I have two planes in the scene.  One plane is a shadow catching plane right under the car with Shadow_Catch_Only checked in its material properties.  The other plane has a background photo applied to it and is behind the car and the shadow catching plane.  I have one IBL light and one Infinite light directly above the car and turned on shadows and ambient occlusion for both.  I did some renders and can't seem to get the contact shadows under the tires to look right. This thread has more detailed info on my scene setup.

So what ambient occlusion settings should I use in the light settings to get a softer looking and better distributed contact shadows under the tires?

The light map size has something to do with the effect as well. Play with that ?

Bagginsbill has an improved shadow catcher shader...

http://sites.google.com/site/bagginsbill/free-stuff/shadow-catcher

Post-work them in. It'll take about 5 seconds.

Why spend so much trouble for such a little thing?

"A lot of screams for so little wool, said the man who sheared the pig."

Quote - Post-work them in. It'll take about 5 seconds.

Why spend so much trouble for such a little thing?

"A lot of screams for so little wool, said the man who sheared the pig."

That's cheating!  I will not stoop so low.   Lol.   Yeah, I was going to do that.  I just wanted to see if it can be improved within Poser first.  It doesn't hurt to learn a few new tricks.

First, are you using Depth Mapped shadows?  If so, don't.  They are useless.

Second, if you're using AO, set the material to AO, not the light.   

if using poser 8 or later, try IBL/HDRI with IDL to get occlusion shadows (no AO).

Quote -  

if using poser 8 or later, try IBL/HDRI with IDL to get occlusion shadows (no AO).

Wow, that looks great.  I'm using the original Poser Pro.  I've been planning to upgrade.  I think I'm finally going to do it.

here's a close-up.  it would look better if I had a good tyre displacement/texture map.  AO can be made to look good, but it's easier to use IDL, hence many users (e.g. me) may have forgotten the right AO settings.

Nice!