The Bolt! - an Electric Go Kart Homeschool STEM Project

Let's not talk about how crazy this year has been... that goes without saying! As a result of all the craziness my wife and I decided that this would be a great year to homeschool our two kids, Rosalie (9) and Vincent (7). Although the school I teach at is "in session" it hasn't been particularly consistent, with the start of the school year being fully remote and now, at the halfway point, we are doing a hybrid schedule. Being home during the remote portion of the year really gave me a chance to spend some quality time working with my own kids in the realms of science and technology. We did a lot of fun lessons during the evenings and weekends that hit upon topics ranging from circuits to rock formation. But, I wanted to do something really cool that would have piles of integrated lessons... in rolls the go kart!

With my kids getting older I thought that it would be a ton of fun for them to have something that they could use to zip around our property with. We have a small 1/10 mile loop trail that would work perfectly with a go kart. Also, since I like to build electric-powered machines I thought it would be a great opportunity to introduce their young minds to the basics of how machines work and how to safely use electricity to get them moving.

Both Rosalie and Vincent were extremely involved with this project starting from acquiring the go kart frame, to grinding the frame down to wiring the control panel, and everything in between. I thought it would be fun to share our adventures building "The Bolt" so that you might be encouraged to try your hand at a similar project.

I put the price I paid for each item in parenthesis, but the price will depend on what you find

1. An old, inexpensive go kart frame ($100)
2. Motor kit ($150 - see picture above)
3. Chain ($10)
4. Voltmeter / Ammeter and Switches ($15)
5. 3D printed control panel (idk... a few cents?)
6. Lights! ($9)
7. New tubes for the front wheels ($12 each)
8. The battery! (free in my case since I was reusing old cells from my electric bike)
9. Battery box and motor mount (scrap wood - so it was free!)
10. Chain guard (scrap metal - free again!)
11. Seat (from a side of the road exercise bike - free, free, free!)
12. Buck Controller ($10)

Once this idea popped into my head and I told the kids about it we scoured good ole' Craigslist for days to find a good frame. The problem I kept running into was that most of the go karts already had a good working engine and cost well over $500, or they were too big and too far beyond the level of repair I wanted to do with the kids. We then stumbled on this little Manco go kart for the low low price of $100! It had all of the needed parts to get ourselves up and rolling; decent wheels, good steering, working brake, and a mostly solid frame. The old engine was totally destroyed so that went on the curb for someone else to tinker with.

Once we got it home we set out sanding the frame down and getting it prepped for paint! We also removed the old throttle pedal / linkage since we would be using the potentiometer throttle provided with the motor.

The kart came with a seat but it was a sheet metal job that was in pretty abysmal condition. Since the kart we got had no suspension, a somewhat comfortable seat would be a great thing to have. Luck came our way when we were driving through the next town over and someone was getting rid of an old recumbent-like exercise bike. The seat on the bike would be perfect for what we needed and I planned on saving the pedaling assembly for a future project building a bike-powered hydraulic apple press to replace our old one we built a number of years ago.

This was the first time my kids got to use a disc grinder. My son was comfortable using the flap wheel but my daughter was all about using the cut off wheel to cut the pieces from the bike frame. Although it doesn't look it, they both are wearing safety glasses while they are cutting.

The bike was originally adjustable so that you could get differently sized folks to fit on it, so we just took the adjustable bits and adapted to our needs. We were able to keep the friction knob to hold the seat in place and use other random pieces of the frame to get it all to work nicely with the go kart frame. We made the whole thing adjustable so that a wide variety of kids (and adults) could fit on the kart. We only got about 6" of adjustability but it was more than enough for our needs.

The kids loved this part... although they did have yellow and silver paint on them for about a week after. We used Rustoleum yellow and silver paint that we already had kicking around the shop. We taped off some parts that we didn't want coated in paint. This was definitely a fun part of the project!

I had originally built a battery for my ebike from a wrecked Ford C-Max car's batteries. The batteries were originally 5.5ah 3.6v lithium ion batteries. Now that they had been put through a wrecked car and used for 5,000 miles on my bike they didn't quite have the capacity they once did. With this in mind I decided to use 3 of them in parallel and 12 in series. I made short sections of wire with 10awg ring terminal connectors to connect each parallel set in series and used some copper stripped from short remnants of wire to connect the batteries in parallel to each other. These batteries have a positive and negative stud on them that use a nut to attach the wire to them so no need for soldering or spot welding. I connected in a 12S battery management system for both balance charging purposes and to ensure that none of the cells would ever discharge below the lowest threshold value. Check out the link above if you want to see more details on how to build a battery using these types of cells.

This ended up being a pretty hefty battery, but it's not going on an ebike, so we just needed to build a box to hold the whole thing.

As for involvement from my kids, there wasn't a ton. I showed them how I was building the battery and we talked about the relationship and difference between voltage and current and did some small demos with LEDs and 3v button cell batteries. I had plans for them to be more involved with wiring the whole thing together in the final phase.

The battery box had a couple of jobs; hold the battery safely and securely, and give solid point for the motor to mount to. The kids helped with this process by measuring the dimensions of the battery and helping with the design of the box itself. They also got the chance to measure the pieces for the battery box and even cut a few. We just used liquid nails and 2" construction screws to assemble all of the parts. The front of the box is removable so that you can easily slide the battery in and out of the case. We used two tension cam clamps to attach the face of the box to the structure. We then attached another piece of maple to the top of the box to stiffen up the location of where the motor mounts. The box attaches to the frame with carriage bolts (round heads on the inside of the box). I attached the old battery box from my bike to the top of the box to hold onto the motor controller and wire connections. This was also connected to the battery box with carriage bolts.

We test-fitted the motor, battery, and controller before we moved on to putting all of the pieces together.

The throttle pedal is actually just a potentiometer on a spring. Surprisingly this was really well made overall but we might add a nice rubber pad to the pedal to make it even more sturdy. We used a grinder to cut out the space for the pedal to sit on the floor pan and then used four sheet metal screws to attach it to the floor pan. The wire was simply routed along the side of the kart and taped tight with gorilla tape.

We used a ruler to line up the chain line nice and straight, marked the holes for the motor mounts and then used some bolts to secure it to the box. The gear that was on the kart required a #35 chain, which we ordered online. We got it somewhat snug before attaching the chain and then used washers and metal plates as shims to tighten the chain up properly. Just make sure to tighten it enough so there is only some inward flex with the chain. You might need to go back and make adjustments again later as the chain stretches... but with this type of access it is super easy.

I ended up using the same little computer I used for both my bike and the bike I built my wife. I like its simplicity and how robust it is for the price. I also ended up using basic snap-in switches to control all of the functions of the kart. There are these two little pockets along the side of the kart that were just calling for a command center, so I printed a simple plate that could hold the computer and a number of switches. I included the STL file if you want to use the same design or some derivative of it.

This was a fun process to do with my kids. The number one tool for this is a good multimeter with a continuity test function (looks like a little wifi symbol). When you contact the two ends of the multimeter together you make a closed circuit and a little buzz goes off on the meter. Since the directions I received with the motor were in Chinese and were printed at size 0.00001 font, I had to figure out all of the controller functions on my own by trial and error. The first step was to get the motor spinning. The throttle wire clearly connected to the mating harness on the motor controller, so that was easy. The three heavy gauge wires on the motor clearly connected to their colorful counterparts on the motor controller, so that was easy. And then there was one big honking wiring harness that connected from the motor controller to the motor, easy again! The only problem was that when I connected the battery power to the controller the motor spun backwards. That's going to make driving difficult for sure. So, I determined which wires from the controller were the reverse wires (this one was labeled!) and did a quick switch wired in to change the direction. Problem is that reverse is speed regulated so although the wheels were spinning the right direction they were going half the speed they needed to. So here is what I did to correct this. I needed the electricity to enter in from the opposite side of the circuit (motor), so I swapped the blue and yellow wires on the motor and controller. So now the blue connects to yellow and the yellow connects to blue. The next thing I did was swap the wires on the wiring harness. In this case I had to swap the yellow and the green. Once this was all done correctly the motor and wheels spun in the correct direction! Although it is a bit difficult to understand him, this guy has a great explanation on how this works.

The second step was using the continuity test to determine which wires did what, if the harnesses were not labeled, and to see how each wire ran out through the connecting wiring harness. That actually was a great thing they included with this kit. A fully sealed 6' wiring harness to connect everything from the motor controller to the switches used to control them. The only problem was that they were not labeled. This is where the continuity test works. With the battery unplugged, I had my kids check each connection to determine where on the controller the wire leads to. They thought this was a lot of fun because it was like a scavenger hunt. We would identify the wire and then see what it would connect to on the controller and then label the wire as such. At the end we were able to identify all but one set of wires that seemed to just go to nothing. We then plugged in the battery and tested what the voltage was at each set of wires. We found two 5v wires for the electronic brakes. Two full voltage wires (48v+) for the computer and the lights, the reverse wires, and the speed limiter wires. There were some for cruise control and even brake lights, but we didn't need these... at least not yet :)

The small computer needs to have a shunt put in place between the negative battery lead and the negative power lead to the controller. It then needs tap wires that run from the controller to the shunt and from the shunt to the small computer. We used one of the negative 48v wires to run from the shunt to the computer and then ran the positive directly to the computer. All of this was neatly found in that wiring harness! The shunt itself was buried in some foam and then covered up to prevent any short circuits. We used 10awg ring terminals crimped onto the wire to attach the wires to the shunt.

My kids loved this part. They learned how circuits work with a real world application, how to crimp connectors on to wire, and how to properly wire a switch. We connected each of the appropriate wires to the switches and to the computer, popped everything into the face plate and then attached the face plate to the frame with some sheet metal screws. We made sure all worked fine and then we shrink wrapped the main wires on the controller to the motor for safety sake. You can see the main wiring harness running behind the seat in the first picture.

Since our power switch for the lights was a 48v power source we needed to either get a light system that could handle that much voltage or needed to decrease the voltage to a more manageable amount. I thought it would be cool to have a simple USB plug available so that we could do all different kinds of things with the cart in the future. I had two extra switches just waiting to be used for something fun! We ended up getting a DC-DC buck controller that would take our 55v and less volts and convert it to a 5v output. We used some great LED lights that were on sale for $9. They are kinda cool since they blink / change color in sync to sounds and or music. We connected the buck controller to the 48v power source with the switch in between them so that we could turn the lights on and off. We then plugged in the USB plug and voila, lights! The kids decided the best way to get the lights all around the kart and we used the sticky 3M backing to stick it to the kart. Yes, they had way too much fun with this part... but so did I. A flick of the switch and we had light!

Well... I didn't want to have any important parts get sucked into the chain spinning at a crazy RPM, so a chain guard was needed for sure. I used some old roofing material from our farmer's market mini truck to make the chain guard. Basically I cut some metal strips, used my tail vise to bend them into channel sections then cut out little triangles so that I could bend the whole thing over and make a curve that fits around the bottom gear. I connected this to the frame with sheet metal screws. I then made the top half and did the same thing, riveting the two together with aluminum rivets. I then cut a flat piece to fit the front and protect hands, fingers, and hair from the Bolt's anger!

It's great getting something moving but it's even better to stop its movement when you want. The kart already had a band brake on it that worked really well but I wanted to be certain that a kid that wants to stop can completely stop. The motor controller had two digital inputs for a complete motor shut down (read... brake). I used a small switch and a random plate that came from something I do not remember. The switch was attached to the plate and mounted so that when the kids push on the brakes the motor shuts off until they release the brake. We tested this multiple times indoors with the frame on blocks prior to taking it outside for the big day!!

Speaking of checking things like brakes. We did a full once over on the machine prior to going outside with it. We did have to put a couple of tubes in the front wheels since they couldn't hold air anymore. Easy enough fix, but of course I had to head out and get the tubes so the kids were definitely chomping at the bit even though the following day's temps were going to be in the teens. We checked all systems on the computer and battery, made sure she was balance charged and topped off and rolled it out into the great outdoors.

What fun! I took it out first because I didn't want to be responsible for a broken arm or worse... and because I was probably just as excited as my kids. Going down the road I easily topped off at 25 with the speed limiter off. The speed limiter switch brings it down to about 12 - 15 mph. We flipped on the speed limiter and for about three hours my kids learned the ropes on control and using the switches around the yard. We have had some really cold weather these last few days and just got 20" of snow, so unless I plan on building a plow for it, it will have to wait until sunnier skies, but I can tell you that my kids loved doing this project with me. They are so proud of the work they did with it and have a blast using it. I hope that this inspires you to try building one yourself and if it's possible, get some kids involved! Happy Karting!!!!

Update: I included a video of how fast the Bolt goes on the road with myself on it (230 pounds). I hit a speed of 24.7 mph.