problem with robot center of turning in navigation

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Hi all

We have modified our P3AT robot in a way it moves with just two front wheels it means we have disabled two rear wheels and the robot moves with just two front wheels. Accordingly, we have encountered with a malfunction in turning of the robot, because of the robot turns with two front wheels instead of all four wheels. Hence I am going to modify the center of turning of the robot from the center of robot to the center of two front wheels. I'd appreciate it if anyone help me to find the solution.

Best Regards Farshid