Those results . . . were dynamic friction,
not static friction . . . .
Yes, but the two are often, though not always, correlated. The standard rule (or standard model) for both static
and dynamic friction (Amonton's or Amonton/Coulomb laws) is that that friction is the product of the load and the appropriate static or dynamic coefficient for the materials in contact, and is independent of the contact area. See, for example,
http://hyperphysics.phy-astr.gsu.edu...frict.html#fri (Article on friction at the Hyperphysics site of Georgia State). Thus an experiment showing that the dynamic coefficient of friction varies with contact area might lead one to at least adopt the hypothesis that the coefficient of static friction does the same.
The truth is that the nice linear model for both static and dynamic friction taught in basic physics and engineering courses only works well for some materials under some conditions. Rubber is not one of those materials. See, for example,
http://www.springerlink.com/content/n30715161g635138/ (abstract of a book chapter entitled "The Influence of Contact Pressure on the Dynamic Friction Coefficient in Cylindrical Rubber-Metal Contact Geometries") which states in part:
As it is commonly know[n], classic Coulomb’s and Amonton’s friction laws, which mainly establish that the friction coefficient is independent of the area of contact, are proven to be not valid in the case of rubber-like materials. See also
http://www3.interscience.wiley.com/c...69929/ABSTRACT (abstract of an article entitled "Analytical and experimental investigation of the static friction regime for rubber-rigid ball contact") which states in part:
The parameters of the static friction regime in terms of static friction force . . . are investigated for a rubber ball/metal flat configuration. . . . The coefficient of static friction decreases significantly by increasing the normal load . . . . Smaller radii of the ball determine a smaller static friction force . . . .
Where rubber is involved, increased contact area (wider tires, larger radius tires, or belts/treads) and lower contact loads often have the effect of increasing the available traction (friction), whether static or dynamic.