Bevan-Pritchard, Marsha asks:

CATAGORY: Mechanics QUESTION: Can you explain to my husband and my brother why thin tires produce more traction than wide tires and why race car drivers prefer wide tires with no tread over thin tires with deep treads. I have tried many times with the typical male response; "you don't understand racing." They seem to think that race drivers like the wide, treadless tires because they produce more traction. Duh!! Can you explain in terms that they would understand? I give up.

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There is a wonderful essay on the physics of racing in a UNIVERSITY
PHYSICS textbook by William P.  Crummet and Authur B.Westin, Wm.  C.
Brown Publishers, 1994.  The essay is on page 160.  The author of the
essay in this text is Ruth Howes and the title of the essay she wrote is
`Physics and racing at the Indianapolis 500'.  It addresses most of the
issues you would like to discuss with your son and husband.  

If you are considering car tires for normal driving on roads that may at
times be wet or snowy or icy then without a doubt you need cars with a
good set of deep treads.  The treads channel water away from the tire so
that the tire can be in contact with the road.  The coefficient of sliding
friction between the tire and the road (needed to propel the car forward
and to brake the car) will drop to nearly zero if a layer of water gets
between the tire and the road (this is known as hydroplaning).  This
dangerous situation easily happens to treadless tires because there are
no grooves to channel the water away from under the tire.  Races are
delayed or canceled if there is water on the track for this very reason.  

Unfortunatly we cant cancel driving to work every morning because of some
rain or snow so for normal driving you are far better off with tires that
have treads than ones that do not.  This begs the question however as to
why race cars have wide tires with no treads.  At first glance it does not
make any sense because in PERFECTLY DRY CONDITIONS (to a first
approximation) there is NO difference in traction between wide tires and
narrow tires.  The reason has to do with the fact that the traction
depends on the number of contact points between the tires and the road.  

If you could look between the tire and road you would see that there are
gaps between the road and tire due to the roughness of both tire and road
so within the area of contact not all parts of the tire are actual points
of contact with the road.  Narrow tires provide less opportunity for there
to be contact points but because the weight of the car is distributed over
a smaller region there is greater pressure on the road under the tire
which helps to create more contact points per unit area than with a wider
tire.  Conversely wider tires provide more opportunity for there to be
contact points but because the weight of the car is distributed over a
larger region there is less pressure on the road under the tires so there
are fewer contact points per unit area than with a narrower tire.  As a
result the number of contact points, hence traction, remains about the
same.  Now if there were a way to increase the contact points by
increasing the pressure of the car tire on the ground or by increasing the
contact points without regard to the pressure or area then one would have
more traction.  

In todays race cars this is done by putting fins on the car which use air
pressure to push the car against the ground to increase the tire-to-road
pressure. Next they use special tires made of sticky rubber.  According to
the essay I read the tires cost about $1000 a set and Indy drivers go
through 12 sets!  The sticky tires actualy act like glue.  During the race
the rubber melts a bit and sticks to the road. the number of contact
points then does not depend so much on the pressure but just how much
sticky area there is. So in this case wider IS better for traction.

OK in summary: for normal, typical, driving conditions in all sorts of
weather [  or for situations where it is perfectly dry but  you dont want
to spend thousands of dollars per 500 miles on a set of tires! :-)  ] use
tires with treads PLEASE! both for your saftey and every one elses too.
On the other hand if you are racing on a groomed, dry, race track then
wide special sticky tires will give you more traction. 

P.S.  It is unfortunate that they (your husband and brother) said "you
don't understand racing."  That kind of a response usualy happens when,
even if they do happen to be correct, do not understand themselves!
Othewise they would explain rather than belittle.  It is also unfortunate
however that you refer to this as "the typical male response".  Please do
not stereotype!  I have found that everybody who uses stereotypes (both
men and women) hurt themselves and the society they live in.  The results
of physics, math and logic is independent of ones sex which is one of the
great things about it. Perhaps with truth and understanding in hand we can
all get along a little better in the world.

Good Luck.

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