I’m practicing some semi-technical marketing talk for Stokemonkey here. Does it work for you?
A direct current electric motor operating at a given voltage will spin up to a certain speed, called the unloaded speed. At this speed the motor produces no useful mechanical power. Once you hook things up to the motor, giving it work to do, the motor slows down. Eventually, if you give it enough work to do, it will stall in an overloaded state. The motor produces no power while stalled, either. Between the zero-power overloaded and unloaded speeds, the power produced can be plotted as a curve.
The most power is generally produced at 50% of the unloaded speed. If maximum power is your only goal, your task is to arrange for the motor to operate at this speed as much of the time as possible. However, you will likely also want to consider efficiency, the ratio between the electrical energy drawn by the motor and the mechanical power it produces.
Just as with power, efficiency at a given voltage is tied to motor speed, and is zero at the over and unloaded speeds. Unlike power, efficiency is highest at a speed well above 50% of the unloaded speed. In Stokemonkey’s motor, it is at about 80% of the unloaded speed.
The band of motor speeds between peak power and peak efficiency is thus only about 30% the total range of motor speeds. Operating the motor outside this sweet zone will always result in suboptimal performance.
Now let’s consider a typical mountain or touring bike. It may climb up a steep hill at only 5 MPH in low gear, but it will come down at closer to 30 MPH in high gear. That’s a 600% spread in wheel speeds, while the rider may pedal with similar speed and effort in both cases. About 30% similar, in fact (say, from 60 – 90 RPM): just like the sweet zone of an electric motor.
By hooking up such a motor to the pedaled cranks of a multi-speed bicycle, instead of to a wheel or subset of the rider’s gearing, the motor can provide good power efficiently over the widest range of speeds that bicycles attain in the real world. Furthermore, driving the cranks in synchronicity with the rider’s legs provides for the rider to constrain the motor’s speed to within the sweet zone constantly, through the normal bicycling skill of shifting to maintain a steady cadence. Stokemonkey is the only product that does this.
This is not rocket science, nor is it controversial; the benefits of multiple-speed transmissions as found on bicycles, power tools, and eighteen-wheeler trucks are widely understood and accepted. Why, then, do the large majority of electric bicycle products have the motor connected directly to a wheel? And why do the few that don’t use only a limited range of gears, excluding those based on triple front chainrings?