DIY Electric Motor Bicycle

Updated April 17, 2018

If you want to zip up hills and cruise against strong headwinds, an electric motor can help you. You can add a salvaged automotive electric motor from a junk yard or repair shop to your bicycle. These motors are powerful enough to get you and your bicycle up hills with ease. Power your motor with a car battery and install an on/off switch.

Find and test a suitable automotive motor. Connect possible motors to a 12 volt battery and check the output shaft speed in revolutions per minute (rpm). An ideal motor will be rated about 1 horsepower (hp) and have an output shaft speed of about 1500rpm.

Calculate the output shaft diameter required for the desired low-load speed of the bicycle. Multiply the desired speed of the bike in miles per hour by 5280. Divide your result by 60 to get feet per minute. Multiply that result by 12 to get inches per minute. Divide by the shaft rpm to get inches of shaft circumference per revolution. Divide by 3.14 to get the desired shaft diameter. For an output shaft speed of 1500rpm and a desired bike speed of 10 miles per hour, this calculation will give a shaft diameter of 2.24 inches. This means that the motor will try to keep the bike moving at 10 miles per hour. If it slows down to less than this speed, the motor will draw more current and generate more force.

Build up the motor's output shaft to your required diameter. The output shaft will usually be around 1 inch in diameter. One way to increase the diameter is to place a pipe and a rubber hose over the shaft, and secure the pipe to the shaft with a machine screw. The hose-covered shaft will then rub against the rear tire of the bicycle with a good grip.

Mount the battery on your bicycle frame. The battery should go as low as possible over the pedals. Cut angle irons to size and bolt together two rectangles to fit over the top and under the bottom of the battery. Bolt the bottom rectangle to the frame of the bicycle. Place the battery on the bottom rectangle. Place the top rectangle over the battery. Drill holes in the corners of the rectangles and insert long bolts to secure the two rectangles together. Place nuts over the ends of the bolts and tighten to secure the battery in place.

Mount the on/off switch on your bicycle. The switch should be a rocker switch with mounting holes. Drill corresponding holes into your bike's handlebars and mount the switch on the handlebars with machine screws.

Mount the motor behind the seat on a swivel so that the weight of the motor pushes the output shaft onto the rear tire. One way to do this is to use a bicycle luggage carrier. These usually attach to the bicycle frame under the seat and have two vertical supports which go down to the axle of the rear wheel. Remove the vertical supports and attach the carrier to the bicycle frame under the seat. Attach the motor to the carrier using its mounting holes and bolts, or secure it with pipe straps so that the motor output shaft presses on the rear tire.

Connect the positive terminal of the battery to one side of the on/off switch by running a wire along the bicycle frame and up to the handlebars. Connect the other side of the switch to the positive motor terminal by running a wire from the handlebars along the bicycle frame and past the seat. Connect the negative terminal of the battery to the negative terminal of the motor by running a wire up under the seat. Secure all the wires with tie wraps. Start pedalling and switch on the motor to feel the effect of the electric drive. Refrain from switching on the motor when stationary, as the motor will draw a very high current and may overheat.


Visit junk yards and repair shops to find your motor.

Things You'll Need

  • Automotive 12V motor
  • 12V lead acid battery
  • On/off switch
  • Drill
  • Bolts
  • Nuts
  • Screws
  • Pipe
  • Hose
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About the Author

Bert Markgraf is a freelance writer with a strong science and engineering background. He started writing technical papers while working as an engineer in the 1980s. More recently, after starting his own business in IT, he helped organize an online community for which he wrote and edited articles as managing editor, business and economics. He holds a Bachelor of Science degree from McGill University.