Chassis & Leg Design:

Shelly's chassis and leg assembly was spec'd out in OnShape. You can see the design here

The main constraint's of Shelly's legs design are lateral rigidity, as well as overall assembly width. The ideal leg assembly would include 6 legs, not 4, however each leg needed to have enough lateral stability that it wouldn't buckle when all of the creature's weight was on 1-2 legs, and in order to accomplish the sculptural direction I was intending, I decided to stop at 4 legs. to obtain the correct width.

In order to achieve this stability, each leg it composed of 3 different co-planar leg-struts. Designing in OnShape allowed me to not only ensure each leg had sufficient stability (by making an assumption + using my design instinct), but also measure out how much specific distance would exist between each leg component in the assembly.

The crankshaft was designed independently.

And the final assembly was constructed to verify tolerances. 

Here is a Google Presentation explaining the initial conception of Shelly + her leg design implementation.

Cutting all the leg pieces was very tedious, and many angles were a huge pain in the butt to cut on my bandsaw because they were so acute. Next time I am going to have the components laser-cut out of 16g sheets and I'll use a truss system to mate them together. Ideally it will be rigid laterally, and also lighter/more precise.
The chassis are quite heavy, I don't remember the specific sidewall thickness but I'm fairly sure it's at-least 1/8 inch. This was the initial design. You may notice from my OnShape design that there is a "jog" in the chassis toward the rear. I had to cut off the back rectangle and raise it 2 inches so that the steering caster wheels would fit and the chassis would be oriented correctly in space.
Drivetrain + Motor v1:

Ultimately I abandoned the gas engine + clutch because it wanted to happily run at like 2,500 rpm. Shelly needed to get down to 250 rpm minimum, so my friend Andrew Renz helped me install an electric ebike motor. This allowed Shelly to actually propel herself at a reasonable speed!

Crankshaft Fabrication:

My friend James Erd helped mill out the stainless steel crankshaft components used to push Shelly's legs. It's undoubtedly the corner-piece of this build, along with the motors/electronics. I have a ton of respect of Jame's process, knowledge, and machinery!

My first and only part drawing I've sent to a machinist :)
Originally I intended for these grade-8 bolts to go through the crank-axles directly through the "center" of the axle. James recommended we "shoulder" the bolts along the top of the axle. I am unconvinced if this is better design, however because this was a collaborative process I was happy to do it a different way, especially if the functional result was largely the same. His argument was that more surface area would be in contact between the axle/bolt/crank-riser, and thus shearing would be less likely to occur. If anything, the orientation of the "notches" in the axle makes it so that you know for sure you are installing the components correctly, since if it went through the middle, minor variances in the alignment would be harder to perceive and it would be more difficult to assemble. It was a good design choice.

James taught me to use a rubber hammer instead of a metal hammer when assembling my tight-fitting metal components. It was one of the best pieces of advice I got on this project.

Leg Assembly & Articulation:

So, in order to achieve the lateral rigidity I referenced before, Shelly's individual leg components need to completely sheath the axel-bolts which hold the components together. I purchased round stock with a very thick sidewall and then used an annular hole bit to cut holes out of the leg components so I could weld the axle collars in. These collars were oversized just perfectly to grab onto the bronze Oilite bushings.

Portions of the legs connect to the 1" stainless steel crankshaft directly (instead of 3/shouldered bolts). These bushings connect directly to the bar-stock and are liable for the sidewall of the leg component to cut into the bronze bushing. I plan to replace those bushing as needed over time, since they are highly accessible and there are only 32 of them.

This closeup is where all of the leg components connect to one segment of the crankshaft. As you can see, I used copper tubing cut to size to space the components appropriately. There is a minor amount of slop which is acceptable, but could also be solved with additional spacers.
This is the brain-box for v1. 2x 36v e-bike batteries from Lunacycle lived inside the box. One was charging, and the other was being used to run the electronics. The power was run into a 36v -> 12v converter, and then again down to 5v for the lights. The motor controller connected to a physical throttle which ran up the steering column through the shell, and to a place where the driver could articulate the legs while steering.