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.


RazEr REV2: Mostly there

Over the past few days I’ve been mostly hanging out at the Georgia Tech Invention Studio. I was nominally there as “guest lecturer”, but I don’t quite think their own 2.00EV is organized yet to the point where I can feel comfortable with that title. All the ‘students’ are actually lab instructors (similar to our MITERS keyholders), so there wasn’t that much ‘teaching’ to do. I did hold some impromptu lecture-like things and generally advised people on their builds where needed (and fixed the waterjet?). Regardless, some… interesting products are coming out of it:

It’s literally twice as long as some of the other scooters.

I’m back now, and one of the first things on the agenda is getting the half-assembled repackage of RazEr up and running. I sort of left this in the middle of construction when I zipped off to Atlanta for the weekend. I had the frame ‘box’ assembled to test fits, but I pretty much had to take it apart again to actually install stuff. My direction was essentially assemble the major subassemblies first (make the fender, reinstall the motor, attach the front end) and then lob the electroncis back in as-is, since it worked fine before.

Here’s the fender in place with its leaf spring installed.The ‘sheet metal work’ was done on a vise, then fitted in place using just tightening screw pressure. 5052 aluminum bends very easily, especially in 1/16″ thickness, so I was literally just leaning on the part to get the bends I wanted. To do the large radius sweep at the top, I bent little by little in ‘facets’ which weren’t drastic enough to be seen as disrete (though you can kind of see it).

Now that I’m a little more comfortable with making sheet metal geometries compatible with other 3d solid parts, I might incorporate it into more builds in the future.

The fender is just mounted on a chunk of 1/4″ threaded rod. Nothing fancy at all this time – no spacers, even. The pressure of the leaf spring alone is enough to keep it in place reasonably.

More progress has been made on frame assembly, with the folding joint  reattached now. I traded the former front end for a new A3 type front that was part of the leftovers from my 2.00EV. It’s substantially less beat to shit and doesn’t wobble as much, and I swear it’s a little taller than the one I had before.

I had a left over new fork from building Straight RazEr (whose wreckage has since been donated to Kramniklabs) which I dug out for this build.

I forgot to take a picture of what’s going on with the 5″ colson wheel, but there is actually a type 1614 bearing bored into each stock Delrin bushing. The Colson comes with a 5/8″ bore bushing  that has a 30mm OD (which presses in to the 30mm bore of the wheel itself). I tried to find a > 30mm bearing that wasn’t of a ridiculous axle diameter so I could bore it into the wheel directly, but gave up and went the other direction instead.

A ‘stock check’ of bearings I had turned up some R8 type and 1614 type bearings. Both were 1 1/8″ outer diameter, which I could bore into the Delrin bushing, but I settled on the 1614 bearing since I easily located a stock 3/8″ bolt to serve as the axle pin.

The job itself was done on tinylathe, which is probably one of the handiest tools I’ve ever worked with.

Moving on to the electronics deck now, I put together the ‘switch panel’ which holds the charge and controller ports as well as the annoyingly bright blue LED endowed power switch. The idea is to have BAT and PWR jumped externally with a Deans ‘patch cable’ so I could jack in a flow-through measuremen device like a Wattmeter if needed. Else, the switch is to serve as the primary turn-on mechanism.

It’s better than the yank-the-battery-connector setup RazEr Rev has used since forever, but I’m wondering how long until this switch falls victim to no-precharge arcing damage like the very first switch arrangement.

The interesting part is on the back. Instead of connecting the switch’s built-in LED to ground directly, I threw a 100 ohm resistor on it. This should prevent the light from exploding right away, as it happens when you try running 12v rated switches on 36 volts… Otherwise, there are just a few select wire jumps which bridge the two Deans ports through the power switch. Note the back-to-back soldered Deans connectors on the right…

With the switch panel done, it’s time to load all the electronics back in. The same shell-less Jasontroller appears, bolted to the aluminum frame directly for some heat sinking. There’s a bit more space for batteries this time, since they can reach all the way under the folding joint, but unfortunately it isn’t enough to actually add more cells – just maybe some padding. If I wanted more battery energy about the only good option is moving to prismatic cells.

With everything wired back to the way it was, I shoved the 3d-printed front endcap on. This was one of those pieces made on the Lab Replicator™.

And the repackaged shot. Unfortunately I gave away my only other black Colson wheel, so it’s gray for now. When I get another one (or get it back), the bearings are transferrable.

This frame rides significantly lower than RazEr Rev – too low, actually. This is likely due to a difference in head tube length between the A3 I got like 5 years ago (for the original RazEr!) and now. RazEr Rev rode slightly ‘nose up’, but this one is definitely nose down. The clearance at the front is about 3/4″, decreasing to less than 1/2″ when I’m actually riding it and the rubber block is compressed.

Not going to work. I’ll compensate by making the two wheel fork sides a little longer. In the mean time, it is rideable, and handles just like it used to except with more stopping. And less exposed wires – check out the 3d printed wire guide at the lower right.