Chibi-Mikuvan: Detroit Recap and The Conclusion

I have the worst go-kart hangover ever.

In the end, the reversing module, spare parts, and electrical system refactor didn’t happen, so I basically went down to the race with Chibi-Mikuvan in its late Wednesday condition. However, all the sciencing and preparation did pay off: Chibi-Mikuvan won 1st place in its 30-lap sprint race. The Endurance race the next day, however, saw my very much still beta-version non-refactored electrical system catch a case of the loose wires from the very bumpy track, an issue which couldn’t be diagnosed in time for the end of the race. The final ranking for Chibi-Mikuvan for the weekend was #6 out of a field of 19.

Also, this picture happened:

We actually didn’t travel down with Chibi-Mikuvan in the open, to avoid things like rocks and bird droppings. Instead, it was tarped over, but the tarp came loose at times and required readjustment, and ultimately had a few large holes torn in it – heavy duty it was certainly not. It was easy to see when the tarp was loose: the intrigued and possibly disgusted stares of passing motorists (and when you’re me, carrying four people and hundreds of pounds of pit equipment, everyone passes you) was the clue that we needed to check the tie-downs.

I’m also fairly sure that if we DID travel with it in the open, we would have caused at least one accident.

But wait! There’s MORE!

In the roughly 1.2 weeks before leaving, the MITERS folks who helped me test became inspired enough to cobble together their own independent entry using literally whatever was standing around in MITERS and upstairs in my shop at the time. I’m sure you’ll hear more of this from the likes of Benkatz and Dane.

Here’s the load-out shot from above as I was scanning the shop for things which I forgot to bring, showing the tarp bubble. The choice of route was straight west on I-90, and then a right turn. I wanted to go through Canada, but figured the 1.5 hours saved was not worth the potentially more than 1.5 hours being hassled by border crossings. Plus, not everyone who came had passports. Sadness – maybe a future vans-on-vans trip into Canada is warranted if they ever hold a race at the future Toronto Maker Faire or whatever. Google Maps said the trip would take 12 and a half hours, but for me it was more like 14 each way.

The Detroit track was made in the parking lot of the Henry Ford Museum using tire barriers. It’s easily the largest PRS track I’ve seen (and I think the series has set up) – it’s easily twice as large as the New York Maker Faire one. For the most part, it was smooth with the exception of three very large bumps that ran perpendicular to the track. I think these bumps were ultimately the cause of the failures I experienced during the Endurance race due to the wiring not being secured well.

One half of pit row. Pit space was tight, and we had to intermingle tools and parts with surrounding teams. This was a shot from the morning, before everyone’s pit became earthquake disaster zones.

We’re contemplating something here… Probably our newly acquired safety bracelets.

An hour or two of open practice was available, so I took the time to get a feel for the track and try out the racing line.

The reason PRS appeals to me as much as it does is how varied the level of technology in the entrants is. It reminds me very much of my other favorite sport of fighting robots – you have personal garage builds, schools, and basically professional shop spaces playing at the same level, and the professionals don’t always win. The rules are purposefully open-ended instead of heavily codified like most motorsports. It’s purposefully and unironically derpy. As I’m sure I’ve made abundantly clear, I’m a huge fanatic about learning via building derpy shit taking initiative and having fun.

Here I am running a qualifier lap. I got the 2nd and 1st best times for qualification, but the 1st place one was negated due to me not stopping in time (it has an integrated brake test). It was no fault of the brakes – my first run, I braked so hard the front discs locked up and one of the Harbor Freight tires got a huge divot eaten into it. So I took it easy – a bit too easy the second time around.

The video of that run is here:

After the qualifying round was a short break before the “Moxie round”, which is a freestyle showoff kind of affair. A few teams acted out skits, one made a simple presentation, and there were some drifting and speedruns. Then there was me:

When I go, I go hard.

In the picture of the qualifying run, notice the small antennae-like structures glued onto the helmet. They’re mounting points (such technical words) for the Miku pigtails seen above. I cut them to 2ft instead of the stock 4 feet to avoid becoming a hazard to everything.

However, all things considered, this was probably the wrong audience for such a display and most people were seemingly more confused than amused – coupled with the fact that I went last (with helping Hack Pittsburgh with their Back to the Future skit, and MITERS with their small goofy vehicle parade), means I didn’t score many moxie points.

During the intermission period, there were, of course, shenanigans. Along for the ride were eNanoHerpyBike (which later gave up a throttle to Chibi-Mikuvan), this contraption the One-Day Kart, and Bentrike.

Here’s MITERS relaxing, right before the heat races began in the afternoon. The field was split into odd and even qualifier positions, so the MITERS entry was in the first race starting near the middle of the field.

Some kind of strategy discussion or trash talking before the 2nd heat race that I was in pole position for. That, or we’re just talking about who to spin out and who to run into the tire walls.

The 30-lap sprint confirmed to me a weakness of Chibi-Mikuvan that I had predicted – that anyone with a DC motor is going to get the jump on me. The Trackstar takes a sweet second to ramp through its starting routine, during which Hack Pittsburgh (in a superbly built custom-bodied DeLorean with Back to the Future theme and everything) was able to get around me. We’re also fairly matched for speed, so it was difficult for me to catch back up unless there was traffic ahead. In the end, it dropped from the race due to a loose wire – letting me take first place. The same fate was to befall Chibi-Mikuvan in the endurace race.

The ‘press shot’ for the weekend, during the 30-lap heat.

Here’s one video of the race, shot from the finish line truss.

A fancy new hubcap!

I discovered during this race that heavy is not necessarily a bad thing. In fact, it keeps you on the ground a whole lot better. I had to slow down more for the turns than some teams to avoid rolling over – I got on two wheels two or three times. This prevented me from effectively using the turns to pass. I may have to reconsider that full-size battery pack for Chibi-Mikuvan in the future.

Both MIT vehicles in one shot during the endurance race.

The endurance race for the first 15 minutes or so was a faceoff between Chibi-Mikuvan and NIMBY‘s Ferrari. There was “some” contact made. Nearing the end, the battery began running down and I started seriously feeling the power dip, so I pulled into the pits, allow them to regain the first position for laps. I neglected to remember that the Hyperion 1420i charger has a charge capacity limit of 9.9Ah – it always stops at 9.9Ah, and I have to manually restart it. I left the battery charging on Saturday, but forgot to restart the charge on Sunday, and so with the sprint race taking out (most likely, due to our science’d data collection) more than 10Ah, it wasn’t running on a full charge.

The starting lineup for the endurance race.


 Chibi-Mikuvan caught in a bit of a traffic jam next to NIMBY. We traded spots a few times.

Around 20 minutes into the race, after the battery change, was when things start going wrong. At first it manifested itself as an intermittency in the throttle – it would work after one bump, but not after the next. It quickly progressed into Chibi-Mikuvan having to be carried off the track and into the pits. I did some short voltmeter investigation and discovered that the 5V throttle wire broke off at the terminal strip.

After re-stripping and pouring hot glue into the area to restrain the rest of the cables better, the next thing that happened was the throttle straight up dying on the second driver 0.9 volts all the time, no response. I harvested the throttle quickly from eNanoHerpyBike and Chibi-Mikuvan got in the running for a couple more laps, but very quickly had to return to the pits because I smelled something funny.

Thinking it was the Trackstar ESC, I shouted for a replacement ESC to be made ready, but it wasn’t that simple – the Trackstar was working great right until the end. I’m not sure what exactly happened but the Arduino Carrier board’s thick ground trace was blown apart, and 14 volts was present on all the 5V rails. The Arduino itself was nonresponsive, probably due to said 14 volts.

In a moment of either desperation or insanity, Dane chopped apart a Harbor Freight 4.5v flashlight, I rigged its battery into a servo tester knob, and then I cruised very slowly like that for a few more laps before something just completely gave out. And that was it for the Endurance race!

The picture above is of the servo tester and flashlight hack. It was taped to the seat back, so I had to drive with 1 hand behind me to gently twiddle the knob. I got used to this style and actually began to try and run the race again until the final, as of yet known failure occurred.

Chibi-Atomic-Jeep (among other names), the MITERS one-week entry, actually did finish the endurance race, but not before busting off a few Harbor Freight tires. I’ll let the others tell that story.

Chibi-Mikuvan didn’t escape the tangles with NIMBY unscathed. I clearly also scrubbed someone’s tire, and that knocked off the catface, which was never recovered. Otherwise, the front body mounts were broken off in a run-in with the tire wall, and I drilled through the body (cringe) and used large zip ties to secure it for the rest of the time. The rear body mounts were redone shortly after completion using E6000 rubber construction adhesive and never had problems again, whereas the fronts stayed epoxied on. The epoxy seems like it was just too inflexible for the task.

Obviously body repair will need to happen, but all things considered it’s in great shape. I had expected far worse. This is all stuff that can be taken care of with some Bondo. I probably won’t be able to get it back running again before Dragon*Con (there is not a race there anyway, just a short exhibition), but I and MITERS alike are making serious plans for Maker Faire New York…

conclusion and next steps

So now that the race is over, how well do I think Chibi-Mikuvan performed overall? Time for helpful infographics in my usual style. As I have said since the start, Chibi-Mikuvan is intended to push several technological and resource boundaries at once, in the interest of anyone else wanting to get into the sport of silly rideable things. Some of it went great, others not so much.

The biggest risk was probably pressing into real heavy duty racing service a R/C car controller, the Trackstar 200A. Granted, it’s for “1/5th scale” R/C cars, which I swear Chibikart is already a 1/5 scale… Ariel Atom with growth stunting problems or something. Chibi-Mikuvan is a 1/3.14 scale R/C car… that’s close enough, right? Checking out the internal construction of this thing along with watching the current draw with the digital dashboard, I’m satisfied that this controller can output its continuous rating at full throttle. In the context of PRS, you’d probably never run 200A at full throttle unless something really interesting is happening.

During testing, I did manage to kill two – one by letting the signal board shake loose (so attach it firmly!) and the other by regenerating into a disconnected battery – instant voltage-induced death. However, it did not miss a beat during the competition, in 90F+ weather, in conjunction with the current limiting algorithm. Too bad everything else did!

One of the things which continually gets me “interesting” reactions from curious builders and onlookers is the gear ratio I’m running on Chibi-Mikuvan – a healthy 20.5:1, compared with the 5:1 to 10:1 of average small vehicles. In part, that was to take advantage of this fist-sized motor in a ground vehicle application. I’ve had 1 person so far ask me if the blue thing was a supercapacitor, thinking the motor itself was under my electronics deck somewhere. Because power is composed of both speed and torque, you get “free power” to a degree by running a faster motor. That’s why R/C outrunners are able to advertise such ridiculous power ratings – in their designed application (aircraft), the motors run very close to their no-load speed so they can indeed be very “powerful”. Chibi-Mikuvan proved that you can indeed run a very small, high speed motor in conjunction with sensorless control if you gear it highly enough.

Sensorless, though, isn’t without its trappings as I mentioned previously in the face-off with Hack Pittsburgh. The sensorless starting routine can still bite you and let someone else get a jump on you.

To use this motor above 1500W or so for an extended time, a water cooling circuit is necessary.

The way I was able to design a 20.5:1 reduction in the rear left corner of the vehicle was by passing the Aquastar motor through angle grinder gearbox. The 9″ generic Chinese angle grinders seem to come with 4.1:1 right-angle spiral bevel gear drives. Now, I can’t vouch for the gear quality, but they seem to be holding up great in this application.

There’s several types – one of which Harbor Freight uses and has a smooth input shaft, and another style which has a Woodruff-keyed input shaft. I modified the Harbor Freight input shaft and the bore of the pinion into a machine taper to get better torque transmission properties, and it worked for the most part, but I had troubles with the pinion gear getting pulled off its taper by the action of the spiral bevel gears: on negative torque (regenerative braking, for instance), the thrust force tends to pull the pinion off the taper.

I ordered two “Truepower” branded ones from Amazon, and they come with Woodruff keyed input shafts. I upgraded the key one size to a 1/8″ thick, 0.5″-circle Woodruff key, and that’s the one that was used during the race with absolutely zero problems. The gearcases were packed full of grease to minimize wear – I wanted to do an oil fill with some differential gear oil, but the gearbox has no gaskets and it would have gone everywhere. I’d recommend the woodruff key version just because it is conceptually easier to interface to. You could cut off the angle grinder motor’s shaft and use the leftover 12mm stub shaft with a coupler, or machine a custom one like I did.

The Ford Fusion batteries came through with absolutely no problems! Dumping 100+ amps is probably not good for the life of the battery, but if Ford’s actually advertising 20-30kW hybrid systems in those things, they then have to be able to push that much current. I gave an in-depth dissection of a 2009-2011 (generation 1) Ford Fusion hybrid battery in its own post (BE CAREFUL! is the theme here), as well as how I packaged them for use.  You can get 28.8v and 16Ah in about 22 pounds the way I assembled them.

A recent (a few minutes ago…) scan of New England yard listings shows most of these batteries starting around $500, which at the rate I used them, is still borderline economical given how many independent packs you can get out of one.

What can I say – the Mac G5 pump and radiator (and my reservoir made of an olive jar that I ate on the spot to get) worked great! The only issue was that the Aquastar motor wasn’t well sealed for most definitions – it leaked consistenly, and by the end of the 30 lap sprint race, the cooling system had leaked dry. A coat of silicone will probably solve that problem, so that’s on deck for New York. The system was silent and effective.

My favorite part, the Hysterical Current Limiter, worked to great effect during both the race and testing. It’s best explained in its own two posts. During the race, I did blow one fuse, possibly the result of running too hard when the weather was hot (the temperatures were nearing 90 degrees Fahrenheit in Detroit that day) – quite metaphorically flying too close to the sun. But I and anyone else could slam the throttle to full without, you know, dying or something. Without the HCL, it takes a trained throttle finger to drive Chibi-Mikuvan, which is why I basically didn’t let anyone else drive it until this was completed. Functioning code for the HCL is provided on those posts/pages.

Ah, the Harbor Freight pink tire. Full disclosure: We cleaned out the nearest three Harbor Freights to MIT of these things, as well as the closest Harbor Freight to the Detroit Maker Faire. I still think they’re a good choice for low powered vehicles. They are grippy, but the rubber is sadly too thin to last long. They’re well cost-optimized for the 0.5mph handtruck or pushcart. The tread is at most 4mm thick, with the tire carcass (plies) barely below that.

The stock hub also has well known durability issues – they crack apart at the weld under sideways pressure (from turning). I made my own hubs and designed these to be swappable, so that was not the issue. In fact, they turned out better than expected on the DMF parking lot – for some reason, their asphalt is less abrasive than ours.

Sadly, I don’t think I’m returning to these – something that I made up my mind about before the race after seeing the results of the “science”. They work, but they simply don’t last long enough. I’d like to get through a race without having to change tires, and that involves moving up to something designed for mobile vehicles. Popular at the PRS race in Detroit were riding lawn mower tires and real honest-to-Go-Kart-Jesus scooter tires, both designed to move under power.

The thing that bit me in the end was my ad-hoc wiring job. Breadboards, multiple cables running into each other, and poorly strain relieved terminal strips don’t make for a high-durability, robust mobile electrical system. The parking lot ruts finished off what the science testing surely started – fatigued wires and unknown shorts and lost connections. With this system dead, I have no choice but to refactor and rebuild it from the ground up. I’m envisioning a single PCB with all the features I need on it – or at the least, prototyping board with actual solder joints!

So what’s in store for the future? Well, nothing for the time being, since Dragon*Con and other projects (not to mention, umm, my shopmaster job) are higher priority. Strictly speaking, the vehicle can run again in its current state with just an electrical system rebuild. Cosmetically it’s beat up, but nothing some Bondo and paint will fix (as well as rebonding the front body mounts properly). Perhaps the highest priority would be repairing that electrical system.  I have a few ideas for drivetrain upgrades, but they’re a long ways off – maybe for next PRS season.

This concludes the Chibi-Mikuvan action for a while. Look out soon for details on how the summer EV Design class has been going, as well as updates on the robots for Dragon*Con!

For reference convenience, I’ve copied over the “cheat sheet” from the introductory post I wrote when I got it running back in May.

Cheat Sheet


I started this project as a museful distraction in October of last year after returning from the New York Maker Faire and mingling with the Power Racing Series folks for the third year. Having seen the league grow immensely, I decided to finally enter something while exploring new and unusual components for hobby builders (my usual MO) while also wanting to see a change away from the “model year bloat” I saw in many teams, who started using heavy forklift motors and other salvaged industrial components. Hence, the focus on R/C electronics and non-lead chemistry batteries.

Work on the project began more in earnest with this season of “2.00gokart“, since I figured I needed to have an instructor vehicle to troll my own students with.

The project was my first jump into making a composite bodied anything, motivated in part by the bodywork repair I’ve had to perform to real-Mikuvan.

Build History

In chronological order up to the previous post, here’s the process of Chibi-Mikuvan creation from conception to implementation:


The project is named Chibi-Mikuvan after its principal design predecessor, the Chibikart twins which were the “prototypes” for the design class I teach today, and my 1989 Mitsubishi Delica known familiarly as Mikuvan.

It has little to do with Chibi-Miku-san though a few large decorative decals would not look out of place on the shell. I’m an avid follower of the crowdsourced synthetic Japanese future girl-pop that you’ve never heard of world of Hatsune Miku and Vocaloid. That’s literally the most concise way to fully describe it, as I have learned over many difficult discussions about what the inglorious shit is it that’s playing all the time in my shop.



  • Top Speed: 25mph (as-geared, Y-termination)
  • Acceleration: to 25mph in < 3 seconds
  • Braking distance: < 30ft from top speed
  • Skidpad: Uhhh, gimme a sec.
  • Clearance: Still not enough for the Maker Faire cable protectors
  • Drivetrain: RR layout, 1 speed, spool axle (no differential)
  • Dimensions: 50″ L, 28″ W, 24″ H
  • Weight: 113lb with battery
  • Seats: 1, though if Chibikart was any indication to go by, up to 7.

Bill of Materials

Here’s the latest iteration of the BOM (5/1/2014 version), which contains at least 95% of everything on the thing, short of the trivial like zip ties. I went into much more detail than the average PRS list; the quality is a little more closer to what I expect out of my students when it comes to found parts and used parts. Everything, to the degree possible, is given a Fair Market Value which sort of artificially inflates the cost a little. While technically over the PRS $500 statutory budget, I believe this is a more realistic representation of the cost needed to replicate this once.

The BOM has 3 cost categories. First is the actual money I spent. I had a fair amount of parts already on hand, but did have to buy things full-price like the Ford Fusion battery pack and the motor & controller. Next is the PRS rules based accounting, exempting some things like brake parts. Finally, what this vehicle would cost under my 2.007 EV Design class rules, where some raw materials are provided to the students so they only need to count materials if they need to be purchased additionally.

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