And for now, my first attempts to master Friction. Some things in life are simple: the implementation, the concept, the experiment and the physics theory. beyond that, I'm on alien grounds. But let's present my current findings first.

Poser implementation

Friction comes in two flavors (static and dynamic) and both can be set in two places, as was flagged already by Laurie and other posts above. They can be set for each Cloth-object-group individually, and for the Sim-Object as a whole. By (un)ticking the box in panel 4, below the Play Simulation button, you can choose which ones to use.
This way you can establish a rough setting first for the whole Simulation / Cloth (remember: those settings address all cloth-objects from the list, there are no settings per cloth-object). Then you can fine tune the behaviour per cloth-object-group.

Friction is important for cloth room. While density (gravity) and air-resistance address the interaction between the cloth and the room itself, friction addresses the interaction between the cloth and the figure it collides with. Simply stated: when the cloth vertices hit the figure in a perpendicular way we're talking collision, and when they interact in a parallel way, we're talking friction. 
So friction is as relevant as collison. Now you (and me myself) can understand why the cloth-object settings in panel 2 address both in the options. 

The concept

When a piece of cloth lies down on the floor or on something else, and a force - like gravity - is applied on it to drive it forward, then the contact of that cloth with the floor will work against it. Up till some limit the cloth won't move at all, when that limit is exceeded it will move but will still apply a force against it. The latter is called dynamic or kinetic friction, and that limit is called the static friction. You might infer that dynamic friction should be quite less than static, which is correct (although Wikipedia reports on some exceptions in the laboratory).

The experiment

Easy to do at home. Clean up a smooth long table, put a piece of cloth flat on one end and tilt the table. Until some tilt angle the cloth will stay put. That's static friction.Then the tilt becomes larger than that, the cloth starts to move smoothly. Thats dynamic friction working against the gravity pulling it down.

So this will be our Poser experiment too. Large long thin box, and a piece of cloth on one end. Start just above the box and give it some (say 10) drop frames to end up on the box.

Now create an animation, say 240 frames in total, and make the box rotate along the Z-axis in the first 120 frames, up till the angle of interest, say 30 degrees. You have to do it this slowly, because otherwise the cloth will get launched. And it's a good idea to zero out Air Damping.

First turn down Dynamic Friction, till its lowest 0.0001 value. Then start playing with the Static friction, till you find that one value that starts / stops the cloth from moving when the box it at its largest angle, and the cloth is on top. Just a fraction less and the cloth starts moving. That 'critical value'. Each angle of interest has its own critical Static Friction value, and vice versa.

At low angles, the values are less accurate to determine. These are mine:

angle static

10 0,050 - 0,045
20 0,060 - 0,055
30 0,1205
40 0,2363
45 0,8744
50 > 1, the max value 

A similar experiment can be done for Dynamic Friction. Same setup, take a Static Friction value a bit below the limit, so the cloth will move but not before it has reached the top at frame 120. Vary the Dynamic Friction value and note the frames (the time) that the cloth passes halfway and the end of the box. Higher friction values make lower speeds and therefore larger pass-by framenumbers.

After measuring the size of the box we can infer the speed, meters per frame or per second, as a result from the dynamic fraction, at that angle limit of the box. This is a shipload of details, so I'm not posting them (yet).

In the meantime, I notices a few effects while playing with the parameters.

• Density has no effect on friction, at the larger angles. This is physically correct. but it does have effect at the smaller angles. 
• The stiffness parameters (fold/shear/stretch) do not have effect, until the stiffness passes values like 400. 
• In some cases with large parameter values, the cloth started rotating while coming down. 

I have no physical interpretation for any of these. It might be something in the simulation algorithm.

The theory

Wikipedia has good info on this. Friction is a force, which works against the force that drives the cloth over the box surface. The friction is proportional to the force which presses the cloth onto the boxes surface.

When the box is tilted, the driving force from gravity reads F = d * S * g * sin(z) for density d, cloth surface S, gravity acceleration g and angle z with the horizon (flat = 0). And the friction reads F = f * d * S * g * cos(z) for friction parameter f. 

At the angle where static friction just prevents the cloth from moving, both forces are equal, and all collapses to f = tan(z), having most values between 0.3 and 0.6 in real nature. Values for f larger than 1 are rare.

For dynamic friction, we've got Coulombs Law stating that the force is independant of the sliding velocity. So, when the cloth moves, we should see a constant acceleration of the cloth with gravity force F = d * S * g * sin(z) minus friction F = f * d * S * g * cos(z), over the cloth mass dS. Hence, the acceleration reads g * [sin(z) - fcos(z)].

This is like free fall, so we might expect a falling speed v = g * [sin(z) - fcos(z)] * t (t for time) and distance s = 1/2 * g * [sin(z) - fcos(z)] * t^2.

Correcting for unit conversions, and noting that the friction parameter is a ratio between forces and therefor unit-less, we should be able to interpret the Poser measurement results.

The issue

Will, my poser cloth room experimental measurements do not fit physical theory. That's it, plain and simple.

The static friction angle vs critical value list does not follow the simple f = tan(z) or anything alike. The dynamical friction values do lead to neat distance vs time relationships, but not the one from Coulombs Law.

Hence my queste for this moment is: what rules do the Poser friction values follow, perhaps even why, and how do they relate to the real world. Suggestions welcome.

- - - - - 

Usually I'm wrong. But to be effective and efficient, I don't need to be correct or accurate.

visit www.aRtBeeWeb.nl (works) or Missing Manuals (tutorials & reviews) - both need an update though