Motor inrush current-power surge only when rapid full stick applied AND one motor group coasts

I have a problem I can’t seem to troubleshoot and would love this great community’s input.

We have a 4 motor drivetrain, two motors grouped: Left, Right. While on the bench and up on blocks (in other words no load on the motors) giving a short forward input on the sticks causes the wheels to move as expected but the left side continues coasting a lot longer than the right side. Checking the device screen on the brain one can see the left motors turned for 4.5 revs while the right side only 4.08 revs. First thought was friction. But checking the mechanism revealed no apparent friction.
And checking the power graph for the four different motors, expecting to find increased power for the motors where friction might be present, revealed no apparent friction but something else.
At full stick deflection, the four motors were average about .5W ranging between .2-.7W. No appreciable difference in power draw-- however , depending on whether the stick was smoothly moved from neutral to full or whether it was slammed forward produced two very different results.

Slowly moving the stick forward the power draw would momentarily reach 1-3W before settling down to .5W. With rapid stick deflection the power would shoot up to as high as 14W (which is higher than I thought possible with 11W motors) before settling back down. While both sides saw this inrush current and power spike the left side, the one that costs for .5 rev consistently saw larger spikes.

When placed on the ground and slowly accelerated the bot would track straight. With rapid full stick deflection the bot would angle left before tracking straight and the same thing happened in reverse. NOTE the increase power and the coasting are confusingly happening on the left side-- but the robot is tacking left when full stick is applied-- meaning either the left motor group is going slower or the right motor group is going fast and this only happens with the full stick deflection.

To summarize:

  1. I have one side coasting (and I checked the very simple code, the two motors are handled identically)
  2. When applying full stick I get a large power spike in all motors but particularly large for two on the left)
  3. when accelerating slowly the robot tracks straight. when accelerating quickly it turns slightly left then tracks straight. But since a competition is almost all rapid starts and stops by the end of a 2 minute match one side of the drivetrain has 50% more revs than the other.

I’ve tried swapping the cables so that the left and right side are reversed and I get the exact same result.

Any ideas?

This sounds a lot like friction. Double check that your spacing isn’t too tight and that there is no rubbing of any pieces, especially on the left. If that turns up no results, it could be a motor or cable problem. Try replacing some or all of the motors and/or cables to see if you get different results. You could also just test the code with 4 motors sitting standalone on a table to see if they all spin normally, which they should. If they don’t, try changing ports/brains/programs (sometimes a program is just doing really weird things). If none of this works, then the final thing I can think of is do you have more weight on the left than the right of the bot?

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Yeah… could be inside the motors themselves.