Environmental Engineering Reference
In-Depth Information
macadamized surface for which pneumatic tyre rolling resistance would be
expected to be in the range of 0.013-0.025 [1]. Radial ply tyres have lower rolling
resistance than bias ply passenger car tyres. For the range given, radial ply tyres
will fall in the range of 0.013-0.015, with slight variation with vehicle speed. Bias
ply, on the other hand, will have typical rolling resistance values of 0.016-0.025
and a more significant increase in rolling resistance with speed.
The SUV tested is shown in Figure 11.3 on the level section of road used for
coast down tests. The road section is approximately 2 mi in length, straight and level,
paved with a gravel and asphalt composite. The tree lines are 30 ft either side of the
road centreline, so minimal influence on vehicle drag coefficient is anticipated. The
beneficial aspect of the tree lines is that testing is accomplished in still air.
The test data were averaged and produced the results, given in Table 11.2.
Table 11.2 SUV coast down test data
Results
High speed: 100-90 kph
Low speed: 20-15 kph
Time
t 1 = 7.52
t 2 = 9.32
Average velocity (m/s)
V 1 = 26.4
V 2 = 4.86
Average acceleration (m/s 2 )
a 1 = 0.372
a 2 = 0.149
From the data in Table 11.2 the aerodynamic drag coefficient and rolling
resistance are calculated using (11.1.5) and (11.1.6), respectively. The results of
this calculation are
C d ¼ 0.399
R 0 ¼ 0.014
These coefficients are consistent with the SUV class vehicle tested, the body
shape and type of tyres. As a point of reference, the rolling resistance of 205/70
SR15 tyres is 0.012. In the next section the impact of towing a trailer will be
evaluated. Generally, it is found that trailers do not increase the drag or rolling
resistance very significantly. The trailer pulled is a covered type that is approxi-
mately the volume of the SUV doing the towing.
11.3 Sports utility vehicle plus trailer test
The same coast down test was performed with the SUV but with a typical covered
trailer in tow. The vehicle has the production 1,590 kg drawbar load hitch
installed that is capable of 159 kg normal force (trailer tongue load). Figure 11.4
is a partial three-quarter view of the SUV with a standard utility trailer that was
used for this test. Table 11.3 gives the specifics on the trailer.
The cross section of the trailer is somewhat larger than the SUV pulling it, so the
drag coefficient will be higher because of this and the fact that the trailer is positioned
nearly a half vehicle length rear of the SUV. The open gap between the rear bumper
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