The planning of the project started in early 2018 and it took until 2019 to gather all the parts and start assembly of the first version. The first edition was built into the frame of an YCF 125cc pit bike. Besides the battery, the specs stayed the same throughout the project.
1000W 36V MY1020 brushless sensored motor and matching controller by Chinese manufacturer Yalu Motor. The kit includes digital twist throttle and key lock.
2x 5S 8000mAh LiPo batteries in series. Later upgraded to single custom 10S 20Ah Li-Ion pack.
Initially I used a light T8F chain to minimize the size of the 74-tooth rear sprocket. Eventually it became clear that a stronger 420-series chain is required.
The motor was relatively easy to install into the YCF frame as the original motor mounts are bolted on and easy enough to modify. For rigidity I added iron brackets on both ends of the motor resulting in 4 points of attachment.
The problem: Excess speed
As per EU and local regulation, this kind of electric bike is limited to 25 km/h, which caused some problems as the 3100 rpm motor drove the bike over 40 km/h even at the comical 9-74 gearing. My resolution was to reduce the tire diameter from 12” to 8” by switching to totally different frame and platform – Skyteam Monkey, a reproduction of the original Honda Monkey.
I managed to purchase a mistreated Monkey with a poor engine, exactly what I needed. The bike was a 2016 with just 3000km on the odometer. After complete disassembly I gave the frame, tank, headlight and other parts a new coat of paint. Unfortunately for the frame, I had to cut out a small piece of the motor mount, but otherwise the swap was relatively easy. I switched to a 420-series chain as that’s what the bike originally used and it’s widely available. Largest and smallest sprockets suitable without modifications were respectively 45 and 11 tooth. These resulted in about 35 km/h top speed, still too high, which meant I needed a sprocket custom made. Fortunately a machine shop was able to help and manufacture a 60-tooth rear sprocket.
Originally with the LiPo batteries the effective range was roughly 10 km and the charging was not user friendly. The batteries do not have integrated BMS and the charger does all the balancing work, meaning each battery is connected by both charge- and balance leads, after which correct program must be selected from the charger. I decided to switch to a custom built Li-Ion pack with 20,4Ah capacity, around 25 km effective range, integrated BMS and dedicated charge port so nothing has to be disconnected in order to charge up.
At this point the bike felt finished and after couple months of riding I put it up for sale. The listing quickly became the week’s most visited one on nettimoto.com marketplace and I ended up selling the bike for decent profit.
For the next iteration: I am definitely looking into building another e-bike and it will be driven by a hub motor. While this mid-drive is better for weight distribution, the chain drive is incredibly loud when nothing else on the bike is making any noise. Hub motor eliminates the chain but adds some more engineering work to the installation. The next bike will probably not be legal for road use as I am considering something between 3 and 10kW for off-road use in a frame of either pit bike or a full sized dirt bike.