Electric Pixie Bike

Electric Scooter + Pixie Bike

This project was an experiment in combining electric scooter components with a micro-sized bike referred to as a pixie bike. I learned a lot of new tools along the way, and created a very unique vehicle.

Pixie Bike Derby

The initial pixie bike frame was created as a project for my club, Cal Poly Bike Builders. We built the frames from scratch, and added hardware from other toddler sized bikes in order to create 5 bikes to compete in our annual pixie bike derby.

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Concept

After the derby, our bikes didn't see a lot of action. I decided to modify the frame of a pixie bike and add a motor and electronics to create a pixie bike electric scooter hybrid. I began by designing a housing for the electronics and batteries that would fit around the existing frame (with some modifications). This housing was laser cut and assembled using tab and slot architecture. 3D printed clamps were used to provide a tight fit to frame of the bike.

Electronics

I initially purchased a cheap 24V, 350W electric scooter kit. I promptly fried the included motor controller due to a lack of documentation. I then set out to make a custom controls system to run the motor. I purchased a VESC Mini, a programmable speed controller that would automatically detect the motor parameters and allow me complete control of the scooter's motion.

I also used a Teensy 4.1 and Arduino Uno during testing, which I later replaced with an Arduino Nano.

Two 12V, 9Ah Lead acid batteries were wired in series to create to required 24V of the system.

12V batteries wired in series

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Initial test using a Teensy 4.1 converting the analog signal of the throttle to digital PWM that controls the brightness of the LED.

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The Arduino reads the analog signal of the hand throttle potentiometer, and converted it to a PWM signal which was sent to the VESC, which in turn ran the motor according to the frequency of the signal. The Arduino also received a digital input from a keyswitch, which allowed me to electronically lock the scooter.

Here the PWM signal sent to the VESC is visible on the oscilloscope.

Perfboard Circuit

In order to compact my electronics system, I soldered all of my connections on to a piece of perfboard, which included socket headers for an Arduino Nano

Rear tail light and electronics cover assembled using individually addressable LEDs and acrylic.

Keyswitch mounted on the top of the electronics housing

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The tail light doubles as a status indicator and power meter. When locked, the light is blue and the handle throttle does nothing. When unlocked, the light is red. As the throttle increases, white fills up the tail light to indicate the power level. In this case the VESC is using the throttle input to limit current allowed to the motor, which is why the motor speed seems nonlinear under no load.

Bike Modifications

Seat Tube

In order to make the pixie bike somewhat more rideable, I extended the seat and steering tube. For the seat tube, I was able to find the same size diameter and wall thickness tube, as well as a steel internal spacer. After beveling both outside edges to get good weld penetration, I was able to tig weld the extended seat tube straight on top of the original. I also then replaced the original small uncomfortable seat with a large wooden seat that was equally uncomfortable.

In both of the above picture, the topmost weld is the one I added in order to extend the seat tube. This was my first real tig weld. While not the prettiest, it's more than strong enough for its application.

Bike frame after extending the seat tube, mounting the wheel and assembling the electronics case.

Fully assembled bike. However in this state the bike was extremely unstable. The large front wheel made it prone to falling over backwards under sudden acceleration. The low handle bars made steering difficult and uncomfortable.

Replacement wooden seat, as well as smaller 8in front wheel. More stable but still uncomfortable

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Steering tube

The steering tube was a bit more difficult to extend. I separated the original handlebars from the steering axle using an angle grinder. I then grinded, filed and sanded the surfaces of both of these parts until they were more or less smooth and free of paint in the weld areas. I used a hole saw to create the cutout in the new extended tube to hold the handle bars. Using some makeshift fixturing, I was able to successfully tig weld the 3 pieces together. The thinwall tubing used in bike frames makes welding very difficult, I blew quite a few holes that I later had to fill.

Fixturing setup for welding the steering axle to the extended tube

Kickstand

As part of my training to be a shop tech, I needed to produce a manual mill and lathe project. For my mill project, I created a kickstand for the bike. This involved squaring the stock, drilling a large bore, drilling and tapping various holes, and cutting the part in half to create the desired clamping effect.

Nearly complete bike with extended steering tube, kickstand and cushioning. However the pedals felt somewhat odd since there was no easy way to transfer torque to the rear wheel.

Foot Pegs

For my lathe project, I turned two aluminum endcaps to fit in the bottom bracket of the bike. A steel tube goes through the center of each, and is held in place with cotter pins. This entirely replaces the pedals.

Final Bike

The final bike is surprisingly very stable and comfortable to ride, and can reach speeds of about 15-20mph. It is somewhat limited by the subpar batteries and low wattage motor. It struggles going uphill, and is limited to about a 2 mile range. Perhaps in the future I will replace the lead acid batteries with a longer range lithium battery.

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