Archive for the 'Project Build Reports' Category


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

Jul 16, 2013 in Bots, colsonbot, derpdrive, Melon-scooter 2, mikuvan, Project Build Reports

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!



Melonscooter 2 Epic Post

Jul 07, 2013 in Melon-scooter 2

Much work has been done on Melonscooter 2 since the last week’s update. In fact, it’s so much that I’m wondering if I should split it up into two posts or not. Since that would be against the tradition of my style of build reports, here; have like 40 pictures. Melonscooter2 is in a state where I essentially just have to put together the electrical system – assembling the battery pack, mounting the controllers, and then wiring everything up.

Last time I left off with the new rear “stern deck” just having been cut from 12 gauge steel and origami’d into shape. It was time to hit up the welders once more.

First step is to atone for my sheet metal sins via clamps. I’ll admit my sheet metal technique isn’t perfect, and that the equipment (an old enormous 10 gauge capacity box and pan type) is pretty clapped out. So, to make the tabs fit into the slots, out come the bar clamps.

After tacking the corners, I removed the clamps and proceeded to scientifically draw metal slugs using a MIG welder.

Next was rigging up the deck on the scooter frame. I had to make absolutely sure it was welded on straight – there’s of course no turning back after a certain point. I again used almost all the clamps in the shop and adjusted things little by little, albeit still visually, for alignment. The rear fork plates are actually not parallel (which could arise from manufacturing or the thing having been crashed at least once in its past life), so I was mostly relying on the existing tube frame.

After initial adjustments and tacking, it was time for some more steel loogies. I welded entirely around the outside as well as on the two sides on the interior.

Next, I turned the frame over and removed the existing starter battery box. It isn’t anywhere big enough to house an actual traction battery, so I would have to devise a custom solution.

The finished stern deck with wheel mounted once again. I had intended on “filling” the fold slots with weld material, but determined that it was pretty unnecessary.

I have a roughly 2.5″ tall space to put a battery box in before the ground clearance becomes too low to be worthwhile. Melonscooter’s lowest point was about 1.25″ fully loaded, which I used as a benchmark. I spent a little while thinking of the approach I wanted to take – custom folded steel box, use an existing steel chassis or some sort, or Landbearshark style waterjet-cut polycarbonate box. Speaking of batteries, I haven’t introduced them yet, have I?

This is a veritable cluterfuck of A123 B456 what letters are they on now?! 32157 type automotive-grade cells. They’re 9Ah apiece and will be arranged 12S, for basically the same capacity as Melonscooter’s former 12S4P A123 pack made of the 26650 type cells. I can’t reuse the Melonscooter pack, which is just fine and functional, because of the height of the cells impacting ground clearance. The cells will be arranged 12 in a row, necessitating quite a battery box that will almost be in line with where the deck ends with the front. I’m planning on equipping this row with some 1010b-compatible cell taps, but I’m also awaiting a shady BMS shipment from my new favorite sketchy e-bike parts store, eLifeBike. We’ll see how that works out first – if I can embed that board inside, then I can even obviate old Melonscooter’s once-per-semester cell checkup.

I decided to build a waterjeted polycarbonate box over folding up a new steel box. Initially, I had even looked around in area military surplus stores to see if any old ammo cases would fit my application. Unfortunately, the only boxes I could find were for 7.62 NATO rounds, which were too long (i.e. tall in the orientation I would need to use the box in). Whipping up a sheet steel box was hampered by the lack of reasonable steel sheets on-hand – I could go out and get several 22 and 24 gauge sheets from the likes of Home Depot, but that’s pretty thin. I’d also need to make a lip with a removable lid. Making up a custom RP’d box was in fact the path of least resistance since I had a spare plate of 1/4″ tinted polycarbonate left over from some robot project that never happened (or perhaps did not happen hard enough). I elected to skip this session of sheet metal lab for now.

In my How To Kinda-ish Maybe Build Everything Instructable, I have a page on “making boxes” – which is something done often with laser cutters and their ilk. So here’s how I went about making this box. Regarding “edge precedence” in the chapter/page, it’s which side I wanted to be able to remove/install the quickest. You could imagine (and be correct in doing so) that I would want to remove the bottom for servicing the battery. However, I decided that having a material-on-material interface for the top and bottom would be the best, since this thing will most likely be shaken violently up-and-down when it attacks the “meh, it’s not a sinkhole yet” roads of the local area, and that the only thing holding in the batteries being a few #4 scres would not be the best scenario.

So I made the two endcaps the “highest precedence” components – to remove the battery itself, I’d have to at least take off one endcap. It would make it less serviceable, but if I am removing the battery that often, something’s very wrong.

What’s that notch in the corner? It’s to clear the kickstand. I could have made a totally rectangular box for easiness, but I knew that other wiring components – distribution, charging ports, switches, etc. will have to go in too, lest they be haphazardly arranged external to this. So one side is about 2″ longer to hug the kickstand and house these parts.

Getting more together now. When I make these kinds of boxes, I make the solid shapes first, then decide which things to tab into each other. The cell models are now in, and I’ve also made a cutout for the Hella battery switch.

Continuing the box design, basically all the corners are accounted for.

And the last two pieces are in. The round hole will be where I mount an actual charging jack, probably made of an embedded Deans or XT-60 connector like on the other vehicles, in a small printed carrier. On old Melonscooter, I had to disconnect the battery itself to charge it. This is just one little layer of refinement.

I made a prototype out of laser-cut plywood just to test for dimensional sanity. Result: Satisfaction. The battery box stops at where the deck stops up front, and while it isn’t nice and curvy, at least it doesn’t stick way out. I moved a few things such as the charger jack around so it wasn’t obstructed by the kickstand as much. Four little ears pop up from the top of the side rails and keep the battery box sitting flat with respect to the frame. I made it this way so it’s easier to fixture the future welded mounts.

I tried to take one shortcut in making the battery box – trying to laser cut it from PETG plastic.

This resulted in what must have been  the most dismal failure I’ve ever generated on a laser cutter. PETG is often advertised as “halfway between acrylic and polycarbonate” – unlike polycarb, it can be laser cut, but not as cleanly as acrylic. And it’s not as shattery as acrylic, but not as strong as polycarb. Well, it also melts, smells like death, and turns yellow halfway as shitacularly as polycarbonate, and takes far more energy to melt than acrylic. It’s not that it wouldn’t laser cut – it just laser-cuts like total unshaven ass. And I suppose instead of smelling like death, it smells like terminal cancer or diabetes.

It doesn’t help that the Epilog 36EXT has an almost-useless gas assist system – instead of, say, a cone over the lens that focuses pressured air into a single stream, it just has a derpy little bent steel tube that kind of puffs on the cut. So, it couldn’t really clear the melted PETG material from its own cut. If I went slow enough that it cut through the first pass, then the melted kerf becomes enormous.

I ended up having to hammer a few pieces out anyway, before just totally writing off this sheet. Luckily, it was a leftover of a previous class run in the IDC space that I’ve been hiding, so I didn’t actually have to spend money on this wreck.

PETG. Not even once. (At least, not without a laser that has a high pressure gas nozzle….)

The battery box waterjet-cut from tinted 1/4″ polycarbonate. That’s much better!

To mount the battery box, I cut up some random steel strap things which were made of 1/8″ thick, roughly 1.25″ wide steel. I literally do not know what these were – they were found in a scraps bin at MITERS.

This was the pilot application of the all-new cold saw I commissioned for the IDC fabrication space.

The steel mounts will each have a hole drilled into them to mate with the battery, and the rest will be welded to the frame. The battery box will help jig up the mounts so all I need to do is tighten them vertically and clamp the whole assembly to the frame.

Here’s the battery box mount prepared after drilling and finishing.

And it’ll go like so!

Back to the welding room for some very quick beads. I clamped the box such that the plates were in position in order to tack them once. Then, to prevent melting the battery box, I’d remove it and finish the welds.

Tacked in place and battery box removed..

…and a fat MIG slug deposited onto each side. That does it for the mounts – this is all they are.

On the same waterjet run that yielded the battery box, I also took the chance to cut out new a drive pulley for the rear wheel. The X-Treme scooter came stock with a weird 8mm pitch chain that nobody uses anywhere except on derpy scooters. Favoring HTD timing belt drive, it was clear that I was going to have to replace this.

The bigger wheel is a double-ended drill bit, a modern cousin to the double edged sword. On the one hand, it’s easier to achieve a higher gear ratio for the same motor speed since the output stage pulley/sprocket/gear can be larger and not hit the ground while turning, but the fact that the wheel itself is larger mostly negates this – your ground speed is theoretically unaffected. What the bigger output stage pulley allows me to do if I wanted to keep the same gear ratio is to use a larger motor-side drive element. This has the upside of lessening the tension in the belt and bending it far less (the curve of a larger pulley is more gradual), and lessening the load per tooth since there are more teeth in contact in the angle of wrap.

Small pulleys and sprockets wear out their belts and chains much quicker because of the increased material flex and decreased tooth contact. Melonscooter was known for going through a drive belt every few months just from them becoming tattered and separating from their rubber backings and breaking the tension elements – these weren’t cheap unbranded belts we were talking about either, it was Gates belts straight from McMaster. I was using a 15 tooth pulley on the motor to transmit north of 1500W most of the time.

With this wheel, I should be able to retain my top speed while using a much larger 22 tooth motor pulley which will have nearly double the number of teeth in contact. I hope to get more belt life this time.  The motor side pulley will be a stock one I have sitting around from playing with gear ratios in the past.

The wheel side pulley, though, is something a little weirder. Notice that it’s made of chunks of pulleys. To save material, I split the outer profile of the pulley into 120 degree arcs, fastened to each other by a ridiculous number of bolts. This was a technique I tried out first last year on a Silly Media Lab Vehicle, and it worked very well. The nice thing about this method is you can quickly build up thick dished pulleys and other elements with rings, without going through 8 plates of metal and generating lots of thermally conductive round pot and dish coasters at the end (hence ruining the point of a coaster).

The critical part of doing this right is overlapping the segments on each successive layer such that there’s not a “parting line”, which would occur if the segments were just stacked one on top of another. That’s why there’s a million bolts around the edge – so I can shift each layer like 60 degrees. In the end, when everything is tight, all the material overlap will approximate a solid pulley to a degree more than what I set out to accomplish.

Here’s the pulley installed on the former sprocket perch.

I actually generated this pulley profile with a custom template part that I made in Autodesk Inventor because of two major reasons: first, nobody seems to make a CAD program that comes with a HTD belt generator. I can’ tell if it’s because HTD is a private brand or what (I, at least, use it to refer to every pulley that has rounded tooth profiles). Inventor has a “T” metric belt line which seems to be an ancient metric trapezoidal tooth profile. I even tried Solidworks (Oh boy, using the CAD program I’m supposed to teach to people…) and it, too, has tons of English belts but only metric T belts.

And second, nobody seems to commercially make a HTD pulley this big. The largest downloadable ones I found were about 70 teeth. My pulley is 108 teeth…

So I grabbed an image of the HTD belt cog profile and made a parametric part in Inventor. With a bit of nozzle offset magic, the belt wraps with no problems all the way around.

What’s left after welding everything that needs to be welded and making the wheel driveable again? Painting!

I’ve never been a big painter or finisher, but if I left the bare metal and welds untreated, sooner or later I’m just going to be riding a small hill of oxidation again. To paint over everything, I cleaned up all the surfaces and used a self-etching primer first on the bare spots. Unaffected paint spots near the welds were hit with a fine grit sandpaper befoerhand to encourage sticking.

Next up were a few coats of black acrylic enamel.

And finally a clearcoat. I have yet to master the art of spraypainting without the “orange peel effect” – a finely textured surface resulting from uneven spray thickness, droplet sizes, time-between-coats, etc. In this application, I don’t really care, but I would of course prefer not to generate it on the van.

It’s bad karma to paint indoors, so I did it the best way possible – right next to the 300CFM laser cutter ventilation fan. Outside that day was approaching 80% humidity – I felt like almost drowning just walking around outside, and my paint would have stayed wet for the next 3 years. The ventilator kept the funky smell from spreading to any other room.

After the paint fully cures, it’s time to start putting things together. First, I still need to assemble the battery pack itself, BMS or otherwise – given that I might not receive the BMS shipment for another week, I might just pitch together some JST connectors for my balance charger. Next, the slated controller, a KBS48121, needs to be mounted. Yes, this does entail putting sensors on the old Melonscooter C8085 melon motor (which I have since re-bearinged, so it should stop sounding like a sandblaster while running!)


DERPDrive: Structural Fabrication

Jul 02, 2013 in derpdrive

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.

The Successor to Melonscooter

Jun 30, 2013 in Melon-scooter 2, Project Build Reports

Ah, Melonscooter.

The unsung hero of my vehicle fleet, it’s the one that actually works most of the time and which putzes me back and forth day to day. It’s a mish-mash of unmatched parts and on-the-spot engineering built on top of a scrapped commercial scooter frame, which has been operational with only a few small service gaps since 2010. Like any vehicle that is more tool than project, it’s also been slowly backsliding partswise as things wear out and I replace it with whatever the hell I happened to have within arm’s reach. For instance, this past few months has seen it devolve from the melon (C80 motor) to an SK3 59mm motor as I realized the bearings on the Melon were slowly becoming powder:

Then, one day in February while pushing through leftover snow, the timing belt disintegrated again – Melonscooter has never seen very good belt life due to me using the HTD belts at way above their safe rated power – and this time I had no spares. So naturally, I devolve even further to chain drive:

The wheel and chain were spare items purchased for that term’s 2.00gokart class.

About the same time, the front of the frame near the steering neck broke a weld and developed a crack. I rode it like this for a while, being extra careful to not put too much load on the joint, and eventually welded it back.

Sadly, Melonscooter finally succumbed to a combination of poor Chinese metallurgy, years of New England road salt, and probably my welding. One day in May, I thought the ride was getting a bit jiggly for some reason. As I pulled onto a sidewalk to investigate, this happened:

Well then. This isn’t very productive.

I’ve been swearing to rebuild Melonscooter on and off for the past year or more, but like a certain other somewhat rusty vehicle is now, it’s just continued working. The entire joint area appeared to have rusted out – I didn’t see it only because it was well covered in paint. The failure propagated clearly from a crack (though not through where I welded).

By this point, the top plate was also cracking from sun exposure, the headset bearings had lost their cages and shields, and I already had another giant hole in the battery box (from going over some sidewalk too excitedly a long time ago) which was becoming its own rust problem.

So I took this as a sign that I needed to move on. While 2.00gokart was wrapping up, I haunted the ever-powerful Craigslist for a cheap donor scooter, electric or otherwise. A week later, I found this thing:

What is it?! A blurry dark picture in someone’s garage and the tagline “scooter $50″ or something to that effect led me, on one of the very first long-distance Mikuvan missions, to this thing. It’s an “X-treme XG-505“, which demonstrates that convoluted alphanumeric naming is not exclusive to the domain of motorcycle manufacturers. Either way, it’s beefy as hell – the steering neck joint on this thing will clearly never fail in the same way. The giant deck is solid 3/16″ aluminum, and the frame is 1/8″ steel plates and what looks like .075 wall steel tubing. It also has dual disk brakes.

Basically, a great (if huge) candidate for chopping up. It was also not running, with engine trouble.

This sounded entirely too familiar, so at the risk of betraying my electric brotherhood, I immediately began tearing the engine off and stripping the frame, readying it for electrification appraisal.

While tearing down the electrical system consisting of a starter battery and related circuitry, I discovered the world that is scooter starting solenoids. They’re basically miniature contactors (slash high current relays), and could be useful if you need something between a 30-40A automotive relay and a 300+A full size contactor.

Starting solenoid also means electric start. What I also found is that the 49-50cc engine kingdom can be found with electric starts that are pretty damn cool – 4 brushed, neodymium magnets, basically like tiny Magmotors Ampflows. The downside is they are often shaftless, being designed to mate to the engine crankshaft.

Still, I do want to mess with these and see what they’re capable of.  I was going to tear this engine down further to extract the electric start, but someone else already called dibs on it so I didn’t want to leave it permanently disfigured.

If you want to mess with these parts, I’ve basically found then using combinations of search terms like “49cc 50cc electric start” or “49cc 50cc solenoid” or similar.

The teardown continues! Did I mention this thing is enormous? The wheelbase is a good 4 or 5 inches longer than melonscooter, which is already pretty long to begin with. If I want it to fit in the same places as before, I’ll need to do some creative basket mounting. I am, of course, designing a basket into this one from the very start.

The last thing to do before I could crack my CAD knuckles was to remove the engine mount. This was done with some selective angle grinding. The engine mount actually stuck through on both sides of the deck’s rear portion; the part on the bottom I couldn’t reach with a cutting wheel, so I literally had to grind the entire thing down to flat – as the result photo shows.

With the ugly metal pectoral fin gone, I began to size up the new “stern deck”, characteristic of Melonscooter, that would also mount the motor and be a splash guard and milk crate holder. The frame’s rear forks are just vertical 1/8″ steel plates with a flat plate in between, but it was very much the wrong width to reuse the old aluminum one from Melonscooter.

While it would have been simple enough to remake the aluminum deck in the correct width, I decided to make a bent sheet metal housing instead. I figured as long as I was most likely going to need to make sheet metal structures for a certain future electric derpy van (temporary or otherwise), I should get some practice. Fast forward to after I was completely done with being occupied by 2.00gokart:

I hopped into the Inventor sheet metal mini-game and, using some crude measurements made with a tape measure and straightedges, whipped up this protoform stern deck. Now, when I say sheet metal, I actually meant 12 gauge (0.1″) steel.  Yes, this is some serious sheet metal. It is, however, the same thickness as the rest of the plate steel on this frame, there’s no additional frame that will be on the underside (e.g. it’s entirely structural), and there could be a few dozen pounds bouncing in a milk crate suspended off the back sooner or layer, so it not only had to be structural to mount the motor, but also rigid in bending as a result.

I think I could have gotten away with 16 gauge (roughly .060), but my intuition is calibrated to working with aluminum and I sure as hell wasn’t going to put 1/16″ aluminum in this application. And I had a 24 x 24 plate of it already chilling in the shop. The other reason was that I could easily mock this up with some 0.1″ thick paperboard stock I also had. This was someone’s laser cutter feedstock, but they left an entire 3 x 2 foot panel of the stuff in the lab.

So using my crude measurement model, I made a paper version and tried it on the frame:

This crude-model-to-cheesy-prototype stage showed me what dimensions were sane, what needed a little bit of changing, and what was totally off base. It turned out the major dimensions were reasonable, so I only had to adjust here and there for looks and clearances.

Back to the CAD world to add mounting features for the motor. The two holes up top are for an eventual basket clamp I have in mind. Those ventilation grates will make sure some airflow is directed over the motor, so it’s not just spinning inside a hollow box.

This little piece adds some more rigidity to the very rear of the deck and also functions as the other component of my eventual basket lock.

Adding some lightening to the other side of the deck which doesn’t need to hold the motor. All of this will be waterjet cut, bent into shape, and then welded.

Like so. Notice the additional slots and edge nibbles that are on the folding edges. Inventor automatically “unfolds” the metal to a 2D cuttable form for me, but I wanted an easier time bending the steel, so I added the slots right on the edge of where the bend is supposed to start to “encourage” the metal there. The triangular nibbles tell me to align them exactly with the teeth of the bending brake.

I used the Giant Brake of Certain Tibial Fracturing in the FSAE/Solar Car machine shop to push this metal around. The machine is pretty clapped out and uneven, so I stuck to one side as best as possible. If I find myself needing more intense sheet metal fab, I might take a stab at repairing this thing too. For now, push-lever-metal-go-bendy is enough.

And the final piece fitted onto the rear with some clear tape. I accidentally left the teeth too far away from the edge of the brake for the last fold – the one with the grille on it. That’s why it’s inset a little from the edge of the side flanges. Fortunately, it was not a disastrous mistake.

Up next: Actually welding this sucker to the frame, and where the hell am I going to put the batteries? To also feature an extensive DERPDRIVE update, since it, too, involves tons of metallic hot gluing.

A Mikuvan Subproject: Operation DERPDrive

Jun 25, 2013 in derpdrive, mikuvan

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.