I used a 4-motor high speed mechanum drive on a robot weighing about ~18 pounds, and it did stall frequently (it stalled just from strafing too much). My recommendation is to either stick to high torque and be a good defensive robot, or if you want to go efficiency use omni wheels on high speed.
No offense but using this configuration is a very bad idea in my opinion, and this is why. At the beginning of the season we had a robot that weighed about 17.5 LB or 7.9Kg and even though all our wheels were about as friction-less as possible we still had it frequently stall on us, and by frequetly i mean every match in the first 45 seconds of driver control. So even though our drive was completely square, and the wheels all could free wheel for 30 sec. we could drive with that amount of weight on our robot.
Just for perspective after this we hired a dietitian and cut our robots weight down to 14.5 lb with omni wheels, but still had unhappy results with our robot continuously stalling. So in summation from my experience anything greater than 13lb with 4 393's set to high speed is probably going to have severe stalling issues.
However if you were to take it down to 14lb or 6.35 Kg then if you put if you implement the smart motor library created by Jpearman and use the current monitor and slew rate you could have a fairly reliable robot with fewer stalls, however I still wouldn't use this configuration myself.
Last year we had a 4 motor high-speed mecanum drive with a 15 lb (6.8 kg) robot that would carry up to 15 bean bags (.5 lb each, .227 kg). It would overheat with excessive strafing, or pushing other bots, but other than that it was fine.
One thing you can try is splitting the 4 high speed motors between the two thermal breakers on the cortex. Motors ports 1-5 and 6-10 have separate thermal breakers, and splitting the motors make the motors less likely to overheat and stall. This may not seem like a major change, but it helped our robot be able to drive with high speed motors without stalling.
This is good advice, just note that the 1-5/6-10 breakers on the cortex are circuit breakers. They break the circuit based on overcurrent, whereas the PTC system in the motors “stalls” the motor when it is too hot (been used too hard too long etc). The circuit breakers in the cortex would stop all motors on that breaker if it detected too much current for too long. So if you were having this problem, you wouldn’t see stalling but every single motor on that breaker ceasing to function.
Your analysis is correct in that if the PTC in the cortex trips, then all 5 motors on that circuit would stop running. However, the same technology is used in the cortex and the motor, they are both PTC devices. The cortex PTC is rated at 4A, that means a current of 4A should not cause it to heat up and break the circuit. The motor PTC is rated at 0.9A, same thing, a motor running with a current of 0.9A should keep running. Both these numbers are only one of the specifications of the PTCs, there are others that determine a minimum current at which the PTC is guaranteed to trip, these are higher. All of the specifications also assume an ambient temperature of 25 deg C, this is one of the reasons that PTCs in the motor trip sooner when the motor is hot.
Maybe since this is still an ongoing issue with us you could tell us some of these teams names and numbers, maybe talking with people that have done it and it worked for them would help.
But anyway from experience earlier in this year that is my opinion, however i did notice that quite a few teams this year successfully avoided stalling issues due to some different measures that don’t seem to ever work for us, so maybe our experience can help others, and it has definitely got me to find out what our robot is capable of.
It was EVERY team last year. The sacks weighed .5 lbs each. To carry 15-20 of them at a time was really common. If you want numbers, go watch any of the Sack Attack Worlds videos. They’re archived on YouTube. At least 200-300 teams there had a 4 motor, HS base.