liveforphysics said:
Can someone give me a reason why anything could be possibly be more efficient than a direct drive hub?
It can have excellent stall rotor torque performance, all the way up to full speed. All drivetrains start at 0% efficiency at 0rpm, but a properly designed hub would be making that torque directly with greatest efficiency due to no additional power transfer stages.
For easy evidence, you can look at solar cars. Most efficient electric drive-trains on earth, and they use direct drive hubmotors almost exclusively.
I look at it like this, if you want 100ft-lbs of torque on the rear wheel, you can make 10ft-lbs at the motor and use 10:1 gearing, but this requires a gearing loss stage due to no power transfer being perfectly efficient. The alternative is to simply generate 100ft-lbs directly. As torque grows at the square of motor radius, you can grow radius until you've achieved the desired torque.
Think of growing radius on a motor like adding gearing. The magnet passes by faster even at low speeds which enables it to reach decent efficiency quickly.