[KenWittlief]

If you have 4 motors on 40 amp breakers then 4*40=160 - so yes, you can easily trip the 120 without tripping the 40's.

4 Gauge wire: against the rules specified by FIRST - your bot would fail a critical inspection. Putting heavier gauge wire on the battery or other circuits reduces the resisitance, and will give you an (UNFAIR) advantage over teams that followed the rules.

Wire melting: where is it melting? at the terminal connection? thats a sign of a bad crimp. You might want to use some solder on the terminal and put heatshrink over it.

Two motors on one shaft - not an easy thing to do - the power/torque curves for the two motors are not identical - if they are not geared exactly right then one motor has most of the load.

Are your controls damped in SW? the worst thing you can do to your drive train (besides stalling the motors against a wall) is to go from full forward to full reverse.

If you did this in your car your drivetrain would be left in the street -but for some reason we think its ok to do this to our bots.

You esp need to be carefull in your auton code, backing up then going forward in one step, or going from straight to a tank turn in one step - can throw the motors from 254 to 0 in 26mS.

"she was a Day Tripper!
one way ticket, yeah
it took me Sooooo long to find out
but I found out!" -Lennon/McCartney