[Jared Russell]

In general, limiting yourself to 40A before you lose traction is overkill. You almost never need to push against a "wall" for more than a few seconds at a time, in which case you can use a much higher figure (easily 2x as much) before worrying about tripping the breaker.

In the drive designs that I have done, I generally first look at things without worrying about the thermal breakers. Assume that you can draw the specified motor stall torque and current for brief durations of time without any issues, and design around that instead. Then, verify that your calculated currents with normal loads and at maximum tractive force aren't going to be major problems. (For a space shuttle, I would model everything. For a FIRST robot I make assumptions and then verify that I wasn't too far off).

If I recall correctly, a 4CIM drivetrain with roughtop and a maxed-out robot reaches a point where gearbox torque and maximum tractive force are approximately equal somewhere around 8-9 feet per second (depending on what efficiencies you assume). We verified this experimentally when testing our drive this season (when pushing against a wall, you could force the wheels to stall by pushing down onto the robot - thereby increasing normal force and therefore maximum tractive force; and you could force the wheels to slip by applying a slight amount of "lift" to the chassis).

JVN's design calculator white paper can help you do all of the above.