We used four motors in our drive system this year, 2 drills, 2 CIMs.
We used the first stage of the drill motor gear box, which matched NL speeds pretty closely between the two.
The drill motor is the more powerful motor. So, we chain drove a set of front wheels off the drill and direct drove the middle set of wheels.
The CIMs were connected to the rear set of wheels.
We had a total of 10 skyway wheels in our drive system (2 sets of duallys per side and one single on the passive axle).
We picked a gear ratio that gave us about 4.1 fps top speed with six in diameter wheels.
We had a ridiculous amount of pushing power, although we could not quite push the entire field boundary when we ran into it in autonomous.
We used the six axle approach, rather than the four axle approach to avoid high-centering on the ramp.
We are in the process of doing some sensitivity tests right now to tune the vehicle speed/pushing power versus motor commands.
As an aside, the drill motors have a distinct assymetry in forward/reverse. More so than the 2002 drills.
Some things which have not been raised in this thread (or others) about speed selection.
1. gearboxes have an efficiency. Torque out does not equal torque in * gear ratio. The more stages you put in your gearbox, the lower the efficiency. (not to mention machining time, weight and cost of gears) In an extreme case of a poorly made gearbox with ultra high gear ratio, the thing can stall your motor without providing any output torque.
In a FIRST system, you have a limited energy rate source. Ie. you can only draw so many amps per second. The less power you throw away due to inefficiencies, the better you are. This is especially true in the drive system, which is probably responsible for 60% of your resources.
Even though you may achieve the same GR with a worm and gear or a bevel gear or a spherical gear or a planetary gear, you should probably only use spur gears arranged in a conventional train. You should choose the smallest diameter wheel to achieve your overall goals.
2. Motors have an optimum efficiency point and a max power point. As you approach the max power point, you draw more current. In the case of the drills, this is an insane amount of current.
This will drain your main battery quicker, heat up your circuit breakers, heat up your motors. As your motors get hotter, they become less efficient. Ie, you start to lose power over time. You also start to degrade your motors. This last effect is seen in the NL current.
Bottom line, you don't want your drive system to just slip at your maximum pushing power. You want to have excess capacity.
Then, if you can "acquire weight" through some kind of redirection (picking up a goal, bins, whatever) you can make use of the additional drive system power.
Because of this excess capacity, you will be running closer to your "max efficiency" point during the majority of operation. Hence, when you finally do get into a pushing contest, your components will be less stressed than the guy who was faster, but who heated everything up just running around.