JunkRat is a character of Chaos! The perfect build for anyone looking for a challenge. This post describes the making-of process from design concept to reality (mostly complete). As with all DropBOB Designs, this Cosplay is made to be functional. The RipTire actually works. It can be controlled via remote control. Read on to see how it works!

I started this Cosplay build on Aug 3rd, 2019 … This is the current status of my build. I used it successfully for Halloween 2019 (success here is defined as: nothing broken, easy and comfy to wear).

I have a few things left to complete (here’s my list):

  • Still missing 7 out of the 8 Retractable Tire Spikes (I’ve completed one of them)
  • Foam Robot Arm: literally just foam, but I can’t find a free Pepakura pattern, so I need to make my own. Should be simple.
  • Make Peg leg easier to wear & un-wear (I really don’t want to use the peg leg unless I strike a pose, but I want it to be durable & realistic when I do use it)
  • Stretch goal (not-necessary-but-would-be-awesome): Add tilt sensors to the tire so that I don’t have to gauge the interior rim position for maximum force. Maybe I should write a post on how this when works physically.

That’s it … 3 measly items … I’ll be able to finish this once I move into my new shop in February 2020. In the meantime, here is the build process up to this point.

The Design Process

Most of this Cosplay design was done on paper. That’s right, like a caveman. Sometimes inspiration takes on many forms and this one worked really well on paper. I added various cross sections to conceptually come up with the fit and finish.

The idea of retractable spikes was one of the most difficult to implement from this imaginary perfect concept. The main problem is that when I imagine things, I do so with an imperfect gauge of friction & flex. I was hoping to have these spikes at least equal in height to the tire itself, but in reality, I had to make them half that size to account for real friction and flex in the collapsible pipes.

The bearing laden ring hub idea was a really good one. This was actually easy to implement and simple to make. The problem with this one was the material used. I opted for wood since it would be light and easy to make with my limited tools. I ended up breaking the rims at least a dozen times per ring … So it’s more wood glue than wood now. It turns out that using wood glue to mend (for my British crumpet) a broken wood piece makes that connection stronger than the original wood connection to begin with. So after repairing the dozen or so “week” points, I have not had to make any further repairs 🤞…

The design of this Cosplay was started in March 2019. At that time I was not sure I wanted to officially use this as my Halloween costume. I know that my time would be tight and that it could be much more complicated of a build than I was prepared to start.

I fully intended to only design using just pen & paper. But I forgot that I needed to 3D print stuff … So I started with the main drive ring.

I also needed some bushings for the retractable springs. So I further continued to add to my 3D design.

And while I was at it, I may as well add the tire & battery for “looks” and for electronics sizing purposes.

Self-Driving Rim build

The design is simple. The inner Rim is expected to stay relatively static because the heavy Lead-Acid battery will be affixed to the bottom. Due to gravity, it will not want to rotate. The inner rim will be connected to the outer ring on bearings with a geared connection to a Motor (or three).

This first thing I did was use a router to cut “perfect” circles (there are bumps, I can tell … next time CNC water jet aluminum would be better).

The bearings will fit perfectly within the groove. This will hold them in place and allow the rings to spin freely.

The circle cutting was easy; The noise was a little loud (sorry to my lovely wife). Oh the NOISE, NOISE, NOISE NOISE … I was able to cut all 4 hoops in a couple hours. Next time I would make a better jig, increase the cutting speed and slow down the feed.

Since I have 2 rings (one on each side of the wheel), I used bungee cords to keep the rings aligned while I drilled 3 holes to connect the inner hubs together. It wasn’t that greatbut it worked.

With the inner hub and the outer hub now connected together, I was able to get my first real life test of how much friction I would actually have to deal with.

Mechanical Drive components

Now that the structure is built, onto the mechanics. To begin with, I need some gears.

Somehow, I was not expecting this to work so well … I don’t know why I doubt myself. The next step is to finish this ring.

Each of these motors produces about 5-lbs of force on the gear teeth of the hub. This is at max power which uses around 3 amps of the battery per motor to do so … I like that they are stepper motors so I can control and hold a position, but I think I could find more efficient regular motors. Something to keep note of if I want to refine this idea in the future.

Electronics Concept and implementation

This was the original design. Simple enough:

*I ended up scraping the A4988 drivers for much more robust and more powerful DRV8825 (2.5A vs 2A and an arguably more robust electrical design … Since I didn’t break these ones yet)

The Bluetooth control via the ESP32 (one ESP32 controller board in the wheel and one in the wireless controller) was somewhat difficult since I’ve never done a direct Bluetooth control before. I’ve done lot’s of Wifi to Server connections … but in this case, I don’t want to rely on any intermediate “Server“. If I’m at a conference I don’t want to rely on the internet so I need a direct Bluetooth line. But, I probably did it wrong, since I am passing a “String” in Bluetooth which I convert back to a number (integer). It’s really hokey pokey here … but I really don’t want to take a “class” to figure out how to do this properly … so if it’s dumb but it works then it’s not stupid. Here’s the Code if you need:

Note to self, I need to add 3-Amp fuses to the circuit, so that I don’t need to worry about short circuits (as much).

Creating the Tire Rubber Structure with Foam

This was one mistake that I really regretted (no going back here, I’m too-far-gone now). If I were to redo this build, I would make sure the tire foam made a perfectly round and flat even surface to roll on (also consider the stability). I would also try to mimic the tire pattern from the game much more (I.e CAD design the shape and then make a Pepakura pattern)

What you see above is the Hub that I will be connecting to my back. I need a very strong and sturdy way of hanging this tire from a backpack. But I also want it to be very similar to the actual character. You can see what It should look like here:

The backpack was simply purchased from a second hand store for about $2 so it was money well spent! I completely tore it apart and only kept the important bits. Here’s how that turned out.

The tire hangs perfectly level. The base provides lumbar support so that all of the load is transferred directly to the hips. Note that this backpack had a metal bar in it (like it was meant to be used exactly for this purpose or something). The metal bar provides perfect weight transfer to my hips.

Tire and Hub Cap details

This part of this tire build was to help stabilize the tire (would not have needed this if I did it correctly to begin with). An “edge trim” foam was added to both sides of the tire to help prevent it from toppling. And then some filler foam was added to fill in the gaps and round it off nicer.

The foam clay that I used was Foam-Mo 300g tub that I got on amazon. The stuff feels amazing. Like a fluffy version of play-Doh, but less messy. Almost a watery feeling. Very hard to describe as there are very few things that feel like this.

Other JunkRat Accessories

I spent less time with making these accessories, but I still had enough time to personalize each and every single pipe canister.

Each one of these has different springs and shape. No, it’s not because I couldn’t remember what they last one looked like … Yes, I was tired. But it was intentional, I promise.

The paint really gives these little guys a bad-ass look. I’m sure some of you can paint good weathering. But, I prefer to actually weather them as I use them (said the lazy cosplayer).

I used fabric to glue down some Velcro with contact cement. I added the leather as an accent. Boom: functional-Watch cosplay-accessory!

This peg-leg still needs some work. But I intend on making the peg leg “Real” looking for photo’s … my leg is bent behind me so I need the correct angle or I can just ‘shop it out.

On my first attempt, the wood cracked. Note to self, look at the wood grain before using a piece of wood.

Ok, this is working better. But the install and uninstall of this leg is a pain. Need’s more work.

Wig Design and Styling

The final item for this cosplay is one of the most important ones. The wig is especially important since it is the first thing you see. The unruly tufts of patchy hair, still a blaze … It’s an iconic item for sure. So I had to make sure this part was done right.

I purchased a short hair blonde wig from one of my favorite Cosplay Artists: Kat.ofalltrades. Her shop here (what do you think, does blonde suit me?):


I then massacred it … literally, with Hot glue, hair spray, paint and wood glue. It took a lot of cutting, trimming, styling to get it to look like this reference.

*Not my Art. I found these online via Google search. References above.

The first model was an old Portal Gun build that I keep wanting to fix-up and eventually sell. But knowing me, I won’t do it until there’s someone interested.

This second model was a little more sturdy. So I continued with this guy.

It didn’t take too long to get the general look and feel that I wanted. The wig held up to my abuse very nicely. Even when I switched to a higher power heat gun instead of a my wife’s hair dryer (pretty sure Kat’s instructions clearly said not to do that).

I wanted to make the wig electronics very sturdy since they will be bent and abused quite a bit. This design should keep these running smoothly for the life of the Cosplay.

For the controller on the wig, I’m using a lilypad. It’s got exactly the right number of outputs and a rechargeable LiPo battery circuit. Done!

It took many hours of styling to get to this point (I understand now why buying this Wig pre-styled is so expensive). I have no prior wig styling experience, so you can do it too! You just need to put the time into it.

I made a foam case to house the battery and lilypad controller. Here’s the Code. I feel particularly proud of such simple code for a fire effect.

Yes, you guessed it …. she’s looking at butts 🍑…

Stay tuned for the next post (things to look forward to):