[Al Skierkiewicz]

As other have stated, your robot is reacting to the friction developed between the four wheels and the carpet in turns. You have to either turn two of the wheels when turning or reduce the side friction while turning. There are several ways to do this, one is to lift two wheels off the floor while turning. Another is to use a special type of wheel where there are rollers on the edge. It grabs in the forward motion but the rollers turn when the wheel is moving sideways. We developed such a wheel and the picutre is in the archive. Our mech engineer, Raul, calls them BUPOD wheels and they worked really great.