Environmental Engineering Reference
In-Depth Information
Table 11.7 Semi-tractor only data
Total mass
kg
9,020
m
2
Frontal area
6.18
Tyre rolling radius, dynamic
m
0.531
Aerodynamic drag coefficient
C
d
0.654
Coefficient of rolling resistance, unloaded
R
0
0.010
11.5 Class-8 tractor plus trailer test
The coast down behaviour of a semi-tractor-trailer rig is very different from that of
a sports utility van pulling a covered trailer. In the case of the SUV and covered
trailer, the ratio of trailer frontal area to vehicle frontal area,
A
fv
/
A
ft
, is 1.034 or only
3.4% higher. In the case of the semi-tractor-trailer the same ratio is 1.533 or 53.3%
higher.
The trailer used in this test is a 48
00
covered and refrigerated trailer having the
specifications listed in Table 11.8.
Table 11.8 Trailer specifications
System
Unit
Value
Trailer style: covered
W
m
2.591
H
m
4.064
L
m
14.59
m
2
A
ft
9.476
Mass
kg
7,045 (empty)
Tyres: (8)
285/80 R24.5
Trailer-to-tractor spacing, fifth wheel
m
0.813
Figure 11.8 shows the semi-tractor-trailer rig with the tractor-to-trailer spacing
adjusted to 32
00
(0.813 m), a spacing according to Reference 4 that provides a
good compromise between minimum beaming effect and tractor front axle 'dive'
when braking with a fully loaded trailer. Close examination of Figure 11.8
shows that with this spacing the trailer mount meets the fifth wheel at about mid-
bogey, or roughly half the separation distance between the tandem axles on the
drive truck.
The combined mass of tractor and trailer is 16,066 kg when empty and
36,287 kg when fully loaded. Weight restrictions in North America are 80,000 lbs
(36,287 kg) gross vehicle weight. This typically restricts trailer weight (48
00
trailer
shown) to less than 45,000 pounds of cargo. Coast down data for the OTR semi
are listed in Table 11.9.
