With the semester winding down (or, perhaps, finally ramping up!), many of the 2.00gokarts are in the process of being wired up and tested. The final product is due next week, and our competition (last year’s video)  is on May 5th!

Some of the students have been industrious and scheduled their checkoffs and inspections early. Here’s a preview of the action that will unfold in a much larger space next week:

Because conventional controls and riding postures are for wussies, apparently. I’m both amused and somewhat terrified at the prospect of there being three (out of eight) karts in which you ride head first. As it was my stated mission to not interfere much with the design and construction of the karts to let students experience as much of the design process, I might have to start padding BurnoutChibi and run interception for wayward karts.

Speaking of which…

Here’s a picture of the aftermath of BurnoutChibi’s motor detonation. As I would later find out, the sparks seen in the video were not the magnets grinding on the can, but rather them cutting up the phase wires.

Here’s a better picture of the ownage. The red wire, in particular, was cut almost all the way through. The annoying thing about this is that the wires were so close to the stator. If they were further out, patching would be a simpler job. I’d have to loosen the epoxy holding the wire stubs in place and also trim the heat shrink selectively.

While I await better motors, i decided to try and repair these. First step was to pop them open. There is a front retaining ring that comes out, then 2 set screws loosen up to free the shaft from the rotor. Then it’s a matter of pushing the shaft out to the right in the picture – this step was done on an arbor press.

Ouch. In total, five magnets broke loose. I figure this must have been a chain reaction where one magnet ditched first, and the resultant imbalance caused can deformations which broke the rest loose.

This is why I recommend motors that have “rotor bearings” or “skirt bearings” to everyone who asks me about them for vehicle apps. Even though it adds a little drag, the distal end of the can is properly supported on its own bearing. The only exception is if the motor is very short, like a more “pancake” style design.

I mixed up a generous dose of long-cure epoxy with glass microsphere (microballoon) filler, to slightly under nutella-like consistency. The offending magnets were pried out, the mating surfaces cleaned, then this epoxy smeared into the new joint. I replaced the magnets and used as much of the remaining epoxy as possible to completely fill in the gaps between them.

Evidently, I didn’t add enough microballoons, as the mixture did sag a little. To keep the cure symmetric, I actually chucked this thing into Tinylathe and ran it on a very low speed for several hours.

After the mixture was firm (but not cured), I set it on a radiator to cure with heat. Luckily for me, the radiators in the building were still on; they were switched off successively as recently as 2 days ago!

I didn’t get a good picture of the wiring repair before, but it basically involved exactly what I described before – carefully scraping away the heat shrink tubing to expose as much wire as possible. The wire was actually all magnet wire, so it would have been difficult to solder. To combat this, I “frayed” each lead as much as possible to expose the maximum amount of magnet wire surface area. Then I cranked the 80W soldering iron up all the way to 850 fahrenheit and literally burned away the enamel by embedding the frayed ends in a big ball of solder for heat transfer.

I think I managed to get back 75% of the red lead. The rest were patched similarly, but did not need as drastic soldering measures.

After the real epoxy fully cured, I reassembled the motor and crammed it back into the left side transmission.

I have yet to ditch a single magnet. Though I figure it’s only a matter of time before the right side lets go…

And with that, BurnoutChibi is ready to lasso its rogue… brethren? Bastard children? Offspring conceived via assistive reproduction technologies? Something. The only thing it does not do very well, sadly, is burnouts! Because the rider weight is basically square in the frame, and is up so high, it really just like to drag the front wheels along even if I’m holding the brakes. The same reason contributes to its severe power understeer (and associated lift-off oversteer!) behavior. Oh well…

In the previous week of work on BurnoutChibi, I’ve fully completed the vehicle but have yet to get it out to really test. This thing really is too damned fast for our indoor.. uhh, test track. A motor quality issue also prevented me from blasting it around in our usual outdoor venue (for very long, anyway). These issues have since been addressed, so it’s almost time for more test video!

As previously discussed, BurnoutChibi is a refit of the derelict Chibikart1 frame into something a little more hair-raising, as if Chibikart 1 wasn’t bad enough already. Since the last update where I had just finished reconnecting the steering, I’ve finished mounting the braking system, the transmission shifter cables and linkages, and also completed electrical hookup. At the behest of some of my students, I completed it in time for CPW last weekend, though the aforementioned motor problem meant it was not out scaring parents and wide-eyed potential freshmen.

Here’s the story in the pictorial form.

I began with a little aside in order to solve the problem of how to mount the two “Sand Castle” controllers. They have no mounting flanges and both sides are made of heat sinks, so just gluing it to a plate would make for some pretty poor thermal design. I decided to come up with a “cradle” that held the two controllers right under a fan for some forced convection  cooling. The fan I selected was out of my plentiful stock of 80mm LED case fans.

This design was an exercise in designing a snap fit for 3d printing. While I could have made the base a little wider and added some through-holes to hold the two halves together, I decided to get creative and dovetail each corner post together. The angle is extremely steep – about 85 degrees – so the whole assembly could be pulled out with force, but otherwise snaps into place cleanly.

…and it’s printed out of PLA.

Yeah, so what if it’s going to melt at about 60 celsius? It’ll just smell like delicious waffles while the ESCs burn.

I decided to try the “translucent light blue” PLA which is sold commonly, and I must say it’s my favorite PLA color so far. It’s not the vaguely jaundiced-rainwater color of natural PLA, and I also don’t like solid color PLA. A tinge of blue helps, but is not overwhelming and makes me think it’s some real plastic.

Putting together some of the electrical deck and testing the fit of the ESCs. Result: pretty perfect!

I set aside the e-deck for a while to return to the transmission and drivetrain.

First order of business is to attach the sprockets to the wheels. This basically entailed making four standoffs which acted as the lug nuts (M6 thread) on one side, and regular 1/4″-20 on the other side. The standoffs hold the sprockets a set distance from the wheel so the chain clears the tires, and also holds them concentric.

Or so I hoped.

There is practically nothing concentric or wobble-free about these shitty caster wheels. I had picked them up since they’re $10 each, but I swear not even Harbor Freight wheels are this bad. While the sprocket seemed to have minimal runout (radial misalignment), the wobble from the poorly stamped wheel rims was incredible.

I literally had to take a dial indicator to the sprocket and hammer on the wheel rims to bend them around. I got most of the axial wobble out of the sprocket this way, but this meant it all ended up in the wheels themselves, which now are a bit “googly-eyed” as a result. It will look hilarious when running.

With all wheels mounted, the frame could finally support weight. It’s definitely lost the Chibikart look a little since it’s so far off the ground (in comparison…). I have an incredible 2.5″ of ground clearance now.

The brake pedal hookup was the exact same as for DPRC. This pedal design doesn’t have a spring return on the pedal side since it is handled by the built-in spring elements in the brakes themselves.

Which, as it turned out, weren’t quite strong enough, so the pedal felt quite mushy and also did not return all the way. I added a long compression spring on each side between the cable stops and the brake lever, and this made the pedal feel much more positive. The brake cables sit in barrel adjusters so the balance could be finely tuned.

Shifting to the back again, I’ve appended the Vex sprockets to the Vex transmission’s VHex output shafts. The Vex sprockets didn’t come with any set screws or other means of axial retention, so for a quick fix, I drilled and threaded three #10-32 screws 120 degrees apart. The three set screws will offer way more retaining power than just one. I decided to forego any other spacers and shaft end-tap screws for now.

 

Here’s a view of the shifter linkage. The mechanism is a spring-balanced cable setup where I provide the pull to shift into 2nd gear, and the spring pushes the shifter back into first.

This was simple enough, but I chose springs which were way too strong initially. I figured “10 pounds of force” at max deflection was enough, but that translated through the cable into the shift lever, times two, meant it was just too hard to throw!

I went to a hardware store and bought several sizes of springs in roughly the same length that were much ‘softer’. The replacement spring is about half the spring rate, and was also too long in that it could not compress enough. The solution to that was to really quickly dremel a few loops off the spring, just  like a good ricer. The shifter now has a positive click as the ball detents lock into place.

Once that affair was taken care of, I routed the chain and moved the gearbox up to tension it (the “goalposts” having slotted mounting holes for this reason). To lock the gearbox in place, I simply tightened the…

… Oh, I can’t reach those bottom socket screws.

Must have bought those hex headed screws for a reason! I was wondering briefly where they were supposed to go on this thing. With the hex heads accessible with a regular wrench, now I could actually tighten the drive up.

With both transmissions hooked up, I spent some time getting pushed around synchronizing the cables. I put another set of barrel adjusters on the shifter cables so they could be adjusted as needed.

What I (not surprisingly) discovered during this push testing is that the brake shimmy is pretty severe. This is caused by combination of factors, two of which include my “kinematically suboptimal” rotor retention method (two screws across a diameter) as well as the complete non-concentricity of the wheels. To reduce the severity of the effect, I had to dial the cables to different tensions. The braking is still effective, but it definitely feels like it’s trying to jerk all over the place.

Ultimately, I’m likely to ditch these drums and go to a disk brake setup with its own guide bearing on the front spindles to maintain concentricity. But for now…

…back to the electronics deck. Here’s the wiring mostly in place with batteries mounted. The batteries are my old 5Ah, 10S sticks. Two of them.

The batteries are secured by Velcro ties and sandwiched between two rigid plastic panels (the baseplate on one side, a 1/4″ thick polycarbonate strip on the other). A 1/8″ silicone rubber pad sits below each battery for shock absorption and more impact protection. Combined, this ought to ensure the batteries don’t move anywhere.

The ESC power leads directly into a 150A fuse junction, and ground has its own big brass distribution block also. Overall, this is the beefiest power system I’ve built since probably LOLrioKart.

At the point, the frame was flipped over for installation of the power electronics deck. The rest of the wiring, including connections to the motors and to the main switch, happened in-place after the installation.

The long run to the power switch is doubled-up 12 gauge wire in each direction.

The only other power side wiring was to make one motor extension cable. With main power wiring completed, I quickly hooked up a HV BEC to provide 5V and a servo tester to convert the foot pedal’s analog 1 to 4 volt output to servo pulses. These two components were heat shrunk and sealed, then attached with Velcro to the top of one of the battery pack plates. The signal electronics for this thing are extremely basic – no fancy signal processing is occurring.  One thing that could happen with this system in the future is converting to electronic shifting, such as with solenoids, upon which I think a system which cuts throttle before the shift and slowly brings it back in would be helpful.

After confirming the functionality of the ESCs and calibrating the controllers, the whole rig is put together.

Here is BurnoutChibi posed next to DPRC! The wheelbases for both vehicles are the same, but BC has a slightly wider track because of the pneumatic wheels. Otherwise, they handle alike and are mututally just as difficult to sit in.

testing

The first few test runs of BurnoutChibi were done indoors, in our Conveniently Circular Building hallway. Due to the extreme acceleration ability of the vehicle, I couldn’t really test it any faster than DPRC or original Chibikart, so we decided to not take video. More testing commenced in an underground garage, then our usual spiral parking garage haunting ground. Unfortunately, I really only got a minute or two of hard driving in before the left motor threw several magnets.

The high speed of the motor caused some serious sparking as the loose magnets scraped the stator and also cut up the motor leads. Unfortunately, the only video that was taken was not focused properly…

The accomplice vehicle is the (still unnamed) tricycle.

Since that test, I’ve reglued the magnets and repaired the wiring, and BC is currently operational. I am currently waiting for a day in Boston / Cambridge when all hell is not breaking loose (in fact, as I write this) to test in the garage again. These pictures and videos will be uploaded when they are taken.

In the past week, I’ve been managing to intersperse bits of BurnoutChibi work between hosting extra hours for the 2.00gokart students as they edge ever more towards completion. On Wednesday, the “Milestone 7″ mechanical inspection occurred, where everyone had to demonstrate their rolling frames with steering and braking. The next steps for the students from here are focused entirely on assembling their electrical system. In fact, two teams have already blitzed their vehicles to completion, and more are surely to follow (parading them around during CPW is a huge motivator). I’m going to make a separate post about the progress of the class later – all I can say for right now is that this year’s competition is going to be awesome.

The first thing I had to do to build a new Chibikart is to disassemble the old Chibikart. Here’s the scene of the crime:

This work left me with a pile of redundant electricals – namely 4 more Jasontrollers and the massive A123 B456 battery. Needless to say, these will probably find their way into some other silly rideable thing.

The plan for BurnoutChibi’s electrical system is actually to use my left over 10S 5Ah lithium polymer packs, instead of making a custom pack or keeping the A123 pack. I decided to this mostly for the power and energy density of the lithium polymer packs (Chibikart 1 weighed 53 pounds because the big A123 bus battery module weighs almost 20!)  as well as the simple fact that said lipo packs have been sitting for almost 2 years, and I really don’t want to see them go to waste. The lipos themselves are from the erstwhile Deathcopter, so BurnoutChibi will surely be the health and well being hazard I envisioned it to be.

The first appendage to the old frame is the new style brake pedal. At this point, I haven’t even removed the old steering linkage yet, but I wanted to see if it would interfere with the new position of said linkage.

I started from the rear with fitting the Vex Ball-shitter transmissions onto the “goalpost” mounts. This whole ‘rebuild’ is essentially replacing Chibikart 1 frame plates with specially crafted DPRC ones. The only difference between this rear corner and DPRC’s is the goalposts!

I focused on getting the motors mounted and the rear end together. Here, I’ve mounted the NTM motors to my NTM-to-CIM converter plates. Eliminating units, the result of this evaluation is something which is basically like a CIM, but 4 times more power dense.

There’s only one problem. The NTM shafts need to have a 2mm keyway cut into them so I can easily used the keyed bore supplied with most FIRST OEM parts such as the Vex transmissions (The fact that I can say “FIRST OEM” is unsettling).

As it turned out, these shafts are casehardened. Wow, Hobbyking, you’re classy now – what this meant was I could not use my single HSS 2mm endmill to machine the slot. Instead, I went on eBay a few weeks ago and bought some 2mm solid carbide endmills. I recommend keeping a set of carbide cutters around for dealing with troublesome materials; the downside, of course, is that they are more brittle and need a stiffer machine setup.

I faced the slight issue of the endmill being too short and the Bridgeport spindle being too fat to reach the nether regions of the  motor. So I did what any self-professed machinist wouldn’t do, and chucked it up in a drill chuck. In my defense, I bought this integral-shank keyless chuck just to do dumb things like this.

I cut the keyway just a little short of actual dimensions because the NTM shafts were not long enough to use the included retaining ring with the gears. So I had to press the key in,and will need some creative gear pulling if I ever wanted to remove these gears.

And here they are mounted. I found the sheer number of hexagonal sockets on the gearcases a bit confusing at first, but now appreciate how versatile they can be.  Chain tension is adjustable using the slightly slotted mounting holes. I inserted locknuts (nylocks) into the opposite side hex sockets, so torque retention will be positive.

Notice how the seat mounts have been turned around. This was necessary because of how big the gearcases were. The seat mounting centers, and overall position, will remain unchanged.

Crawling up the side of the vehicle, I reached this build’s star attraction: The gear shifter. This came together amazingly well, and the feel of the ball detent plungers is extremely satisfying.

Heading up front, I popped out these new steering knuckles. In keeping with the tradition of doing the least possible work, these were specifically designed as drilling operations in a 1″ aluminum square barstock. The four flange holes will be where the drum brake mounts.

Continuing work on the front end, the drum brake mount has been attached and the new narrower steering…ears? are mounted. I’m not sure what to call them on Chibikart. They’re too short to be A-arms or wishbones.

Recall the new steering linkage arrangement – the crank arms are basically socket wrenches that fit over the hex head bolts. Motion is transmitted via giant set screw in the steering knuckle. To ensure positive engagement, I machined a deep flat into the hex head bolt shanks and picked flat-bottom set screws to maximize the contact area. To retain the crank arms, I center drilled a hole and threaded it for a retainment bolt. Otherwise, the crank arm is thinner than the bolt head and will be free to float about 1/16″ or so.

I moved on to chopping up the 90mm drum brake to fit up front. The mounting method I ended up devising would have been fine with keeping the giant torque arm, but the design would be cleaner without.

To maintain the cleanest possible lines, I brutally slashed the housing with a Dremel cutting wheel.

To attach the drum brake itself to its mount, I first had to machine the little round spacer which adapted the 14mm bore of the brake housing to my 1/2″ bolt wheel spindles. I sandwiched the brake housing between the mounting bracket and the spacer so it was reasonably centered. Next, it was a quick drill press job using the mounting bracket holes as a drill template. The steel housing on these brakes is just thick enough to hold a few threads of #10-32, so a socket cap screw was screwed directly into it through a standoff.

The mounting bracket itself involve one sheet metal bend to create a spot which will eventually anchor the brake cable. Well, I managed to bend it the wrong way the first time. Heating up the aluminum with a torch and carefully bending it back the other way worked, but the metal still cracked on one side. I had a buddy on MIT FSAE lay a quick TIG bead across it (see the irregular texture where the sheet metal arm bends left).

The brake drum mounting itself is what I’d call “kinematically suboptimal” very nicely. Basically I squished the slightly tapered stamping flat on a hydraulic press to get a flush mounting face on the bottom side. Then, two standoffs which each have a small shoulder that is precisely fitted to a mounting hole keep the drum attached to the wheel. On the top side, the standoffs have a 1/4″20 thread so I can use already available button head screws to retain the rotor. On the other side, the standoff is tapped M6 X 1 to interface with the original wheel lugs bolts.

The concentricity, needless to say, is less than stellar, but turned out way better than I had anticipated. I’m likely to replace this whole rig with a custom machined aluminum dish that has M6 x 1 holes tapped into it so I can just dismount the whole tire without causing loss of alignment. The brake does scrub, but only slightly and intermittently, and works very well otherwise. I have no doubt that this thing can lock up and skid.

And the front end is basically together.

Work now will move to the rear again with assembling the drive wheels and sprockets. I have an order of brake cables and associated parts coming, so I hope hooking up the whole drivetrain and shifter this week is a possibility.

Continuing on my last post, I’ll talk a bit about how Chibikart is getting a radical design departure from my usual silly vehicles, and later on some about what the hell I’ve been up to for the past week or so. These days, like last spring, my time during the day is heavily  devoted towards herding 2.007 students in some way, including my own “victory garden” of 2.00gokart students who have seemingly become accustomed (read: spoiled) to work late into the night like I do. I’m clearly being the best of influences for my impressionable undergraduates. With my spare time being more fragmented and less conducive to sustained building, I’ve been revisiting some things which I’ve mentally tucked away for better days.

Anyways, to recap, BurnoutChibi is my proposed refitting of the Chibikart 1 frame, which is currently sitting derelict waiting for motors I will never regain the patience to rewind, to a form which offers some more excitement. The reason for the upgrade is twofold – first, I’d like to bring it up to “Chibi 2″ standards, but more importantly, to up the power and show how an inexpensive sensorless drivetrain should be executed. The last time I showed the design, it was a simple one-stage design that was basically the DPRC with more power, and pneumatic wheels.

As I was designing the drivetrain reduction ratios, I began thinking more and more that having 1 speed, even in an electric vehicle, is just suboptimal. With the power levels that the selected drives were capable of, I could reach about 18mph and smoke some rear tires. But that would be it – the fastest it would ever go would be said 18mph. Yet just power-for-power alone – power dissipated by air resistance at speed versus the mechanical power produced by the motors at 50% load – the drives have enough punch to get me to nearly 40 miles an hour. If I were to gear for that speed, it would accelerate very… gently. And that’s only in the ideal case of the motors being able to sustain the high current draws needed to produce enough torque to get there.  Regular permanent magnet brushless motors are really quite limited in the ranges of speeds and torques they can achieve alone.

I’ve wanted to build a multi-speed vehicle for a while, because it still just seems like a better idea. You can shift the peak power and efficiency speeds around depending on your load requirements. Recalling Ben’s successful über-trike build pushed me even more towards that direction – that thing actually has 8 speeds using a bicycle hub gearset as the transmission. I was interested in even having two – burnout mode, and do-more-interesting-things mode.

It was easy enough for me to cook up a custom “shifter” design, but what better opportunity again to test out new parts on the market? Once again, I turned to Vex Pro for the answer. I’ve been eyeing their “ball shifter” (which sounds somewhat painful) transmissions since they were put up on the website.

Many a drivetrain in FIRST have been built using the classic AndyMark shifter transmissions, ever since the drill transmissions went out of style, and AM was the first place I went to when I was looking for COTS 2-speed options. What I didn’t like about them (both now, and years ago in FIRST) was how huge they were. The large, open frame sheet metal design is easy to manufacture, I’m sure, but there was no way I would have stuffed one onto this frame.

The new VEX transmissions seemed to be better packaged, and used a much more compact shifting mechanism. Inside its hollow shaft is another shaft with a round lobe on it that can slide axially. It pushes out on one of two sets of steel balls seated in the outer shaft, overall resembling a rachet wrench’s ball detent. So, depending on where the lobed shaft is pulled, a different set of balls is pushed outwards, acting like a 3-point spline. The output gears have little cavities that the balls lock into. If the lobe is not present under that set of gears, the rotation of the gear on the shaft will naturally shove the balls back towards the center. Pretty nifty.

Seeing no obvious complaints on the FIRST grapevine about them, I’m assuming they’re working pretty well for the competition and are robust enough to shuffle some 120 pound wimpy robots around. But can they stand up to moving a Chibikart at inadvisable speeds, while transmitting enough torque to break traction on asphalt?

I had my doubts – the ball shifting solution seemed like a great alternative to the AM Shifters’ dog clutches, but I trusted the metal-on-metal pushing of the dog clutches far more. Primarily because the dog clutches transmit torque at a greater radius – such that the stresses in the materials are much lower for the same level of torque transmission; and that the engagement is extremely binary – either engaged, or not. The ball shifters seemed to have a much greater potential of being caught “between gears” if one or more balls don’t release.  But maybe that is just a problem if I push so much power through them the ball detents deform.  I was already aware that shifting under power was basically out of the question – there’s no synchromesh devices on any of these things, and even in a real manual transmission car you wouldn’t hold the throttle down while shifting gears anyway.

So, with this many questions about whether the part would be worth anything, it was clear that I was going to have to get myself a set of Vexboxen. Because if nobody in FIRST is going to break them, I might as well.

Or, hold on… time to get myself another set of Vexboxen. I already have a VersaPlanetary that I’ve dissected and taken pictures of and am still waiting on a reason to do a Beyond Unboxing post on – they’re quire nice pieces of kit.

I downloaded the CAD model of the whole gearbox off the Vex website and immediately started cutting it up. First off, nothing was constrained – the solids just floated around, the constraints being broken by the export to a generic format. So I spent half an hour “reassembling” all the gearbox parts.

Next, I removed everything I didn’t care about – namely, all the hardware and the encoder mounting stuff. And the pneumatic cylinder that is supposed to run the shifting shaft. I was intending to cook up a mechanical linkage of some sort, since I didn’t want an air system on this build, no matter how air actuated brakes would have been.

I replaced the CIM motor with my NTM 5060 model using an adapter plate that will be machined. Both motors have an 8mm shaft, so interfacing with the supplied gears shouldn’t be an issue. I took the opportunity to raid eBay of some 2mm endmills to make the keyway in the NTM motors.

A quick fit test to the frame and… My goodness these things are huge. Luckily, when mounted upside down, the resultant sprocket spacing was acceptable.

Notice how these gearboxes are all designed for 2 CIM motors. I have no qualms, if this experiment is successful, of dropping 4 NTMs on this thing and upping my motor sprocket size for even more ludicrous performance.

I flowed some virtual metal around the mounting points to generate these two-pronged mounting adapters. The big gap in the middle means I can still access all the important motor mounting screws. Else, these gearboxes did not seem to require any other additional mounting – even the Vex website just recommended using their stock L-bracket mount.

The output shaft being a Hex of Convenience and Marketing Exclusivity, I decided to just get a 22 tooth hex-bore sprocket from Vex. Okay, Vex, you win this time.

The final drive ratios for each speed, then, are 18.17:1 in low gear and 8:1 in high, for a resultant speed of only 14mph in low gear and 31mph in high. I would have preferred a speed spread of less than 2.0x with a wider low gear, but hey, that’s what I get for not designing the thing. But the extra-tall low gear will definitely prove my hypothesis that sensorless drives can be successful if they are highly geared.

To actuate the gearbox, I needed a mechanical hookup. I was concerned with how much force the shifter shaft needed to operate – the installed shifting mechanism is a small pneumatic cylinder that is capable of nearly 30 pounds of static pull. Clearly this was not required for operation, because that would be ridiculous. After studying the mechanism in CAD more, I determined that it really should not take much force to actuate if I am not applying power at the same time. The actuation method would just need to be very fast to prevent the balls from skipping slot to slot.

I briefly entertained electronic shifting using big solenoids. The required travel was only 1/2″, which is well within the range of big open frame solenoids I could find for cheap. What drove me away from that was finding said solenoid that had a continuous power dissipation rating. Generally, industrial solenoids are rated for only 10% or 20% duty cycle – in other words, 1 minute on then 9 minutes off is 10% duty cycle. More frequently, the “maximum on time” is also specified, usually also 1 minute. Even though electric shifting would likely have been quicker and only required a button instead of running linkages or cables, I was more into the visceral mechanical solution and not particularly interested in cooking solenoids.

I elected to use a spring-balanced cable sort of mechanism with 10 pounds of return force to shove the shifter back into the starting gear. Luckily for me, the lowest gear was associated with the innermost position of the shifter shaft. So, it was easy to find a spring on McMaster-Carr which qualified for the needed return force at the needed stroke (about 1/2″).

What I could not do was find one which also worked with the dimensions of their included shifter coupler. The black object in the center of the gearbox between itself and the standoff-mounted plate is my own quickly whipped up coupler design, which let me fit a spring (not shown in image) between it and the rightmost black flanged doobob. This spring will try to keep the gearbox in 1st gear, so long as I am not tugging on the cable to keep it in second gear.

Some cable adjustment will be needed for sure to ensure synchronization of shifting.

On the other end of things, I put together a simple lever using mostly McMasterables. Did you know McMaster sold random knobs and levers? Now you do. Oh, and little ball detents already loaded into threaded bodies. Super simple instant two-click gear shifter!

This and many other reason are why, this coming Dragon*Con, I am hosting a panel session on how to shop on McMaster, among other places.

Time to mince some metal soon! I ordered this stuff last week. I have yet to assemble the gearboxes, but they seem legit. The casing is heavy fiber reinforced nylon (or fiber reinforced something or other), and the gears are… sticky. Seriously, whatever magic coating they put on these things, it straight up sticks to my hand. Repeatedly, even. I can flat-palm a big gear and lift it straight up off the table every time. Hey, aren’t you supposed to make gears slippery?

The picture quality is so lacking compared to my usual ones because after 5 years of nonstop service and over 11,000 pictures, my free-to-me Fuji S9100 has finally bit it. Cause of death? The USB port just straight up fell off inside and likely shorted something. So, back to the phone camera.

Oh, the thing was secondhand, too, so in a prior life it probably took even more pictures. This thing has been such a brick that I am actually eyeing its slightly newer successor, the S100FS, or even the current generation X-S1.

All of my projects go through periods of ups and downs. I either end up demoing them too hard, or they get “temporarily” parted out for some valuable component only to never get put back together, or the weather stops being terrible. Given that the winter is letting up and the sun has emerged above the horizon, it’s time to think about the small silly electric rideables once again.

Almost a year after its construction, the story of Chibikart has played out not unlike many Hollywood celebrities and pop icons. After a meteoric rise to fame based on a single hit. which spawned a wannabe or two, Chibikart bungles a live performance but pulls it together just enough for the next show (At the bottom… though I still stand by everything I said in that dribbling polemic). Afterwards, however, Chibikart is never really the same, and sinks into a life of decadence and smoking magnet wire enamel. With my desire to never rewind those damned hub motors again, it seemed like Chibikart was doomed to live under a table forever:

…with his washed up co-stars…

But another season of 2.00silly-rideable-thing dawns, and as such, I must have an Instructor Vehicle. The Instructor Vehicle is totally out of spec and over budget, because Instructor Privilege. I had to decide what to do with Chibikart. It was easy enough to start on another bunny-brained scheme since I have more than enough parts to conceive any rideable, drivable Eldritch abomination, but when I had the semi-working hulk of one of my projects sitting right there taking up space, I found it hard to justify anything else.

So what do I do with it? This was an equally hard decision. I could just rewind the hub motors one last time and be done with it. Chibikart is actually quite fun in hub motor form. I could make, instead, 4 kitmotters to demonstrate the concept in real life even better – one of the ideas on my Infinite Time*Money Back Burner is to make a to-spec Power Racing entry using Kitmotters, since machine time and labor are not budgetable items.

One of my favored ideas from the beginning was to show how to do a sensorless drivetrain “right”. I spent much time in 2.00gokart scaring students away from using R/C style controllers because of their notorious unreliability and lack of current limiting and start-from-zero predictability. Yet at the same time, I wrote an entire section on this in the latest Scooter Power Systems instructable.

The way I see most people use R/C motors with sensorless ESCs in vehicles is in direct contrast to the guidelines mentioned in the Instructable. They’re generally the biggest (or bigger than necessary) motors you can buy, hooked up to a low or moderate gear ratio calculated from load necessity or cruising speed. While this is perfectly fine for DC motor drivetrain design and also sensored BLDC/AC motors, with R/C controllers it’s just asking for trouble. Huge current bursts will be drawn during the starting regime because the motor might not “start” in the right direction, or if it does, needs to expend tons of power to produce the torque needed to accelerate. More often than not, the first thing that goes is the motor controller, since they’re invariably small and unprotected.  It’s been my assertion for a while that a 50mm class R/C motor (typically 1500 – 2000 watt) is more than enough power, if used correctly, to move a person, but usually I see people gunning for the bigger 63mm and 80mm motors. Hell, that’s even what I run on Melonscooter. And precisely like I warned against, I go through motor controllers like crazy and really shouldn’t try using all 6000+ watts of power – it’s literally enough to get me to highway speeds and I have enough fun already flying by parked cars at 20-25mph.

So, perhaps a more noble goal is to troll everyone by making a properly proportioned sensorless R/C power system with typical hobby parts. I’d pair a 50mm motor with an oversized (for overhead) R/C controller, but instead of gearing sensibly and picking the slowest (highest torque per ampere of current) motor, I’d head the opposite direction: An unreasonably fast motor coupled with a double-digit gear ratio. Remember, R/C controllers love to spin things which are going really fast. In the combat robot world, this technique has a nickname: “Gene-ing” it, based on the habits of one particular builder to overvolt small outrunner motors dramatically and gear them down very low to get tremendous power into his weapons. The whole idea would be to divide down my apparently inertia to the motor so far that even the jerk the controller gives it is enough to move me a little.

I next spent some time on two sites. The first was Hobbyking, shopping for potential motors in the 50 to 60mm range. The second was the still-very-useful Torque and Amp-Hour Calculator whipped up for roughing out Battlebot drivetrains over a decade ago. I was aiming for a few goals, in no particular order:

• Top speed of about 20mph at my desired system voltage of 37v, to try and conform to at least one of my own rules!
• Gear ratio in the 10 to 20:1 range. Something easy to accomplish with  two relatively simple stages – such as 3:1 and 4:1.
• Wheelspin current of about 100 amps or less. This was important – I wanted the wheels to actually break traction at a current which I could find an R/C controller to run ‘continuously’. I mentally derate these things by a factor of 2 or more when juggling numbers. This means that the wheel should never just fully stall out

What this came down to is if I apply a step throttle input to 100%, the motors will have a high enough mechanical advantage to start consistently each time, and as soon as they do, they have enough torque to just break traction.

That‘s my current limit. Doing a burnout. That’s how it should be.

Hence, this project was named BurnoutChibi.

motors

At the end of the night, I arrived upon my component choices:

On the right, the NTM 5060/380kv. This was pretty much the only thing available that satisfied my requirements – any slower and I was going to go back into undergeared startup nightmare world, and the few motors that ranked higher in RPM/V in this size were very expensive (by comparison) “competition” motors. The cost of the motor? $36.

I’ll be amazed if it stays together.

On the left is a Castle Creations Hobbyking “160A HV” controller. Gee, it sure looks an awful lot like an older Castle controller – I’ll call it the Sand Castle!

These things have piqued my curiosity for a while, but I’ve never really bought one to chop up. It turns out they are thermal epoxied together anyway, so it’s not like I can chop it up without permanently breaking something. I blame Apple.

While I could have gone with my usual trusty Sentilons (which I know the operating characteristics of well, have decent startup ability, and enough FETs per bridge to let me sleep at night), I decided to take this opportunity to try something new. If the Sand Castles fail, I have 4 left over Sentilon ESCs from the late and great Deathcopter project. The reason I’m running 37v (10S lithium polymer cells) is because I have two giant 5Ah 10S “sticks” left over from the same project. These Lipos are big enough to beat someone senseless with and probably light them on fire at the same time. You R/C aircraft people are nuts.

Two of those packs ought to be plenty to feed two NTMs flying at top speed. Oh, I’m sorry – did I mention I got 2 sets of the above?

tires and brakes

To dump multiple thousands of mechanical watts onto a Chibikart drivetrain would result in the tiny, stubby caster wheels being ground into a pile of elastomeric powder. I’d need something vulcanized and beefy to stand the increased power, but I also didn’t want to go to 8″ pneumatic tires which are the most common size for small EVs. It just wouldn’t fit the character. Chibikarts have to have small wheels.

I decided to try and get some 6″ pneumatic caster wheels. The 6″ pneumatic wheel is something which should exist, but is difficult to find; when it is found, it’s generally expensive and part of a caster already. Harbor Freight doesn’t sell any, and McMaster is generally unhelpful and ambiguous about which one of their 6″ casters is actually pneumatic. For the record, I have a sample of 2717T41, which became the basis for my hunt. I figured if McMaster sold it, there was a cheap and generic verison of it somewhere.

The answer came from Northern Tool, a  Harbor Freight function-alike of somewhat higher market segment. They have a 6″ pneumatic caster wheel replacement for Somewhat Less Price. I went ahead and snagged 4 to see if they were identical.

Also pictured above is a 90mm drum brake I got from Monster Scooter Parts. One of the missions I set out right away to complete was to find a good front wheel brake solution. While both Chibikarts have had rear brakes, this basically precludes the chance of doing any burnout whatsoever. The last time I could do a brake standing burnout was LOLrioKart, and I kind of miss that.

I started out trying to find a small enough disc brake, since I prefer those when I can fit them in. The smallest disc brake setup available stock is 120mm – a bit under 5″, which would have been too close to the ground; I’d be riding on brake rotors if the tire ever went flat or someone bigger than me got on this thing. While I could have just machined a custom brake disc, I decided to be adventurous and see what a cheap scooter drum brake is like. I have plenty of experience with cheap scooter band brakes on front wheels from LOLrioKart’s first front brake attempt, which ended with me discovering what the Capstan Equation meant in real life (Band brakes grip extremely asymmetrically with respect to direction!)

I settled on these and got some samples because I haven’t ever seen them before and was interested in exploring a new part For Make Better Glorious Nation of Silly Go-Karts. It’s my understanding that drum brakes also grip asymmetrically, though to a much lesser degree than band brakes.

As luck would have it, the two spanner wrench holes on the drum line up pretty much perfectly with the hub lug nuts on the wheel. Well then – that pretty much seals the deal.

What I’ll probably do for attachment is make two standoffs that replace the lug nuts with posts I can screw the brake rotor in from the other side with. Two of them will have a pilot “lip” to seat in those holes. I’ll shim up the bore to be concentric, then drill the other two holes using the wheel itself as a template. I’m sure two standoffs might work, but using all four possible locations is just more robust.

This is what the assembly would look like in real life. Instead of using the long built-in torque arm, I’m going to cut it off for compactness and instead mount the brake body itself with a circular bolt pattern. There appears to be enough space between the stamped housing and the brake shoes to drop some #10 or 1/4″ button-head screws into.

I went ahead and made a representative model of this assembly in Inventor. The bore of the brake housing is 14.5mm, so to use with a 1/2″ axle, I’d have to make another spacer with a seating lip.

The next thing to do was to take Chibikart 1′s CAD file and rip everything the hell out of it. Structurewise I was going to turn this into a DPRC, which I engineered a more robust brake pedal for.

I started adding in the new wheels and testing steering geometry, but soon ran into the curious issue of Chibikart being wider than it was long. The pneumatic wheels and brake combination were threatening to push the width out to 30″, because they were just so much wider than the skate wheels!

Some innovative compacting on the steering linkage was going to be needed.

I pushed in the “ears” on the frame a half inch or so, such that the flanged bearings were almost against the 80/20 side rails. Instead of a DPRC-style spindle which is a bolt embedded in a block made of stacked layers, I went for ultimate compactness and designed the spindle blocks to be made from 1″ aluminum square. Like the current arrangement on Chibikart 1, the ‘axle” is a bolt which is tightened into this block.

The hard part was the steering linkage itself. Part of the reason why I made the ‘ears’ so far out in the first place was so I could get enough linkage travel before the ball joint hit the frame. With scooting the steering axis inwards so far, I would only have gotten maybe 15 degrees of wheel travel in either direction. Besides seriously messing with the steering linkage geometry, the other solution was to move the linkage out-of-plane with the frame. I decided to try this first. Basically, the spindle block is solidly connected to the kingpin (which is now a hex head bolt), and on the other side, a big crank arm with a hex bore cut out of it sits like a wrench over this hex head, retained by an axial screw (not shown). This way, the linkage is moved fully under the frame and I can once again have proper steering geometry without  compromising angle.

This configuration might in fact offer too much angle – there’s not a physical hard stop in the system any more, so I might actually add something else later to prevent linkage overextension.

Back on top, I spent a while editing this sheet metal part in-assembly to get the lineup of the cable anchors correct. This plate is essentially an extended version of the “endcap” found on DPRC’s front wheel spindles, with a bolt pattern that engages the spindle block. The mounting bolt pattern for the brake housing, not yet shown, will also be put into this part.

I whipped up a quick 4.16:1 gearbox using VexPro gear models. Vex has gained my interest immensely after debuting their new line of “bigger bot” parts this Spring for the 2013 FIRST season, ostensibly to compete with the likes of AndyMark because FIRST is now big enough to sustain two parts houses. I actually have a VersaPlanetary box bought completely for the purposes of dissection, and intend on writing it up for Beyond Unboxing one of these days – they’re nice, I must say.

The NTM motors have convenient 8mm shafts already, so any gear that can go on a FIRST CIM motor can be fiddled onto this one.

The new rear end of BurnoutChibi nee Chibikart 1 is basically a DPRC back end with a different motor adapter plate. This time, the 4.16:1 gearbox hovers above the main 3:1 chain drive, with the usual chain tensioning method. At this 12.5:1 ratio, the top speed is right at 20mph and the per-wheel burnout current is something like 80 amps. Not bad, but not exactly good either.

So right now, Burnoutchibi is essentially DPRC on some serious roids, with pneumatic tires. By itself, no matter what, it’s not that exciting any more – I’ve built enough things like this already. And by this point, it’s maybe a little hackneyed. The next post will be about what I intend to do about that…