Environmental Engineering Reference
In-Depth Information
Vehicle acceleration
80
80
1
0.447
V
n
, 1
60
40
20
0
0
0
50
100
150
200
250
250
0
V
n
, 0
Time (s)
Figure 11.9 Coast down simulation for full semi-tractor-trailer
Table 11.10 summarizes the comparison of tractor only to tractor plus trailer
coast down test results.
Table 11.10 Semi-tractor-trailer comparison data
Unit Tractor
only
Tractor
þ
trailer
empty
Tractor
þ
trailer
loaded
Total mass
kg
9,020
16,066
m
2
Frontal area
6.18
9.476
9.476
Tyre rolling radius, dynamic
m
0.531
0.531
0.531
Aerodynamic drag coefficient
C
d
0.654
0.58
Coefficient of rolling
resistance, unloaded
R
0
0.010
0.010
It is interesting to note that aerodynamic drag coefficient is relatively unaf-
fected by cargo (as would be expected), but rolling resistance is surprisingly 30%
reduced with a fully loaded (to legal limit for North America) trailer. The aero-
dynamic drag coefficient
C
d
~ 0.58 is typical of tractor-trailer rigs having some
degree of aerodynamic streamlining such as rounded features, roof fairing
if installed and particularly cambered leading edges on the trailer (as shown in
Figure 11.8). Rolling resistance of the tyres is distributed approximately three
quarters to the belt, and the remaining quarter is split nearly equally between the
shoulder (portion of the tyre between belt and sidewall) and the sidewall itself. In
Table 11.10 the rolling resistance,
R
0
, is less underrated load partly due to the
somewhat higher ambient temperature during the road testing and also due to the
tyre construction and its degree of wear (worn tyres have
R
0
approximately 20%
less than tyres with full thread). Natural rubber truck tyres, in general, have lower
rolling resistance than synthetic rubber.
The lower rolling resistance obtained for the fully loaded tractor-trailer semi is
difficult to completely explain. Certainly some of the reduction is due to higher tyre
