BICYCLE ELECTRIC AUXILIARY POWER UNIT

This is a simple method of getting that extra boost of power when you are climbing a hill or just want to coast home. Easy to build by sacrificing an old power tool and using some easy to source parts. Totally uncomplicated as it utilizes friction drive. I used an old Ryobi One+ 18 volt saw, as all my tools are this brand, and I have lots of batteries and chargers. It is wise to use the largest capacity battery made for the tool.

Parts and Tools

Working battery powered tool, battery and charger. ( Panel or reciprocating saw) If you don’t have one, check out local pawn or charity shops.

Hook up wire (same gauge as used in tool)

Piece of 1” I.D. auto heater hose

3 gear clamps (large enough to go around motor and rear bike strut) Also one to attach battery holder to frame

Heavy duty caster (wheel same diameter as motor)

Zip ties (various sizes)

Epoxy glue

Drill, various drill bits

vice

Wire stripper and crimper

Crimp connectors

Hack saw

Phillips or Torx and straight blade screwdriver bits

Take the tool apart. It will require either a Phillips or Torx screwdriver. Separate the halves. Pull out the motor and on-off switch. Cut the leads to each and to the battery holder. Use a hack saw to cut away the battery holder halves and put them back together using screws or epoxy. Save the switch and battery holder for later and prepare the motor as follows:

The caster wheel will be held in place with a pin. Secure it in a vice and drill out one side of the pin with a large bit. The pin should fall out and the wheel can be removed. Check the size of the wheel bushing. You need to drill it out so it fits tightly on the motor shaft. Secure it with epoxy glue.

Check that the motor can fit on the strut with the drive wheel touching the side wall of the bike tire.

Slit the rubber hose and place it over the strut to provide spacing, cushioning and a good non slip fit for the motor. Secure it in place with the two gear clamps making sure that the drive wheel fits firmly against the bike tire with no slippage.

Secure the switch to one handle grip with zip ties so it sits in a position where it can be operated safely.

Gear clamp the battery holder securely to the frame so the battery can be removed easily for recharging. Connect one side of the motor to one of the battery holder leads. Connect the other battery holder lead to one side of the switch. Connect the other side of the switch to the remaining lead on the motor. Use the wire stripper to bare the leads and use crimp connectors to join the wires. Zip tie the wires to the bike frame. Be sure to leave enough length at the handlebar so you can still steer the bike.

Plug in the battery. Put the bike on the kickstand so the rear wheel is off the ground. Keep hands away and hit the switch. If the wheel turns the wrong way, reverse the leads to the motor. Make any last minute adjustments. Take it for a ride. It is wise to get the bike moving before you switch on to reduce strain on the motor. Watch the attached video.

Note: I built this as “proof of concept.” Next steps would be to replace the switch with a speed control. (Available on similar power tools.) Also, either make a regenerative circuit so the battery charges when freewheeling using the motor as a generator- OR, mount the motor on a moveable bracket so it can be disconnected from the wheel when not needed.