Broke-Ass Mario Kart – Build Log 02

New Motors!

I decided to source more reliable motors than the ones I found at Active Surplus. I love Active, but they have an “as is” policy on a lot of their used equipment that made me nervous. Also, some of the motors were missing pretty essential things like brushes and they don’t have any fixtures on them to easily attach to a chassis or hubs to attach wheels.

I reached out to wheelchair repair companies around Toronto in the hopes of finding something more reliable. Wheelchair motors are incredibly heavy, so you could easily spend a few hundred on the shipping costs alone.

I shot off e-mails to about 10 companies, but I only heard back from Jay Wenmann at MEDIchair. MEDIchair and Jay were incredibly helpful, so I owe them some love. Jay said that they had motors in stock, but they could still be swapped into a chair, so they were fairly pricey – $250 each! He suggested that if I waited, I could probably get a pair with tires for about $200! Sure enough, Jay contacted me a few weeks later with an incredible deal on a beautiful pair of used motors.

MEDIchair is located out in Oakville, so I signed out an Autoshare car and drove to meet Jay in person. I asked Jay if he would be comfortable with other people contacting him in future to see if he had more motors. He said that he often had motors that were viable to resell and might even be less expensive than the ones I got – that’s an amazing resource – Jason Wenmann, Positioning and Mobility Technical Specialist, 905-825-5335, or jason at halton dot medichair dot com!

The two motors I got were caked in mud, dirt, dog hair and coffee stains. Similarly, my bike is covered in coffee stains, so I felt a connection to the unknown previous owner. I imagined how the motors would have fit into someone’s life and all the things that the motors enabled that individual to do. I felt they were cherished artifacts, so I spent a good hour or so scrubbing them down – they cleaned up real good!

The motors perform well enough with 12v, but they rip at 24v. I’m using 2x12v sla batteries (like car batteries) per motor connected in series – the voltage gets added for a total of 24v per motor. I calculated the speed with 24v, without a load, to be about 5.86 mph or 9.4 kph – walking speed is about 5 kph. I was especially pleased that I calculated the speed using PI on PI Day. Here’s how I calculated the speed:

  • Count the revolutions per 30 seconds
  • Multiply by 2 for rpm
  • Calculate the circumference using the diameter and the formula πd
  • Multiply the RPM by circumference for the distance per minute
  • Multiply the distance per minute by 60 to get the distance per hour
  • Convert to the appropriate distance unit. In my case, inches to miles and kilometers

The motors have a brake built in that’s engaged by default. Jay and I tested the motors before I left MEDIchair and we determined that the brakes release when you push a minimum of 15v through them, so you need a lot of power for the brakes to unlock and turn. I’ve seen online that many wheelchair motors allow the brake to be removed, but that doesn’t seem to be the case with these.

The first “chasis” for the Kart was built using wood from the blue pie trebuchet that was part of the CTT Pi Fighter 2 project. The motors were attached to angle steel brackets that were bolted into the wood plank. Unfortunately, the brackets extended beyond the back of the wood plank and the wood itself was very dry and kept splitting. The front wheels were two small furniture castor wheels that were screwed into a piece of wood and affixed to the front of the plank.

I noticed early on that the rear portion of the chassis was facing a lot of torsional stress. The angle brackets that extended beyond the rear got twisted fairly quickly with the ridiculous torque of the motors. I added a piece of wood to help support the brackets, but the wood split apart because of the screws and dryness.

On Saturday, March 22, 2014 I sent the following text message to my partner:

I remember laughing with giddy delight when I was first on the kart and it moved – I made a powered vehicle!

The second run of the motors was on the sidewalk in front of our office. Sitting on the kart, I flicked the switch on my “flux capacitor” and the kart took off. The kart hit a tiny bump in the pavement and the front wheels were knocked off, but the motors are so powerful the kart didn’t even slow down. I hurriedly reattached the wheels and tried again – the entire back of the chassis basically blew apart under the torsional stress. Then the front wheels blew apart. I had to start again. As lovely as recycling the old wood is, I had to work with better materials.

I rebuilt the chassis using new wood and angle brackets. Sadly, because the motor block is such a strange shape, it’s only attached to the wood with a single line of machine screws – that means that there is room for the wheels to move outward as the kart drives. This puts a great deal of strain on the chassis and will have to be remedied before the kart is finalised.

Here’s a second running of the kart inside. I crashed into a few things in the office.

It travels fast and hard – it’s amazing to ride.

Here’s the kart spinning in a circle of shin shattering doom:

One Motor from Christopher Lewis on Vimeo.

To control the motors, I ordered two Pololu High-Power Motor Driver 24v23 CS circuits. The major concern when working with motors is having enough power and the high amperage that the motors draw. High amp circuits can easily melt wire, heat up, etc., so you have to make sure you have the proper gauge wire and everything is properly rated. Beyond that, there isn’t much to it – don’t cross the streams.

With motors, the amperage changes based on the load. The Motor Drivers have a preferred amp limit – 23 amps or so. Much higher and I’ll need to add a heat sink to keep them cool. This presents a really funny issue that I haven’t got to yet. I can turn the motors on or off, but I can’t use the Drivers to steer until I’m sure it’s safe. I need to be sitting on the kart with my multimeter in my lap and driving it to test the amperage draw. Then I can safely attach the Drivers and figure out how to control them with an Arduino.

I’m working to add a joist between to two motors to counter some of the strain. I got to borrow an Angle Grinder – that was fun. Here’s a video of me cutting angle steel to brace the two motors together. FIRE:

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