Drivetrain Basics

[Chris is me]

Rather stupid questions ahead! Consider my rookie year and the only year I was a student was 2009, so I never got the normal drivetrain lessons I assume anyone else has.

All other things (rate of acceleration, climbing over large bumps, swerve drives, etc) aside, say I'm building a one speed drivetrain designed to drive at a decent speed and to push for a few seconds here and there, like I had to with 2791. I've picked up this from Chief:

1. The two ways to maximize pushing force are to maximize robot weight and to use wheels with a high CoF.
2. When pushing, you want your wheels to be traction limited so that your wheels slip as your motors draw 40A current; this way you do not trip the PDB's circuit breakers.

With those two things in mind, I ran some numbers to figure out how to best gear the Toughboxes we were using. Of course, being a genius I ran the numbers for an 8 CIM drivetrain, not a 4 CIM drivetrian, so the robot was torque limited. Not living that one down anytime soon.

Running the numbers again, I had a lot more difficulty getting a drivetrain that used roughtop while remaining traction limited at 40 amps current draw. Something like 4 feet per second would end up being your maximum speed if you followed assumption 2 to the letter. So I have some questions...

1. Is assumption 2 wrong? Do teams get away with pushing using a drivetrain and pulling 60 amps or so for the 1-2 seconds you're pushing? Do they rely on needing less than "maximum" pushing force since once your opponent starts moving the force goes way down due to the change to kinetic friction?
2. Do people use roughtop tread if they don't plan on direct pushing matches? I mean, it's used everywhere, and while I'm willing to bet most drivetrains from all the top teams shift between a traction limited and torque limited gear, I bet at least some don't plan on doing a lot of pushing. Is it used on normal drivetrains for high resistance to pushing, even if they don't have a method of becoming traction limited?

The main thing stopping me from saying "yes" to both of those are reading posts by people like Paul Copioli and whatnot that advocate being traction limited. The particular one that stumps me is way back from 2005:

Originally Posted by Paul Copioli

We determined early on that we wanted to be a fast capper so we wanted a pretty decent top speed (9.5 ft/sec). We also knew that teams would want to block us after we received a tetra from the auto loader, but before we actually left the auto loader. Given this assumption, we also wanted to maximize our pushing power without shifting gears.

Given our desired top speed, desire not to shift, and our pushing force desire we calculated that the 6 motor drive would give us what we wanted.

If we eliminate the FP from the drive, we would either need to reduce our top speed (increase gear ratio) to about 6.5 ft/sec or change tread material to decrease our coefficient of friction with the carpet in order to not trip breakers in a pushing match.

(I use this one for reference since my team was using a very similar thought process when selecting transmissions this year)

I read this post and then got really confused. Even lowering robot weight to 130 (pre-bumper 2005 robots) got me numbers way different than in this post for drivetrains that run at that speed under load. So, maybe my math is completely off and you can get 6 fps out of a drivetrain that's traction limited, and more with an FP in there, or the motor specs were lowered since then, or something.

So basically, can some of you smarter people give me some tips for gearing drivetrains in the future?