Introduction: Easy DIY Electric Motorcycle Conversion

By Museum of Future Sports
About: The Future Sports Academy is a San Francsico-based STEM education wonderland. Our community initiative offers ongoing educational programs, workshops and after school leagues focused on engineering, design, co…

We started with a Honda Rebel junker and a dream - to make a practical, zero emissions vehicle for commuting in San Francisco. After consulting with some plans available online which required chopping the frame significantly, We decided to figure it out for ourselves. Armed with a basic metal shop, we methodically convert the Honda rebel 250 into a clean quiet bike in about a week. It is registered and insured in CA as a modified 250. (if the power system were 24V or less, no registration is required).

The old rebel rolling chassis provided us with many of the needed parts - wheels, tires, brakes, etc... which was a real bonus - but it also supplied us with many rusty, broken, stripped or missing parts. In fact, most of our difficulties were because of the old parts, not because of the conversion itself. Without these annoyances, the conversion would have taken about two and a half days.

With an onboard 48V charger and 3-prong wall plug hardwired on, recharging is easy anywhere there's a wall outlet. Charging time depends upon how depleted the batteries are, but usually they were topped off after 5-8 hours.

Step 1: Gather Parts, Tools and Materials

We scored a blown Honda Rebel 250 on Craigslist for under $500. It had a blown engine, but included all the other parts needed to make the bike safe and street legal: Lights, brakes, wheels, shocks, etc... I had a set of 4 deep cycle batteries from previous projects. We purchased most of the other AC components from Electric Motorsport in Oakland and also consulted a set of plans from 21 Wheels. Ultimately we had to redesign many aspects of the project and develop our own solutions.

BASIC PARTS LIST
Honda Rebel 250 rolling chassis
Perm PMG-132 electric motor
(4) 12V 50Ah batteries in series
36V-72V PWM controller
5 k potentiometer (i.e. twist grip throttle)
48V AC charger
48 -12V down converter OR additional small 12V battery (for lights, signal and horn)
#4 welding cable and lugs
Custom sprockets

BASIC TOOLS
Welder
4" angle grinder (with cutting and grinding wheels)
Drill press
Metal band saw
Sawzall
Standard auto shop tools, wrenches
Heavy duty wire cutters / crimping tool

RESOURCES
Honda Rebel service manual
Make Magazine Gear calculator (for choosing sprocket sizes)
EL Chopper ET builder's plans - outdated, but a great starting point
Friends who can help

Step 2: Stripping the Frame

The first step is stripping the frame of all components. This is fast and fun and only requires basic hand tools. Save all the parts. Keep a jar or bucket handy to collect all the nuts, bolts, washers, etc... You will need most of these bits later.

Once everything is removed from the frame, clean it thoroughly. Be sure to remove all grease, dust, dirt. Lightly sand any rust spots.

Step 3: Removing the Motor Mounts

The frame requires some modification to accomodate the AC motor and batteries. Fortunately, this metal work is fairly simple. ( the plans suggest "chopping" the frame and extending it by several inches to accomodate all the batteries, but we decide to keep the original frame dimensions and distribute all the batteries around the bike. Two batteries will go into the engine compartment, but two others will ride on the back of the bike, like saddlebags.

Using a combination of 4" angle grinder and Sawzall, remove the existing motor mounts points and tabs.

Step 4: Cutting the Swing Arm

The swing arm modification is probably the most challenging part of the metal work. It requires a hole to be cut out of it to make room for the motor. Many people have correctly commented that cutting into the swingarm can seriously weaken it, potentially causing problems. This is partially true, but the final design takes that into account - The placement of the AC motor helps to strengthen and reinforce the swingarm stiffness.

This step requires precision - the motor must sit perfectly in-line with the rear wheel, so that the chain travels straight between them without any flex or twisting caused by misalignment. We used both a square and straight-edge to mark the cut lines with precision.


Step 5: Fabricating the Swing Arm Motor Mount

Once the hole is complete, and the motor fits snugly into it, we measure and fabricate a mount. Cutting a rectangular piece of 1/4" plate steel, then measure and remove a concave semi-circle, resulting in a custom mounting plate that holds the motor precisely in place.

Two holes are measured and cut for mounting bolts, then the piece is welded to the swing arm and the motor attached.

Step 6: Battery Tray Fabrication

Using steel angle iron, we fabricated three trays for batteries. One tray holds two batteries in the engine compartment, while two smaller trays are mounted like saddle bags on the rear of the bike.

The original design for the battery trays used beefy 2" angle iron to insure that they could support the weight (about 50 lbs. each). This was not a great choice. It added far more weight then needed. A redesign of the bike uses slightly smaller batteries, which helped in several ways.

A newer re-design of the bike uses smaller batteries. Now all four batteries could fit into the engine compartment - eliminating the need for the two rear saddlebags. The new design also replaces the 2" steel with 1/2" angle iron as well as gigantic zip-ties. Overall, this new design saves a ton of weight, which helps to offset the smaller range of the smaller battery set.

Step 7: Painting the Frame and Parts

Now is a good time to paint the frame. Fabrication is done and everything fits together.

Mask off any bits of chrome and clean the metal once more. Use a rust inhibiting spray paint and coat the frame lightly with multiple coats of paint. Allow each coat to dry between applications.

Step 8: Gears, Sprockets

This step can a little tricky. You need to calculate some things to try and determine the best arrangement of teeth on the two sprockets - one on the motor (the Drive sprocket) and one on the rear wheel (the Driven sprocket). The performance of the bike will be greatly affected by slight differences in gear-teeth number, so we used a gear calculator, like this great one provided by Make Magazine.

Once we knew the ideal sprocket sizes we purchased them online from Sprocket Specialists

Step 9: Assembly and AC Component Placement.

Once the fabrication of the frame is complete, reassembly of the bike can begin. Start with the basic components: the forks, swing arm, wheels and battery trays. With many of the usual parts of a motorcycle missing, it will definitely look different as you reassemble it.

The new electronic components need to fit somewhere, including the motor controller, charger, fuses, etc... I found this old cooking pot and decided to fit the parts inside of it - It looked odd and dangerous, but somehow appealing.

Step 10: Wiring

This part is challenging. Wiring the new 48 v. AC system is complex, plus the original 12V power system is still needed to run the lights, horn, brake lights etc... There are several different ways to proceed:

One: Keep the old and new electrical systems separate, and run the old system off a small rechargable 12V battery. This option keeps the wiring simple, but requires maintenance of two separate battery/charging systems.

Two: Integrate the two systems by using a DC-to-DC converter, which steps down the power from 48V to 12V This is a more complex wiring set-up, but can be maintained with a single charging system, which is much more convenient on a day-to-day basis.

We chose to integrate the two systems together, so we relied on several sources of information to guide us, including the original Honda Rebel service manual, the suggested wiring diagram from the 21 Wheels plans, consultations with the guys at Electric Motorsport and quite a bit of guessing on our part.

It took a few tries, and we burned out some fuses and bulbs along the way. But ultimately we got the wiring working reliably.


Step 11: Conclusion - UPDATED!

The final layout and design of the bike was ok, but needed some work. After some time on the road, we decided to make some modifications. First to go were the four big, mismatched 80Ah batteries. These were never a good matched set and always caused unreliable performance. I decided to replace them with four sealed, golf cart-style 50Ah batteries, which were smaller and lighter, but stored less energy.

However, the new batteries were small enough that all four could fit into the engine compartment - eliminating the two heavy rear saddle bags ( and all that weight). Using smaller gauge steel and gigantic zip-ties to build the new battrey trays saved even more weight. Sadly, the new design meant there was no room for the cooking pot that held the electronics, so those had to be remounted in an available spot.

The final design looks cleaner, weighs a lot less, but holds less power - a trade off that didn't result in much change in the speed or distance of the bike, which always fluctuated around 35-40 MPH, and 15-30 mile range per charge.

Visit Gomi Style for videos, plans and instructions for many cool projects.