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.


One thought on “BurnoutChibi Reassembly: Inside the Vex Ball Shifters; RazEr as Art”

  1. A few of the photos near the end didn’t show up for some reason, particularly the finished product. I declare pics or it didn’t happen.

    Also, congratulations on being awesome.

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