Straight Razer, Part 2: It Goes Straight!

Straight RazEr made an appearance today at the August Swapfest, alongside tinykart, in the usual MITERS weird-vehicles-at-Swapfest fashion. The previous day was a steady effort to finish the vehicle that lasted about 28 hours – great, because the day before I slept what amounts to an entire day so that puts me about where I should be…. right? Overall, I declare this thing done. It works great for what it was designed to do: go really fast in a straight line. The minimum speed dictated by the pickup of the Turnigy controller is something like 8 miles an hour. The acceleration is even more brütal and uncontrollable than melon-scooter after that.

I have yet to clock the top speed of this thing since there wasn’t enough of an open stretch (that’s totally flat) to hit top speed, but the numbers show a top speed of about 35 miles per hour.

Since the back half is structurally done, it’s time to mount the front. The front end of this thing is stolen off another Razor A3. They come up for cheap on eBay all the time, and honest is the fastest way to get a folding joint, handlebar set, and most importantly the steering neck, for small scooters. The A3 and other front suspension Razor scooters also have the advantage of removable front forks, meaning you can swap yours for an ugly blob of plastic too.

The downside? There’s like 10 scooter back ends at MITERS now. Two of them made it into Fanscooter – perhaps it’s time to start making custom vehicles that only use the back ends. \

Here’s a rolling frame only shot of the thing. The first thing that I noticed is that the front end is low. Now, while this gives the cool drag racer look, it will be very hard to turn properly. RazEr rEVolution had the same problem; both arise from the fact that the Razor A3’s folding joint has a taller riser tube (that meets the steering tube) than the regular A series.  The difference is about an inch of length.

I ended up switching RREV to a standard A folding joint while keeping the modified A3 fork. I might have to do the same for this if I want it to be remotely rideable – then again, the point is to not do anything besides go straight, right?

Alright, back to work. I chucked the motor in the Bridgeport (actually, just the shaft) and put the requisite flats on the shaft for proper set screw interfacing. I also had to remove the faceplate and redrill/tap the mounting holes for M6 X 1 screws.

Why? Because I keep thinking these Turnigy 80mm motors have 6mm mounting holes. In reality they have M5 threaded holes. I even specified clearance holes for M5 screws in the model of the vehicle. So clearly I’ve measured this before, but for one reason or another ordered 6mm bolts.

Well, as long as I have 6mm bolts, might as well use them.


The sprocket is a stock McMaster #25 sprocket with a 3/8″ bore. I bored this out to 12 millimeters, then made a spacer that put it exactly where the model dictated. Because way more power than specifications allow will be flowing through the chain drive, I wanted to make sure those sprockets are well-aligned.

Here’s the completed drivetrain. As usual with waterjet-cut sprockets, this needed a bit of a brute-force run-in period to “deburr” the sprockets and flatten out the taper a little – this time, it was accomplished just by kick-scooting around while the chain was under high tension.

Ihad to edge-machine the large drive sprocket because it was too thick. This was done by gripping the inside bore using an almost-closed lathe chuck with standard inside jaws – I was lucky that the bore was large enough to fit over said chuck.

Next, I put together the wheelie bar. This was easy, since it amounted to making some oversized standoffs. The small skate wheel is on an 8mm axle which is end-tapped and mounted to the side bars.

With the drive and motor mounts complete, it was time to start wiring up. Slightly bad picture, but I cut off the bright red handlebar grips and replaced them with a twist throttle and matching other-side-grip-thing. In retrospect, I should have kept the thumb-style throttle I usually use. But again, it’s supposed to be stupidly uncontrollable.

The battery is mounted to the underside of the top plate using sticky-back velcro. I added some strips of insulation foam on the bottom such that the bottom plate is also pressing up with some force when it is closed – no batteries are hanging by only velcro here. The balancing leads will usually be stored in the position shown, but periodically I can access them through the slot in the side.

Note to self: wires are big,

I didn’t really think of how much volume the wiring would occupy when I designed the Upper Deck of Turnigy Mounting. As it turns out, alot, especially if they are 12 and 10 gauge silicone insulated wires. There’s almost no space inside and I definitely had to push down on the top plate pretty hard to close it up. I can’t imagine the thing getting any airflow in this condition – so maybe it’s time for an air scoop on the top!

The electronics on this thing are utterly stock and simple – as close to the way my Scooter Manual dictates as possible. This is the cheap Hobbyking servo tester chopped and screwed to accept the analog throttle voltage input, and heat shrunk. There’s no added filtering or complications on it.

The electronics are mostly just stuffed inside behind the battery, though the big 5v BEC module is attached using velcro too. More strips and blocks of foam were added over these parts so they are also supported by the bottom plate.

And a shot of the totally finished back end, from the motor side. I like the way the motor is mounted – barely visible, and from afar looking like it’s just part of the frame.

And at Swapfest!

My “impossible to ride” conjectures were proven mostly right – turning is difficult because the vehicle can only tilt a few degrees from vertical until the low front end bottoms out. It does not handle any terrain. And oh yeah, the acceleration. Dear god, the acceleration. Test video will come soon – the videos shot today were mostly unimpressive because of terrain and people. I need to get into a basement freight corridor or a parking garage and just beast it.

Oh yeah:

This is what happens when you slam down the front end from an epic wheelie launch, if your front fork is made of shady 3d printed plastic.

Oops. Guess I’ll be changing that.

Straight RazEr

I’m going to take a break from updating all the 17,000 other things that are being worked on this week to announce yet another new thing. It’s a ground vehicle this time, and you can tell because it’s almost done already and isn’t lingering in software and half-assed broken hardware hell like the two air objects. I’ve in fact been working on it the most this week. But because the dev work on this vehicle has been going on for a while and includes things done months prior, this post will be abnormally long and pictureful.

Back in April, I went to the Véhicules Ecologiques et des Energies Renouvelables (EVER…it’s like CERN) conference with RazEr rEVolution in tow as a presentation prop, a part of Shane and I presenting somewhat legitimately on the construction of small electric vehicles as an educational initiative (and because it’s cool). At the “two-wheeled time trials/drag race/meetup” event after the conference proper, the hub motor scooters performed admirably against full size electric bicycles being used with pedal assist. Now, this being a totally unorganized “race”, I’m not complaining about the fact that the pure EV lost to the human-assisted EV, but I started thinking of how to build the most stupidly fast small scooter there can be, just to troll any further drag racing events.

Back in the hotel room, I immediately begin CADing what would be known as the Straight RazEr project. The idea was to stuff melon-scooter‘s power and sheer uncontrollable launch into a frame not much larger than RazEr. The vehicle would clearly no longer be hub motor propelled. I was fine with that. The final vehicle weight would still be 18 to 20 pounds and it would have a design top speed of 30 miles per hour, and use 5 inch wheels

This is what I came up with before I passed out in the hotel room.

I pre-emptively modeled in 5 inch Colson wheels because of their softness – durometer 65A – and width. This thing was going to look meaty with the 2″ wide wheels, and it would also have great improved ride over RazEr’s concrete-hardness wheels. But that was about as far as I got. The CAD sat untouched for several months after that because I figured I already had enough things to build. EVER faded out of memory and the reactionary desire to never, ever be beaten by anyone at a small EV race again tapered off.

Until a few weeks ago.

After my presence in Atlanta for the Mini-Maker Faire was confirmed, and wanting to bring to the table a little competition for my Georgia Tech rival, I decided to dig this design back up. Melon-scooter itself would be too large and unwieldy to bring. It’s also my general commuter vehicle, so it’s kind of dirty and beat up.  The design progressed in a few days from the above mostly-geometric representation to:

Now we’re getting somewhere. I’ve put in some prospective hardware and modeled the drivetrain. Most frame stitching and t-nuts have been added too.

The overall length of the vehicle is around 28 inches from a vertical plane tangential to the front wheel to the back of the frame, not counting the wheelie bar. I guess that’s a curb-to-curb length. The frame is a little thicker than RazEr rEVolution to give more breathing room to the batteries – an identical-to-RREV 12S2P A123 (WTF BBQ TLA) battery.

A little more modeling and a closeup of the rear end. I’m going to try a little harder to make this waterproof – one bad trait of melon-scooter is its very rudimentary open-air motor and drivetrain, making wet operation of the vehicle very uncomfortable. I’m not concerned about the electronics, but the belt, tire, and motor all sling water and road garbage over my back and legs.

To this end, the controller will sit near the motor in a little tray-like internal separation device that separates it from the elements. This will be a 3d printed piece. On the other side of the wall sits the battery and assorted switchgear.

A size comparison with RazEr rEVolution. The front forks are just copied and pasted for representation only. The vehicles are functionally the same length, minus the epic wheelie bar of Straight RazEr and the bulk of the rear end. I like to think of it as a epic power hump.

last week

We jump into fabrication with the usual picture of waterjet abuse.

I had purchased several large plates of 1/4″ aluminum for cheap on eBay the week prior, so it only made sense to just blitz it all out in one day. There was enough 1/8″ aluminum scrap in the machine room to avoid having to use any new metal.

These cuts all feature the assembly trick I used on Make-a-Bot, which is purposefully offsetting the waterjet nozzle such that the entire part is made smaller (and internal features bigger). The total taper across this machine (which does not have a fancy 5 axis head or tilty thing) on a 1/4″ thick part with average quality is about 0.004 to 0.005″. Therefore, setting the nozzle roughly 2 to 3 thousandths inwards makes everything slip-fit together nicely without sanding or filing.

Of course, I had a derp moment.

Some times, the lines on the surface of a part model are in fact just another part sticking into it and overlapping in the software. It is not in fact a slot you modeled to accept the part. With enough inattention, this flaw makes it through to production.

Result? Oops, need to manually make a slot there. This is a retention plate for the folding hinge, so it’s a pretty damn critical piece to mess up.

Test assembly of the frame…

I also made a battery for this thing concurrently with refinishing the RazEr battery. This time, the balance cables were routed more intelligently – I actually like how they run between the valleys of the cells. This ensures no wire is hovering or being mashed against a cell interconnect.  This battery was also wrapped with Epic Heatshrink – now from Hobbyking. The pack was charged and balanced by 4chan. That thing is so handy I can’t imagine how I lived without it.

While all this was going on, MaB was pushing through the internal separator device. I printed it in 2 halves, since a one-shot would have ended in sadness. There is some corner pulling on the ends, since it’s inevitable in a part this big. Considering that it spanned the entire build surface, I think MaB did quite well.

Here’s the ISD mounted in place.

I’m using a custom plate sprocket this time, so I designed a hub that links it to the 5″ rear Colson wheel. It was carved out of 2″ aluminum round in about 3 hours counting numbskulled mistakes and running back and forth between 2 shops exchanging parts, tools, and measurements.

Because this was a relatively large piece needing good finish and tolerance, I elected to use Bertha, the auto garage lathe (now 1 of 3). It’s a Rivett 1030 type machine that sounds like a small diesel truck. It’s refreshing to knock off 0.200″ in a single pass for once.

Before starting on the heavy metalwork, I set MaB printing the Epic Front Fork, similar to RREV’s fork. There are some slight geometry differences to accomodate the wide 2″ Colson wheel.

This is what happens when you update your part but not the exported DXF drawing for waterjetting. Oops – one bolt circle on the sprocket, newer bolt circle on the hub.

For some reason, this took me three tries to get right. The second set of holes were foiled when I realized my indexing fixture wasn’t fixed to anything. I had forgotten to tighten the table bolts on the milling machine.


The third set of holes worked.

While MaB was finishing the front fork, I made these bearing sleeves for the front wheel. I could have maybe… I don’t know, used flanged bearings? The Colsons have a weird 1.185″ internal bore, and I chose to kept the wheel stock instead of boring the wheel to something more common and using stock bearings.

Here’s both ends of the scooter. Maybe I can be hardcore like Amy and make a tinyscooter that has that exact wheelbase. Hmm…

Notice the cement holding the fork together. This build failed because the filament tangled while I was machining and caused one missed layer. I caught it about mid-way through and forced the Z axis back down one layer, but the adhesion ended up being poor. This fork has been designated bad, and is only being used for modeling and fitting purposes. It’s not seeing the street – not with a glue joint holding the axle in.

Oh yeah, axles – they’re stock McMaster-Carr steel shafting, drilled and tapped appropriately. Nothing too special there.

With the battery made, drivetrain mostly done, and electronics being bone stock, this thing ought to be running pretty soon.

Straight RazEr is named after the antithesis of the safety razor, the original shaving tool/murder weapon, the straight razor. And the fact that it will be really good at going straight – and that’s about it.