First draft of Sawppy the Rover‘s wheels exposed some problems that needed to be addressed in follow-up revisions. The first goal was to push the motor assembly further into the wheel so it does not protrude beyond the width of the wheel, to avoid collision with ground obstacles. The second goal is to change its proportions so it looks more like wheels on our inspiration Mars rover Curiosity.
After a little research, I found that Curiosity’s wheel’s width to diameter proportions are about 30% wider than our first draft. This is great news – by widening our wheel to match Curiosity proportions, the motors will end up inside the wheel, solving both problems at once.
When the second draft wheels were printed and assembled, it accomplished both goals. The wheel look like Curiosity wheels, and now only the suspension arm protrudes beyond wheel width. I then printed a few more copies at different wheel and spoke thicknesses… and again this was premature. There was a big problem staring at me in the face. Emphasis big.
When first draft was designed, I specified wheel diameter of 180mm. This number came because I wanted to see if it would print well on a 3D printer with 200mm x 200mm print bed and 180mm seems like a good value leaving a 10mm margin on all sides. Now that I have Curiosity rover dimensions in hand, I could answer an important question: If the model has proportions faithful to Curiosity, and wheels are 180mm in diameter, how big would the rover be?
The answer: a little longer than 1 meter, a little wider than 1 meter, for overall footprint greater than 1 square meter. That’s big! In fact that’s too big to navigate normal household hallways and doorways, or even fit in my car for transport. So for the third draft, the wheel diameter was shrunk down to 120mm. This still leaves enough volume inside the wheel for our servo motors, and results in a rover that should be an interesting size but not so large to be unwieldy. 120mm also happens to be the print bed dimension of some popular mini 3D printers, enabling them to print these wheels. (In theory, at least.)
From left to right, the first through third drafts of Sawppy the Rover’s wheels.