My Go Kart

A small electric vehicle.

A few years back, I came up with the idea, that I wanted to build a miniature vehicle with two seats. Since then I have embarked on a journey in which I have learned a great deal and gathered a lot of valuable experience.

To be honest, when I started on this project, I totally underestimated how much time and money it would take. This was mainly due to the complexity of the project. However, it has been completely worth my while and it certainly helped me develop a feeling for how long these things take. On this page I will explain how I built the go kart and why I did things the way I did. My hope is to inspire you to build something similar.

The build

First I decided to build a platform to work on. I settled on 1m x 1,75m. It's constructed of 60x45mm beams making up a rectangle with two extra cross bars. I then screwed on some wooden planks to create a floor to stand on. At this point I realised that I needed to make the tires flush with the side of the go kart and to do that, I would have to saw through the outer frame, thereby destroying the entire structure. So I bolted on two more beams lengthwise and closer to the middle. This drawing shows final the structure:

Steering

The next step I took was the steering. It took me 3 to 4 attempts until I got it right, but my final design is strong and does the job extremely well. My main advice is, unless you can weld, don't try to make the steering spindles yourself.

My first attempt was to use some 90° angle brackets to screw some spindles I had made out of metal pipe to the side of the inner beams. This construction was much too weak and it all bent and broke immediately. Next I tried making it stronger without any success. I then moved on to mounting essentially almost the same type of spindles on a square shaped metal rod and bolting that to the bottom of the go kart. Here's a picture of what it looked like:

This design actually held up pretty well, but it was messy and I thought I could do better. So I figured it was best to order professionally made spindles and then mounted them with some very thick metal plates, which were screwed to a wooden cross beam, which connected to the inner lengthwise beams. This worked very well and is extremely strong. It's actually the only part of the go kart, which (until now) never needed any fixing. Here are a few more pictures of the final design:

I think this would be a good time to give a small bit of advice. I have found it extremely useful, when working on a big woodwork project such as this one, to limit myself to only two different screw sizes. I usually refer to them as "single" and "double" screws. By choosing the sizes of these screws carefully depending on the beams you are using, you will see that these are the only screws you will ever need. Single screws should have a length almost as long as the shortest width of the beams and are used for attaching plates and other things to a beam. Double screws should be long enough to screw two beams to each other, but once again not too long. In general I think it's useful to get screws with a wide diameter, even if that means you need more and bigger pilot wholes. I always recommend SPAX screws with a T head. The screws I used on this project were: 5x40mm and 5x80mm (T20). For these screws I used a 4,7mm bit to drill the pilot wholes. This may seem trivial, but it actually simplifies things a lot and saves huge amounts of time, because you don't always have to look for the right screws, drill bits, screw bits, etc. It also allows you to buy the screws in bulk which saves a lot of money too.

Drivetrain

At this point my go kart possessed two out of four wheels and I came to the conclusion that the logical next step would be to add the remaining two wheels. This also meant figuring out the rear axle, sprocket and break. I realised there was no way I could machine all these parts by myself like I had planed at the very beginning of the project. Unfortunately, there are very few possibilities to obtain parts for go karts in Europe, so despite high shipping costs, I ended up buying all the parts from an American supplier called BMI Karts, who I am honestly very pleased with.

This is a list of the main components I needed to complete the drivetrain:

You should be aware, that these are only the parts I used. You will most likely need very different parts and sizes.

The first challenge was mounting the axle. I drilled holes in the inner lengthwise beams, for the axle to run through. I made the hole bigger than the diameter of the axle and then mounted bearings so that the axle does not touch the wood. To do this I bolted two flangettes with a bearing inside to a bearing hanger. I then screwed the bearing hangers with particularly thick and strong screws into the wood.

At first I simply tightened the set screws on the bearings in order to stop the axle from sliding from side to side. However, I soon realised, that this wasn't sufficient. I constantly had to realign the axle. Two lock collars set against the bearings solved the issue.

Before attaching the axle to the frame, I had to decide on where to place the sprocket and the disk for the break. Then while sliding the axle through the bearings, I added these components. Both the axle and the sprocket or break disk have a keyway (a shaped channel). Using a fitting piece of metal, called keystock, these two channels interlock and stop the components from moving independently from the axle. Next, using set screws I locked the sprocket and break rotor on the axle.

To finish the break, the only thing that remains is to attach the break calliper. This is no big deal, but it has to be parallel and very well attached. You don't want it to come loose when you need it most. That could be bad. Now I needed a way of activating the break. I'm using a pedal, but I will cover that later.

The Motor

Now for the most important component: the electric motor. I chose to use a winch motor, that I found on eBay. It's supposed to produce 4HP. The key to this motor, is that it's a series wound motor, which means that the stator and the rotor are both electric magnets (coils), where as with PM motors one of the two is in fact a permanent magnet and the other a coil. In retrospect, I probably would have been easier to have used a PM motor for several reasons:

These are all things I learnt the hard way, because I just started to build without considering the advantages and disadvantages of each motor. I guess the lesson to learn here is do your homework thoroughly and then build. Anyway I stuck with the motor and carried on. However much of what I will describe is the same regardless of the type of motor.

As for attaching the motor, that totally depends on the shape of the individual motor, but it is important to keep in mind the intense rotational forces the mount will have to endure. In my case I attached the front of the motor to a wooden board with a whole for the motor shaft, then supported the motor from below at the front and back. I also added a metal band to hold it down.

Attaching the smaller sprocket to the motor shaft followed basically the same procedure as for the axle. One thing to keep in mind while attaching the motor and adding the sprocket, is the chain and the distance between the two sprockets for the chain to have the right tension. Finding the perfect amount of tension can be quite tricky.. The chain cannot slack or it might jump over teeth or even come off the sprockets completely. On the other Hand, if it has absolutely no slack at all and is too tight, it will probably break. Some people add a third sprocket to give it the right tension.

Needless to say, the chain and the sprockets need to match in terms of size. I used a #41 chain and sprockets. Gear ratio is also something to consider. I used a 10 tooth and a 45 tooth sprocket, which means a 1:4,5 gear ratio. This is important because it means that the wheels will experience 4,5-times more torque, but will turn 4,5-times less. The seller for my motor recommended a gear ratio of 1:10, but I couldn't use such a big sprocket, because it would have had a bigger diameter than my wheels. I could have solved this by using a crank shaft, four sprockets and two chains, but frankly, that would have been too much fuss and this gearing seems to work fine for now.

Batteries

Batteries are a big subject when working on the electrical system, as they will provide the power. I am using plurals here because most likely you will need more than one. The first thing I decided on was the voltage I was going to use. This is mainly dependant on the motor and speed controller you are using. The motor I am using is rated for 12V to 24V. I found a Curtis speed controller which can run on voltages between 24V and 36V. Therefore I chose to use two 12V batteries in series for 24V. Most commonly one will use Lead-Acid batteries for this, because they are comparatively cheap and can have a large capacity. If you are willing to spend a lot of money, you might choose to use Lithium Ion batteries. These are lighter, have greater capacities, can source more current, can be charged more quickly and can endure more charge cycles, but they are a lot more expensive.

Lead-Acid batteries are the same as the type of battery in a car. They are heavy, but can deliver a lot of power. The main problem with these batteries though is that they are normally not suitable for constant use. In other words, they cannot be discharged too much. They are designed to deliver a large amount of power for a short period of time when the engine starts and then to be immediately recharged to a full state. A deep cycle means to discharge a lot or most of the batteries capacity and then recharge it again. Because of the chemical process in Lead-Acid batteries, deep cycles will quickly destroy them. Luckily there are batteries that can endure deep cycle use. These are slightly more expensive, but it's important to use these. They are usually called marine, solar or simply deep cycle batteries.

In my go kart I am using 88Ah batteries.

Arduino Control Unit

The Go Kart is controlled and monitored by a system consisting of an Arduino Due with a touch screen and several ATtinys. The motor controller can be disabled and the Go Kart thereby secured by means of a passcode. Below are HTML and Javascript simulations of the lock and main view on the touch screen.


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More coming soon!