Category: derpy vehicles

anything silly and rideable.

BurnoutChibi Repair and Revival!

chorl

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!

 

Loose Ends and Tag Closing for Bits of October: Site Updates, Chibikart and Mini-Jasontrollers, New Expensive Things!

chorl1 Comment

Now that the season of Dragon*Cons and Maker Faires and everything else has finally settled down, I’ve reached the curious state of having nothing to do with my life, being between large builds in much the same way you’d be between coffees or meth hits. My day to day activities revolve around managing the IDC (excuse the cheesiness) fabrication facilities, of which there will be some updates shortly, and monitoring & mentoring the classes running in the center, including the renowned How to Make a mess out of Almost Anything. I’m not a TA for the class per se, but part of the process of making sure the shop isn’t lit on fire is some times giving extra attention to those who would be most likely to do it.

That isn’t to say that my life is entirely empty and devoid of meaning. I’m tending towards taking the downtime to fix up my eternally problematic go-kart children, starting with Chibikart2. During some hard running at the Powerwheels race, I lost one of the Jasontrollers to Sudden Jasontroller Death Syndrome, a fairly common failure mode for them when they are over-run. The failure is always gate drive destruction since the circuitry is so fragile, and always not worth repairing to myself because it involves replacing so many small shitty transistors. Next up on my list after this is probably to add the electronic solenoid shifting to burnoutChibi and finally get rid of my super-rigged cable linkage. I’ve also been collecting many prospective parts for the “Chibi-Mikuvan” project, so stay tuned for a massive Beyond Unboxing the likes of which have never been seen!

But first, by popular request, I’ve added Pad Thai Doodle Ninja and Colsonbot CAD files to the References page. PTDN’s files are only made of 3D printable frame parts, but Colsonbot is the full bot – you’ll need Autodesk Inventor or a compatible viewer for anything but the STL files. All of the details on these bots are available in their respective build threads.

Onto Chibikart’s controller update. Like the dual controller mount I made for BurnoutChibi, I designed up a two-mini-Jasontroller snap-fit mount which also holds an 80mm fan. Essentially the same idea of BurnoutChibi’s. I was planning to current-hack these controllers to 40A, and for sure they will need supplementary air cooling.

 

The mount was printed on my Up machine, and this is about the largest object I’ve found it can handle reliably. It came out well, with minimal warping. I sincerely recommend the Up (now on 2 Plus!) to anyone thinking of getting a small hobby-class 3D printer.

Short of popping it in the Dimension, the Up is my go-to for structural parts. The ABS formulation they use is a higher hardness/toughness than the soupy generic stuff you feed to RepRaps and Makerbots. I was concerned about the snap fits being too aggressive and snapping off themselves, but they turned out to be just on the side of the acceptable line.

The mini-Jasons were cleaned up of unnecessary wires, leaving only the motor phases, power, the Hall sensors, throttle, and the ‘regen brake switch’ which may or may not be wired in in the future. The regenerative braking on these things is a fixed low current on-off kind of affair, so it’s not very helpful.

I plucked the 80mm case fan from stock – there’s nothing particularly special about it.

In the past, I’ve current-hacked these things with a blob of solder on the current sense shunt, but it’s such a bad hack and is unreliable – I’ve actually had the blob melt back off before. To remedy this, I began looking for large current shunt resistors packages that fit in between the leads of the existing shunt. This is the result – for a mini-Jason, a “2818” (.28″ long, or so) package current sense resistor is a nice fit. One that is 8mm (“31xx” – “35xx”) will fit even better and not require much solder bridging would fit better, but I could not find any that were not also square in shape – rectangular, the long way, is preferred.

I actually had this hack vicariously tested by Daniel (YAMEB) a while back. These shunts are 5 milliohms (not 10 – I measured erroneously the first time), so it took a nice sandwich of 10 milliohm resistors to get my 40 amps. The exact part number I used was WSHA-.01CT-ND, and it has a 5 milliohm brother in the form of WSHA-.005CT-ND.

I cleaned up the floorpan of Chibikart after removing the old Jasontroller – it was positively disgusting and filled with 2 years of floor grunge buildup, plus mud and dirt from running at two slightly wet Maker Faires. The new installation drops right into where the old controllers used to sit, after redrilling some mounting holes.

Systems wired back up. The first test drive was without the fan hookup, and without the sensors connected.

To rehash, these controllers “self-calibrate” sensors if you connect them and then run once to full speed. I couldn’t achieve this on the ground since the vehicle never really reaches “full speed” in the space available, so I had to freewheel it, being mindful of the 4700rpm-ish commutation limit. After one power cycle, the controllers had learned the sensor configuration and Chibikart could apply “static pressure” to something again. To get a good transition between sensored and sensorless, the sensors have to be aligned properly first (check out Equals Zero Designs’ page where I have an actually well documented example.) – and that’s all you need to do, not actually try and optimize their timing position.

This was, of course, the important part.

Now, the 12v PC fan could not handle 24 volts, so I just dropped a giant 40 ohm resistor in line so the fan only saw about 15v. This resistor surely dissipates more power than the fan actually removes…

With two motors on 40 amps, instead of on ~25 before, Chibikart2 is way more fun. Not, say, tinykart Black Edition level fun, but it is far more peppy. The small Colson wheels are starting to reach their traction limit.

I hit 1.1kW on indoors testing, and there is much room for improvement yet. Because the controllers are doubtlessly still running constant current before I run out of hallway, the power will only increase with vehicle speed.

Say, I haven’t garaged something properly in a long time (mostly because said garage was under repair construction this past summer). Maybe it’s time to take Chibikart back to its proving grounds.

Next, some of the ongoing facilities improvement projects that I have going on in the space! The place is kind of like I-95 around New England – always looking like someone’s working on it and the construction shifts every once in a while. I swear, though, it’ll be over soon – just like they say in Connecticut (In my six years in this area, I have never once driven through 95 in CT without hitting some kind of construction…)

First up, a full size Shopbot – the full 5 x 10, gifted by the Architecture department. I’ve been itching to have one of these for a while – with an 1/8″ carbide bit, they’re practically mechanical waterjets! Expect some Shopbottables to emerge on my end soon due to the “It’s the closest tool next to me and I don’t have to ask anyone to use it” effect. It will be very handy for producing Chibi-Mikuvan’s body panels since they’re all larger than what can be stuffed into the laser cutter.

Above, Media Lab students operate the machine as part of the MAS.863 “Build something big” week.

Next up, the legendary Form 1. Full disclosure: There’s four Form 1en in the space at the moment – this one is “The Lab’s”, and the rest belong to researchers and classes residing in it. Four. That’s more forms than Formlabs (okay, probably not), but the Form 1 density must be up there.

The Form is a SLA-like machine which can hit much higher resolutions, but the  material isn’t too strong – it’s an acrylic resin, so it has some mechanical strength, but does shatter and snap. Dat rez tho…

These are some of Brian Chan‘s insects. Check him out on Shapeways! I also printed the crab, lobster, and some other doodads from his collection.

Of course, with every 3D printer that makes it in here…

The  model is “Pillared Miku” though I used the version without built-in pillars – the Form software generates its own support lattice.

Now, moving up in the Expensivity scale is our latest acquisition:

 

An Objet24 (By Stratasys™)! This is just contributing to the slow rounding out of 3D printer technologies in the space. Objets are incredibly high resolution, very nice, and very expensive. This unit was purchased used from a local company for only $7,000, but you’d easily eat up that much per year in materials alone. The Objet Goo comes in 700 gram jugs that each cost $300-350 and up.

And this is the entry level machine.

The Objet technology combines SLA (light cured resin) with inkjet style nozzles so it can control the deposition very finely. No giant bubbling cauldron of goo here. It also has its own Windows XP computer built into it.

Now, I know XP is pretty much the OS that saw the Internet grow up with it, but this machine was built in 2011….

…and even worse, it requires a very specific network setup to talk to. Objet-Stratasys (ObSys? Stratajet?), I’m going to publicly shit on how bad the Objet communication infrastructure and software are. I should not have to configure a point-to-point LAN, disable Windows Updates, and disable firewalls just for it. The whole setup procedure gives me the vibe that they had to ship the machine and had 1 day left to write the drivers, so took whatever the developer’s computer was at the time and just made that the exact requirement. That, or given Objet is an Israeli company, probably just opens your computer up to direct monitoring by the Mossad.

I’m amazed I didn’t have to start Space Pinball and log into Pandora before the printer would communicate.

The slicing software is also slow, prone to crashing, and has an inconsistent UI. For such a beautiful piece of hardware, the software end of it seems so incredibly rigged.

Of course, the first thing to do with every 3D printer that makes it in here…..

Yeah. This was like a $30 Miku given how much of the material I used.

This corner of the room has been reconfigured to become what we now affectionately call “printrgartn”. The Form 1 is immediately off to the right, as is the Replicator 1. I’m trying to commission an Up for the lab (in supplement to my personal machine).

What’s absent, sadly, is a powder printer. I need to do some Powder Print Affirmative Action here.