Sawppy Wheel Traction Has Downsides And Upsides

Sawppy copied the rocker-bogie suspension system from Mars rovers Curiosity and Perseverance, who are destined to roam Martian terrain. But Sawppy on Earth mostly roamed flat ground instead of uneven terrain. This made the suspension largely superfluous except for contrived demonstrations, adding complexity and weight that is not strictly necessary. But I love it anyway, as a tribute to our Martian robotic explorers.

Another tradeoff Sawppy inherited from the big rovers are the wheels. Sawppy’s wheels surfaces are designed to mimic that of Perseverance rover. Grousers (raised ribs on the surface) designed to scrabble over sand and rock struggle to find grip on asphalt or concrete. This is not a problem for Curiosity and Perseverance as there is no asphalt or concrete (or carpet, or tile…) on Mars. But Sawppy struggles on made-for-human interiors.

Most wheeled vehicles on Earth use rubber tires of some type for traction. Many different varieties are available, optimized for different surfaces. Mars rovers do not use rubber tires because rubber (both natural and synthetic) would quickly break down in the Martian atmosphere. They are also quite heavy, and weight is a constant enemy for anything launched into space. Which is why Martian rover wheels are lightweight thin-shelled metal constructs designed just tough enough to handle all known Martian terrain. Unfortunately, Curiosity’s wheels have been torn up by some unknown Martian terrain. Lessons learned from engineering tests led to redesigned wheels on Perseverance that should better handle its voyage.

But none of that are of concern for Sawppy rovers here on Earth, so various rover builders have explored improving wheel traction. Chris Bond replaced the wheels with RC monster truck wheels, similar to those on JPL Open Source Rover. Steve’s Tenacity rover got some rubbery overshoe to fit over standard Sawppy wheels.

But increasing traction could also magnify other problems making them worse. With my wheels, a Sawppy rover can be tolerant of minor steering angle misalignment, the wheel will just slip sideways a bit as it rolls. With high-traction wheels, steering angle misalignment would start pulling the wheel towards/push it away from the body, twisting the suspension geometry. This has proven to be a problem with JPL Open Source Rovers and their high traction rubber tires. This is not very noticeable when driving short distances, but minor steering alignment eventually become a big problem for longer drives.

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