A RobotDynamics::Math::SpatialAcceleration represents a 6D acceleration vector with associated reference frames. It inherits from RobotDynamics::Math::SpatialMotion, and it is important to note that it will also inherit the RobotDynamics::Math::SpatialMotion::changeFrame method, but for an acceleration vector to change the frame it is expressed in, additional information is needed that is not available to the changeFrame method. Therefore it is important to note that for spatial accelerations, the changeFrame method will only return a correct result if there is no relative motion between the expressedInFrame and the new reference frame. If there is relative motion, you need to call the provided RobotDynamics::Math::SpatialAcceleration::changeFrameWithRelativeMotion method where you are able to specify the additional quantities required.