The major motivation to build Micro Sawppy Beta 3 (MSB3), leaving MSB2 behind, was the increasing complexity of MSB2 wheels and I wanted a rover that is simple to build. Switching to using commodity TT gearmotor plus its associated wheels for the rover’s six wheels goes a long way. By using a gearbox that is already designed to support and drive a matching wheel, we eliminate custom modification and any worries about robustness of said modifications.
When I looked inside a TT gearbox I didn’t see any ball bearings, but I did see support structure that should suffice for a rover at this scale. At a very minimum, the rover would be as robust as every other robot design that use these wheels.
But like everything in design and engineering, there are tradeoffs. The biggest disappointment (and why I was reluctant to use these things to begin with) is that poor ground clearance. Comparing MSB3 against MSB2 and MSB1, we see ground clearance has degraded with iteration. There were good reasons to take each of these steps but it’s not the direction I wanted to go. Even with feedback that this feature isn’t ranked highly on many other people’s rover builds. Fortunately(?) I think this is as far as I have to compromise on this topic, at the moment I don’t know of anything that might further degrade clearance for future rovers.
Another problem is the fact these wheels don’t much resemble real Mars rover wheels. I was really proud of Sawppy’s wheel design and how I was able to scaled down for MSB1 and MSB2, but now I’m leaving them behind. While there’s the possibility to replace these generic wheels with 3D printed rover wheels, it would only be cosmetic. Given the geometry here, I don’t see a good way to restore functional ground clearance. But at least these wheels come with rubber tires, which was one of the biggest non-Mars-authentic feature requests for Sawppy.
A TT gearbox offers three attachment points. Two near the motor and one at the opposite end. While it would be most robust to use all three points, for MSB3 I decided on simplicity and used only the two close to its motor. The mounting bracket is angled to meet up with the steering axis, which is aligned with the middle of the wheel so the wheel could pivot in place.