Back by popular demand, B-line’s fleet mechanic, Bill Murdock goes in depth about the inner workings of our trikes and the challenges that comes from an electric-assist system.
One of the challenges in developing an electric-assist trike is the integration of the motor with the conventional bicycle drivetrain. One method is to use a hub motor laced into the front wheel, but this approach has a few limitations that make it unsuitable for our needs. Instead, our trikes use a sophisticated transmission that incorporates multiple drive chains, hubs, and an automotive-style differential to transmit power to the rear wheels. Although complex, this arrangement offers a wide range of gearing possibilities and permits the use of a high output stand-alone motor. An intermediate “mid-drive” hub allows for either motor assist or pedal power alone through a standard 8 speed cassette:
- The primary drive, a standard bicycle-type system, transmits pedaling effort in the typical fashion through a cassette freehub mounted to the mid-drive hub. A single chainring up front and a wide-range 11-32T cassette gives eight usable gears.
- The secondary drive, a 420 motorcycle chain, connects the electric motor to the mid-drive through a modified single-speed freewheel positioned inboard of the cassette. The freewheel functions as a one-way clutch, allowing the mid-drive hub to turn at a faster rate than the motor. Without this feature the motor would be overdriven (causing drag/resistance) if the road speed exceeded the maximum RPM of the motor, an effect similar to engine braking in a car.
- The final drive, also a 420 chain, transmits power from a cog mounted to the end of the mid-drive to a sprocket on the differential housing. Like a fixed-gear bicycle, these sprockets are always turning as long as the trike is in motion.
With every stage incorporating its own set of cogs there is enough gearing flexibility to suit any application. The characteristics of the motor’s output can be maximized with different sprockets to give the best torque, and the trike’s top speed can be tuned with the final drive gearing. This kind of compound gearing keeps the motor in its optimum RPM range for the vehicle’s load without directly impacting road speed.