Game Development Reference
In-Depth Information
Keep in mind that the values in Table 8-2 are theoretical maximum velocities. The Boxster
S can't really reach 299.3 km/hr in sixth gear. According to the manufacturer's specifications,
the top speed of the car is “only” 266 km/hr . The reason the car can't reach the theoretical
maximum velocity in sixth gear is because the car is also subject to the decelerating forces of
aerodynamic drag and rolling friction.
Equation (8.13) can also be used to calculate what the engine rpm value will be after a gear
shift. If the car is shifted into a higher gear, the gear ratio is reduced. If the velocity of the car is
assumed to be constant before and after the gear shift, the engine rpm level will decline because
of the lower gear ratio. The new engine turnover rate,
Ω e ( new ), will be equal to the engine turnover
rate before the gear shift, Ω e ( old ) , multiplied by the ratio of the new gear ratio to the previous
gear ratio.
gnew
(
)
Ω= Ω
(
new
)
(
old
)
k
(8.14)
e
e
gold
(
)
k
For example, if the Boxster S shifts from first gear to second gear at 7200 rpm , the new rpm
level of the engine after the gear shift will be the following:
2.2
Ω=
7200
=
4147
rpm
(8.15)
e
3.82
This is an effect you've probably seen quite a bit. If you are driving a car and shift from a
lower gear to a higher gear, the rpm level of the engine falls. The opposite is also true; if you
shift from a higher gear into a lower gear, the rpm level of the engine will surge.
Manual and Automatic Transmissions
There are two general types of transmissions. With a manual transmission the driver must
make all of the gear shifts manually. An automatic transmission is one where the transmission
shifts automatically. When the automatic transmission will shift varies from transmission to
transmission but usually is dependent on the car velocity, the engine turnover rate, and the
load being put on the engine. If you wanted to use an automatic transmission in your game
programs, you could just specify when the transmission would shift. The automatic transmission
in the author's 1997 Toyota Camry shifts when the engine turnover rate is between 3000 and
4000 rpm .
Aerodynamic Drag
A car in motion is subject to the force of aerodynamic drag. As we know, drag acts in the oppo-
site direction to the velocity vector of an object, so drag force will cause the car to slow down.
As shown in Equation (8.3), the drag force on a car is expressed as a function of the air density,
velocity of the car squared, the frontal area of the car, and a drag coefficient.
To evaluate Equation (8.3), we need to determine the drag coefficient and frontal area of
the car.
 
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