### URDF coefficients of friction mu1 and mu2 question

Can someone explain what these coefficients of friction mean? From looking at this tutorial it says that `"Here mu1 and mu2 are the friction coefficients of the wheels."`

http://www.ros.org/wiki/urdf/Tutorials/UnderstandingPR2URDFGazeboExtension

However, when looking at the PR2 urdf for the wheels these coefficients are fairly large (`100.0`

). That seems extremely high for a coefficient of friction.

The coefficient of rubber on dry concrete is `0.6-0.85`

for example.

Thanks in advance!