Re: car tire friction & contact area

[Joseph Bellina <jbellina@SAINTMARYS.EDU>]

This topic is always interesting because what really happens at the
microscopic level for most materials is more like the macroscopic tire.
The textbook rules for friction say the force depends on the normal
force and not upon the area.  In fact the maximum force depends on the
area of contact because the more area the more bond formation, the more
the maximum shear strength.
All materials have rough surfaces...the earth is smoother than a
billiard ball if you define smoothness as the maximium variation in
diameter divided by the diameter.
When rough surfaces are pressed together they first make contact at the
highest points.  As the normal force increases the surfaces are smashed
together increasing the contact area, hence the number of bonds, and
hence the maximum shear force.  Since the area in constant is a very
small fraction of the geometric area, it appears that the force does not
depend on area.
Seems to me that is what happens with rubber tires...find ways to
increase the bonding area, and you increase the maximum shear force.
However, if the temperature rises enough to affect the shear modulus of
the bulk material, then failure occurs not at the interface but in the
region behind it...so you leave rubber on the road.

Does that help or have I missed the sense of the question?

joe

On Mon, 19 Apr
2004, GJ
Schade wrote:

> I did an archive search on this topic and only found somewhat vague answers,
> so let me try to get the discussion going again.
>
> We learn in my high school physics classes that the amount of area in
> contact between two surfaces does not affect the amount of sliding friction
> between them.  Invariably, every year a student asks why are wider car tires
> better at handling, etc.  I respond, somewhat vaguely, that the tire is
> rolling over the road with little or hopefully zero slippage, and therefore
> it is a different phenomenon.  One student this year, showed me a car
> enthusiast listserv thread on the same topic.  One posting stated that Ff =
> mu * Fn only applies experimentally if mu < 1 and rubber versus road has a
> mu > 1.  Is that really the case?  Does anyone have a high-school level
> explanation for wider car tires that I can pass on to my students?
>
> I copied and pasted the address for the car enthusiast listserv thread
> below.
>
> Greg Schade
> Physics/Chemistry Teacher
> Parkway West HS
> St. Louis, MO
>
> http://www.team-integra.net/forum/display_topic_threads.asp?ForumID=10&TopicID=76204&SearchPagePosition=1&search=rubber+coefficient&searchMode=allwords&searchIn=Thread&forum=0&searchSort=dateDESC&ReturnPage=Search
>
> _________________________________________________________________
> Check out MSN PC Safety & Security to help ensure your PC is protected and
> safe. http://specials.msn.com/msn/security.asp

Joseph J. Bellina, Jr.               574-284-4662
Professor of Physics
Saint Mary's College
Notre Dame, IN 46556