Selecting Tires for Race/Street Use and Adjusting
Tire Pressure to Optimize Handling
by: Miroslav
Ovcharik
The tire you use is one of the most important
suspension tuning choices you will make in building
and racing your car. Even a car with a
professionally-tuned suspension will not be able to
clock fast laps or handle well on a poor quality or
inappropriate set of tires. The major performance
factors to consider when choosing tires are
compound, contact patch, and sidewall height.
A softer compound tire will have more grip but
will wear out quickly while a harder compound tire
will have less grip but will last for a relatively
long time. Your choice of compound is mainly
affected by the type of racing you plan to do and
your budget. For serious racing, you will need a
soft compound tire to be competitive. If you plan to
do mostly street driving and/or do not have the
money to frequently replace tires, go with a harder
compound.
The second factor to consider is tire contact
patch, or the area where the tire makes contact with
the ground. You want the biggest contact patch that
you can get. When more of the tire is in contact
with the ground, the tire can exert more force and
allow your car to achieve higher cornering speeds.
The contact patch is determined by the width of
the tire and the tread pattern that is on the tire.
(Usually the width provided in the tire
specifications is the overall width, or section
width, of the tire, not the width of the actual part
of the tire that touches the ground. However, some
manufacturers will provide the actual width of the
contact patch, which should be slightly smaller than
the overall width of the tire.) If the tire has very
little tread, the contact patch will be larger
(since there are fewer gaps on the tire surface for
the tread), and the tire will have more grip. A tire
with no tread, used only for racing, is called a
slick. Slicks will maximize your car’s cornering
ability but are illegal and dangerous on the street.
As a rule of thumb, if you plan to drive on the
street, use a tire with at least three vertical
treads. These are often referred to as semi-slick
tires.
You can use your choice of tire width to change
the handling characteristics of your car. For
instance, if your car tends to understeer, you can
increase the width of the front tires to improve
front grip. Also, if your engine produces
substantial power, you should try to maximize the
width of the drive wheels to make sure you can get
the power to the ground.
The third factor to consider when choosing a tire
is sidewall height. A tall sidewall flexes more than
a shorter one. Therefore, a short sidewall tire will
provide a more precise steering feel than a tire
with a tall sidewall, but using a taller sidewall
makes the car feel more forgiving and less twitchy.
Also, a short sidewall tire makes the ride harsh on
the street. For a dedicated race car, this is
obviously insignificant.
Tire pressure can be adjusted to change the
handling characteristics of your car. By increasing
tire pressure, you cause the tire profile to become
more round, which decreases the size of the contact
patch. You also cause the spring rate at that corner
of the car to increase since you are making the tire
harder and, therefore, “bouncier” by filling it with
more air. Both of these results take away grip from
the tire. If you increase front tire pressure, the
car will become tighter (added understeer), and if
you increase rear tire pressure, the car will be
looser (added oversteer)
Lowering tire pressure causes the tire to sag,
which increases the size of the contact patch. In
addition, it decreases the effective spring rate at
that corner of the car and makes the tire rise in
temperature. The additional rolling drag (or
friction) produced by the sagging of the tire is
responsible for the temperature change. These
factors provide the tire with more grip. If you
decrease pressure in the front tires, the car
becomes looser, and if you decrease pressure in the
rear tires, the car becomes tighter. The temperature
increase may be desired if it will allow the tire to
heat up to recommended operating temperature.
However, overheating the tire can lead to loss of
grip, “chunking” (large pieces of rubber tearing off
the tire), and generally faster tire wear. Before
making any tire pressure adjustments, find out the
recommended minimum and maximum pressures for your
tire from the manufacturer. Also, experiment with
tire pressures by changing them in small increments
until you find the ideal setup.
Visit my website for more information about tire
selection:
www.240edge.com
About The Author: Miroslav Ovcharik
I have been an automotive enthusiast throughout my
life and have participated successfully in various
amateur racing series. I specialize in tuning the
Nissan S platform cars, particularly the US domestic
market Nissan 240SX. Visit my website
http://www.240edge.com to get information about
240SX performance modifications.
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