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!