Setting friction params

Ok guys, is any of you open to explain this in a simplified matter, or on a specific example? We all usually need friction for wheels; so If you have a wheel, turning on the **x** direction. You would want to have friction on the **x** direction. and on the **y** also so the wheel does not slip on the sides. So **mu = mu2**. But if you want to model an **omniwheel**, you would like the y direction to have no friction, so the wheel can actually slip that side. So you set up the **mu2 = 0** In case your wheel is not oriented on the specified axes you can change the direction with **fdir1**. I keep on seing people advising friction of 100 or some other values that are not comprised in the **0..1** interval specified by the **sdf documentation**. What is the catch with that? I played around with the friction, but apparently for the same settings, my wheels react differently from a run to another. I try to simulate a 3 omniwheeled robot base. And I've just come to realise there is no friction in the sdf code generated for gazebo from the xacro file written. code URDF: 100 1.0 0true generated code SDF with gw sdf -p: 1e+061 A part of my question can be also found [here](http://answers.gazebosim.org/question/1512/what-do-the-friction-coefficients-mean-and-why-are-they-so-large-in-the-drcsim-atlas-urdf-files/), but nobody actually answered that part.