DERPDrive: The Assembly; Plus, Some Other Neat Parts: IG32 Gearmotors and Scooter Pulleys

It’s been a little while! My entire previous week was spent preparing and organizing the big summer MIT-SUTD go-kart race which went down on Sunday. Yes, that’s a thing. The summer class is not totally over yet, so that report will come shortly.

In the mean time, I want to pay attention to something which has been a little neglected over the past month as I devoted more resources and time to making sure the class ran on schedule and people were able to finish up. That something is DERPDrive, which, given that I found one of my structural frame tubes being used as a go-kart wheel chock the other day, was pretty damn forgotten. Luckily, it’s now assembled and already through one attempt at a test fit for which I could not conjure enough macho to accomplish. It’s general knowledge that I look just as home in a Miku costume as anything else, and sheer manpower is not one of my notable traits. The test fit will be repeated once I can get a team-lift going.

Here’s what all took place.

After watching the paint dry, DERPDrive was unceremoniously shuffled to one corner of the shop as the students’ build season really started kicking in. Over the course of a week or two, all kinds of mayhem landed in my parts box – from safety goggles to tools to other people’s drivetrain parts. One thing that I keep trying to instill in high-intensity overachieving engineering students is how to pick up your own droppings as you work, and how throwing all your tools from the day into the nearest bin doesn’t constitute cleaning up in the least. This occurs with varying degrees of success.

Since the primary mechanical parts of this thing had already been machined and ready weeks ago, all I needed to do was assembly. This went quickly:

In this configuration it has already been mistaken for a motorcycle jack or some kind of pressing tool.

The next steps in assembly included mounting the astronomically huge bearing blocks. Into these 1.25″ bore blocks will be mounted a solid (unless I feel like saving about 2 pounds out of over 100) 1.25″ steel keyed shaft. The holes next to it are for a 3/4″ steel auxiliary shaft, and holddown method for all of them is 1/2″ fine-thread bolts and grade-mismatched-because-that’s-all-i-could-find hex nuts. These things are surprisingly cheap on the surplus market.

They’re also built for serious slop. I assume the target market is people who assemble entire production machines in a cave, with a box of scraps, because every part of them is adjustable.

The mounts are slots, so they can slide back and forth. And the bearings are captured in spherical housings, so you can have shafts that are just eyeball-aligned. So that’s how Tony Stark did it.

The next degree of freedom to adjust is centering the 11″ go-kart wheel and hub in the center of the swingarms. I mounted the wheel on the hub and eyeball-aligned it, then clamped the hub in place. When the wheel is mounted, I can’t reach the keyway clamping screw , so this adjustment needs to be done beforehand. Normally, these hubs are used on the very end of an axle, not in the middle.

The Big Axle assembled. Notice the overlay on top of the frame jacks – this is 1/16″ 80A neoprene rubber glued to the steel. A thin compliant layer ought to increase the amount of friction generated by the jacking force. That’s the intention anyway – I don’t think I will have “detaching issues” when the wheel is down with a few hundred pounds on it.

Now this thing is getting scary. The 52 pound D&D sepex motor is now mounted, and I’m really, really having trouble wrestling this thing around on the bench. It is just now dawning on me that this might be a little more hardcöre than necessary.

But just think of how awesome of a go-kart it will make had already made! This motor has had an interesting MIT tenure for sure.

The midshaft is now mounted. The little sprocket for the final stage is behind the larger one in the front. Between the two stages, the total gear ratio is 8.66:1, which should yield a top speed of about 15 miles per hour (for a certain high field current of the motor – field weakening, a feature available in the Alltrax controller I’ll be using, will produce an artificially high top speed if acticated).

While the motor is capable of much more power to push the vehicle faster, recall that I’m gearing for a high enough thrust force to get the van out of my parking garage, not to do really awkward silent cruises in front of local night clubs while implying that I have something which hangs down really low.

This whole rig now weighs north of 110 pounds, and getting it down from the bench was probably one of the most precarious situations I’ve found myself in.

It’s time for a test fit…

The crime scene. The plan was to back DERPDrive, facing the right direction, under the two frame rails it will squeeze between, and then hold it in position while I tighten the jack screws.

Alright, I definitely didn’t think this through very hard. Minus the jacks, that’s roughly what the assembly’s going to look like once installed. I accidentally found out that my spring preloading setup works really well when I trapped the swingarm against a jack and was still trying to lower the whole thing.

After a multitude of jacks and bottle pistons and failed attempts at bench pressing the thing up to where it needs to go, I decided to give up before dropping the entire van on myself. This is gonna be a 3-hoodrat effort at the least, and I might need to fiddle my way back onto the lift. I got it almost there – but the whole “hold this thing with one arm while trying to reach the jack screws” thing was just not happening at all. This will certainly be troublesome in the future when I have to deal with 1000lbs of batteries and 200 pounds of Siemens motors. Now we know why I build little things most of the time…

What’s nice, though, is finally getting off the bench. Can you tell which half of this bench I parked one of my project heaps on for a month?

More DERPDrive will come after I secure a test fit!

slightly past unboxing: IG32 gearmotor, timing pulleys

I’m not sure I could call this section beyond unboxing, since there wasn’t much to unbox, so let’s roll with slightly past unboxing then?

It’s August, and there is exactly 1 thing I do in August and that is robots. I need to get my fleet gear for Dragon*Con, which is basically in three weeks including travel time. Yep, I’m in that situation again.

Here’s the situation. Überclocker desperately needs a new top clamp arm actuator since the previous one was so damaged at Motorama (see the bottom of the event recap). The leadscrew got pretty chewed up since it was the first thing to hit an opponent that really sunk deep into the fork, so it would bind at the top of travel. And it had more than enough bottom travel, letting the top clamp actually poke out under the fork, which was just unnecessary. Plus, the oddball chain drive I designed is only getting worse tension-wise, and it really has to go.

I wanted to rebuild the clamp actuator using a stock gearbox (so I can have multiple on standby) and not using those damned chains. It should be much lighter than the current half-a-drill setup I got going on, and not nearly as powerful, because there’s no reason to need a full 550 motor on grabby duty.

I already have a fast-travel (1/2″ per turn!) precision leadscrew and nut, so I’d just need to find a motor with higher ratio to get a manageable clamping speed. Going from 0.1″ to 0.5″ per turn would mean a motor that spins 1/5 as fast, or is 5x more geared down. This puts me in the neighborhood of a 30:1 gearbox with a 25,000 rpm motor (18v drill motor overvolted to 7S lipos, or 25.9 volts).

I took recommendations for reasonably nice gearbox, and one of the candidates (recommended by Jamisong) was the IG series from Super Droid Robots. I’ve seen these before, and had been eyeing them for a while, but never had a reason to buy one since I’d been well-pampered by chopping power tool motors. They seemed like a nice compact solution that I could merge with a 400-class motor for less weight.

So I ordered three for kicks, adhering to my 2n+1 Rule of Procurement for Stuff I Can Afford.  One for using, one for backup, and one for fucking around with!

Here they are! Cute little setups, really. The RS-380 class motor it comes with is quite possibly the mildest wound motor I’ve ever ran. The gearbox itself is constructed from a few die cast aluminum parts, and the ring gear is steel with an aluminum over-sleeve-wrap-thing.

…but that’s all I have to say that’s productive and nice.

What is with you people and plastic gears?

The first stage is bullshit plastic! Ostensibly it’s for ‘noise reduction’, but all I see is cost cutting. I will gladly pay like $5 more for some metal in there.

Well there goes any potential of overdriving the motor significantly, or replacing it.

The difference in gear strength is incredible. So you go from 2.5mm thick plastic gears to 5mm thick steel. That’s way, way more of a torque increase than the plastic can handle. If those 2.5mm gears were steel, too? Certainly, then the progression of torque makes sense. But this is just cost cutting, one that happens in enough gearboxes today to piss me off.

So for $20, I’m not going to argue at all with what you get – I’m sure these things work for exactly what they were designed for. But damn, I’m not going to put up with this for actual robot use. Glad I got three – because my intention for the bot is to combined two of the gearboxes into one that has all-steel gears. The untouched one can be for emergency backup or something where I know I’ll have to go extra sissy on the throttle.

Next up on the list is something interesting I found on TNCscooters while I was putting together a weekly student group order list.

It’s a big timing pulley! Specifically, it’s a 5mm HTD pulley, 80 teeth, that has a center thread which mates with most threaded scooter hubs. These seem to come in three sizes.

Even though the material looks like very crudely cast aluminum, this is still good to see existing because typically, you can’t find HTD drive pulleys in very large sizes from industrial suppliers. At least not in a form which could mate to a purchased wheel in this sector of industry.

Here’s an example of the kind of rim you can thread these pulleys onto. The thing on the left is a typical 8×2 tire rim (example 1, example 2). For non-regenerative vehicles (which do not perform motor braking), you’d probably want a strip of teflon tape or other anti-seize tape on the threads, lest over time these two things become one piece of metal. For regen vehicles, there’s not much choice but to use some light threadlocking adhesive to prevent the pulley from unthreading upon motor braking.

If you aren’t inclined to use the cheesy cast aluminum thread, then those four little hole pilots in the center near the raised hub is the exact same spacing as the four lug nuts on said rims! So, you could do it standoff style by drilling through the pilot holes and bolting it directly to the rim.

This is a typical arrangement of the two parts. On the left side thread, you’d mount a band brake rim or brake disc adapter or similar.  This ought to help those who want a synchronous belt drive like melonscooter but have found that commercial HTD pulleys are a pain to interface to without machine tools.

So concludes another episode of Slightly Past Unboxing! Coming up next are a full report of this summer’s go-kart shenanigans, and a catch-up of what I’ve already been doing with the robots in preparation for Dragon*Con.


Loose Ends Roundup for the Week of the 14th: Adafruit Trip Summary, DERPDrive Painting, Melonscooter’s Battery, and What does a Colsonbot Do?

Here’s another one of those posts where I report up on like 17 things at once! Running (this time wholly my own – no more protection afforded by the likes of 2.007!) the summer go-kart class for the MIT-SUTD collaboration has been one hell of a time sink, so I can only get small incremental things done at any one time.

We begin first by recapping what all went down to get me on the Adafruit Ask an Engineer show this past weekend. The trip to NYC all started as a group desire to just hang out in the city for a few days; so I contacted Makerbot and Adafruit Heavy Industries Co. Ltd. to see if I can swing in anywhere and check them out. Sadly, Makerbot is too pro these days to afford a random visit to their production facility, but Adafruit gladly obliged with an invitation to their web show.

This trip was actually slated to be the very first major long distance haul for Mikuvan. None of us really expected to end up in the city – more like broken down in Rhode Island somewhere. I made sure to pack all the tools needed to service anything short of catastrophic driveline failure, and picked up a new compact spare tire (the stock full-size spare having rusted out seemingly years before, which I took in to get scrapped) beforehand from Nissenbaum’s up the street here.

I’m proud to say that it went down completely without incident. Now I have even less of a reason to dismantle the powertrain, right?

I even looped a new A/C compressor drive belt beforehand (came without one) to test the state of the air conditioning coolant circuit – and to my utter surprise, it blew totally cold. So there we go – all the amenities of a modern car with 9000% more “What the hell is that thing?”. By the way, the A/C still runs R12.

Above is a picture of the van right after arrival in Flushing, Queens.  The only downside, of course, is that it has juuuuust enough horsepower to climb the Whitestone Bridge at about 50mph constant velocity with the gas pedal floored. Horsepower is not something hastily-modified JDM cargo vans are known for, but the electric version ought to fix that. I’m aware the speed limit on the Whitestone seems to be 30mph, but the crowd of delivery trucks and NY-plated private cars huddled around me seemed to beg to differ. I’m sorry, everyone, for having no power whatsoever.

Anyways, Nancy sums up our discoveries about Adafruit well. I no longer think they are made of magic and open-source genome unicorns, but infinity work and dedication.

On this trip, I confirmed the engine oil consumption as about 1 quart per 700-800 miles highway driving, and more like 500ish-miles local (with more cold-starts and short driving trips).  This is a staggeringly high amount, but I don’t think most of it is burning up. During my pre-trip inspection, where I recorded all fluid levels and made sure things weren’t jiggly and double checked my brake rotor-pad-shoe-drum-line-fluid conditions (since it should at least be able to stop, nevermind go) I discovered some fresh oil slicks near the bottom of the timing belt cover and that area of the engine block. This tells me that I probably have a leaking crankshaft front oil seal, and could explain the terrible condition of the timing belt discovered prior to Operation: BAD TIMING. It also tells me the current timing belt might not live that long anyway. The exhaust does emit a brief burst of smoke when cold-starting after a few hours of sitting, so it could indicate a number of other things worn, like the valve guide seals which were suggested by more automotively inclined buddies. I’m willing to write it off to 20+ year old poorly maintained engine. The oil itself does not show excessive signs of burning – the shade isn’t particularly dark, nor does it smell like burned fuel significantly, so I’ll say that most of it is just physically leaking out.
The fact that I hauled ass a total of 450 miles without any hiccups is amazing in and of itself, I think…


Hey, if I’m not going full-on electric right away, let’s at least check in on the thru-the-road hybrid shop-pusher module. DERPDrive hasn’t moved an inch in the past few weeks save for painting (in the same round as Melonscooter2), and that process looks kind of the same:

I picked up a handheld sandblaster from Harbor Freight (this one) to pluck all the rust and scale off the welded steel tubing quickly. Along with a jug of 80 grit aluminum oxide, it took maybe an hour or so to reduce the major frame parts to fresh steel. Here’s a picture of the blasting in progress. By the end, I’d created a small ejecta ring of sand, and I was basically covered in sand in every place imaginable. To supply the blaster, I borrowed a 25 gallon compressor from the IDC shop.

I hung up the parts using picture hanging wire and gave them three coats of the same etching primer used on Melonscooter space a half hour apart. With some of the lessons learned from Melonscooter’s frame, and a bit more advice from more legitimate painters, these parts came out far more even in the end than the scooter frame.

Next up were three coats of black (the same black, again, as used on Melonscooter since I bought like 5 cans of the stuff). Notice how I started during the daytime and it’s now the dead of night. There’s still some “orange peel” areas, but overall, everything dried totally smooth. I ran out of clearcoat, so DERPDrive won’t get the same crisp and shiny finish (But you’re never supposed to see it anyway…)

The finished parts after sitting in cooler, drier air for a day or two.

After the paint fully cured, I began adhering rubber strips to the front and rear of the structure, the parts which will be jacking on the van frame. These are some moderately hard (70A) and thin (1/16″) BUNA rubber strips I bought, being attached with contact cement. A thin layer of compliant material will aid in the attachment in a way two metal on metal contacts cannot – especially given that I won’t be able to torque down the jackscrews fully given that the van frame is still some pretty wimpy stamped steel rails. Again, if this doesn’t work out (like I start popping spot welds), I’m just drilling through everything and attaching them with rivet nuts.The C-clamps are to keep the adhesive fully engaged with the welded steel parts.I hope to assemble DERPDrive soon – I got into another one of those cycles of opening up multiple project threads, unfortunately…


The only work I’ve been able to get in on Melonscooter2 recently has been constructing and balance-changing the battery pack. I also prepared the motor controller, a KBS48121, and most other chunks of wiring for immediate installation. What I have been missing is the timing belt and pulleys – I ordered them last week, but of course waiting for shipping is the killer here. After I receive these parts, everything ought to fall into place quickly.

This is the battery pack in the middle of assembly. I waterjet-cut some 1/32″ copper bus bars for the task. One of them, to the left, has a chunk cut out of it to act as a last-ditch +250 Fuse of Oh Shit Amps. Unfortunately, I had used the wrong design equation values to make the cross section – I think this is actually good for something like 800 amps. Oh well…

Check the fully assembled pack. I added two 6S independent balance leads just to check cell voltages with for now – I hope this pack will be maintained infrequently enough that just cracking open the battery box and alligator clipping to it every few months is enough. Worst case, now I have one of these guys that I’ll make a balance lead jack for. These cells were in wildly varying charge conditions, so I had to spend a day or two just pushing buttons on balancing chargers, but now they’re all within 20-30 millivolts of each other.


Colsonbot… Colsonbot..

Does whatever a colsonbot does

Can he spin? Can he win?

No he can’t! He’s a wheel.

The Battlebots crew up here has reached critical mass. Full disclosure: The real reason for testing Mikuvan to New York City and back was so I can take it to Pennsylvania and back this weekend! The event in question is the PA Bot Blast, and the MIT crew will comprise myself, Dane, Jamison (whom I welcome to the MITrap), and freshly dragged into the craze, Ben.

If I thought trying to wing it up a bridge with only 4 people was bad, then climbing the Allegheny Mountains with four people and robots is going to be really adventurous!

Colsonbot has been in planning since a joyous all-hands dinner at Motorama 2013. Basically, the idea is to build an entire fleet of 3-pound “beetleweight” class robots and sprinkle them about the arena  as a “multibot”, or multi-part entry, to cause trouble and mayhem. Oh, and they’d all be shaped like wheels.  They would be otherwise functional “shell spinner” type bots, but the shell itself would be made of a popular robot drive wheel, the Colson Performa.  I was basically tasked with whipping up a “mass produceable” prototype which we can make a box full and show up to any event with.

I’m proud to say that’s now well under way. To extend this post even further, here’s the work that I’ve done on the Colsonbot front in the past few months. Bear in mind that this sucker has to be ready to run in like 4 days. Luckily, all the parts are on-hand and ready, so I’m only doing some mechanical assembly work.

The way I planned Colsonbot is as a design which could be a successful shell spinner on its own, if only I didn’t put such a silly bouncy rubber shell over it. The drive should be 4WD for stability and traction, and the weapon drive should be as reliable as possible, though not necessarily the most powerful. Under all reasonable circumstances, it should keep rolling! Basically its strategy is to get smacked repeatedly and just roll away.

This is the basis of Colsonbot, a 6×2″ Colson Performa wheel. Typically you’d find these on 30 and 60lb (if not larger) bots. They were a staple of the early 2000s 60lb and 120lb pusher wedge – they paired well with the popular EV Warrior motor and some power wheelchair motors, so they were used widely by new builders. Now that the new builder typically starts in a smaller (e.g. 1 through 30lbs) class, they are less commonly seen than their smaller brethren in the 2 to 4 inch range.

One of the first things I did was to core out the Colson to as far as I thought was reasonable. This process should be repeatable for everyone in on this build, so I didn’t try making any fancy contours. The main body of the bot was consequently limited to about 4″ diameter x 1″ height, with an extra nub on top where the hub of the wheel is normally molded.

Check out those molding voids – someone just did not care at all. Typically, injection molded parts are rejected if they contain voids inside – a result of gas bubbles evolving in the material from impurities or just shitty sealing. However, an industrial caster is hardly a precision application, so I guess this is fine.

The nub in question. I found that the bore of the wheel was basically ready for two FR10 bearing (flanged R10 bearing with 5/8″ bore and 1 3/8″ OD) back to back, so the shaft support was potentially great. I hollowed out the bore as far as I was comfortable with given the Colson’s pseudo-spoked core.

Cored vs. stock, with FR10 bearing. If you actually want to buy these, be aware they are rarely sold as “FR10″ (in the vein of FR8 1/2” bore bearings, which are very common). Try searching G10 or FR2214 bearing instead. By the way, these are exact swap-ins for the horseshit bearings in common Harbor Freight wheels, like these or these (my favorite!)

This is where the fun part starts. Time to try stuffing an entire robot drivetrain into the hollow cavity of the Colson! The only motors short enough for the job were the Sanyo-type “micro” gearmotors sold by a number of places, including Pololu. Literally no other common robot motor (i.e. which we could all buy a bundle of) could fit, even in an “offset” 2WD application, while leaving enough space for the weapon motor and batteries, at least to my sophisticated (…apparently..) specification. I have my own doubts about how robust these very tiny motors will be given the high-impact application they will be in, but we shall see. I purchased a handful of 30:1 units for testing.

After some component shuffling, this is what I came up with. It’s actually shaping up to be a great bot. The four motors are placed in a nearly square wheelbase for best handling, and the weapon motor is off to one side. I decided on a spring loaded slide assembly to keep constant pressure on the shell, which has not been modeled yet.

The hardest part about this thing is the battery. I wanted to fit at least a 1Ah, 3S lithium battery into it, but sadly there were just no options available which could fit in the space required. I had to settle for a 800mah pack from Hobbyking, and even that (as you’ll see in a bit) was pushing it.

Wow, now we’re getting somewhere. I’ve designed this frame to be very quickly blasted off on a 3D printer. As a result, it’s actually the most product-like thing I will have built, yet. The body is all plastic with lids and snaps covering the important bits.

Now with more colson and other parts. The left part of the frame is where the motor will mount – it will be on a little dovetail slide assembly.

This is the mechanism modeled in more detail. I typically just model big blocks and geometric representations of parts until I get to them in earnest. The motor will have a “tire” made of rubber O-rings mounted around the outside. The motor in question is a Hacker A20-50S, first generation (i.e. without the obnoxious tailcone) that I have a few of thanks to my weird airplane friend Ryan. It was the only motor I could get in short order that was short enough yet had enough power. In the”mass production” Colsonbot, this will be replaced with an equivalent Hobbyking shady outrunner.

After the big mechanisms were settled, I began hollowing out cavities for other components and making wire guides.

Here’s a picture of most of the guts installed. The master parts list rundown is:

  • Leftover Turnigy Plush 18 for the weapon controller
  • Hacker A20-50S 1Gen for the weapon drive
  • Vextrollers for main drive
  • Hobbyking T6A receiver guts for the receiver
  • Z800 3S 20C pack for the battery

The center axle is a 5/8″ fine thread bolt with the head machined down for fitness and hollowed out for weight. I don’t think there will be any problems with robustness for the joint between bolt and plastic frame.

I’ve moved onto designing covers and plates here. The motors mount only using the frame members to clamp them in place. They’re square and of a known length gearbox-wise, so this was actually quite easy. It is the same system in use on Pop Quiz 2 to clamp its own 4 Sanyo-style micro motors.

With the battery cover done, it was fine to export everything as STLs and 3D-print all the parts in ABS plastic.

I popped these into a Dimension 1200SST and ran out the last bits of a cartridge with it. I would have tried this on our shop Replicator 1, but just had this sense of hopelessness from the amount of weirdly sticking-out parts.

Test fitting parts now. The motors snap right in – I could almost just run these as-is without the bottom cover!

One issue I found was with the 3/4″ Dubro airplane wheels I bought. I’d never drilled them out before – Pop Quiz 1 used the same wheels back in 2005, but with their stock 2mm bores. It turns out their hubs are no more than about 3.5mm diameter in the center, so when I drilled them to 3mm to fit the Sanyo-style micro motors, there was nothing left to drill and tap into.

Well damn. I quickly whipped up a set of 3/4″ o-ring wheels to be 3DP’d to get around this issue.

Remember the battery? Hobbyking’s dimensions should be considered to be +1mm in all directions in the worst case. I designed this battery compartment using their given dimensions, but when I actually got the battery, it didn’t fit!

Just barely, however. The heavy plastic wrapping they use to shield the pack against punctures sort of got in the way. So what do you do in this case? Cut the damn thing up and just use the 3 cells totally naked. Hey, they’ll have some thicker plastic armor once in the bot anyway. I intend to do this to the 3 packs I got for this thing as spares.

Colsonbot should be all together in the next 2 or 3 days, so definitely stay tuned for this one!



DERPDrive: Structural Fabrication

Continuing on the DERPDrive after a quick melon break, here’s what all happened to get DERPDrive to an almost ready-to-install (mechanical) state. Bear in mind that at this point, the thing’s been sitting on a handtruck for a week and a half, waiting for the weather to stop being incredibly humid and spontaneously rainy so I can go outside and sandblast and paint the whole thing. I got a little wimpy sandblasting gun from Harbor Freight the other day, so I can move to finishing it (and test fitting!) as soon as the weather window opens up.

Last time, the pile of parts was reaching critical mass, just waiting for a day when I can hide in the shop to put it all together. It coincided well with the welding work on Melonscooter2, so there will be an update on that soon too.

Step 1 was to section the large tubing sections into the proper lengths. To do that, I meandered down to the FSAE & Solar Car & Mexican Grill shop and used the 10″ coldsaw. This saw is on-and-off maintaned, and luckily it’s currently in an “on” period where the blade actually has teeth. Get a load of the color of that coolant! Machine coolant, especially the new vegetable-based biodegradable stuff, actually spoils pretty fast if left unused and unchilled. I was told it was changed “like a few months ago, I think”.

Whatever, it was still oily and didn’t smell like the local greasy Thai food place, so it ought to do something.


Tubing and rod stock sectioned to length and ready for the next step, drilling.

I designed this assembly to be thrown together quickly from square tubing with holes drilled in it, so there’s no fancy fishmouthing or angled round tubemancing here. Fine positioning was accomplished on the venerable MITERS Bridgeport.

I bought the two sizes of hole saw I’d need to cut the larger holes. These Home Depot class hole saws are really designed for wood only, and these few holes completely destroyed them. That “Bimetal” must be “horseshit” and “pot castings”.

Drilled, sanded, and deburred. There’s only one thing left to do…

Time to join metal. This post should really be entitled “How to work in 4 shops at once”, because that’s what happened. No one space I was working in had the right combination of everything to do all the jobs needed. Up in the IDC, I really have no heavy equipment at all, but a universe of hand tools and a laser cutter, so I can do the assembly work. In MITERS, there’s everything but welding and sheet metal equipment, and the hand tools are in ass condition. And finally in the FSAE/Solar Car/Pastries shop, there’s welding, big machines, and sheet metal tools, but everything’s just barely maintained and there are no welding jigging and setup tools anywhere.

That’s one thing which buggers me about MIT shopdom in general – everyone would rather have their own spheres of influence and fiefdoms than one well-manned, well-equipped place.

Anyways, here I am invading the D-Lab where they have a very high end welding setup with actual clamps and whatnot, for rigging creations using very high end third-world bicycle frames.

I began with the TIG to join the swingarm sections together. This went well enough – I would actually show my product in public in front of people who, like, know how to weld. But there was one thing which kept me from finishing the job with TIG – it wasn’t fast and dirty enough. Yeah, sure, TIG can let me weld an aluminum can onto a fairy-sized airliner…

…but for something like this where I’m beasting into thick walled steel tubes with no real need for pretty or even incredibly strength, the ability to draw a huge loogie of metal in 10 seconds and be done with it was far more appealing. The MIG welder in the space was much, much larger than the little dinky one that was in MITERS, and the feel was a world of difference. This translated to some very nice looking loogies.

Above is my setup to put the frame tubes together after having finished the swingarm. I used almost all the available clamps for maximum rigidity in trying to prevent warping. Overall, everything came out pretty square.

Next up was attaching the motor mounting plate to the swingarm. This was once again a dance of clamps, using the trunion tube and the folded flanges of the 12 gauge sheet (the same sheet that Melonscooter’s bits came from!) as fixturing spacers.

Here’s a mockup of the assembly after the major welds were done.

During this mockup, I discovered that I welded on the back rail completely backwards. Like, utterly backwards. Both upside-down *and* facing the wrong way. Phenomenal.

A trip back to the mill to grind through the remains of my 3/4″ hole saw, which by this point was cutting more like .800″ polygons of constant width, solved this.

With the frame done, it was time to finish the things which attached to it. To make the leadscrew nut trunion assembly, I took the 3/4″ Acme hex nut from Surplus Center and machined it down to 1 1/8″ OD most of the way, then stuffed it into the hole.

The nut was then welded in place. This joint is of questionable metallurgy, since the nuts are made of 12L14 steel. 12L14 is well known in machinist circles for parts that need to 1. sink and 2. be magnetic – it’s not very strong, and the (very trace) lead content technically makes it impossible to weld because it forms big globules and makes the weld porous. However, opinions seem to differ – some say it can be welded just fine if the material is preheated (which I did with a propane torch for the additional reason of the section thicknesses being very different), others say it cracks and destroys itself immediately.

It seemed to go down just fine with preheating. I wouldn’t, say, put it in space or something, but no matter how starship-like Mikuvan looks, it should, unless the circumstances were most unusual, stay firmly planted to the ground.

To attach the endcaps, which are 1/4″ waterjet-cut donuts, I just MIG welded a huge bead around the perimeter…

…and finish-machined it on the 19″ LeBlond, the only machine with a chuck big enough to swallow the protruding Acme nut.

With the trunions complete, I next turned to the jack, the floating half of the frame which would be pushing against the van ladder frame.

This thing is made of a few chunks of threaded rods and 2 standoffs, which I machined in the same session as the trunion endcaps. The standoffs shown are actually made from chunks of leftover 3/4″ shafting from the same order. They serve to align the jack in the stationary frame. The long threaded rods to either side are what will be providing the force.

The other part of the jack is made from some plain steel tubes that the threaded rods insert into. Aligning this whole setup for welding was therefore simple: put it together like it’s supposed to go, then weld it. The base of the tubing was welded from both the outside and inside of the frame, since by welding the back rail incorrectly the first time and being forced to redrill, I’ve opened up a way to get at it from the other side. Strength and concentration-of-stresswise, this is probably for the better.

Here’s the entire frame completed.

Moving on, the last link in the system – literally, since the frame is one and the swingarm another – is the leadscrew. I needed to put a hex or other drivable shape on the end of the leadscrew so I can crank on it with a power drill or ratchet to raise and lower the assembly (automatic electronic raise and lower would have been funny, but overboard and unnecessary). To start, I machined the leadscrew down to something which was fully round.

Other machined parts include that chunk of 3/4″ steel hex which will be the driving end, and the preload spring retainer on the left, made from a leftover chunk of 1.25″ shafting.

I began by welding the hex onto the end of the leadscrew. For this precision operation, I went back to TIG.

Next, I threw this on a drill press and drilled a few shallow radial holes. Then the holes were filled with plug welds to fuse the material together in those spots like inserted pins would do the same.

The excess weld plug was ground off and the end of the screw machined for prettyiness and consistency. I might have overdone it on the plug welding a little, judging by the deformed hex, but it still fits a deep 3/4″ socket easily.

Here is the finished leadscrew assembly. The J shaped piece is responsible for lifting the assembly back up. In case it’s still hard to see, imagine the tube fixed and the leadscrew being slowly pulled back away from the camera. The spring would compress and cause the hook of the J piece to move along with the leadscrew. This compression is what forces the 5th wheel into the ground to give it traction.

To lift the assembly back up, the leadscrew is cranked back towards the camera, the spring relaxes, and then the force is transmitted into the J piece which now hooks the tube from behind. Because the swingarm is only going to weigh about 75 pounds, the return mechanism doesn’t have to be as hardcore.

The J was made first by bending in discrete “facets” on the big sheet metal brake, then heating it up with a torch and beating it over the tube until it was a little rounder. Recalling the CAD model, it has a big slot where a round hole to pass the screw would otherwise be, since “beat on with hammer” is not considered a precision operation by me at this time (but wait until I start doing bodywork…)

The observant will notice the tiny thrust bearings (by tiny I mean 3/4″ bore) which provide for free movement of the leadscrew relatively to The J while still transmitting force into it. The whole sandwich is retained by a giant E-clip, which can’t be seen from this angle.

Next chapter: Sanding and painting this thing in a fashion which would reflect what I need to do to properly repair the body rust after patching it. That’s why I’m even taking steps at all to make this thing not a rust ball on its own – I figure if one little chunk of the project would help me practice for others, so much the better.  The same sort of thing has to happen on Melonscooter’s frame too.

A Mikuvan Subproject: Operation DERPDrive

I’m going to take a quick break from being too sissy to start on rust repair work to begin a thread for something which has been planned since the beginning when I got the damn thing. As I keep telling myself (I swear this is still true), the end goal of this project is to fully electrify Mikuvan with a Siemens 1PV5135 motor, Azure Dynamics DMOC645 inverter, and a stack o’ batteries from everyone’s favorite undead alphanumeric battery company. When I bought the van in non-running condition, this seemed like an immediate possibility; at the time, neither I nor anyone on the trip were auto mechanics, just your average Battlebots-buildin’, scooter-ridin’ hoodrats.

Well, now that it’s running just fine for some reason, that enthusiasm has been admittedly damped a bit. Taking it out of commission now to drop the engine and transmission out would mean potentially months of MITERS no longer being able to haul hundreds of pounds of shelving and materials on a whim:

We can’t have that, now, can we. But I’m a little too heavily invested parts-wise in this project to never let it see the light of day.

Here’s the trouble: There is a gap of about 1 mile between the shop with a 2-post lift and my actual, legitimate parking spot for this thing, with a rather steep garage entrance ramp in between. I can’t hog the lift or the patch of space underneath it for months on end while working it, and I would hate to ask for a tow or push from someone else every time it needs to move. If electrification started in earnest, there will definitely be a period of time when the vehicle will have absolutely no remote possibility of moving under its own power.

From the start, I pondered ways to real quick rig up a temporary electric drivetrain that could exist wholly independently of the vehicle and basically jam itself under it to move it gingerly around. Ideas were thrown around ranging from what basically amounted to a two-man push-assist made with welding wheelchair motors onto a stick, to hijacking the rear driveshaft directly and basically going parallel-hybrid. At times, the thought of seriously manufacturing a “car tractor”, like a smaller aircraft tug, marketed towards shops and yards was considered.

What I didn’t want it to become was a science project of its own. It had to be quick and dirty, existing just to scoot Mikuvan in the dark of night between shop and spot. It could move at 5mph for all I care – it had to go all of 1 mile, but it had to have enough torque to shove the whole thing up a roughly 20 degree slope.


I consulted the low-orbiting cruft cloud that is the N5x complex and came up with a few candidates for this job.

  • Basically gluing a power wheelchair to it. 10″ wheels, 24v motors upped to 36 volts, and basically 5 miles per hour it was. I had my doubts that the motors would even have enough thermal load capacity to make it that mile. It would definitely be easy. The downside? Not even theoretically enough torque to push the thing up the entrance ramp to my parking garage, and I won’t be able to get enough speed out of them to take a run at it either.
  • Eteks everywhere. Between all the electric vehicle shenanigan hotspots, there must be like five brushless Eteks (now known as Motenergy ME0907s). One would have been more than enough power, but it would require external gearing (slash chain drive). I also don’t have a brushless controller big enough to make this worthwhile.
  • Cap Kart-Van hybrid. The giant D&D sepex motor (Hey guys, how fucking hard is it to give me one damned web catalog with all your motors on it? What is this, 1993?) of the legendary Cap Kart was dismounted a while ago to be used as a dynamometer load by someone that said something about solar cars. Like the fate of many projects at MIT, it never got remounted, and has been sitting on a bench since. This thing, a D&D ES-101A-33 type, is pretty much capable of moving a Geo Metro or something independently, with a peak power capability probably north of 20kW.

Controllerwise, I mined up a working Alltrax DCX500 from the defunct Vehicle Design Summit group, whose materials have been slowly diffusing back into the building’s various tenants. Running at 48v and up to 500 amps and paired with the D&D motor would make a respectable power system on its own – and certainly one hell of an pushing attachment. Parallel hybrid is looking reeeeeal good right now. Needless to say, this combination, with its appeal to my sense of unnecessary overkill and having just the right amount of potential disaster, won the appraisal round handily. The power source would be taken care of by one of the prospective alphanumeric battery modules – we’re not talking Model S class driving range here.

I also scavenged back from MITERS one of my old 11″ (real) go-kart wheels which was going to make it onto the never-built Super LOLrioKart back in the day. At this rate, I might as well just hang Cap Kart, whose carcass is hiding in a corner, off the tailgate and be done with it.

I ran some quick numbers and found that the D&D motor would only have needed around 4:1 of gearing to shove Mikuvan straight out of the garage while pulling 500 amps. Unfortunately this would have also resulted in a go-kart-like speed of about 45mph once I was done with exiting. Appealing, but I would also like to avoid piloting something without power steering or braking at those speeds. An 8:1 reduction would cut the speed to around 25mph with the ability, given enough traction, of shoving Mikuvan straight up a wall. Now, 25mph is plenty to keep up with traffic and have nobody notice that something might be a tad off.


The next question was where to put this complication. For that, I turned to the underside where my spare tire was hiding:

Emphasis on was – the spare tire was basically the first thing I removed and disposed of since the rim was almost completely rusted out. Dismounting the tire and hanger uncovered this pristine area between two parallel frame rails in the back – the “#6 Cross member” and “Rear End Cross Member” according to the manual. These things are (as it turns out) monocoque construction but with a discrete frame structure, so it’s not totally unibody like modern minivans tend to be.

Here’s a better look from under the lift:

(It’s also the only spot on the underside that isn’t covered in filth.)

This spot seemed to be begging for a weird action movie attachment to be installed in it. It’s located very close to the rear axle, so I wouldn’t need to build in tons of compliance and “suspension” travel. It’s out of the way of the possible design and manufacturing exercise up front. And parallel frame rails.

The only downside I could see was that I might want to hide the Siemens motor in that spot some day, but I think by that point I’ll have a justifiable reason to leave it on the lift for a little while. That, or give it a nosewheel.

The dimensions were also pretty handsome:

The width between the rails was 15″, with another 10″ ahead of that before the rear differential bulb. The rail depth was about 3″ and the distance from the underside of the floorpan to the ground, with the vehicle parked on a level surface, is 18″. Width was pretty much arbitrary.

the mechanism

I spent a while musing about what kind of mechanism to mount everything with, and how to attach it to the frame. I didn’t want to weld anything in (making it permanent, at least from my traditionally welding-free building methods), and wanted to avoid drilling and bolting if at all possible.

Not knowing how strong the spot welds holding everything together actually are, I decided to pursue a jacking type of attachment. The structure of this device would push itself against the two frame rails hopefully with enough strength to resist the loads of the motor cranking on it. This was going to have to be a very strongly braced connection, since I’m basically mounting a fetal twin EV to the underside.

If it turned out that I was going to pop welds or bend sheet metal, I would just bail out to drilling and bolting using blind insert rivet nuts into the frame rails.

I began by hopping into Inventor and sketching out what would basically be going on:

I made the basic mechanism in a sketch, first using lines only (or just the essential “bones” of the mechanism), then fattening it up with representative motors and wheels. In this graphic, the big circle is the 11″ go-kart wheel and the smaller circle is the D&D motor.

At this point I’d basically settled on making most of this contraption from welded steel tubing. My usual modus faciendi is to waterjet-cut some plates and throw them together, but I’m guessing that the majority of fabrication on the final vehicle – motor mounts, battery boxes, additional structures, etc. – will be welded, manipulated steel sheet and plate joined to tubing, so what better than to practice?

The mechanism of raising and lowering is an extremely simple single-swingarm, almost like a motorcycle rear end, with what would be a “shock absorber” in a real vehicle application being an adjustable leadscrew. That way I can crank the wheel down and continue loading it against the ground to take weight off the rear axle.

And this mechanism in the lowered position.

With the basic mechanism loaded in my head, I started embodying it in 3D. This is the tube structure that will be welded up. 2″ square tube make up the swingarm, 1″ square and 1×3 rectangular make up the framework. All 1/8″ and 0.1″ wall – in other words, 1,000x more heavy duty than the van itself. I’m fine with that – this shit is cheap.

Using the 2D sketch mechanism info, I transferred mounting holes to the 3D model. The four holes are mounts for some beefy pillow blocks to hold the wheel driveshaft and the intermediate shaft needed to complete the 8:1 mechanism in two stages (I can’t achieve that in 1 stage without going to ridiculous sprocket sizes)

I’ve moved onto adding models of the D&D motor and wheel. The dimensions are obtained from calipering the real world items.

Added pillow block models and also one idea for performing the frame jacking. The pillow blocks are giant cast iron jobs from Surplus Center – maximum cheapness per bearing.

The jacks are giant turnbuckles used in reverse to provide compression force. But wait, aren’t turnbuckles only designed to add tension to a system? Yes, hence the hugeness. The long skinny sides of turnbuckles make them ill-suited to pushing against a load – they’d rather buckle apart. I figured that making them enormous would mitigate this issue for the clamping forces I’d need. These are 10,000 lb turnbuckles from McMaster, who fortunately pried a CAD model from the legacy U.S. company that is making them so I did not have to drop $70 to find out otherwise.

I wasn’t too set on the returnbuckle idea, but for the time being I settled on the rest of the mechanism and assumed a jacking method will exist.

Turning my attention to the leadscrew linkage, here’s some shots of the trunion design. The trunions will be made of some chopped up 1.5″ diameter steel tubing with welded endcaps. The nut in the center there is a standard 3/4″ Acme steel nut, the kind you use to hold steam valves together, and again purchasable on Surplus Center for a guava and two potatoes.

The underside is where it gets a little interesting. So here’s what’s going on – As the wheel contacts the ground, the blue spring will compress with every further turn of the leadscrew, adding “preload force” down on the wheel. If the wheel hits a pothole or something, or I drive off the entrance ramp, it can dip town and maintain traction, avoiding awkward fake burnouts.

If I need to crank the wheel back up, then the J-shaped hook applies pressure to the backside of the swingarm trunion (the long round tube in the center) and so the whole assembly can float back up. When the spring is compressed, the hook moves away from the trunion a little.

What this doesn’t do is add upwards compliance, say a speed bump or armadillo in the road (because Cambridge has a wild armadillo infestation issue – ask any long time resident). However, the path I intend to take is pretty free of obtuse bumps. If the wheel hits an obtuse obstacle, the forces should be transmitted handily into the ladder frame. Should be. Those insert nuts are looking delicious right about now.

I knew coming back to the previously handwaved mechanism would make me smack myself for even thinking of it. Here is a new jack design made only of welded tube, threaded rod, and nuts. $150 cheaper and probably less shady. The forward (right side in the image) bar is free to move in and out of the tubes, kept from moving in only by the two nuts jamming against the tubes. If I need to expand the width, then I just crank on the two nuts.

This design was frozen after a few days of not looking at it, during which I instead watched the Singaporean students try to design kart drivetrains using 4,000 RPM/V motors. Which, mind you, is totally possible if you don’t mind using a 100:1 gearbox or something, but your handling could suffer.

construction begin

Here’s the pile of big parts as of last week. Motor, sprockets, bearings, a bunch of related hardware…

…and this pile of steel, primarily foraged but also ordered from Speedy Metals. The huge shafting, in 3/4″ and 1 1/4″ sizes, came from Surplus Center to match the bearings.

Why such huge shafting? It’s because as it turns out, 1 1/4″ is a standard American go-kart wheel axle diameter. I found a cheap hub on eBay which matched the wheel perfectly and converted it to a 1 1/4″ shaft.

I’m guessing the 32mm standard size is the Irritatingly Close But No metric size for the same application.

I also tried something a little different sprocket-wise this time. I normally waterjet my own sprocket profiles, but with the assemble-from-COTS-parts mantra of this build, adapting them to the drive shafting would have meant that custom flat plate sprockets were pointless. Instead, why not buy commercial flat plate sprockets? From Surplus Center, large sprockets get cheaper as you move to these “welded hub” versions. For $20, you can basically have any sprocket size and hub bore/feature combination. The final output sprocket, of 50 teeth, gets the huge 1 1/4″ keyed bore, and the smaller intermediate sprocket will ride on a 3/4″ keyed shaft.

I’m going to spare the welders the pain of seeing my handiwork, but let’s just say that “MIG-over-TIG” was an acceptable ditch plan. It’s often said that in TIG welding, the best welds look like a stack of coins. Mine look somewhere closer to a stack of rabbit droppings (Part of the problem, as I remembered/was reminded, was that I was trying to weld these sections using a 100 amp TIG welder and a tungsten too small to even take that current).

With the parts buffered and ready, it’s time to attack the structure itself. There’s much welding metallic gluing ahead; the next post will focus on the construction of the structure and machining all the little round things that go into it.

In typical fashion, I spent a few minutes thinking of ways to name it as close to an Internet meme as possible, and the result is Detachable Electric Rear Powerdrive , or DERPDrive for short. I wish everyone the best while facepalming.

Also, I found a nice sample of first Legendary Derpy Van, the Toyota Van, while cruising through Cambridge back streets avoiding traffic one day. If only vans were like dogs or guinea pigs.