Game Development Reference
In-Depth Information
Drag Coefficients for Motor Vehicles
The drag coefficient for a car or other motor vehicle will depend on the shape of the vehicle.
A sports car will have a lower drag coefficient than will a garbage truck. Typical drag coefficient
ranges for several vehicle types
1
are shown in Table 8-3.
Table 8-3.
Drag Coefficients for Some Typical Vehicle Types
Vehicle Type
Drag Coefficient
Sports car
0.27-0.38
'60s muscle car
0.38-0.5
Sedan
0.34-0.5
Truck
0.6-1.0
Tractor-Trailer
0.6-1.2
Motorcycle
0.5-1.0
While there may be some variation in vehicle drag coefficient due to its Reynolds number
and other effects, for game programming purposes, you can assume a constant drag coefficient
for the vehicles you are simulating. The drag coefficient for the 2004 Porsche Boxster S that we
have been using as an example in this chapter is 0.31.
Frontal Area
The drag force, as expressed by Equation (8.3), is a function of the frontal area of the car. The
simplest estimate of the frontal area of a vehicle is the product of the width and height of the
vehicle. This method assumes that the frontal cross-section of the vehicle is rectangular. In
reality, sides of most cars are sloped so the true frontal area is less than the product of the width
and height. One way to account for frontal area slope is to multiply the width and height of the
vehicle by a factor between 0 and 1. For the car simulator we will develop later in this chapter,
a factor of 0.85 is used to compute the frontal area of the Boxster S.
A
=
0.85*
width
*
height
(8.16)
The Boxster S has a width of 1.78
m
and a height of 1.28
m
. Using Equation (8.16), the
frontal area of the vehicle, for purposes of computing the drag force, would be 1.94
m
2
.
Rolling Friction
As you learned in Chapter 7, rolling friction is a force that resists the rolling motion of an object.
While it is referred to as friction, it really is a contact force caused by the deformation of the
object and the surface it is rolling over. As shown in Equation (8.4), the force due to rolling friction,
F
r
, between the tires and the ground is equal to the normal force exerted on the object,
F
N
,
multiplied by a coefficient of rolling friction,
m
r
. If the car is traveling over flat ground, the normal
force will be equal to
mg
.
