I’m happy to say that Chibikart has exceed alot of expectations.

Does this mean I finished it, finally? Yes. The build has taken in total about 3 weeks from announce to first ride test, which is rather short for one of my usually long and drawn out projects, but I was helped greatly by previously-made parts and an easy-to-assemble frame…..and not having to build a controller for it. Here’s how the remaining parts went down. The obligatory hoonage video is at the bottom.

First order of business was making the rear wheel anchor blocks. Unlike the front wheel which has tapped threads in the block (due to the need to pass the steering kingpin), the rear wheel is a fully bolt-through configuration, with the socket head 1/4″-20 bolt sitting in a counterbored hole.

With that part finished, I could finally put Chibikart on four wheels…

There was much push-riding involved. The steering was found to be very nimble and light , probably because of the very low drag wheels and high lever ratio. This thing also rolls. Because the hub motors have so little cogging and the wheel is moderately hard, it’s almost as good as a ball bearing caster wheel. I spent a few minutes finding “low spots” in the IDC lab space hallways, because Chibikart would actually start rolling towards them if pointed in the right direction.

I’m also glad to see that the wheel mounting scheme does not deflect much under even large white guys riding it (myself being relatively small and vaguely Asian). Granted, this is all on a smooth linoleum floor, and the real structural test will be when it goes outside.

Now, to do something about the baseplate upon which all the electronics will be mounted. It’s waterjet-cut from a 1/8″ polycarbonate sheet. I put in some slots for nylon/velcro strapping in order to hold the battery, and some holes to mount the Jasontrollers, but otherwise planned on freehanding all the necessary distribution electronics and other parts. I also cut out some test parts for the planned mechanical brake, but those are not yet installed.

So here’s the big generically orange block that was in the CAD model. It’s a 10S2P A123 battery module using 32113-type automotive cells. The total capacity of the pack is about 300Wh. That means Chibikart has an asston of battery for such a small vehicle. Now, while I do have giant Turnigy lithium sticks left over from the giant quadrotor of doom which are smaller by volume, this random surplus pack fit the frame so perfectly that I had to use it. It’s also more enclosed and has an internal BMS.

I didn’t take many pictures of the wiring process this time, because it’s very simple. In the upper right is a one-in-four-out terminal block which conveniently splits the battery input to exactly the number of power connections necessary. The throttle signal from the pedal gets split at a terminal block into the four Jasontrollers, each of which is otherwise only hooked up to power, ground, and motor. That’s it – 8 wires. That’s what I like about these things – they’re so bone simple yet effective.

Granted they also come with about 8,000 other wires which perform random generic electric bike functions (like pedal sense assist, cruise control, just not regnerative braking for some reason), and for all intents and purposes they don’t exist for this project.

At the pictured stage, I tried Chibikart in 2-wheel-drive mode with only the rear motors connected. As expected, starting from standstill is a little challenging because the motors have so little torque relative to my inertia. However, any movement at all is enough to cause the Jasontrollers to lock in and begin applying drive torque – even wiggling back and forth in the seat. They have quite an effective startup routine for what they are and how much they cost.

I expected that with 4 motors, it will either be better (all the motors contribute to starting torque) or totally useless (the controllers “park” the motors at the start in different directions, they fight eachother, and I have to perform an in-situ hip thrust to begin moving). Afterwards, I sacrificed some spare IEC power cords for their 18-gauge, 3-conductor hearts and extended wiring runs out to the front motors. Turns out, either situation can happen depending on where the motors stop – go figure.

I measured the current draw under acceleration at the motor for a totally stock (unmodified and unopened) 350W Jasontroller, and it was about 22 amps. Running at 32v, that means a maximum power throughput of 600W… Since the 36v native design can in fact run unmodified up to ~44v, it means that these controllers are a rare instance of some shady eBay Chinese product being underrated.

Here’s the “press shot” of the whole thing. I haven’t weighed it in yet, but it “feels” about 40 pounds. I must say I’m very pleased with the full 4-motor performance of the thing – I had expected that it would be on par with the skate motors (or at least, their very power-limited incarnation in the skates), but for some reason these are way peppier. This is probably because of their higher ampere limit (~20 amps per motor) and sliiiightly better efficiency. It remains to be seen how Chibikart handles an entire day of running outside, like at Swapfest as MIT vehicles tend to be debuted, instead of inside the cool, smooth, air conditioned hallways. Also, that lawn tractor seat is actually quite comfy.

And as promised, the video!

The short story:

• Frame size: 34″ x 18″

• Motors: Custom-wound and packaged direct drive hub motor, 300W peak each*

• Wheels: 100mm 87A skate wheels

• Battery: 32v 9Ah lithium iron phosphate pack

• Controllers: 350w-class Mysterious Chinese Sensorless e-Bike Controllers (“Jasontroller”)

• Top speed, theoretical: 26mph (voltage & motor RPM/V & wheel diameter)

• Top speed, realistic: 21mph**

• Actual top speed: To be determined?!

*30 second “peak” rating at 20 amperes

**Factoring in conservative estimates for air drag, and motor resistive losses at-speed, smooth and level ground assumed.

 

Alright, so the past few days have been spent mostly waiting on AmazonSupply McMaster orders and watering my seedlings who are now taller than I am, so I haven’t been able to tool on Chibikart as much. I wanted to get the rear two motors mounted ASAP, but working on the braking mechanism has kept me from finalizing the design for the motor mounting blocks. Therefore, I’ve been focusing more on the front wheels and getting the steering hooked up.

First, though, what the mechanical braking scheme will look like:

While I’ve certainly built plenty of vehicles which featured nonexistent or substandard mechanical braking, it’s just a bad habit to get into. I’ve decided to repurpose the Razor scooter fender brakes that I have collected through parting out quite a few scooters (and other people having done the same). Chibikart’s wheels are conveniently the same size as the standard Razor A and A2 scooter, so it was just a matter of reduplicating the mounting pattern for the brake pivot (relative to the wheel axle) on my own structure.

The more difficult part was actuation. Initially I was going to weld a steel tab or something to the steel fender to use as a pull crank for a standard bike brake cable (with return force provided by the stock torsion spring of the scooter). However, I decided to try something a little different. The cable would instead be wrapped around an offset circular Delrin (or similar plastic) cam, so if I pull the cable, the cam swings down and pushes on the brake. The (rather stiff) torsion spring provides restoring force. This execution ended up being alot cleaner than the design which involved the mysterious welded bell crank. Also, sweet rear fenders.

Onto steering!

Yes, that’s a Kurt vise speed handle. I found it eBay for like 10 bucks, and it was essentially the correct size and everything!

The steering column itself is made from a section of 3/4″ OD, 0.040″ wall chromoly tubing, droppings from the FSAE racing team. I sandpaper-finished the tube on a lathe to take it to the proper slight undersize in order to fit into the steering bushings. A 3/4″ steel hex shaft cutoff was machined down to the ID of the tube, and is retained via a clamping shaft collar.

Chibikart’s steering is just a tower of shaft collars. The leftmost one is a bottom retaining collar and prevents the shaft from being pulled upwards. The one to the right has a built-in mounting flange that I used to attach my “Pitman arm”, the driving link in a standard linkage steering setup. I wish I had known about these things when making a certain other kart’s steering arm. That one was more metal than I think exists in this entire frame.

finishing those damned motors

Over the past few days, I also had a few stretches of 100% HARDCORE NONSTOP MOTOR WINDING. Finally knocked all those motors out… My hands are unhappy, but nowhere near as bad as when I had to wrestle 20 and 22 gauge solid – that stuff takes more tension to get right, where as much of the wire tension on this build was supplied by my little winding jig.

The motor torque constants are scribbled on their cans. As I finished each motor, I lathe-o-mometerd it to obtain its BEMF profile, and the Kt was estimated from that.

Back to steering. Here’s one of the “swivelly block axle anchoring doobobs” which I’m sure have a real name I cannot remember at the moment. They are simple chunks of milled aluminum with a 1/2″ hole through them. A 1/2″ steel pin with threaded ends fits in the hole, and it rides in the flanged R8 bearings. There is no torque transmission to the axle at all – it’s just a slip fit, since the linkage will push on the block directly. I installed around 0.1″ of shims in between the block and the R8 bearing, but I’ve found out that this is not enough – under the weight of a rider, the aluminum block still digs into the plates above and below it!

The reason seems to be that adding shims to the stack seems to only succeed in pushing the bearings out of their holes. I should have designed with the flanges on the inside to prevent this – it seems obvious now, but it’s definitely a manifestation of 5am Engineering Syndrome of which I am probably the patient-zero for.

The linkage itself is bone simple. Those chromate plated ends are control rod ball joints which are convenient because they have a right-angle stud ready to bolt into a linkage. The arms on the axle blocks themselves are “square shoulder plain rod ends“, screwed into the blocks and then locked in place with a jammed nut. The linkage proper is a section of 1/4”-28 B7 threaded rod, not even in tension-compression arrangement.

I like this linkage alot. It gets the point of steering linkages across while being simple to build. The downside is it that is not a true Ackermann style steering linkage – to get that kind of inside-wheel-turns-more behavior, the wheels must be positioned further out so the steering arms could be mounted inboard from the steering kingpin, a design tradeoff I wasn’t willing to make. It is approximately Ackermann for the extremes of its travel because the center linkage is shorter, causing one side to approach toggle quicker than the other.

What I learned while making this linkage was that nylock nuts are in fact nuts in a shell. I needed a non-locknut in 1/4″-28 really quick, and wasn’t sure where to find them. However, I had a bag of 1/4″-28 locknuts. I machined off the nylon part in hopes of finding a plain nut, but instead I discover it’s a threaded insert inside a formed steel shell!

Now, maybe not all nylocks are like that, but this was one of those “oh so that’s how they make it” moments.

No, I still haven’t figured out how they got the ball inside the ball joint.

After receiving my order of axle bolts, here’s one of the front wheels mounted!

I tried to sit on this 2-wheeled arrangment (2 front wheels) and tested the steering, which is when I discovered the scrubbing problem. I’m realizing that “more shims” isn’t the answer in this case. I’ll either have to flip the plates (bearings on the inside) and shave 1/16″ off each side of the blocks, which may or may not actually be possible, or perhaps just mill off a thin layer of everything but a small section in the center to make a virtual shim without changing the thickness of the block substantially.

Or just, you know, pour cutting fluid into it and let it sort itself out.

Next up on Chibikart, attaching the baseplate, making the last two wheel mounting blocks, and throwing some electrons at the thing!