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Jeff Ives
12-03-1994, 08:34 PM
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