Introduction: Free Electric Propeller Bike Mod

About: I am an engineer focused on recycled batteries, electric vehicles and renewable energy. I am always working on engineering projects I can’t stop.

Update Instructable with test ride video at link below.

Video of Bike and Propeller test.

I wanted to put an electric motor on my bike. I didn't want to mess with a chain and sprockets so I build an electric propeller bike. It is easy to install and remove. It sounds cool and its easy to use the pedals, the prop or both.

I used a free car electric motor that I fixed, free laptop batteries that people threw out and a home carved propellor.

Step 1: Parts Needed

Parts needed
Electric motor (car rad fan, AC compressor)
Battery 100Wh + (laptop batteries work best)
Switch (door bell switch over 10A works)
Wires (I used 14 gauge)
Some wood (to build the mount and prop)

Step 2: Carving the Prop

First I used excel to calculated the angles needed at the different prop stations.

I choose a diameter of 20 inches because thats about how wide I am. I don't want the prop sticking out and hitting stuff as I ride my bike.

I just want the propeller to push me around 20 to 30 km/h. So I want a pitch that work well at these speeds. I started with a 6 inch pitch. From my excel sheet a 6 inch pitch is about 27km/h at 3,000 rpm. Most large motors operate around 5,000 to 10,000 rpm. The prop will operate most efficiently at an angle of attach of about 3 degrees. The air will also accelerate as it is sucked onto the prop. So I am thinking that it might be more like 5000 rpm when the bike is doing 27km/h.

I used pine 4x1 boards to carve the prop. I decided to have a 2 inch tip width and 3.5 inch root width. I used the excel sheet to Calculate the drop every inch along the radius. I marked the drop with a marker. I then started carving using a box cutter and homemade draw knife. The motor side of the blade is curved and the other side is flat. I used 80 grit sand paper to get a very aerodynamic blade shape.

Step 3: Attaching to Motor

Every motor shaft is different. I found with the motor I used it was easiest to JB weld a wood mount to the shaft.

I cut 2 circles using the hole saw. I glued the shaft in the center and glued a 3.5x3.5 inch square of 3/8 plywood to it. I then used 4 screws at the corners to attach the prop to the motor.

Step 4: Batteries

If you use laptop batteries like I did you will need over 6 cells (one laptop battery).

Best would be 3 batteries or more (over 18 cells).

You can see how to build the battery pack below.

Step 5: Motor Mount

You can make a sturdy mount out of wood for most motors.

I had to use a 3 inch hole saw to cut 2 half circles to hold the motor.

Step 6: Balancing and Testing

It is important to balance the propellor very well. The propeller is rotating very quickly so a small imbalance will cause a lot of vibration.

Vibration will damage the motor and steal a lot of power.

To balance, mount the prop to a shaft. Place the shaft on two objects of the same height. The heavy blade will rotate down. Remove some wood and balance it again. Also balance the hub.

Test run the motor/prop and made sure it pushes.

Step 7: Control Switch

For the control switch I used a 10 A doorbell switch.

I like this switch because it switches off when you stop pressing it (much safer).

I used 14 gauge wire to wire the positive to the handlebars and through the switch. The negative was directly wired to the motor.

A larger switch may be required as the motor can pull more than 10 A.

Step 8: Mounting to the Bike

In my first test I mounted the motor and propeller to my cargo crate. This was not strong enough.

I made a mount that uses hose clamps to fasten to my rear carrier vertical tubes. I drilled holes in 2 2x1s to put the hose clamps through. The motor mount was then screwed into the ends of the 2x1s. This was very secure.

I only mounted the motor here because I wanted to keep my cargo crate attached. The motor should be mounted so that the prop sees clean air if possible.

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