LEGO MINDSTORMS Robot Inventor
Center Drive Platform
This robot vehicle layout has the two drive wheels located in the center of each side, with the weight balanced so that almost 100% of the weight of the robot is on the drive wheels. This provides good traction and very good maneuverability.
There is a small balance/drag wheel in both the front and the back of the robot for the extra weight of the robot to balance on, although only one will balance wheel will touch the ground at a time if the surface is flat and smooth. As the robot's weight shifts during driving, the balance can shift back and forth from the front to the rear balance wheel, but almost all of the robot weight remains over the drive wheels for good traction and accurate turning.
You can build onto this vehicle platform to make larger robots that can benefit from this layout, such as the Gun Sentry Bot.
For a faster (geared up) version of this wheel layout, check out the Sprinter.