If you run a lower voltage it has to draw more current (amps) to get an equal amount of power (watts) So when the current is higher you get more heat.. That is why those SC trucks that are restricted to 2S have to use huge fans to keep the motor cool. I would recommend to get a better motor. The Castle 1800 is somewhat undersized for this weight to my opinion. Either get a motor with better efficiency or a bigger buggy motor.