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Electric Bicycle Project

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Image of completed motorized bicycle

Living in Boston as a college student, it is not necessary nor environmentally friendly to own a car. The public transportation system in Boston is easy to use but costs money, and walking can be convenient but it is quite slow. As a result, one of the best ways to get around is by bike.

Biking is often faster than a car during rush hour in Boston, and is a good form of cardio, but at the end of a long day it is sometimes a struggle to have to bike to run errands or go two miles from campus to get home. Over the summer of 2021, I modified a road bike to be powered by an electric motor to overcome this struggle. 

In order to motorize my bike, I carried out the necessary dynamics and energy calculations to determine which components to purchase to achieve a desired speed range. When I ride my bicycle for exercise, I usually average between 10-15mph; At this speed I knew I would be able to safely operate my bicycle, so this is the goal speed I was designing my electric bike to reach. Another factor in this speed choice was that if my electric bicycle went faster than 25mph, it would be classified as a motorcycle and I would have to get it registered and insured (which I did not want to have to do). 

With my design requirements defined, I was able to purchase the necessary motor and electrical components to satisfy my needs. Pictured here are the main components: a 250 watt electric motor, a 36 volt 14 amp hour battery, and my unmodified 10 speed road bike. With these parts in hand, I utilized my mechanical design skills to prototype and machine parts to fix the motor to my bike and configure it in such a way to allow for both manual and electric riding.

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a) 250 Watt electric motor, b) 36 Volt Lithium Ion battery, c) unmodified 10-speed road bike

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Render of 3D printed hub fixture to attach motor freewheel to rear wheel hub.

Pictured here is a 3D printed hub which I designed to allow me to fix a second free-wheel driven by the motor to the rear wheel. This design enables both manual and motorized riding.  
After a series of mechanical issues (primarily related to the $80 bike I bought on Facebook marketplace, not my modifications) and many test runs, I was able to get the motor to drive the bike while I was sitting on it. 

Here is a video of the first time I was able to ride the bike with the motor. This was a partial success in my book, but as the video shows, there were still some issues with this version as it was very slow and the drive chain repeatedly slipped on the motor hub. 

Video of first ride on motorized bike. Camera viewing of motor fixture assembly to diagnose lack of power/chain slipping issues.

After another couple weeks making tweaks to the system, swapping out the drive chain and adding some tensioning mechanisms, I got the system working smoothly and was able to conduct a full speed test of my electric bicycle on the streets. 

Using an exercise app on my iPhone, I rode my bike along a street near my apartment. I determined that the top speed of my bike was around 13.1 mph, exactly in the range for which it was designed. 

With the bike's performance evaluated, I did some additional work to make the bike ready for daily use in the elements. I waterproofed electrical connections, acquired a more robust housing for the electronics, designed a sleeve to protect the motor chain from the pedal chain, and added additional fixtures to keep the motor from slipping on the frame. 

My motorized bicycle is now complete. I use it almost every day to commute to and from campus and running other errands in the city. Typically my commute to campus would take 25-30 minutes by foot, but now takes an effortless 10 minutes on my bike.

Video of first full road test of motorized bicycle

Overall, I am very satisfied with the bike's performance given the effort I put in. This was a side, personal, project for me and I only worked on it in my spare time. In total my electric bike cost just over $300 in hardware with the main expenses being the 36V battery, 250 watt motor, and bike itself. This cost is very reasonable given the fact that many commercially sold E-bikes cost at least $1000 and I am a college student living on a budget. 

While I am satisfied with its performance, I may return to this project to make further modifications to improve the top speed or the overall riding experience (such as minimizing the sound of the motor). I also may add a speedometer to get a better analysis of the bike's performance. Aside from initial component calculations, the assembly of this motorized bike has been very iterative and trial and error based; with each iteration I learn what factors relate to bike performance and continuously making improvements to the overall design.

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