Re: Automobile tires and friction

[Joseph Bellina <jbellina@SAINTMARYS.EDU>]

The microscopic view of friction shows that the force depends on the
numbers of local bonds formed across the interface.  In real large and
rigid objects there is little correlation between the geometric area and
the actual contact area, since the real surface is rough...hence the claim
of area independence.  Incidently the dependence on normal force occurs
because all objects deform to some extend and so as you increase the
normal forcue you squash the material together more and so there is a
larger area of contact.
I suspect that since the tires are much more deformable, the geometric
area correlates better with the real area of contact, and hence there is
an area dependence.
Incidently to give your student a feel for roughness in rigid objects.  If
we define it
as the ratio of the range of radii divided by the average radius, the
earth is smoother then a billiard ball.

joe

  On Sun, 24 Feb 2002, Michael
Bowen wrote:

> A student asked me last week, during a friction lab, why, if the
> coefficient of friction is roughly the same regardless of the contact area
> between two surfaces, wide automobile tires are commonly considered to have
> "better traction" than narrow ones. I couldn't come up with a good answer.
> Is the superior traction of wide tires a truth or a misconception? Does
> "traction" for tires depend, at least in part, on factors other than mu?
> Your comments, please.
>
> --MB

Joseph J. Bellina, Jr.               219-284-4662
Associate Professor of Physics
Saint Mary's College
Notre Dame, IN 46556