From Spadout, one of our largest sponsors:

Climbing Rubber Friction Test

Summary: Spadout published the first publicly released, third party climbing rubber friction test. The test was designed by physicist Steven Won and tests the top 9 climbing rubbers on granite slab and a climbing hold.

Methods

We measured the angle at which the rubber sample failed (slid) on both granite and a flat plastic hold. In both cases the range between the best and the worst rubber was less than 6.5 degrees.

What we tested

We tested all "popular climbing rubbers". This is defined by rubber featured on at least one climbing shoe that is sold by at least 5 online retailers.

How the tests were performed

There are nine popular climbing rubbers on the market today. We purchased one pair of shoes for each type of rubber. The soles were removed and 2x2 inch samples were cut. A weight was taped to the top of each sample. The samples were placed at the same starting location on a granite slab. The angle of the ramp was increased until the sample failed. The angle was measured.

The same process was repeated on a flat climbing hold. Twenty tests were done for each rubber type (10 for plastic, 10 for granite). Both the left and right shoe's rubber was tested to verify the test was consistent. We also did a “pull test” (using a spring scale and a 5 lbs weight) and received similar results.

Full Results
http://www.spadout.com/r/climbing-rubber-test/

I like it, generally, but it does have a weaknes in the test- it does not measure under real loads (IE. 100+ lbs/sqIn) and perhaps moreso, it never does address the property of the material under heavy loads (IE edge rolling/perversion).

Softer materials liek this mold to varrying degrees to a surface, and interlock in ways other than frictin under very heavy loads.

It occurs to me that using a far greater weight and a force gage to measure the slip-stress on the same sample might be more accurage, but admittedly, harder to administer.

Other reviews have dome a Durometer (Shore test) reading. It's published in the mags and I bet one could find it if looking back far enough. I used to get both major American covers, so I'm not sure which one.

Seems to me the Red Chili shoes I've tired are visciously sticky, and hold small edges well. Wonder what rubber that was? That was on granite.

I also noticed the HUGE difference in ratings for indoor Vs Outdoor. Wonder what sticks best to small sloping Eldo holds? I dunno.

All that said, I once had a pair of Mythos soled with XVS on one, Stealth C4 on the other. I was surprised that I was unable to ever tell the difference in the gym (they were indoor shoes). I don't think pure friction was ever the key when I was near my limits.

The only thing that seems to make a difference on indoor holds is if you can get your foot to bite and not roll on those really tiny holds. Straight friction basically never matters the way that it does on a Tuolumne face route.

The test document is dated 2005.

How many of the rubber formulas have changed since then?

How many manufacturers have introduced new named formulas?