With the front half of LOLrioKart done for now, it was time to work on the powertrain. I don’t have a plan yet for the pedals, levers, knobs, and switches in the front. This includes the all-important front brakes, which I have to figure cable logistics for. I will return to them after all four wheels are connected to something that is (or will be) controllable.
Since UPS seems to have taken off early today, I wasn’t able to get the parts needed to fully finish everything that I wanted. Such a shame, because with a few hours of rigging, I could have wired the motor up and went on a (linear, fast, most likely very short) test run.
Let’s begin.
With my new 12 inch digital calipers!!!!, I measured the center distances between three critical points. First, from the rear axle to the shopping cart’s rear horizontal undercarriage tube.. Second, from said frame tube to the bottom rear basket support tube. And lastly, from said rear basket support tube to the rear axle.
Since I knew the side lengths of this triangle, all the necessary angles could be solved for.
Here’s a cool trick if you ever need to find the center distance between two rods or two holes, and don’t have a convenient handy-dandy centerline gauge handy.
Measure straight around the outside of the rods, or the farthest points on the bores. Measure again between the insides of the rods (with the ID side of your caliper), or the closest-together points on the bores. Add these two distances and divide by two.
Sure, experienced machinists are probably giggling over this, but it was one of those COOOOOOOOOOOOOOOOL I LERNED SUMFIN moment for me. No more eyeballing.
With critical dimensions determined, it was time to buy a few Jolts and 5am-engineer something (Like I resolved never to do again.)
The proper thing to do with any new design, especially one of questionable workability, is to prototype and proof the concept. Prototyping allows a more concrete observation, and subsequent interpretation, of your idea. Usually, prototyping is done with materials and tools immediately available.
Luckily for me, that happened to be leftover 1/4″ MDF and a giant lazer. I whipped up this 1:1 scale mockup of the motor-and-differential mount to see if my dimensions were actually correct. I did the same thing for NK5 before Dragon Con. It’s handy to prototype with a low-valued material, so you don’t bust money setting a flawed design in stone….
… or 1/2″ 6061 aluminum plate. After making sure the Etek actually sat in the mount and the mount actually sat on the kart (and by extension, the Etek on the kart), I headed back to the Media Lab with a half inch aluminum plate.
Only around MIT would you see someone sprinting down the street with a 3 foot long slab of metal. Nobody was decapitated in the process. An hour (40 minutes of which was trying to jiggle the 2D outline file such that it would actually cut properly – I blame the 5-year-old layout software) later, I had the semi-final, waterjet-cut parts. Semi-final because they still needed…
Threading, boring, and related secondary operations. Here’s everything thrown together as a visual. I used the boring head on the mill to clean up the bearing pockets. Some mounting holes were threaded 3/8″-16.
Also, I had to toss the rear mount back on the mill to add and/or lengthen some cutouts because I miscalculated the placement of some Etek features by a few degrees of rotation. Too lazy to drag the 200+ pound full size rotary table onto the mill, and not having a big enough head for the dividing fixture, I just manually interpolated a circular pocket.
It worked pretty okay.
Okay, so what’s the deal with those weird cutouts right next to the motor? I’m not seriously hanging a 25 pound high performance motor capable of gear-stripping, chain-breaking, shaft-shearing torque with a high-inertia steel disc rotor from a narrow cross-section of a material with a finite stress cycle life, am I?
That’s where those little Pac-man things come in. Here’s the whole transaxle test-assembled with hardware I found lying around. The Etek is indeed hanging on by one screw.
It is a well known fact that two objects cannot physically occupy the same volume in three dimensions. Since we happen to live in a (mostly) 3D world, I couldn’t actually install this transaxle onto the kart if the cutout wasn’t present (i.e. there was only a hole the diameter of the frame tubing) because it would involve passing it through the tube first.
Life isn’t Autodesk Inventor. So, the cutout slips over the tubing, and Pacman closes the gap from the other side. The two pieces are securely screwed together with 3/8″ cap screws. A circle of the same diameter as the frame tubing, 1″, is left in the middle. I will axially secure the transaxle on the frame tube. with some big 1″ two-piece shaft collars.
And it’s mounted.
This whole assembly is some Serious Fucking Metalâ„¢. It weighs in somewhere north of 35 pounds, more once I make new shafts for the differential (I didn’t expect to be building something around it – oopsie) and add real hardware (including giant steel standoffs for the Etek).
The observant will notice there’s nothing attaching this assembly to the basket mounting tube. It ended up that the basket itself was welded to this tubing with enough overhang that I could not easily use it to actually mount anything pointing downwards.
Unfortunately, this meant that the rear axle itself had to become a stress-carrying member, or else the transaxle would have no means to react against moment loads (torque). Having your motor spin freely around a mounting point is no good, since that means your wheels don’t spin.
I figured loading the axle like this was okay, since no less than ten ball bearings support the axle: four in the differential, two in the metal frame, two in each outer bearing block.
Granted, they’re cheap-ass ball bearings.
Despite not being joined in the middle, the axle should be just stiff enough after the addition of two giant keyed/clamp shaft couplers to give a bit of “bounce” compliance to the motor – beneficial – but still transmit power and keep the gears straight.
Give me some slack. This isn’t going into space.
Not this version, anyway.
So here’s the profile of LOLriokart with transaxle installed.
Wait, couldn’t I have put the motor under the rear frame tube? It would drop my center of gravity!
Yeah, by maybe an inch. If I had put the motor closer aligned with the bottom of the frame, I would have no place to mount the batteries. The batteries combined weigh around 80 pounds, and I want them as low in the frame as possible. Currently, they fit widthwise, four packs, lined front-to-back, between the side tubes.
So what can I do without my shaft couplers, 3/4″ shaft stock, motor mounting spacers, proper-length socket-head cap screws, and other things that UPS didn’t get to me today?
BATTERY MOUNTING TRAY!
The plan is to whip up a cage that drops slightly under the bottom of the tube frame and that mounts to said tube frame. If I’m crafty, I’ll also work in a flat mounting surface for the electronics, right above the batteries.
The battery packs will need to be woken up after 8 more months of slumber (one metered in at an amazing 3 volts!).
This is the last weekend of IAP. Next week, I’ll have to start worrying about all that “preparing for the semester” bullshit.
And the week after that, 2.007 starts.