First off, most gearboxes used for drivetrain applications that you’ll encounter in FRC won’t be planetary gear systems. Please don’t treat them this way. The only common ones that are would be the VersaPlanetary from VexPro and the Banebots Planetaries from Andymark. As you can see, their names literally note the fact that they’re planetary gearboxes.
To better understand motors and gear ratios, you can look at this resource here: ](http://curriculum.vexrobotics.com/curriculum/mechanical-power-transmission)
It may be from VEX EDR, but the same principles and concepts transfer over to FRC. Take some time to explore beyond that unit to learn some other fundamental topics commonly used in robotics as well.
Once you understand the underlying concepts behind gear ratios and gearing, you can use the JVN Design Calculator to make better educated decisions on how you gear your robot.
For example, the Toughbox Mini used in the Kit of Parts drivetrain is a two stage, single speed gearbox. Stage means the number of reductions in the gearbox to get the desired output. The way these work is much more straightforward than a planetary gearbox.
Assuming that you’re using a standard setup for the KoP drivetrain (6" HiGrip wheels, 2 CIM motors per gearbox) and a max weight robot (154 lbs which is a 120lb robot + 10lbs of bumpers + 14lb battery), you get something that looks like this: ](http://imgur.com/a/jou7U)
Note that all the salmon colored cells are spots that are customizable. When designing, you can experiment and plug in different gear ratios, motors, wheel sizes, and more to see the resulting speed and current draw.
On the left there’s 2 stages of reduction: 14:50 and then 16:48 which has an overall gear ratio of 10.71:1. A ratio of 10.71:1 using 6" wheels results a free speed of 13.02ft/s and an adjusted speed (assuming a speed loss of 81%) of 10.55ft/s, which will be approximately the speed your drivetrain will go in the real world.
Note that you have a pushing current draw per motor of [EDIT: 71.14 amps], which is pretty safe. [EDIT: It’s best to make sure that your PDP breakers can tolerate your current draw, see post below for details] Current draw is distributed among your motors, so having more than one for your gearboxes is usually pretty safe. Stay with two CIMs per gearbox (total of 4 for your drivetrain) since having three CIMs (with a total of 6 for your drivetrain) can cause your roboRIO to brownout at higher speeds.
A common rookie mistake that I want to point out with gearing is that they’ll select very large diameter wheels in order to go “fast” (usually 8" or more). However, its harder to turn larger wheels and it significantly increases the current draw on your motors when your drivetrain is stalled (sometimes they don’t even move). Stick to smaller wheels and you’ll experience much more success on the field.
Hope this helps!