Introduction: Electric Moped. Blade Runner Inspired Space Bike.

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When I was 17, I read an article in Wired magazine that claimed we would have affordable, street legal, hydrogen fuel cell powered motorbikes for $2000 by 2012. A wildly specific claim. They had even included a render.

I never saw that bike hit production, but the dream for a clean city bike lived on.

Recently, I noticed the cost of electric motors and lipo batteries rapidly approaching my price range. When I saw bikes like Onyx, I decided to build one for myself. This post represents the 30,000ft view of what I learned, and what it takes to build one for yourself.

Attachments

Supplies

  • Angle Grinder
  • Welder of some kind (capable of welding 1/4")
  • Basic electronics kit (i.e. voltmeter, soldering iron, etc)

Step 1: Find Yourself a Frame

Craigslist and Facebook marketplace are littered with 70s and 80s era mopeds, scooters, and dirtbikes. If you've never purchased an older vehicle online, the first thing you'll notice is that a high percentage of those vehicles won't be titled. Ideally, you just purchase a titled vehicle, and move on with your life, but if you're trying to save money, a title-less, barn-find can be just the thing. There are two paths here:

  1. Buy a titled moped. Transfer the title into your state (if purchased out of state).
  2. Fill out a bill-of-sale with the owner, and either and go through the Title acquisition process in your state. (generally not an easy process)

Some things to look for in a used frame/wheel set:

  • A tires with wire spokes (rather than some kind of alloy wheel). You'll need to lace the hub motor into the rear hub.
  • Rear shocks. A hub motor will add significant weight to your rear wheel.
  • A convenient place to weld on a battery tray (this will make more sense in a few steps).

Step 2: Spec Your Bike

Once you have your frame, you can start making some decisions on the hub motor, battery size and voltage you'll need to get to your desired speed and range.

In a standard combustion engine car, you generally have a transmission that allows you to use different gears to achieve different torque/rpm combinations, but with a hub motor (unless you use some kind of bike cassette setup), you're limited to one gear. Electric motors have a nice torque curve starting at low RPM, so we just need to find the sweet spot.

Time to make a few calculations.

Torque

To determine what motor we need, we'll need to know how much torque our motor needs to deliver. It doesn't need to be 100% accurate (we'll end up adding some head room anyway). We're just looking to get in the ballpark. For this:

Acceleration force + Grade Resistance + Rolling Resistance = The total force your motor needs to overcome (comfortably)

I used the formulas from this reference to calculate the "tractive effort" I would need to reach 40mph in 10seconds (~60 N.m in my case).

RPM

To calculate your target RPM, you need to know:

  • Your tire circumference
  • Your target speed

RPM = [[(top_speed_mph) * (5280 ft_per_mile)]/(tire_circumference_ft)]/60min

My target RPM came out to ~850 (assuming a load). With no load, let's add some padding and say we need a no-load RPM of 950rev/min.

I knew I wanted to purchase from QSmotor (given the good results I had seen from a friend's electric motorcycle), so given my required torque and RPM, I went shopping on their site. All reputable motor manufacturers will have good specs published (or by request), and QSmotor was more than happy to make suggestions based on my specs.

I ended up purchasing the KLS7218S Kelly controller along with QSmotor's 2000W hub. It fit all of the specs I had and was relatively inexpensive (around $600 including about $200 worth in shipping). Not bad.

Step 3: Build or Buy a Battery

You have a couple options here.

Buy

There are plenty of 18650 battery pack suppliers out there for all of the standard ebike voltages (36, 48, 52, 60, 72), but they can be kind of expensive. Luna cycles, for instance sells great batteries, but you'll end up dropping $550 on a 52V battery. Smoke em if you got em.

Aliexpress is for sure a source of more affordable batteries, but quality is much harder to guarantee (IMO).

When looking for a battery, make sure:

  • The individual cells have known specs (i.e. continuous current, max current, etc). Even better if they're a known reputable manufacturer like panasonic or samsung.
  • The continuous current of the battery is sufficient for the motor chosen.

Build

Your other option is to build the battery yourself. There's a bit of a learning curve, but...

  • You get a high quality battery, much cheaper
  • The battery you build can have an unusual shape (to better fit the frame)
  • You can always expand your battery later

There are a ton of DIY battery build Instructables out there, but here's the parts cost breakdown (for comparison's sake) for a 52V, 13.6aH battery:

Which comes to a total of -- $306.06. Not too shabby, and if you build a second battery, it will only cost $180.

Step 4: Weld on a Battery Tray

Your battery is going to need a place to sit. Once you've determined the size of your battery, it's time to break out the welder. Anything that can weld 1/8" should be fine. The battery tray doesn't need to be fancy since it will sit beneath the battery cover (i.e. the tank).

Angle iron and some flat bar steel from Home Depot will be plenty strong for the application.

Once you've got the tray welded together, weld or bolt it to your bike!

Step 5: Build a Battery Cover

The simple version - Use the existing gas tank from the used moped you purchased.

  • Break out the angle grinder a cut a whole in the bottom of your tank
  • Sand down the inside of the tank
  • Paint the inside of the tank with non-conductive primer

The more complicated version - build a battery cover

  • When designing your battery cover, make sure you leave enough room for electronics, and any other components that need to stay dry
  • Add a hinge to one side to allow for easy battery removal (you'll be doing this a lot).

Step 6: The 12V System

Every street-legal moped needs some combination of turn-signals, brake lights, and headlights (depending on your state). Our 52V battery is great for running the motor, but a little too high voltage for anything else. For that, we'll need a 12V regulator. There are probably other good 12V regulators out there, but golf cart regulators work great for this size application. I was trying to go for a blade runner look and like the exposed heat sinks, but most people tuck these away within the tank (or within a compartment below the tank).

Wiring

When designing the body of your bike, make sure that you leave enough room for this regulator, and for your wiring. Ideally, this is a sealed container that prevents any kind of moisture from getting in. 12V golf cart regulators are generally safe to expose to the elements, but you'll need some sealed place to make all the connections for you lights, fuses, etc.

Fuses

Fuses are cheap. They:

  • Provide some safety (electrical fires, body shorts, etc)
  • Protect expensive elements (like the motor controller)
  • Force you to think about how much current should be running through each component.

Lights and switches

  • The easiest thing to do here is purchase a handlebar mounted turn signal/headlight switch, as well as some fork mountable turn signals.

Step 7: High Level Wiring

Every bike and every state will have a slightly different set of requirements, but if you want to ride on the road, you generally need brake lights, head lights (and possibly reflectors), and turn signals; although I still can't figure out if hand signals in lieu of blinkers are 100% legal state to state. Including them for good measure.

The diagram included shows the rough wiring of components, but a couple notes here:

  • Not all controllers will support regenerative braking, but if they do, you'll have a few connections directly to your motor controller (pinout will vary controller to controller)
  • Both the throttle and regenerative braking toggles can use either hall sensors or potentiometers for the control signal. Make sure you buy the right one for your particular controller (some controllers support both).

Step 8: Lacing the Hub Motor. Build/Buy.

Once you've received your hub motor (or the digital drawing), it's time to order some custom length spokes. These will transfer the rotational force of the hub motor to the wheel itself.

Buy

Whatever online store (or physical location) you buy your hub motor from will likely also sell rims, pre-laced with the hub motor of your choice. You'll pay a little more, but if you can find a rim that works with your bike, it's probably worth it. There are a lot of places where your spoke measurements can go wrong.

Build

For this, you'll need to break out a ruler and take some measurements. Namely:

  • The Flange Diameter: which is the furthest distance between two spoke holes (center to center)
  • The Effective Rim Diameter: the distance between the end of the spoke (where it sits in the rim) to its opposite

There are a few spoke length calculators, but I can only vouch for this one's accuracy.

Hub lacing is another deep hole (that probably deserves its own Instructable), but there are a handful of good Youtube videos out there that cover it. A few important notes:

  • A heavy duty moped rim (or a similarly heavy rim) is necessary to support the weight (i.e. no bike rims)
  • Hub motors often come with 13gauge spokes, but heavy 12gauge would be better for the weight of a moped.

Step 9: Paint

You can absolutely take your frame to be painted professionally in a shop, but if a professional paint job isn't at the top of your priorities list, a good old rattle can paint job will do just fine. A $20 paint job doesn't necessarily have to look like butt. Not by any measure an expert, but a few notes that improved my paint jobs:

  • Be diligent when sanding. If you can feel irregularities with your hand, you'll see them under the paint.
  • Keep your can warm by resting it in warm water (don't let any of the water get on your paint job)
  • If you don't have a well ventilated indoor space, don't sit under direct sunlight, or in excessive humidity
  • READ THE DIRECTIONS. Time between coats, cure time, priming directions.

Step 10: Legal Stuff

State to state, legal requirements for mopeds can be wildly different. Most of it pertains to top speed, registration, and light requirements. Make sure you find your state and read these as well as find the full text for your state's moped laws. In Pennsylvania, one large showstopper is not having a VIN (or a title), preventing registration. The process for this:

  • I can't speak to all states, but I would think the process would be similar state to state. In order to request a VIN in Pennsylvania, you need to visit a DMV capable of enhanced safety inspections and prove that your moped meets all of the state requirements for a moped.
  • If you purchased the frame used, but it didn't come with a VIN, make sure to bring your Bill of Sale.

A second consideration was top speed. My design speed was 40mph, but in Pennsylvania, a moped can only legally have a "design speed" of 25mph. Fortunately, some motor controllers (at least Kelly and Sabaton) have built in "eco mode", which allows a few max RPM settings based on a switch input. On my bike, one of the control switches just tells the controller to limit my max speed to 25mph (based on the MAX rpm of the motor).

Step 11: Drive It Around!

Hopefully this gives a good overarching introduction to the steps/costs involved in building out an electric moped. The whole process can seem like a lot at the onset, but taken one step at a time, it can be an immensely satisfying project. When I started this project, I didn't know how to weld, make batteries...honestly most of the steps in this Instructable. It's a little rough around the edges, but it feels very mine because I designed and built it. It's a great introduction to anyone interested in fabrication, electronics, or design. You don't need to be an expert to get started.

Good luck!

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