Based on past experience building with aluminum extrusions and 3D printing, I was confident I could build most of the rover structure. The construction technique I used had been evolving through the Luggable PC projects and was further refined by building Sawppy. It ended up as a Hackaday.com article to be shared with the world.
But the wheels? That was a question mark early on. 3D printing a rover wheel was an important part of hitting our $500 price target. If we have to buy commercial remote-control monster truck tires and wheels like the baseline rover, that would take a pretty big bite out of the budget.
Besides cost, we also want our wheels to resemble that on the actual rover, with extra brownie points if we can also replicate some functionality. Curiosity’s misfortune is my luck in this case – thanks to unexpected wheel damage on the Curiosity becoming a news item, there are lots of online pictures of rover wheels for reference. I also had the chance to take a close-up picture of a model of Curiosity in the lobby of a JPL building.
We see six spokes, and the shape of each spoke shows an integration of two distinct mechanisms. The outer ~20% of the wheel stands at a shallow angle to the outer rim, making it amenable to bending to absorb shock. The remaining spokes stand perpendicular and can still help with shock absorption but will be much more reluctant to do so.
Another important detail is that the wheel mounting point towards the outer far edge of the wheel. So the spokes must be simultaneously soft to up-down forces but strong against twisting forces to make sure the wheel surfaces stay flat against the ground.
The real rover’s spokes were machined from titanium and their shape isn’t something we can duplicate with hobbyist level 3D printing. However, we can copy the general profile which will give us some of this two-stage shock absorption mechanism. And by extending our imitation spoke across the width of the wheel, we gain the desired strength against twisting forces without using titanium machined parts.
This is pretty good for a first draft, but there’s more fine tuning to be done to this design.