BurnoutChibi Reassembly: Inside the Vex Ball Shifters; RazEr as Art

Quantity of Working Chibis: 2

I’m now back to no longer having any dysfunctional vehicles! So I bought a new one.

This…. is my life.

It’s a Razor “e-Punk” minibike similar in flavor to herpybike (Left), but much smaller. But it still uses standard 200×50 scooter wheels, so it lends itself to all kinds of potential shenanigans. I have no immediate plans for it, but at $20 for the wreckage, it’ll be around when I or an eager froshling have them. It’s actually engineered pretty well in the back end with a proper spring-loaded belt tensioner and sliding motor mount and all, so motor swaps are easy, and the battery tray can be repurposed for other components.


To continue the repair of BurnoutChibi, last time I finished making the new front wheel assemblies. Over the past week, I put the rest of the new front end together, and the thing is now operational again.

I also have a (sadly a bit brief) word about the redesign of the shifter mechanism – it hasn’t changed much physically, but this time I put just a pindrop of Design Mojo into it and the functionality was greatly enhanced. However, it was both short enough of a hack, and risky enough in my mind that I didn’t take a picture of the actual installation, but enough should still be present.

First up, to attach the new wheels to the frame, I had to machine a new steering upright (or knuckle) since the offset distances changed due to the new brake arrangement.

I took a liiiiiiiiiiiiiiiiiiiiittle more care in machining them this time, from 1″ 2024 barstock. You know, care when tapping so the first 8 threads weren’t stripped or something. The axle spindle is a 1/2″-20 bolt, and now the Set Screw of Kingpin Retainment is a 1/2″-13 thread instead of a 3/8″-24 thread. The latter, which were used in the first blocks, tended to wiggle loose since they didn’t have enough tip area.

The new upright, with brake caliper mount and caliper, and installed with kingpin. These calipers are a bit different – beefier – than the ones I actually designed up last time, so they came too close to the kingpin nuts. I had to space the kingpin out of the bottom with more bronze washers (I just gave up and have a small box of them now), then use a thinner jam locknut up top.

Hmm, the toe angle is a little wrong…

I hadn’t uninstalled the old steering follower links yet, so they’re still hanging off the tie rods.

Wheel mounted on one side. I can tell it’s better already. The caliper can be adjusted out slightly using the knurled screw such that there’s very little disc scrub but still rapid engagement. The right side disc does have some runout, possibly from the hub being slightly warped from welding. A post-machining operation to smooth them out would have been beneficial. The left side has almost no visible slop.

Here’s the installation from below showing the steering follower link attached to the tie rod.

Back up on all wheels! I stole the Hella switch from Burnoutchibi to borrow for… something. I’m not sure, but one of the fleet lost a Hella key (probably Melonscooter2), and I did not have a spare at the time. I since ordered a stash, so I picked a new one.


A closer view of the new front end arrangement. I test drove it like this with the worn out shifter (holding it in gear like I usually had to do anyway) to confirm and tune the brake operation. Since the two brakes were mechanically ganged together (fixed distance travel), I had to adjust the cable in and out to get the two engagement times as close as possible. This is one inherent disadvantage of multiple ganged cable brakes instead of hydraulic ones.

The shifter was having a harder and a harder time holding gears before the front tires blew out, and after this front end repair they were not particularly any better. I decided to break down the Ball Shifter transmissions completely and inspect them for damage.

I cracked the transmissions back apart to check for excessive wear, but the shifter mechanism is hidden in the shaft assembly in these, so I’ll have to go deeper. I never really tore deep down into these during the initial build, so here it is!

One thing I noticed right away is that the motor pinion and first stage gear were wearing through their hard anodized coating. Meaning now I’m going aluminum-on-aluminum. I suppose it’s ultimately a consequence of building these transmissions to survive a FIRST season or two. Even though the gears are 7075 alloy and would likely be stronger than a typical carbon steel gear, aluminum doesn’t wear nearly as well as steel. One thing that could be done is swapping the 14 tooth Vex aluminum pinion with an Andymark steel one; pinions tend to go first over larger gears.

So far, none of this has been the cause of any issues, nor do the teeth look substantially thinned, so I left it alone. \

I took the big retaining clip off the center shaft, and here is the result. The two shifter gears slide off, and then a bunch of little balls fall out. Well, nothing’s bad looking – the ball splines have some “Oh God It Hertz” divots, but nothing major. The balls aren’t deformed, their crossdrilled ‘tunnels’ weren’t smeared. So why wasn’t it staying in gear?!

It turns out that my shifter throw distances were incorrect. When my shift plunger hit its first gear hard stop, it was too far in. The action of applying torque forces the internal round lobe further into the transmission, but since it’s backed by a hard stop, it “works” anyway. The result when I shift is that the plunger stops too far short to properly engage 2nd gear, and the application of torque forces the balls radially inwards, tugging the internal lobe back. What I found was that I could not actually adjust the cable travel out to meet 2nd gear properly.

I’m thinking I must have missed one axial dimension, or applied it in the wrong direction, because the Vex CAD model does actually line up with what I got in real life; I had at first assumed the model was different. The proper plunger offset in first gear is 0.25″ from the inside face of the plunger bearing (on the far left end there) and where the hollow output shaft begins. My original plunger design was only 0.2″.

The travel was confirmed to be 1/2″ as measured center to center on the ‘ball tunnels’, so only my endpoints were potentially incorrect.

So, dammit, I’m gonna redesign myself some shifter plungers. On the left, the old style. On the right, the new one. Making the plunger stick out further required moving some of the geometry around to be compatible with existing spring lengths.

The difference between the two parts; old one on the left (too short offset), new on the right.

New variant shifter plunger installed. Result? Success – the balls not pushing on the ramped part of the internal lobe means no translation of radial force from torque application to axial force.

So, tl;dr start at 0.25″ and end at 0.75″.

Even though it did “work” with the existing shifter knob, the aluminum detents were getting extremely worn out; there was basically no feeling of which ‘gear’ you were in. This has, again, been the case for a while, and I had contemplated going to an all-electric servo shifter or some other fancy electronic method instead. But, as long as I was warming up my 3D printer to make the new plungers, I decided to apply a little flexural mechanics:

This is a drop-in flexural detent shifter module that replaces the steel ball plungers with a solid “spring” and two speed bump detents. I roughly calculated the force needed to click the solid spring’s contact surface over the detents as about 10 pounds at the knob – this is quite a lot, but at this point I wanted to be a little paranoid. It will probably get smoother with time and use.

The estimation method was:

  • Calculating (read: FEA) how much force it took to push the little ball up around 1mm, about 15 pounds force.
  • Dividing by the cosine of the roughly visually estimated tangential angle that the detent is acting on the speed bump at “about 60 degrees ish, kinda”which yields a “How hard do I have to pull sideways to generate x pounds of upwards force” number (30 pounds)
  • Taking that through an estimate of the coefficient of friction of ABS plastic on itself (0.35) to yield 10.5lb force
  • Noting that the lever ratio is roughly 1 to 1 between the handle and the point of contact, so let’s call it 10 pounds.

Various forces will come together to make that better or worse – it will, for instance, definitely get worse as time goes on when the speed bumps wear down and decrease the angle of action. If I grease it, it’ll definitely lessen the actuation force.

This is probably not going to last very long, but at least enough to get some grins in, and it was put together in one evening. I do want to machine the ‘shift gate’ version eventually.

This is the installation. There’s 2 different colors involved since I went back and changed the yellow piece a bit to force the machine to fill in the region solidly; before, the profile was too thin and the machine did not add any infill, leaving the spring area hollow and weak. By that time, I was printing yellow stuff for students.

The device as it stands now. This thing has been a total blast to drive around now that the front wheels actually, you know, roll properly. And the brakes are sensitive enough to decelerate extremely consistently. Actually, they were so sensitive that I almost brake’d myself off the front handlebar. A strong return spring has since been threaded over the cable in between the actuation arm and the cable stop on each side to alleviate this.

I went outside to see if I could “manually ABS” the new front brakes – it IS possible, just not very practical since the vehicle’s mass is so low. I’ve been practicing cadence and threshold braking on occasion in pre-widespread-ABS-era Mikuvan out of curiosity and an abundance of caution.

As usual, it’s missing good test footage. I’ll need the weather, availability of photographers, and mutual desire to go out and test stuff in this season to build up first. Inside, the vehicle is severely traction- and space-limited, and indoor testing video won’t really be worth watching.


As implied in the title, I’ve managed to somehow break into the design gallery scene:

What?! Yep, that’s RazEr REV2! On display, in a fancy architecture design gallery in Boston. Fancy.

How the hell did that happen? Long story short, the IDC’s population has a large Architecture and visual arts/design minority, and two of the researchers happened to be affiliated with the BSA. All of my small rolling contraptions seemed to be a fit for their “Urban Mobility” display this winter and next spring, so I was solicited for potential display items.

Originally, they wanted Landbearshark as a stark contrast to everything sensible. I almost agreed, but with the season of subarctic melancholy on the horizon, wanted to keep it around for shenanigans.

What’s nice and practical, looks reasonably well finished, and completely useless in the snow? RazEr.

It gets its own Fancy Display Platform complete with ipad scrolling some of the build pics.

There were other Fancy Hardware examples too. You may remember the Tribey from Mt. Washington. And the green thing in the background is an Lemelson-MIT Prize winner. Winning at what, I am not too certain. The display runs through May 2014 and can be viewed at the BSA public gallery.


BurnoutChibi Repair and Revival!

Chibikart just can’t seem to catch a break.

Even in non-hub-motor form, it’s been sitting in a pile of its own wreckage. During the late August summer go-kart finale, I raced against some of my own students and TAs, and about midway into the race, the left front wheel just completely locked up, skidded, and blew out. It’s even in the video:


I never really got a chance to drive it well, since it was built during the rush of organizing the two kart classes, then rendered mostly dysfunctional right afterwards. Since then, it’s been living alternately under the bench and on top of it, reducing my moral authority to command people to clean their workbenches to near zero.

I originally used the scooter rear drum brakes as a parts examination, and came to the conclusion that no. But, it’s not really their fault – most of the failure is from the fact that the wheelbarrow tires are completely and utter trash – they’re out of true in every possible fashion! It’s almost as if they were not designed to be used on go-karts or something. Imagine that.

Anyways, I’ve had a mind to swap Burnoutchibi over to front disk brakes almost since I tried stopping with the drums for the first time. I figured that if the solution involved a custom non-shoddy hub for the tires anyway, then there’s no point in trying to machine or adapt a custom drum when a brake disc is just a flat chunk of metal.

That effort finally got rolling last month some time when I began to fiddle with my left over brake calipers. The brake calipers I speak of are the generic $10-20 scooter calipers commonly found on small gas and larger electric Chinese scooters, the kind retained by the likes of Monster and other scooter parts places. They’re simple, but get the job done.

What I found in the course of my research about them is that nobody has ever modeled one in 3D. That I know of, anyway. Nor does an original model seem to exist at all. I’m strongly of the opinion that these things were copied once, then everyone else just duplicated dimensions without really thinking about what’s going on.

And so, desiring a model which I could line up in CADspace instead of assuming things about those mounting points, I set about correcting this issue by virtualizing the brake caliper. Now, these things don’t have a SINGLE straight edge to datum some dimensions off of, so I had to resort to tracing over an image:

I took a very long-zoom picture of the caliper, end-on, and used it as a the backdrop to a sketch full of freehanded splines. The tight zoom is to flatten out perspective so I could capture all the features on top of each other. I didn’t do it perfectly, as some of the mounting screws might show.

This sketch became the basis of several features that used the profile information, in conjunction with real life caliper measurements, to generate some solid model parts of the caliper. Critical dimensions such as hole spacings were found physically and then those dimensions overrode the raw picture traces.

And there it is, a model of that which has eluded modeling. These dimensions ought to work for most generic small caliper brakes – later on, I show which ones I ended up getting more of, and even though they looked different, the holes and spacings were the same.

I’m putting this file up in the References section as brakecaliperrh.zip, in which are Inventor files, Solidworks files, and a generic X_T (Parasolid) for importing to other CAD platforms. Hole dimensions are exact to 0.1mm, and the axial spacing and thickness of the parts represent worst-case scenarios on purpose, such as designing to clear parts around them will definitely clear the calipers in real life.

To verify, I grabbed a chunk of virtual Tinykart from Shane and dropped my caliper model into the ad-hoc physically located model they made for it. Result: Pretty much an exact match!

The next step was to adapt the new brake caliper to the front end of the kart. The placement of the holes and generation of the adapter plate was driven entirely off the position of the caliper. I made midplanes and mating axes to space the caliper out where I wanted. I was going to make a custom brake disc anyway, and the brake disc diameter was chosen based on not being so large as to hit the ground if the tire goes flat, using as much of the brake pad surface as possible, and being large enough to clear its own hub mounting bolts. Ultimately I decided on a 100mm (or 4 inch) disc.

A new ‘upright’ block, which the wheel spindle bolt tightens into, is required because the axial spacing of everything was so different. This set was made hastily and sort of out of square anyway.

This is the new combination hub. Brake disc mounts on the little end, and wheel mounts on the big end. The tube in the middle is a 3/16″ wall DOM steel tube with slightly machined ends. It’s basically a smaller, cuter version of the average commercial go-kart hub, like some of these. The discs are to be waterjet-cut, then welded to the steel tube.

I would have cranked these out of some aluminum billet, but I wanted to pilot this method for Chibi-Mikuvan, which will use a slightly larger version of this.

The planned assembly overview.

The idea is to strip the shitty rolled steel hub completely out of those wheels, using only the stamped rim halves and bolting them to my own hub which will, in the best of circumstances, be more true-running and less… I dunno, shitty than the stock wheelbarrow-grade hubs.

An overview of the new assembly to be manufactured. No matter what, I couldn’t get the disc brake design to have the same front track (width) as the older version, so this one will have slightly wider track at the front – but only a half inch. Not enough for anyone to notice but my alignment-obsessed self. That’s the one downside of using disc brakes with these smaller scooter- and handtruck- type pneumatic wheels. Unlike a car tire which is deeply dished on the inside to fit the hub, steering knuckle, and braking hardware without it all sticking out, here you usually just have to deal with a scrub radius of 2 or 3 inches.

(Unless you want some seriously wild camber. Stancekart?)

Brake rotors, hub components, and the caliper adapter all cut out from 11 gauge cold-roll steel (0.120″). To the right are some replacement steel steering links. The aluminum ones, while they worked in principle, started smearing their hex bores out eventually so the steering backlash began growing with each test. I decided to just make a weldable version; the geometry is otherwise identical.

The beads were dropped with a MIG welder for expediency. I machined each steel tube end to be a slight press fit in order to keep the pieces from warping while being welded.

An interesting feature I noticed about the steel tube I got from Speedy Metals was that the outer and inner surface were both very shiny and tight-tolerance. This is in contrast to most DOM style tubing where there is a visible weld seam on the outside and inside, and the sufaces are dark with oxidation. This makes me wonder if it was actually extruded – I’ve heard of extruded steel but wasn’t sure if it was a common process.

So far, this only applied to the 1.5″ OD, 3/16″ wall stuff I got for BurnoutChibi – the other sizes that came in the same order for Chibi-Mikuvan are normal looking.

To cut off the stamped rim from the hub, I took a boring tool to it, cutting on the back side with the machine running in reverse, until it just popped off.

Using some 1″ OD by 1/2″ ID aluminum extruded tube, I made the internal bearing spacers. The faces of the spacer were turned down to leave a little shoulder so the whole thing wasn’t rubbing on the bearing seals and races. There’s no reason the spacer has to be so thick radially, but the close match between spacer diameter and hub ID means there’s less playing “Guess where the spacer is!” when shoving the axle through.

Also shown are the non-trashy R8 flanged bearings to replace the very trashy stamped steel lawn mower bearings which I swear use MIG welding spatter beads as their balls

Finished hubs with holes tapped. The wheel side threads are M6 x 1 to reuse the bolts they came with, and on the other side, the brakes are #10-32. Yes, mixing metric and U.S. threads. Yes, I am a lazy american.

Wheel mounted…

And both wheels completed.

The hub is designed such that the small projecting boss of the steel tube is the right diameter to align the center hole in the wheels. This contributes immensely to true running. Combined with the flat hub and real bearings, the amount of wobble in the wheels is now minimal, and practically none at the brake disc.

Moving onto the new steering links, this is how they fit onto the existing kingpin bolts.

And they are welded the same onto them. Now there’s no more need to constrain them from the bottom with a bolt and washer, and ideally this system will have no backlash…

What I have to do now is to remachine the upright blocks and then mount the wheels. At the least, then I can try a test drive to see what difference it makes.

Next on my list with Burnoutchibi is a rebuild of the shifter mechanism – while the principle was sound and it worked great in the short tern, the aluminum ball detent slots rounded off pretty quickly and right before the race, it was pretty hard to keep in gear. I’m going to move towards a spring steel bar (incidentally the same spring steel bar that I made 12 O’Clocker’s front legs from) that can bend in and out of slots, like a lawn mower shifter gate. Since there’s only two require positions, it’s not exactly difficult to come up with a pattern for it!


A Preview of 2.00Gokart and Finishing BurnoutChibi

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…

Finishing BurnoutChibi: Transmission & Drivetrain, Controller Mounts, and Wiring

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.


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.

BurnoutChibi’s Steering and Braking

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.