[Al Skierkiewicz]

P,
If your front wheels are on a turntable so that they both turn about a central point then the turning radius is the distance between the two closest wheels. That is, when the turntable turns one of the front wheels comes closer to one of the back wheels. If you have enough travel in the turntable so that in a turn the back wheel does not move, that wheel is now the center of your turn. The radius then becomes the distance between the stationary wheel and the closest front wheel. However, your robot frame and the other wheels are still larger than that so the space you inscribe is the distance between the stationary rear wheel and the furthest point on your robot opposite that wheel. If the front wheel turntable is able to turn to a point where you could line up the center of both front wheels and that imaginary line where to intersect with the back wheel, I am guessing your robot would run into the trailer and that would become the limiting factor in your turning radius.