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  • Friction: responses

    Colleagues;
    Recently I posted the following query:
    During my class discussion today on friction, that is, the
    force needed to overcome friction is proportional to the
    normal force and the coefficient of friction--and not the
    surface area--a student asked why then do performance cars
    have wider tires? I muttered something about stability
    and center of mass and base of support. Does anyone have
    any other explanations, such as softer tires (higher
    coefficient of friction) need more surface area to decrease
    wear and improve tire stability?

    Instead of posting the individual responses, I will summarize
    them and add my own findings. In general, most of the
    responses were speculative, and centered on factors such as the
    nature of the tire-road surface interface, heat dissipation,
    tire deformation and elasticity, stability, and a myriad of
    other environmental and engineering constraints. The 'best'
    answer suggested that the tire problem did not fall under
    the standard Coulomb (dry) friction parameters, thus using
    tires as an example was comparing apples to oranges.

    Indeed, further digging supports the latter statement.
    According to Engineering Mechanics: Vol. 1, Statics (2nd ed),
    JL Meriam and LG Kraige, Wiley and Sons: New York, 1986, the
    coefficient of ROLLING RESISTANCE, while analogous to the
    coefficient of static or kinetic friction, is really an
    entirely different beast. It would be most difficult to
    describe fully without a free body diagram, but is a function
    of many factors, including, but not limited to: road and tire
    deformation and the resultant pressure over the area of contact,
    elastic and plastic properties of the mating materials, wheel
    radius, speed of travel, and roughness of the surfaces. Meriam
    and Kraige state, "... depends on many factors which are difficult
    to quantify, so that a comprehensive theory of rolling resistance
    is not available."

    Moral of the story? There are two: 1) Theory is just that, and
    2) Stick to wood blocks on inclined planes.

    Thanks to all who wrote.
    Jeff Ives, Ph.D.
    Dept. of Exercise Science
    Campbell University
    Buies Creek, NC 27506
    jives@camel.campbell.edu
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