Hello everyone, I have been a member here for a few years now and have mostly just been researching and seeing what everyone is up to. After seeing this post https://endless-sphere.com/forums/viewtopic.php?f=28&t=88595 by Vertigo I was hooked and had to have something similar. After a year in design, I have finally started production and want to share my journey and where I'm at now. I am creating a series of blog posts on my website and will slowly be updating and adding new posts as progress is made.
My Website: blog.jktobin.info
I am an engineering student at Arizona State University and have been using what I have learned and the tools available to me in hopes to make a high performance, reliable, and sexy electric dirtbike. Using a combination of downhill mountain bike, electric, and custom components I want it to be lightweight and versatile. By making this post here I hope I can get some views on my blog posts and be able to answer any questions you may have. I have done extensive research and engineering on all aspects of the bike and want to share what I have learned. There is still a lot left to do and I will be keeping track of my current progress and replying to questions here on Endless Sphere.
Motor and Controller
The motor I am using is the ME-0907 brushless DC motor w/ thermistor by Motenergy. Originally the motor was designed to power a golf cart and produces peak 26hp with .0959 ft-lbs of torque per amp. With a maximum amperage of 220 amps for 1 minute, the motor itself will produce 21 ft-lbs of torque without any gearing. At 72v it is considered a high voltage motor and with proper gearing should be able to move the bike along at 50mph.
I know the importance of a good controller and I did not want to skip any corners here. I chose a kelly controller due to their high build quality and available support. The KLS7275D sinusoidal controller can output a consistent 200A and a boost amperage of 500A! This controller should be able to provide the motor with more power than it could ever need but will allow room for future upgrades while also keeping heat to a minimum. The controller, originally for a golf cart as well, has a lot of features I plan to take advantage of. ABS, regenerative braking, reverse, and customizable torque curves were some of the features that drew me to this controller.
Currently, the battery is my main focus and is taking a lot of my time and energy. Originally I planned to make a simple pack, spot welded together and then encased in heat shrink. After a lot of research, I realized that this was not the best way to go and was far from the best design. Tesla's 18650 pack engineering was very interesting to me and seemed bulletproof. Copying their design using a plastic spacer with aluminum bus bars and individually fused cells, I designed a battery that fit my frame very well. I am going to end the description of my battery design here as it is not finished and still has a lot to go. However, the wiring and electrical systems that are part of the battery will need some explaining.
Looking at the 18650 cells avialable, I chose the LG HG2 cells. This gives each battery 3000mah and a continuous 20A discharge rate. These numbers put the entire pack, which consists of 10 parallel groups and 20 series groups, at 30Ah and a continuous discharge rate of 200 amps. With a high amperage output pack like mine, few BMS units allow over 200A to flow through them and with my goal of regen braking I was unable to find any that would even come close to working. To solve this I decided to cut some corners. With large cell packs maintaining a consistent voltage during discharge and regen braking is much easier than on a small pack. Therefore, I decided that for discharge and regen I will not be monitoring individual cell voltage. Instead, I will be relying on my controller to cut power when the overall pack voltage is too low. With no protection there I needed protection for charging in which I am using a Supower 150A BMS. I will not be charging anywhere near 150A but the BMS was originally purchased to provide both discharge and charging. This solution is not ideal but will still allow me to have a reliable battery that can be charged on the bike.
A powerful, fast bike that cant takes a hit or hop a couple of curbs would not be very much fun. With the added weight of the electrical components over a real bike, I needed high-performance components that will keep up. Using full-blown motorcycle parts would be too expensive and extremely heavy. Therefore, I chose to go with the best downhill mountain bike components I could buy. The front forks are 2017 Fox Factory 40 forks for a 26" wheel. With a heavier spring, the forks should handle anything I will throw at it while still being extremely lightweight and durable. For the rear shock, I chose to use the fox DHX2 with a 650lb spring. Determining the spring rate for the rear took a lot of time and a couple of programs to make it happen. Starting with a program called Linkage X3 I designed my rear suspension geometry to produce a progressive spring rate that increased near the end of the stroke. This was to help prevent a bottoming out scenario which with the added weight of the bike would not be good for my rear shock. After getting the geometry correct I took a few measurements and plugged them into Fox's Mountain Bike Spring Rate Calculator. This made it possible to choose a spring rate that would still be rideable but allow for the correct amount of sag.
Wheels and Tires
The wheels and tires for the bike were more work than I thought they would be. I needed a strong, beefy wheel that would take a hit but still be able to mount up to the existing components I had already picked out. The solution to this was found in an article on electricbike.com. Using moped rim I could mount larger tires and have a stronger rim to ride on. Using the Yamaha Play Bike Front Rim from a TTR125 and the Shinko SR241 tires I was able to make a very nice wheel and tire combo that looks awesome. Sadly the moped rim did not quite line up with the mountain bike hubs currently available on the market. In the front, I needed a 110mm boost hub with an ISO disk brake mount. In the rear, since there will be no pedals, I needed a 150mm front hub. Both of these hubs by themselves are not very popular and were difficult to find.
Onyx racing, a company out of California makes specialty, high-performance hubs that were exactly what I needed. After contacting them I was able to purchase the hubs I needed. Now the hard part, finding spokes and nipples that will fit the motorcycle rim and the mountain bike hub. Thankfully someone had already attempted this! Holmes hobbies sells oversized nipples that fit into the moped rim and taper down to a standard bike spoke size which is exactly what I needed. Using Sapim's strong spokes for the added strength, I had the wheels built by a local wheel builder who was more than excited to take the project on. I think they turned out awesome and are the best-looking part of the bike so far. [will update with pics soon].
Another pinch point for this project was the drive-train. I wanted to use a chain for its durability and strength, plus chains are much more readily available than belts and allow for more freedom in design. After reading an article on chain and sprocket design and doing some math, I decided to use a #40 chain with a 5:1 gear ratio. Sadly, the largest available sprocket at the time that would support a #40 chain was 60 teeth. That meant that my drive sprocket could only be 12 teeth. This, according to my research would be too small of a sprocket and can cause binding. Due to budget and time restraints, I chose to go with it anyway. Currently, there is a 72 tooth sprocket that is available which will allow me to have a 15 tooth drive sprocket and may be the way to go after testing.
For durability and availability, I chose a steel rear sprocket. This was an issue because from the supplier it weighed over five and a half pounds! With that weight the entire wheel, tire, spoke, and hub assembly weighted less. To mitigate this I decided to lightweight the sprocket using topology optimization. For a round disk, it was quite simple and I ended up having a local shop EDM the sprocket which removed almost two and a half pounds giving the final sprocket a weight of 3.12 pounds. I plan to possibly remove more material as the sprocket is still quite heavy and feels plenty strong.
As previously stated, I plan to give the bike the ability to regen brake. This is especially useful in hilly areas and will only give me more range. Mounting the rear drive sprocket to the same ISO 6 bolt location that a standard disk brake would mount to I did not have any options for a mechanical rear brake. The rear brake will exclusively be regenerative and will have a hand control on the handlebars similar to the front brake. For the front brake, which does most of the work, I went with the Shimano ZEE brakes and Ice-Tech 200mm rotor. This combination of a hydraulic brake and a well-cooled rotor will give me a lot of stopping power out of the front wheel. With the features built into the controller, I should be able to achieve braking performance similar to two mechanical brakes.
With my engineering experience, I have begun the process of designing and manufacturing a PCB that will be the central power distribution for the bike. From my 72v battery, I plan to have a 12v rail that is used to power an Arduino, lights, and a horn. I have always been a fan of digital control and am surprised at the number of vehicles and parts that are still produced today that are analog. Following my interest, I purchased two IXFN360N10T-ND MOSFETs. These MOSFETs can handle 360A a piece and with two of them in parallel, heat should be kept to a minimum. Having an on-board Arduino nano will give me complete control of the ignition, lights, and any other electronics on the bike. This will allow me to do cool things like use a serial addressable LED strip as the tail light and directionals, switching between red and amber, or control secondary circuitry at the push of a button. Since I couldn't make it simple, I have embedded an RFID antenna into the handlebar to activate the bike's ignition with the presence of an RFID chip that I wear on my wrist. I hope this will be a cool addition to the bike and give it that extra little pizzazz and reduce the possibility of it being stolen. (I am aware that this will drain my battery if the battery is left plugged in but it's so cool I don't care)
This by far has been the most complicated and difficult thing I have had to design in my engineering career. There are so many things to consider and without a team of people behind me, I was forced to take all of them into consideration. Starting with the wheelbase and head tube angle of the bike I slowly began building a frame to hold all of my components. There were many iterations of the frame as I learned more about frame design. Slowly I began to find out what is important and what is not. The frame will be manufactured from one inch 0.095 wall aluminum tubing with a machined head tube. The swingarm of the bike will be made from two-inch by one-inch rectangular tubing with CNC'd axle blocks and pivot points. These sizes were determined after extensive FEA simulations. I am having the frame manufactured by a local motorcycle shop that creates custom choppers and other tube frame street bikes.
To begin designing the frame, I determined some of the specifications I wanted the final bike to have:
Seat height: 33"
Handlebar height: 40"
Ground clearance: 14" (half the overall wheel diameter)
Headtube angle: 67 degrees
Through a lot of iterations (not all documented), I arrived at my final frame design that meets most of these specifications:
Some of the biggest challenges were making changes to one dimension and not causing catastrophic changes to others. With this final design, I feel that I have a good balance between seat height, ground clearance, and suspension performance. Here is a picture of the sketch overlayed on the bike. (mm are easier to design with, sorry)
There is still a lot left to do before this is a finalized bike. The battery will need more design changes before it is ready for production and needs to be perfect as it is a very crucial part of the bike. Currently, I do not have a seat for the bike and don't plan to add one until the rest of it works as it should. This should be a simple addition and will make the bike look much more complete. The frame is currently being manufactured and will need a lot of refinement once that process is finished. Mounting holes will need to be added for things like the controller, battery, and motor.
Overall I am extremely happy with the progress I have made and am excited to ride this thing once it's done! I will do my best to answer any questions and throw as many recommendations as you can at me, I need them. I will hopefully be making a second post on the battery to gain more insight on my design.
Moped Rims for Ebike: https://www.electricbike.com/moped-rims-tires-hubmotors/
Chain and Sprocket Design: http://gearseds.com/documentation/deb holmes/2.5_Chain_drive_systems.pdf
Holmes Hobbies: https://holmeshobbies.com/
Shock Spring Calculator:https://www.ridefox.com/fox_tech_center/owners_manuals/08/WCeng/Content/mtbspringratecalculator.html
12awg Silicon Wire:https://amzn.to/2H3krs2
6awg Silicon Wire:https://amzn.to/2YXIaA3