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Not sure...
Based on the characteristics for those LEGO motors, I'm not sure that your experiment should have worked out the way you think it did.
The larger motor has about 4 times the stall torque of the smaller motor. If this is the case, then the larger motor should have been able to back-drive the smaller one when you stalled the output shaft.
In a standard open differential, the same torque is always put out to each side, regardless of the output speed. This is why a car with an open differential rear end will get stuck even if only one of it's driven wheels is in a mud puddle or on a patch of ice. The proposed reverse differential drive should ensure that both inputs sides are supplying the same amount of torque - regardless of speed.
But... If the two motors can not supply the same stall torque and the output shaft is stalled, the torquier motor must back-drive the other motor. This will increase the back EMF of the stronger motor, reducing its torque output. It will also (I think) create negative back EMF on the weaker motor, INCREASING its output torque.
With one motor having 4X the stall torque of the other in your LEGO design, I think that you should have seen back driving of the smaller motor. There are two things that could have kept this from occurring (assuming the transmission was correctly assembled). First is that the battery you used could not supply enough current to the motors when you stalled them out. Second is that there appears to be some thermal protection for the gray motor, a thermistor that would cut the power when the current gets too high.
Since it looks like the red motor doesn't have the thermal protection, I would repeat the experiment with two red motors, but gear one of them down 2:1 before running it into the diff. Also make sure that you have new batteries, one for each motor.
In closing, I'll repeat what I said in one of the other threads about this transmission. If you gear the motors so that their stall torques (not their free speeds) are matched prior to entering the differential, this drive should work. I think that this set up is more electrically efficient that a matched speed (locked differential) drive - but I'm not sure that it makes sense in terms of bang for your buck. That's not necessarily what FIRST is all about though, so build it if you can.
Sorry this was so long.
Good Luck!,
P.J.
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