Friday, October 4, 2013

tinyKart Black: Some Nightkarting and 4WD Thought Experimenting

Now that  tinyKart is back in action, I've rediscovered the fun of running it around the parking garage lot late at night when nobody's around. I feel like the new paint job is more fitting for some night karting as well.

New venue, new test drivers: Zack driving, with Mitch providing chase lighting?

It's still running a mismatched motor pair, with an EMP 6374-200rpm/V motor on the left rear drive and a Turnigy SK6374-170rpm/V motor on the right rear drive. I attempted to balance out the torque and speed ratios using the maximum current and speed (voltage) settings in the Kelly controllers, with the left side at 75% current (90A) and 80% speed (32V) and the right side at 60% current (72A) and 93% speed (37V). Not that I think you would feel the mismatch anyway.

One thing that you do notice when poorly matched are the disk brakes. tinyKart has only front wheel braking, so if the braking forces are not well-balanced, it will pull the steering to one side. It was for this reason that we used this recumbent tricycle dual brake lever, which pulls both cables with equal force. Equal cable tension doesn't exactly equate to equal braking force, though, due to different amounts of travel in the brake calipers themselves and different friction in the cable housings. 

So after a half hour or so of trial-and-error tweaking and almost crashing into curbs, the brakes were balanced. When they are working properly, they are quite good. (The 160mm disks might be a bit overkill.) I do want to get the rear drive regenerative braking working too, though; not for more braking force, primarily, but for taking some traction load off the front wheels so they can do a better job of steering into a turn. tinyKart does have a considerable braking understeer, though it's usually complemented by a tiny amount of power-on oversteer, as demonstated in the video above.

I don't plan to leave the kart in its current mismatched-motor state for long. The left motor shaft could suffer the same fate as the former right motor shaft at any moment and the new right motor is transmitting torque through the feeblest of set screw flats at the moment. As a result I haven't been able to use the new Kelly controllers to their full potential yet. Time for a motor upgrade.

These are a new flavor of grapefruit-class motor from Alien Power System. With Leaders Hobby gone and HobbyKing switched over to the 59mm SK3 63xx motors, this is one of the only places I've found that still carries true 63mm outer diameter motors. And for a reasonable price, although still 50% or so more expensive than Leaders Hobby was. They do not have a can bearing, which means they might very well resonate themselves to death at high speeds like tinyKart's original SKs. But beggars can't be choosers. If I found a dodo bird walking around I wouldn't be like, "Damn, it doesn't fly."

They are well-constructed:

Nice double-sided magnet retaining provisions and seemingly good epoxying.
Clean and dense windings with no loose strands.
Total weight is just under 800g and the line-to-line resistance is 29mΩ. All good specs for a 170rpm/V motor. (I haven't confirmed the Kv just yet, but even if it comes up a little higher I would not be disappointed.) And maybe the most interesting feature:

A keyed 10mm shaft!
These motors are purpose-designed for electric long boards, so they provide an actual torque-transmitting feature on the motor shaft: a 3mm keyway! This makes my adventures in custom long-shaft machining and set screw escalation moot, I think. A quick FEA check on the aluminum pulley hub convinced me that even a short 3mm key should be okay for transmitting the full motor torque (6.7Nm at 120A).

Safety factor of about 4.
My mind has trouble accepting this result so I will probably also use the set screws I spent so much time on. But at least I don't need to machine a custom shaft for these motors. I can go back to using a separate idler shaft sticking out of the other side of the pulley to go to the outside bearing plate.

I should be able to get the new motors on and re-time the sensors this weekend. But in the mean time my mind became occupied with an interesting hypothetical test.

4WD Upgrade Feasibility

A long time ago, before the design for tinyKart was underway, I had designed the rear drive modules. (In fact, all the pictures of tinyKart have a prefix of "dm" for drive module instead of "tk" for tinyKart, since the media folder was inherited from this parent project...) The drive module was pretty simple: a motor (almost any face-mountable motor) mounted to a plate, belt drive, a tensioner, and a cantilevered scooter wheel on a custom 17mm aluminum shaft. But back then I didn't design it as a rear drive...I designed it as a steerable drive module for something like a giant Twitch.

Anyway, the drive modules' design got re-purposed for tinyKart's rear wheel drive. But recently I started thinking about whether it could have worked on the front wheels too. The conclusion I came to was, mostly, no. At least, not with disk brakes. The belt drive itself could work, but the brake disks would move to the outside and that would require significantly more structure to mount the caliper.

So, looking for any way to put the drive and brakes on the same side of the wheel, I sketched this up:

Drive Mod v2.0?

Starting from tinyKart's existing front wheel modules, I made only two very minor changes. First, I added a 75-tooth, Module 1, 8mm face-width gear to the brake disk hub. I originally thought of having it replace the hub entirely, but the hub is threaded for interfacing to the wheel rim and it's easier to simply pocket out the gear and fix it on top of the hub:

Pocketing out the gear saves a lot of weight too.
The diameter of this gear is limited by the brake caliper moutning. A gear ratio of 75:18 would match the rear drive pretty well, although there's no reason why they have to match.

Second, I replaced the normal brake caliper mount with an extended brake/motor mount that also holds an intermediate idler gear for achieving frame clearance for the motor.

Shown with the brake disk hidden.
My mind keeps coming up with reasons to dislike this spur-gear design, and then eliminating them almost as quickly. It's a fun game to play:

Spur gears? Really?
Yes. Belt will not fit in the small gap between the disk and the caliper mounting plate. Any other solution would involve lengthening the spindle shaft, which as all sorts of nasty side effects, or a complete redesign. (Either putting drive and brake on opposite sides, or something something even more drastic like a live shaft design.) Chain would work, but  it would be tough to install. Also, I have an deep-set aversion to chain drives after many years in FIRST robotics. I got some inspiration from this drivetrain for an Eco-marathon car I saw in Singapore:

But they are noisy!
Well so are chains and square-wave-drive BLDC controllers.

But the idler gear is stupid and inefficient. It looks like a 2.007 robot drive train.
The efficiency hit is negligible, compared to direct gear drive. It's certainly no worse than a belt drive. Chain might be more efficient but I kinda doubt it.

Such a thin gear can't handle that much torque.
It can, I did the math. Module 1 gears (shown in CAD) are borderline. But for sure Mod 1.5 or an equivalently beefy inch gear with the same face width would work. That's also assuming full motor torque, same as the rear drive. That's certainly not necessary and probably not even desirable, for other considerations such as battery life. A 40/60 or 33/67 front/rear torque split could work just fine.

The motor shaft isn't doubly-supported. It will break, or deflect so much that the gears will go out of mesh.
It does pain me to leave the shaft unsupported, but the maths don't lie:

Safety factor of 2 at 120A torque. Guess where it would break if it did....
Max deflection is 0.02mm.
Safety factor of 2 on the shaft is based on 1020 steel. I don't actually know what the shaft is made of, and the dynamic loading could in theory go higher than 2x static. But I did not model in the gear hub which will spread some of the bending moment closer in to the root of the shaft. And again the front does not need to run 120A torque, see above.

But the idler gear will for sure need to be doubly-supported. It can't have a 10mm steel shaft.
At first I thought the idler gear was safe because the forces on it would be balanced. But this is only true of the separation forces. The torque-transmitting forces actually add up and put even more bending load on this gear's shaft. But, the idler gear could potentially be on a dead shaft that is screwed directly into the mounting plate. So the moment arm will be much smaller than that of the motor shaft. This method would involve putting bearings in the idler gear itself. The details of this are TBD.

What about the aluminum plate? That could bend enough to un-mesh the gears.
True. I state without proof that it can be designed and engineered not to, even if the current CAD placeholder would.

It's impossible to assemble.
It was. There would be no way to put the motor pinion on from the outside. So instead I made a slot in the mounting plate into which the pre-pinioned motor slides. Then, it may take some specially-modified right angle hex wrenches to secure the motor in place. But it's no longer physically impossible.

So yeah, I started out very skeptical of the spur gear drive mod, but now I'm fairly convinced it would work. The appeal is that it can be tested on tinyKart with very little modification. That doesn't mean I'll actually do it - the cost and weight of two more motors and controllers isn't really justified. Also, the power to run the front wheels has to come from somewhere. The choices are either shorter battery life, more battery weight, or less rear wheel power. So for now it remains a thought experiment.

Though it would be an interesting proof of concept for future vehicle projects.........


  1. If we end up building a 4WD version it should be called tinyKart: the don't wear sandals edition. Also, while it may not be suitable for tinyKart, I think a motor-integrated differential would be interesting to make...something broadly applicable like the rear drive "modules" on tinyKart.

    1. It's already a little scary to have your feet resting in the metal guillotine of the front plate while the ground goes by inches away at 30mph.

      Yes, an ultralight one-motor transaxle would be awesome. Single state gear reduction to the differential. Output would be two CV-jointed shafts so it can be used in front or rear, with or without suspension. It would have to be limited-slip though, or it wouldn't perform as well as the dual independent motors, I think.

  2. I wonder about the design space where you merge the brake disc and drive torque transmission (pulley/sprocket/gear) functions. Are there any advantages?

    1. It definitely makes for a nice, thin, light solution. I thought about ways to combine them even more, such as putting and internal gear on the inside diameter of the brake disk, but the geometry just didn't quite work out. The width of belt required to transmit the same amount of torque as a bit limiting - everything would have to get pushed out 5-10mm compared to the spur gear solution.

      They're all just crazy ideas in my head for the moment. I don't think the added power would justify the added weight and complexity on the current design.