Tuesday, July 12, 2011

tinyKart: Round 1

By popular demand, more picture of crazy vehicles, and less math:

Extrude...thin feature...mid-plane...
That's the size of tinyKart. So I don't know, maybe it's not really in the tiny class, more like miniKart. But whatever, we'll still call it tinyKart since it's going to be 1/5th the weight of Cap Kart.

Step one in the tinyKart fabrication adventure is the acquisition of a large amount of waterjet-cut aluminum plate. The front half of tinyKart is a sandwich consisting of two 1/8" plates with 1" 80/20 t-slot extrusion in the middle. In addition to this, the rear frame, motor mounts, steering system, and brake caliper mounts are made of a combination of 1/8" and 1/4" aluminum plate. None of the waterjets that I have access to can handle 30" lengths of plate, so the entirety of the plate stock was sent out to the Big Blue Saw. A week or so later,

Kit Kart!
It's always fun to see your 3D model almost instantly become a real thing.

Maybe we'll get in the 80/20 flipbook again.
The only problem is we ran out of t-nuts. But the frame is together enough to get a feel for the size. It's a good deal smaller than Cap Kart, but it doesn't look tiny. In other words, a reasonable person would probably call it a go-kart, not a tinyKart. But whatever.

The motors also arrived, so of course I took them apart:

I was a little disappointed, but not really surprised, by the overall construction quality. The magnets, which are the source of most of the negative reviews for this particular motor (Turnigy SK6374-170), actually seemed to be pretty well glued. But the windings are very loose and in fact on a couple of the motors, one or two strands of wire were sticking out into the air gap. So while the motors were open, I took the opportunity to epoxy all the windings in place. I also tested the resistance, and they were all right around 23mΩ, line-to-line, which is a lot lower than the specs say. And there were no shorts, so that's good.

The next order of business was to machine the shafts, which Max, et. al., made short work of:

These shafts are made of 6061 aluminum and the entire weight of the kart and driver are transmitted through them. They start as 3/4" stock and are turned down to 17mm to fit the special replacement bearings for the 8" scooter wheels we're using. The rims get sandwiched on and the shafts are bolted to the chassis with a single 1/4-20 cap screw.

The load carrying capacity of this configuration has been verified in simulation and also by the more scientific method of jumping up and down on it:

The rims needed only minor modifications: For the rear wheels, the brake mounting threads were machined off to make the overall width of the kart as small as possible. For the front wheels, the drive pulleys were machined off to save a bit of weight:

And the last little bit of machining for Round 1, boring out the steering reinforcement plates. These are the 1/4" plates that hold the bearings for the steering kingpins. They are probably the coolest-looking parts of the front frame. I decided to bore them out all at once, which Charles and I decided was a 2 on the scale of bad ideas from 1 to Eating at Bullet Train:

Turned out fine.
Here's the frame at the end of Round 1:

It's certainly come a long way since the tape version on the floor. But there's a lot more work to be done on just the mechanicals, including all of the steering system. To get a quick estimate of how we're doing on weight, I hung the frame and a box of many of the parts that are to go on it from my favorite shady hook scale:

17.63kg, or just under 39lbs, including the batteries and motors, and the weight of the milk crate.. What's missing is only the seat, four tires, the two motor controllers, the steering column and wheel, the brake cables and lever, and any other miscellany that we decide we need. So, 50lbs looks tough but still possible.

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