How to Make an Inexpesive Electric Bike
Most nerds have dreamed of building their own electric car, although this could be a glorious DIY project (this guy did it), its also can be extremely costly due to the current battery technology. A much more achievable goal is to build your own electric bike to whiz around town at slightly less hair raising speeds. Its a good time to do it too, manufacturing technology is catching up resulting in cheaper parts. Mass manufactured hub motors and lithium polymer battery packs have helped bring down the cost. If you follow the tips herein you can do the project for a few hundred dollars. But First, lets manage those wild expectations.
The Electric bikes two major component systems.
Frame /Standard Chain Drive - Yes, that's right ladies and gentlemen you need to start with...a plane ole' bicycle. Although nearly any frame will work, you want to avoid road bikes with skinny tires as they complicate the subsequent motor implementation. Also, the frame material is also a concern. Aluminum frames may be lighter but may crack and fail due to vibration and other fatigue. To minimize this risk, It is recommended that you use a steel frame. The good news is that steel frames are generally cheaper anyway. Keep in mind that you may be carrying in excess of 40 lbs strapped, bolted, and zip-tied to your new metal steed so having shelve and rack space (i.e over the back wheel) helps. If your considering building an electric "beast" over 500W that would do over 20 MPH I would also highly recommend getting a frame with a disk-breaks so that you don't go flying into a ravine.
2. Power system -As mentioned before, integrated "wheel motors" or hub motors are used now commonly on electric bikes. These designs greatly simply the power transfer to the wheel by BECOMING the wheel. The practical implantation looks like this.
If you want to take the fastest lap around your block, great, get the baddest motor (and supporting electronics) you can afford, but enjoy the nearly unavoidable 4 hour+ recharge time after your thirty seconds of fame. Electric bikes are a balance between duration and speed due to limited battery capacity.
You should think of the electric drive on the bike as an electric "assist" and not the primary mover (you are). It's easier to think about it in terms of energy. One gallon of gasoline contains approximately 33.7 KWH (kilowatt hours) of energy. To put this into perspective, your home air conditioner uses about 5KW to just run for a single hour. Generally, you would be LUCKY if you fit one KWH of batteries on the back of a bike. Therefore, your total energy here is very poor. It goes without saying that if the electric bike concept is going to work well you either need to stuff it with expensive batteries (thus blowing the point of this project) or use energy as wisely as possible. So how do we do the latter?
We size the entire power system to a comfortable cruising speed and thus obtain a more usable distance for a given energy. Everything here is an optimization problem with the systems tuning point generally set as the most range possible. If you really want to nerd out on the numbers head over to the e-bike simulator at ebikes.ca to see how varying the motor, controller, battery and even the wheel diameter very the efficiency and duration. We really had fun playing with the calculator and seeing the large effect some small parameter tweaks can have.
The power system is the electrical part of the bike composed of the motor, speed control, battery, and throttle grip. Here are some things to know when spec'ing out these peices for your new toy.
Battery Voltage - This is set by the battery itself. Just like a car battery is about "12 volts" whatever battery you buy will have a nominal voltage. For E-Bikes, this is generally set at either 24V, 36V or 48V. For the sake of a balance in cost and simplicity we recommend you build with a 36V system. These batteries are available on ebay for about $150ish. Try and get at least 12aH at 36V. More if you can afford it. Keep in mind that when ordering batteries from foreign manufactures the posted ratings may be somewhat exaggerated.
Motor Wattage- Again due to the optimization of things its hard to define exactly what this parameter does but a good rule of thumb is that larger riders need more power. A 200lb adult on a 500w motor can tool along at around 20mph on a 36v system. If you have been dipping into the girl scout cookies, maybe you need more power here. For our test case we go with 500W.
Motor Controller and throttle- Often these days this is purchased with the motor but can be found separate on amazon and eBay. Just be sure that the controller is rated for the voltage/wattage of the motor and battery. The controller comes with a twist grip throttle and switch.
Now that you have all the pieces wire it all up, (wiring will vary on the controller used) and head off. Here is a quick video of someone who used this method to make a very inexpensive e-bike. This will give you a good idea of how it comes together.