Despite all the fun I had riding the first electric bike, there were a few drawbacks. The bike was geared way to high. This cut my acceleration dramatically. On top of this, the battery could only output a peak current of 20 amps which underpowered the motor. There were also physical drawbacks from the bike itself. The bike was a road bike which made the ride uncomfortable on anything other than pavement. So for this next iteration I was determined to iron out all the issues.
The Design
Similar to the first bike I wanted a mid-drive power train in order to keep the bikes ability to shift. However, this time I went with a full suspension mountain bike, which no left space on the interior for the motor. This forced me to place the motor beneath the frame.
Motor Mounts
In order to create the motor mounts, I fabricated a wooden jig that would hold the motor, bottom bracket and bike in line. The jig also allowed to me check for clearance to ensure an adequate fit .
Final Brackets
With the jig I was able create a motor mount which attached to the shock and main pivot. These mounts where MIG-welded together and had custom Z-brackets at each connection point.
Drive Hub
Another aspect where this bike differentiates from its predecessor is the drive hub. Similar to the previous bike there was a freewheel in crank arms allowing for power to transferred to back wheels without the need to pedal. However, this bike also allowed the rider to pedal without turning the motor. This was accomplished through a left hand thread freewheel. This freewheel was mounted to the motor via a threaded boss (pictured to the right). This boss was then press fit on a hub which bolted to motor.
Electronics and Cosmetics
For this bike I stuck with the same motor, a 48 volt 1000 watt hub motor. However, this time I went with a fully programable Kelly Controller capable of 72 volts at a 120 amp peak. The current was later regulated to peak at 80 amps and have a continuous current of around 50 amps.
Batteries
To fuel this controller I created two lithium ion battery packs. Each pack was made up of 80 18650s in configuration of ten series and eight parallel. The cells were spot-welded together using a cheap welder off Aliexpress that I modified to prevent the welder blowing its MOSFETS. I then soldered on a BMS to allow for safe and balanced charging. This resulted in each pack having a nominal voltage of 36 volts and a total capacity of 24 Amp Hours. These packs were then ran in series giving me a total of 72 volts, 24 amp hours, and a peak current of 80 amps.
Controller and Battery Mount
Similar to the previous bike, I decided that the best place for the electronics to be mounted would be draped over the top tube. However, the set up from the previous bike could not be used as the cases and controller spot were too small. I started by 3D printing a couple clamps that would hold on to the top and down tube. this all for a rigid connection to the bike as I had three points of contact: two on the top and one on the bottom. From here I built out an aluminum frame that cases and controller all bolted too. I then added a wire management box to keep all the wires organized.
Final Thoughts
This was a major upgrade from the first iteration. The bike was much more powerful than its predecessor. At peak power the motor was outputting seven times as much power for a total of 6720 watts. This upgrade in power combine with the reduction in gearing resulted in a bike that had a ton of acceleration. On top of the acceleration, the upgrade in voltage resulted in a much higher top speed. On flat ground the bike reached a speed of 44 mph, a huge upgrade over the 28 mph of the previous bike. However, there still a few drawbacks. For one, the bikes “suspension” was terrible. I was still forced to stay on pavement or face having an incredibly bumpy ride.
While the bike did not accomplish everything I dreamed it would, I learned a ton of useful skills. This project taught me to MIG weld, gave me a ton of knowledge on lithium ion batteries, and gave me an even deeper understanding of electronic circuits.
