The Desk Chair
As a high school freshman, there is a definite need… a need for speed. This was evident when my friend Nathan and I sat in our desk chairs in our auto shop class, pretending to drive in sports cars while sitting in those chairs. It was a moment of immaturity, but also a stroke of genius; we realized that we had to motorize some desks. With that, we picked our own desks of choice (they were not stolen, contrary to popular belief) and headed off to my shop to create our own version of a desk-chair-go-kart thing.
Starting the Project
It all began with buying many parts and welding up the base frame which the desk would get welded to. This adventure started in 2016, before lithium got really popular, and for that reason, four lead acid batteries were chosen, they’re cheap, crank out current, but are very heavy. The choice in electronics would change drastically for my future projects. As eBay and Amazon would slowly fill with parts and new tech from China, vehicles would only get lighter faster, and if not the same price, cheaper than what was used for this project.
Pictured is the 36 volt motor that got over-volted to 48, the matching speed controller, brake rotor from a bicycle, some throttle/brake pedals and the frame itself on the floor
Axle and Brake Hub
This project was one that forced me to use our machine shop in the most sophisticated way than I have ever used it before. For context, we have a 2 car garage that was converted to a workshop, and contains a manual mill and manual lathe, more than enough to learn on. The first task was to lay out the axle, 1’’ cold rolled steel bar was used, it is easily in tolerance for the bearings used, and very machinable. It was parted to length, both ends were tapped to hold retaining bolts, and a keyway was milled in the necessary spots to keep the brake and drive sprocket from rotating.
The brake disk hub was quickly sketched out, and turned with a shoulder to keep the disk concentric on the rotating axis. I took it to school and keyed it using a ¼’’ broach. Holes were drilled and tapped, to hold the disk, and one for a set screw to keep it from sliding on the axle.
1/4’’ keyway being cut for the brake disk, drive sprocket, and drive wheel
Finished brake hub, final step is to broach a keyway into the bore
Almost fully assembled axle, missing bearing blocks and drive+free spinning heels
Electronics and Weight Distribution
The electronics setup is just four 20 amp-hour SLA batteries in series, going into a generic 48 volt 1000 watt brushed controller. The controller specifies that it allows 30 amps peak, so realistically the peak power is closer to 1500 watts. The battery tray was welded in an effort to be the most compact possible, as well as keep as much weight over the rear axle, being over the rear axle is an important note.
Ideally this kart would utilize a differential, but those are expensive and would require one brake per rear wheel, for this reason, I went with a one wheel connected to the axle, while the other spins freely on a bearing. For this reason, as much weight as possible is needed to ensure good breaking.
Controls
The most difficult part of this project was setting up the “gas” and brake pedals. To turn limited vertical motion, to high travel horizontal motion offset by 2 inches was far more difficult to design. It is best explained with pictures, and with minor modification of the existing pedals with my trusty MIG welder, it worked smoothly and was not over-responsive. The brake pedal was directly routed via cable into the bicycle disk brake caliper. The throttle cable attached to a bracket I welded and machined, accounting for the modified offset thumb throttle.
The Final Product
With the wiring tucked away, batteries charged, and chain lubricated, the Desk was ready for its maiden voyage. Luckily this first drive was recorded by my friend Nathan, and has been attached. During our testing, where we tried to find out the maximum capabilities. We confirmed it could take 3 people reasonable distances, and a maximum of 27 miles per hour. When the batteries were new, had a potential range of about 10 miles (conservative driving). In the future I would consider converting to a brushless system as well as lithium batteries into this thing when I have the time and motivation. Until then, this project can serve as proof that ancient brushed power trains can still get around.
What Makes This Project Significant?
This project taught me to weld, taught me to use the lathe and mill for accuracy within .001’’, how a brushed dc motor worked and why it is limited and inefficient. Along the way to the final product pictured above, I had spent hours sifting through forums just absorbing all the information I could to apply to future projects. From learning about different types of drive systems, the pro’s and con’s to using a chain over a belt, all the way to how to spot weld a lithium battery pack together. At the end of this project, Brushless motors and lithium pre assembled batteries started to become more and more available and therefore cheaper. The next electric project would be sure to use this new efficient cheap tech. This was a big mark in my high school career, as it made both of us famous, as we commuted daily in our unorthodox methods of transport.