Setting up a wheel joint

Can I get some advice for setting up the wheel joint? I'm trying to use the differential drive plugin. I got most of this from the mybot tutorial, and it works, but I'm having two issues. First, I can't seem to add friction to the joint. When you set the speed back to zero, the robot keeps coasting along, as if there is friction, but very slight. Nothing like a geared motor car robot would have without disengaging the transmission. I've played with the settings for dampening and friction for the joint, but even high values don't make a difference. Second, the robot only turns slightly. I'm still trying to understand what the differential drive plugin does, but it's not very intuitive. Here is the main .xacro for the mybot And here is the xacro for the wheels and joint. Gazebo/Blacktransmission_interface/SimpleTransmissionEffortJointInterfacehardwareInterface/EffortJointInterface1