[Jon Stratis]

You know, i had a big long post drawn up here explaining the principles behind it all, and then i realized i was explaining it for an ideal, frictionless environment. I'm not a Mech E, so take this with a grain of salt, but here's what i remember from class years ago... The short answer is any time you turn you're going to have some scrub on your wheels. Your ideal turning radius is determined by the angle the trailer can make with your robot (at that angle or wider the trailer wheels will scrub the least. So by this definition, your center of rotation is going to exist outside the robot.

the way your center of mass and center of rotation work is like a pendulum. You have a long arm with a big mass attached to it, swinging around a central point. The longer the arm and the bigger the mass, the more effort it takes to rotate a certain number of degrees around the center. In those terms, the center of mass for the robot doesn't really matter.

What does matter, however, is how much scrub you generate on the robot's wheels. A robot that has a center of mass on the front edge will have a tendency to "power slide" in a turn - the trailer will whip around and pull the back of the robot with it. Similarly, having the center of mass at the extreme rear will give you a similar experience, as the trailer imparts a rotational force to the robot. A balanced center of mass would give you more direct control.

So, what's better? That's up to your drivers. The examples i gave were for extreme examples, where you have a bare chassis with a 100 lb weight on one edge. Most likely with most robot designs this year, you won't be able to artificially shift the center of mass enough to make a noticeable difference (by adding a 15 lb steel plate, for example).