From: welty@balltown.cma.com (richard welty)
Subject: tires, the laws of physics, and reality as we know it
Organization: New York State Institute for Sebastian Cabot Studies
Date: Mon, 12 Oct 1992 16:03:33 GMT

i guess it's time for another entry in the frequently asked questions
list.

one of the most frequent sources of misconceptions, suprisingly enough,
is college freshman physics.  often, students are required to take this
course as a preliminary to going into some other corner of science or
engineering, so it is the last physics course they ever take.  the common
mistake is to confuse freshman physics with reality.

freshman physics is a useful basis for understanding reality, but anybody
who confuses frictionless surfaces and ideal massless objects with
the way the the world really works is asking for trouble.  else, why
do you think that engineers spend 3 more years in materials and statics
and dynamics classes, and physics majors spend 3 more years (and possibly
more of grad school) learning the differences between freshman physics
and reality?

the conclusion that tires cannot acheive more than 1g of grip in lateral
acceleration is one of those notions that comes out of a freshman
physics view of the world.  the problems iwith it are abundantly obvious
once pointed out.  we've already agreed here that cars with aerodynamic
assists to increase downforce can exceed 1g fairly easily; i'll not spend
time on this.  the other problem is this:  the freshman physics analysis
of friction is for idealized smooth dry surfaces.  tires and asphalt
or concrete roads are not smooth, and they are not rigid and flat.

the way that tires grip on dry pavement is as follows:  once at operating
temperature, the tire will deform to match the irregular surface of
the pavement.  the ensuing resistance of the tire to acceleration will
be as a result of sheer forces _within the rubber_; the rubber that
has been forced into depressions in the pavement surface will wish to
stay there, causing this situation.  the grip of the tire will then
be affected by several phenomenon: the extent to which the tire deforms
(this has to do with the operating temperature of the tire and with
the compound used), the size of the contact patch (which can increase
the amount of rubber which is resisting sheer), and the extent to which
a the rubber in a tire resists sheer.

any sort of liquid or fluid (water, oil, glycol) can alter the equation
markedly; note that i qualified my remarks above with the word `dry'.

the way that snow tires work is inverted; the rubber blocks bite
into the mud or snow, and then the mud/snow resists sheer.  normally
a soft sidewall is used so that the car can to some extent `float'
above the traction surfaces.  narrow snow tires therefore allow
the weight of the car to be concentrated on a smaller contact patch,
hopefully acheiving more bite.  ice racers and winter rallyists
actually sometimes go below 80 series down to 90 series tires to get
more concentration of weight.

richard
-- 
richard welty                            518-393-7228
welty@cabot.balltown.cma.com
  ``if you can read this, mario, you're too close''
    -- bumper sticker seen on a CART safety truck
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