Contact Area and its Relation to Friction?

[Joachim]

Grippy non-linear materials like natural rubber, when on relatively smooth surfaces, can have a higher effective coefficient of friction at lower pressures--so for a fixed robot weight, larger contact patches can give higher friction becuase the rubber of the contact patch is under less pressure.

For an experiment showing the non-linear coefficient of friction of rubber (higher coefficient with lower load on the interface) see http://www.tuftl.tufts.edu/files/asu..._Testing.2.doc particularly graph 1 and graph 3. See also http://www.robotbooks.com/robot-materials.htm toward the bottom of the page, where you find the statement:

"The confusion here comes from the fact that rubber has a very unusual property. The more lightly it is loaded, the higher its apparent coefficient of friction."

Of course carpet can change everything, so you need to experiment for yourself.