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I understand that you want to calculate odometry for Ackermann steering based on a frame that is centered between the rear wheels. But for planning purposes, the base_link frame is more convenient if it is in the middle of the robot's footprint. (The local map works like you want, for example.) Best to define base_link in your URDF as the middle of the robot footprint (however you define that), as well as a frame for midway between the rear wheels at a fixed offset from base_link, perhaps rear_midpoint_link. Then you can publish odometry relative to rear_midpoint_link. (Or transform yourself prior to publishing.)