Originally Posted by duckman996
I found that as long as you geared properly the Epic based motors get faster as the com decreases to about 7.2 mm - then they just overheat and run strong for 2-3 min... after that, they die off quickly in a race.
That is the same experience I've had with the common Trinity motors. However, you can get around this by shaving both the leading and trailing edges of the brushes. As the comm gets smaller, the edges of the brush wrap around more comm surface area. This causes more overlap of the brush between the comm segments as the comm rotates. Another simplified way of looking at it is that more overlap usually means more heat with the increased RPM.
Shaving the edges of the brushes will reduce the comm overlap back to a level where the efficiency picks back up and the motor doesn't overheat. But you have to keep in mind that you also need to increase the spring tension some as the comm gets smaller. All of these things can be quite hard to master for consistency though.
donwhit is correct about the CP motors. I can run the comms AND brushes quite small without losing power. I'm not sure, but I think the CP motors don't heat up with the increased overlap since the amount of brush edge touching each segment changes as the comm rotates (round face versus rectangle face with long flat edge). However, I do have to spend some time with each motor on the CE dyno to find out how much to change the spring tension as everything gets smaller.