This calculaotor can get you close
Lightning Rods Power Graph:
/ how to motorize a big heavy vehicle/ Hub Motors / How much power do you really need?
Mid-drives can be done in several different ways.
The picture is of a Metropolis two speed internal geared crank set, that sends only a small part of the torque through the pedaled crank at 60 rpm. Most of the torque is sent to the rear wheel by a Gates belt. Bicycle drive trains cannot take the full torque that a large motor will produce. Hard starts on a cargo bike is a very bad idea.
To drive the left side crank with a three speed Sturmy-Archer gear hub (as a geared mid-drive). Use a chain from motor to hub sprocket, and a chain or belt from a pulley mounted on the flange of the hub.
|This motor drives the crank from the left side|
and the rear sprockets from the right side
|old tech gearless hub motor|
When driving the cranks, so that they move at human speed with a 'gear-less' hub motor, it is less efficient than what you would get when using a fast motor with a two stage reduction, like a 'geared' hub motor, because motors were made to move fast, faster and even faster (depending on the voltage and the way they were made).
The one thing I don't like about mid-drives is that they use up your drive chain faster than human power alone (100-150watts). That is one reason I would drive the wheel directly with a belt.
I like the idea of powering the rear wheel directly through a two gear hub. Even if you should use a larger motor, with ability to run more amps at a slower rpm.
A Nuvinci N171 is possibly the only hub gear that could handle the torque form a 2800 watt motor, when climbing an 8% grade with 350 to 400lbs.
|Nuvinci hub gear with power input|
|Direct to wheel through a gear hub |
-SRAM Dual Drive, 3-speed Internally-Geared-Hub
(with 4 derailleur sprockets retained).
these are not strong enough for more than 300 watts