Used to transfer power and torque to create speed. This gear is the first gear after the motors pinion gear, and usually takes the most abuse. If you find the power output is lacking even with a fresh battery, the spur gear may need to be changed out. Avoid High Torque moments like when your wheels are stuck and can't move. Don't use the motor to try to free them up. Walk over and free them by hand. The Motor and pinion gear will move but if the wheels are stuck, the Spur Gear won't move, and the motors pinion gear will strip the points off of the Spur Gear or worse.