URDF coefficients of friction mu1 and mu2 question

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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!