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- Mechanical
grip
In order to develop a good race car, we want to develop maximum
grip in all directions, with minimum drag. To do this, we must
consider mechanical grip separately from aerodynamic grip. While
aerodynamic factors affect cornering mostly at high speeds, mechanical
factors affect cornering at all speeds.
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- Because
their effects are masked at high speeds by aerodynamic downforce,
we generally examine and develop the factors which affect mechanical
grip at slow speeds, in hairpins or other slow corners. Race
teams sometimes use skidpads for exploring their cars' mechanical
grip, sometimes even running without wings so they can optimize
mechanical grip without the influence of aerodynamic factors.
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- Once
we've optimized mechanical grip, then we can tune the aerodynamics.
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- Slip
Angle
The angle of a tire's travel in relation to its centerline is
known as its slip angle. Since a race car spends much of its
cornering time near the limit of adhesion, understanding slip
angle is crucial to understanding and optimizing the car's behavior.
As the cornering forces on a tire increase, it begins to follow
a track which diverges from its centerline. The difference between
the tire's path and its centerline is known as its slip angle.
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- As the
slip angle increases, the grip available from that tire increases
- to a point. After the optimum slip angle for a given tire is
reached, the grip available begins to decrease as slip angle
increases. Go very far past this optimum slip angle, and you've
lost control.
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- The
slip angle characteristics of each tire differ. Graphs of these
characteristics are available for some tires. If we could choose
an ideal tire, we'd like one whose slip angle graph showed a
gentle increase to optimum, and, more importantly, a gentle decrease
after optimum. The more gentle the slip angle curve, the more
forgiving the tire will be to drive. A tire with rapid falloff
of grip after optimum slip angle will be difficult to drive;
it will seem to "let go" without warning.
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- For
rear wheels, the tire's slip angle is roughly the same as the
angle of the tire's travel in relation to the car's centerline.
If the car is turning right, and enters an oversteer state, the
rear tires' slip angle will increase as the car gets more and
more sideways.
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- Note
that once a wheel is locked under braking, and the tire is simply
sliding, consideration of slip angle becomes rather irrelevant.
The same is true once a car has entered a spin, or the driving
wheels have been broken loose by engine power and are spinning
under acceleration.
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- Our
objective in developing a car setup is to find one which allows
the driver to easily find the optimum slip angle for all tires,
and keep the car there as much of the time as possible in corners.
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