There are two main reasons to use a 2-speed gearbox:
Faster Sprint Times – Low gear is used for higher acceleration and high gear is used for faster top speeds, with a typical shifter spread of around 2-3x. This allows you to get from point A to point B faster than a single speed gearbox with the same motor power. In order to see any significant benefit though, you need to be sprinting a far distance (more than half the field length) and implement auto-shifting code. And even then you’re only saving a second or two each run. Nowadays it’s a lot easier to just throw 6 Falcons on your drivetrain and get more or less the same sprint time with much less complexity.
More Pushing Power – High gear is used for most of the match and low gear is used on the relatively rare occasion that you need to push another robot out of your way, with a typical shifter spread of around 5-6x. For games with wide open fields this isn’t necessary because you can avoid defense with speed, but it can be useful on more closed-down fields where you would expect to see defense in high-traffic areas. A very slow low gear allows for maximum pushing power without causing brownouts or tripping breakers.
Because of the difference in shifter spread and overall speeds, you generally need to pick one reason or the other for using a shifter. It’s hard to find ratios that give both benefits, and if you do you’re likely not getting either benefit in full.
For 2020, we chose to use a shifter to get extra pushing power. A number of times in 2019 we were hampered by defense where we could neither drive around it nor push through it. In our strategy discussions this year we saw a few points on the field where we were afraid that may happen again, so we decided the shifter was worth it. Our drivetrain was designed so that most of the time we would stay in high gear (18 ft/s), which was optimized for sprinting ~15 ft without shifting. When needed we would shift into low gear (~4 ft/s) which was optimized to give us the maximum pushing power at a slow speed without tripping breakers or causing brownouts. A spring return was designed to return the gearbox to high gear in case of pressure loss, but it was never implemented. We found this to be useful to help beat defense at the one event we attended, using it 1-2 times per match on average.