I am using Gazebo 7 and I want to use the Cart model with front steering available in Gazebo also with RViz.That is why I am trying to make a URDF version of it. The front wheels of the cart are connected to the chassis with the use of UNIVERSAL joints that rotate in 2 the y and z axes. However there is no UNIVERSAL joint type in URDF. Which joint type of URDF is the equivalent and do you konow of any examples/tutorials that use it?