On the multiple friction problem:
I would have used the physical way.
Frictions are defined against a "opposite material friction" of 0, so as if the other side does not have any friction.
What happens when two different materials with friction > 0 are used, is that they push each other up (so from that point of view, any of the 4 possibilities is just wrong. It won't land in between in normal cases. You know that most likely from experience in reality. Tires against Tires is much higher friction than Tires against Ice ^^)
I've seen different approaches to that (there is no generic one as in physics, the actual surface structure makes a diff), but basically you have max(fric1, fric2) + a*fric1*fric2 where a bases on the surface structure of the 2 in relation to each other.
I would have used the physical way.
Frictions are defined against a "opposite material friction" of 0, so as if the other side does not have any friction.
What happens when two different materials with friction > 0 are used, is that they push each other up (so from that point of view, any of the 4 possibilities is just wrong. It won't land in between in normal cases. You know that most likely from experience in reality. Tires against Tires is much higher friction than Tires against Ice ^^)
I've seen different approaches to that (there is no generic one as in physics, the actual surface structure makes a diff), but basically you have max(fric1, fric2) + a*fric1*fric2 where a bases on the surface structure of the 2 in relation to each other.