Pass parameters to xacro from launch file, or otherwise

Is there an example anywhere of how to actually pass a value from a launch file to the xacro files? I found a number of things here and there, but nothing clear as to what you need to do in the launch file and then how to get and use the value in the xacro. I'm just trying to spawn two robots in a single world with a couple different parameters. I'm using kinetic. I've examined these bits of documentation but I can't figure out how to weave them together. I've experimented with enough that it would be clutter to include. http://wiki.ros.org/xacro (5) http://wiki.ros.org/roslaunch/XML#substitution_args http://wiki.ros.org/roslaunch/XML/arg Here is what I have right now. robot.xacro ... ... robot.gazebo ... falsetrue100left_wheel_hingeright_wheel_hinge${chassisWidth+wheelWidth}${2*wheelRadius}20${ctopic}mybot/odom_diffdriveodomfootprint ... robot.launch ... ... Thanks