Mid-drive motors like the BBS02 use the existing drivetrain and augment your bike pedaling with electric power. They achieve this by spinning the front chainring directly, which then drives the chain and rear gears, spinning the rear wheel and moving you forward. From the bike's perspective, it's no different than having someone that can pedal many times faster and with much more power.



Without going into a huge amount of detail, the size of the chainring determines how much torque (rotation power) you get from a specific RPM (rotation rate). A smaller chainring will allow you to accelerate from a stop more quickly and will put less strain on your motor, but will have a lower top speed. A larger chainring will give you a higher top speed, but the motor will work harder to achieve that (which translates into more heat and battery drain). For guidance, I've broken the sizes (measured in the number of gear "teeth") into three general categories:

Small (30T) chainrings work best for riders who are regularly climbing huge hills (mountain bikers)

Medium (40-44T) chainrings are suited for riders who ride a mix of terrain and are not racing downhill

Large (48-52T) chainrings work best for flat terrain and riders who want maximum top speed

The Bafang kit comes with anywhere from a 44-52T chainring, depending on availability. You also have aftermarket options like the Lekkie Bling Ring and other "narrow-wide" chainrings. The major advantage with these is chain retention - that is, they reduce the risk of the chain flying off the gears. More discussion in a later step.

Follow along around 4:12 in the video for tips on how to fit the chainring to the motor. For reference, the default Bafang ring should "dish" inward, towards the motor (see picture). Definitely test how the ring fits before tightening any bolts.

One of the only complaints I have about the Luna Cycle kit is that the included bolts were too large. You can see in the picture above (and this video) that the ring actually rattles, and the bolts are long enough to scrape the motor housing if used. I solved this by purchasing some Origin8 Single-Ring Chainring Bolts (full disclosure: I didn't email Luna Cycle for replacements as buying the new bolts was quicker). You can see in the final picture that the new bolts fit just fine.