This is dependent on your design objectives. 2 speed shifting gearboxes are desirable because they allow for design criteria which are often conflicting with a single speed gearbox. There are kind of 2 “schools of thought” as far as the design criteria each gear is intended for.
-Low gear is intended for pushing matches, and in some cases fine movement (e.g. bridge balancing in 2012). You want this gear to be traction limited such that the wheel slip condition (which is when torque the output from the gearbox overcomes the static friction of the wheels on the floor) occurs around or below 40 A per motor. With 6 CIMs, you might need to begin to consider the Main Breaker 120 A limit, but be sure to check out the spec sheet and understand how long your breaker will last at max current for the drive train.
-High gear is what you use at all times when you’re doing anything other than pushing an opponent or doing fine motion which is less effective at a higher speed. The aim should be to minimize the travel time for a given sprint distance that aligns with your style of game play. It’s nice if this is traction limited, but don’t lose too much sleep over the actual current draw numbers at your traction limit since you should never hit those with good, practiced driving.
-Low gear is intended for completing game objectives with a short sprint distance, and again some fine movement tasks. The gearing should be chosen to optimize that sprint distance. You want this gear to be traction limited such that the wheel slip condition (which is when torque the output from the gearbox overcomes the static friction of the wheels on the floor) occurs at some value which will allow you to be in a pushing match for some amount of time. You will also use this gear when in pushing matches.
-High gear is is intended for completing game objectives with a farther sprint distance. The aim should again be to minimize the travel time for a given sprint distance that aligns with your style of game play. Similarly to School A, it’s nice if this is traction limited, but don’t lose too much sleep over the actual current draw numbers at your traction limit since you should never hit those with good, practiced driving.
The merit to School A is that it’s often easier on the drivers to have a simple dichotomy of which gear to use when. When you’re mindset is “Pushing match = low gear, Every other situation = high gear”, it’s hard to go wrong. It is also good peace of mind to never worry about losing power during a pushing match, especially in years like 2014 (well…almost never :rolleyes:). Chris is spot on with his 6 ft/s suggestion if your design objectives align with School A. Depending on your efficiency and your CoF, 5-7 ft/s is a general range that gets you down to 40 A per motor at your traction limit for a full weight robot with battery and bumpers.
If my memory serves me correctly, one example of a School B design is the Killer Bees’ robot in 2013. Their robot was a floor pick up machine and would often times pick up discs from the floor during teleop if the opportunity presented itself. However, there weren’t always discs on the floor. In that case they would drive to the opposite end of the field to get discs from the feeder station. Playing the floor pickup role was a short sprint distance objective, and playing the feeder station role was a longer distance objective. They couldn’t necessarily know going into each match what role they would play, and sometimes it would change throughout the match, so having a separate drive train gear ratio for each style of play was an elegant solution. It seemed to work well, they were World Finalists after all.
In general, drive train gearing is a trade-off between how long it takes you to go from point A to point B and how much current you’re pushing through your breakers. When going above about 15 ft/s, you start to reduce your pushing force and initial acceleration in high gear at the expense of additional top speed (assuming a full weight robot plus battery and bumpers).