[InFlight]

There's been quite a few gearbox threads that you could look at. We used vex ball shifters, custom AM super shifters, and various AM single speeds (Nanobox, Toughbox, etc); all with good success.

Lubrication and attention to detail when assembling gearboxes is critical.

We generally choose our gearboxes based on the drive type, and wheel size, and desired speeds.

For a CIM
Wheel Speed = (4455rpm/60 sec) * ( 1/gear ratio) * (wheel size/12")*3.14

For a shifter size your high speed gear ratio to ~15 ft/sec and the low speed will just fall out of the high/low ratio. For a single speed aim closer to 12 ft/sec.