[Tristan Lall]

Originally Posted by Chris is me

You are right in that the first factor is the shaft size - obviously the pitch diameter can't be smaller than 8mm.

For the more adventurous, you can turn down the shaft diameter, and add an aftermarket gear (like the ones for the RS-series motors). You could also cut gear teeth in it directly.<sup>1

1</sup> For the most adventurous, if you really want to go crazy to prove the point that we aren't limited by the shaft diameter, and aren't averse to some creative fixturing and metallurgy, you might even be able to machine it, then harden it in place, and finish-grind the diameter (to maintain concentricity) and the teeth (to clean up the profile). That would permit a very small, strong shaft. This isn't remotely easy or cheap, and it's virtually inconceivable that any FRC team has ever tried it. You'd need a lot of very direct heat and a lot of heatsinking ability to avoid cooking the varnish in the motor or demagnetizing the magnets, while still changing the phase of the steel at the tip of the shaft. Then you'd need to quench it fast. And finish grinding of gear teeth essentially requires a custom fixture, which you'd have to build. (Note that if the next set of FRC rules have a definition of modification that allows disassembly of motors, this becomes a fair bit easier.)