Hello Friends,
I am trying to use URDF to model a 4wheel differential drive vehicle. It is
more like Pioneer 3AT.
Tutorials explains basics but I still had not understood some of topics. I
have some queries.

1. How can one model differential drive behaviour in 4 wheel robot. I had
tried modifying erratic.urdf.xacro for this purpose but without much
results. I am attaching erratic.urdf.xacro.

2. Do one need to write some kind of new driver for such kind of vehicle.
erratic model make use of p3d .

3.. in <link> tag, origin is defined w.r.t. which coordinate?

I have one doubt about stage as well which might be very silly :-) . Stage
is 2D simulator. Do it treat environment and other entities as 3D and
visulalize it as 2D?

Thank you
NItin