The main issue with the previous bike was “Mountain bike”. The bike was cheap Walmart bike with fake suspension. On top of the terrible suspension the brakes were terrible. As a result, for the final bike I went with a true Down Hill bike with proper suspension and disk brakes.
The Design
Similar to second iteration I created the motor holders using a wooden jig. However, unlike my previous wooden jig this one was much more rigid. This added rigidity allowed for a more precise set of motor dropouts.
The Motor Mount
The new motor mounts where a huge upgrade over the previous generation. These holders featured sold steel arms that bent to the mounts rather than using z brackets. They also included an upgraded motor slot. In the previous mount the slot was cut from eighth inch steel. This version however, was cut from quarter inch. A much beefier dropout system.
Drive Hub
A shared feature of this bike and its predecessor is the Drive Hub. All three version of my electric bike have had shared a freewheel in the crankset. This freewheel allowed for power to be transferred to rear wheel without the need to pedal. However, It would not be until my second iteration where a third left hand freewheel was added. This freewheel was mounted to the motor and allowed the rider to pedal without turning the motor. This feature was only useful in the case of my battery dying which, with the size of the battery, was a rare occurrence. 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
Batteries
For the motor I chose a 48 volt 1000 watt hub motor. I later over-volted the motor to 72volts increasing the motor’s RPMs. For the controller I used a fully programmable Kelly controller. Through the controller I fed the motor 72 volts that had a peak current of 80 amps and a continuous current of 50 amps. This resulted in the motor having a peak power 6720 watts.
To power the set up I stuck with the same battery packs I had built for the previous bike. These batteries were created from 160 18650s. Each pack consisted 80 cells configured so that they had 10 cells in series and 8 in parallel. This resulted in each pack having a nominal voltage of 36 volts and capacity of 24 Amp hours. To create the packs I used a modified spot-welder to weld all the cells together. I then finished the packs by soldering on a BMS in order to have safe balance-charging. However, in order to use the batteries in the bike I had to discharge unprotected. This was because the batteries were connected in series in order to give the controller a nominal 72 volts. While that may seem unsafe I was able to mitigate the danger by programming a voltage cutoff in the controller itself.
Controller and Battery Mount
Since I was using the same batteries and controller from the previous bike I was able to keep mount. However, I was forced modify the mount as I was using a new bike. The top and down tubes both had a larger diameter this forced me to design and print new clamps in order for the aluminum frame to be attached .
Final Thoughts
This bike was everything I looked to create with my first iteration. The bike was the perfect vehicle to get around town. Having a nearly 7000 watt motor combine with low gearing allowed for a ton of acceleration. On top of this, by overvolting the motor to 72 volts I was able to reach a top speed of nearly 45 mph a huge upgrade over the original designs 28 mph. While the speed and acceleration were on point with previous version, where this bike really shined was its ability to be ridden off road. The added suspension was amazing I was able to transition from pavement, to dirt, to grass without any hiccups.
However, despite everything, all the power and suspension upgrades, there was one glaring issue. The bike was simply not made to go as fast I was pushing it. In a traditional downhill set up when traveling at break neck speeds the rear wheel is freewheeling. This means that the chain is not spinning. However, in my set up the chain is constantly under power. As a result my top speed was limited as the chain would simply fly off when trying to do anything over 40. If I were to make another iteration I would not start with a bike. I would start with something more suited to the high speeds such as a dirt bike or a pit bike.
