The contact incident marker is defined such that the x-direction is always in the slip velocity direction. I'd like to limit the friction force to only act relative to a direction cosine given by another part, and so break the contact friction into x- and y- forces in the contact incident reference frame to accomplish this.
Think of a box rolling on a surface with roller bearings, where the surface is inclined down at 45 degrees relative to the rotation axis of the roller bearings.
