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- Tire
Compound
Since softer compounds generate more cornering force, we always
want to run the softest compound we can. However, since softer
compounds heat up more, and wear out faster, we sometimes have
to run harder compounds, especially in races. The only way to
find this out is by trial and error, monitoring temperature and
wear. If the temperature of a tire gets into the tire's critical
zone, then we have to use a harder compound on that corner.
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- Note
that changing the tire compound can affect the balance of the
car, particularly if we change compounds at only one end of the
car. On ovals, it's not uncommon to run a harder compound on
the right front, or even both right side tires, since these tires
do so much more work. If we go from, say, a soft to a medium
right front, we'll have less grip on that corner, and we'll have
to find some way of increasing the grip from the front end, or
else we'll wind up with understeer. The car will want to slide
straight off the track instead of turning in.
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- That
brings us to ...
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- Balance
When a car is cornering, we'd like it to be balanced; that is,
when it reaches its limits of adhesion, we want both the front
tires and at the rear tires to begin to slide at the same time,
at the same rate. If the car is not balanced, either the front
tires will begin to slide first, and we'll slide straight off
the track, or the rear tires will begin to slide first, and the
car will spin.
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- For
the car to be balanced, the grip available at each end must be
proportional to the percentage of weight at that end of the car.
In other words, if the car has 60% of its weight on the rear
wheels, it must generate 60% of its grip from the rear wheels.
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- Many
factors affect balance. As we have seen, tire compound can impact
the balance. A small change in tire pressure, by increasing or
decreasing the grip available from that end of the car, can also
change the balance.
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- Weight
Transfer
The amount of weight transferred onto each tire during cornering
can also change the balance. This turns out to be a useful fact.
A good rule of thumb is, the more weight that is transferred
away from a tire, the worse that tire will grip. This rule does
not apply in direct proportion to weight transferred to a tire;
a tire having more weight transferred to it will not gain as
much grip as a tire having the same weight transferred away from
it.
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- In other
words, if we are going through a certain corner with 250 pounds
on the right front, and 200 pounds on the left front, and we
make a setup change that puts 270 pounds on the right and 180
pounds on the left, we will lose some from the total grip being
produced by the front end of the car. The increase in grip on
from the right front will be more than offset by the decrease
in grip from the left front.
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