Thanks! Maybe you can help me understand the oft-claimed mid-drive climbing advantage.
Look at enough official mid-drive+derailleur bike specs, and you'll see that most have chainrings larger than any of their stock cassette cogs. Torque-wise, this puts the mid-drive motor at a mechanical disadvantage in every gear. Also makes the both motor torque and mechanical power subject to drivetrain frictional losses.
The only reason this happens is because some mid drive motors have a secondary housing - that contains the gear reduction off the motor - which sticks out on the drive side and in turn widens the Q factor out to the right. Thats why BBSHD crankarms have an 18mm offset on the non-drive-side... to even out the pedals underneath you so they are centered again.
The existence of that secondary housing also means the chainring moves outboard. Which in turn means chain alignment is biased towards the outside of the cluster, onto the little cogs. You can't even reasonably get to the big cogs oftentimes. SO the solution to that is to have a chainring which covers overtop of the secondary gear housing like a lid on a jar thats a bit too big, and provides inboard offset to bring the chain line back to normal. This loose fit over the 'jar' can only go so small as a result. The limit is 42T. If you buy a Lekkie replacement motor cover for a BBS02 or a BBSHD they are a bit smaller and reduce the size limit to 40T. Thats the entire reason you see big chainrings on BBSxx motors, and would also see them for example on ToSeven motors.
My Smash has a Cyc X1 Pro on it, which does not have a secondary gear housing to fight with. If I recall correctly I put a 34T hiding behind the primary (kart) drive chain.
My Big Fat Dummy has a BBSHD, but thanks to the long stays on a longtail, the angles that come from that allow me to get away with murder on chain alignment, and I can use a 36T front chainring without alignment issues. You can see in the closeup how the ring is only barely bigger than that secondary housing just behind it. BTW when I finish with this post I am going outside and fitting an 11-51T rear cluster on that bike.
And here is a BBSHD with a replacement motor cover that allows me to bump down the front to 40T.
And here's a real good look at that inboard offset. This is a Luna Eclipse ring which has the most offset of any BBSxx ring on the market. It looks as if you could go smaller if you look at the secondary inside the chainring, but you can't.
Why is this better than applying the same motor torque and mechanical power directly to rear hub? What am I missing?
Gears help the rider climb a hill. We all learned this before ebikes, the first time we tried climbing a hill without shifting. They do exactly the same thing for an electric motor that is using the drivetrain to do its work just as you do.
Something
@Rexlion related above pointed out a caveat I hadn't thought of, though. He test-rode a 250w Bafang mid drive motor and found it ... sucked. Which points out that mid drives are not repealing any laws of physics. If they have no power to start with, they aren't going to do much, just like a weak hub motor. Here in the USA our legal limit is triple that of a 250w EU motor (even though those EU motors typically peak at more than double that) so I'm not accustomed to seeing or dealing with such a weak mid drive.
I mentioned earlier that Bafang motor power is overstated. Here's what I mean: A BBSHD is rated for 160 Nm. Sounds enormous compared, for instance, to a Bosch CX's 85 Nm. But Area 13 put a BBSHD on its dyno and found that 160 Nm at the axle translated to 58 Nm to the ground. Its still a lot, but nothing like what you would think by reading the specs. If a 48v/28a BBSHD is only putting down 58Nm, whats a 20a / 36v BBS01 putting out? Not much. What I see very often in EU cargo bike conversions are 48v BBS02's, which from what I hear creates really good middle ground insofar as power is concerned.
