Chief Delphi - pic: Concept 8wd Drivetrain

Quote:

Originally Posted by JVN (Post 772603)

What I AM questioning, is why we let people get away with loosely referencing a machine like this as proof that "more wheels = more pushing force" or a similar argument. ESPECIALLY if "the physics" doesn't support such an argument.

Anecdotal non-quantitative arguments are driving me batty... That was my point.

The standard physics rule for both static and dynamic friction (Amonton's or Amonton/Coulomb laws) is 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]).

But this model works well only for some materials under some conditions--and rubber is not one of the materials.

Here are a couple of literature references:

(1) 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."

(2) 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."

Where robot weight is fixed and rubber or rubber-like drive contact surfaces are used, increasing the drive contact surface area--such as by more wheels, wider tires, larger radius tires, and/or belts or treads with many bogies--gives lower contact pressure, and often increases the available traction (friction), static and dynamic.

Of course this does not prove, for a given drivetrain on FRC carpet, that more wheels equals more traction. But there is a recognized effect that appears under the right conditions (and a physics-based model to characterize it is explained to some degree in the article text of the second reference, for those who can access it). So it is at least rationally conceivable that the proposed 8 wheels would give more traction.