This was a fun robot to build. It's our 4th. It's the most agile one we've built so far. The 2 wheels mounted on the center access provides great maneuverability. I'm using small non-driven castor wheels in the front and rear. I use a simple toggle switch to toggle between fast / slow modes for outdoor / indoor driving. It's way too fast to drive indoors in fast mode. In slow mode, the microcontroller (propeller) just applies a maximum speed it sends to the speed controller. The Scorpion speed controller from robot power works great with the big 24V motors. I have a 25A fuse on each motor just as a precaution.