Wheels are rotating but robot is not moving

I am working on a simple simulation that I cannot seem to get working. I am able to send a Twist message to the robot and the robot's wheels will turn but it doesn't move anywhere. I tried setting the `mu` values to the wheels to very high values (`100`, `10000`, etc.) but it has no effect.  Snippet from urdf macro: Gazebo/Black The full source code I am using is on my GitHub here: https://github.com/nick-paul/mybot_gazebo_tutorial To reproduce the error, clone the repo and run roslaunch mybot_gazebo mybot_world.launch then send a Twist message to `/mybot/cmd_vel`. Thanks for any help! Edit 1: It looks like the wheels are making contact. If I set the Twist to a high value and let it run for a little while, the robot will begin to slide around very slowly. You can see that it has moved a little bit every time this GIF loops: