved by replacing the gear reduction cover on the motor. These are very nice but are more expensive and harder to install.
What size chainring is ideal? The ideal chainring size can be illusive because you are making tradeoffs. The Bafang motors want to run fast and perform at their best with smaller chainrings while the TSDZ2 motors tend to top out at a cadence of 90 to 100 so a somewhat larger chainring for riders that like a fast cadence can be better. If you just want overall lower gearing for mountain biking a 30-38T chainring is ideal. However, if you have chainline issues then you may want to go with a 42T and make sure you have ultra-low gears in the back in the 40-46T range. If you are a speed demon and want to feel like you are contributing and getting a good workout at higher speeds you may want a 48-52T chainring. I say "want to feel like you are contributing" because even if you are pedaling very hard the wind resistance is so significant that the motor will be doing the lions share of the work but you will still get a good workout.
Should I upgrade my chainring? This question may be answered for you if you choose a different size chainring than the one that comes with your motor. If you want higher or lower overall gearing you need an upgrade. Even if you decide you actually want the size that came with your motor, you may still want to get an upgrade. The upgrade chainrings made by Lekkie and others are typically CNC 7075 T6 hardened aluminum and are lighter, better looking. They also have a significant advantage by using a narrow/wide tooth profile. This means that every other tooth is wider or narrower to fill the entire space in the outer and inner chain links. This has the advantage of holding the chain onto the chainring so that it doesn't come off. If your bike had a front derailleur then you probably never had this happen because the front derailleur holds the chain on. Without the front derailleur you may find your chain frequently or infrequently coming off of the chainring. This is no fun. Some bikes never have that problem. We sometimes advise that want the size of chainring that comes with the motor to just run with the stock chainring to start out and then if they have their chain coming off they can upgrade to a narrow/wide tooth profile chainring to help solve the problem. Chainguides can also be used and if your rear derailleur has a clutch that will also help to keep the chain from popping off.
What about IGH hubs? All of the above applies to your chainring consideration except if your chain is properly tightened it won't ever pop off. You may still want a chainring upgrade to get higher or lower overall gearing or to trim weight and have a better looking bike.