R = 0.90
Figure 21. Relationship between British pendulum number and Illinois skid
not. Another advantage of this device over the
of the pendulum. Test method ASTM E 303 (1995),
BSRT is that it can be used to determine the fric-
Standard Test Method for Measuring Surface Fric-
tional properties of coarse and open graded
tional Properties Using the British Pendulum Tester,
mixes. As with the BSRT, this test has been stan-
uses the BSRT to measure the frictional properties
dardized by ASTM under E 707-90 (1995).
of the microtexture.
As with the BSRT, the friction value is read
The result from the BSRT is the British pendu-
from the scale and is presented as the variable
lum number (BPN), which relates well to skid
numbers obtained in the field (Fig. 21). (The term
Class I in Figure 21 refers to an Illinois DOT road
classification system.) Sometimes, the BPN value
is converted and reported as a polished stone
value (PSV). The PSV value is the BPN value at
the end of 6 hours of testing.
The advantages of this device are that it is por-
table, has a low initial cost, and can test in differ-
ent orientations. Its disadvantages include that
results from coarse macrotexture are question-
able, it can only simulate low-speed skidding
To maintain consistency between different
dard aluminum surface. Good agreement has
been reported between two testers calibrated
with this procedure.
NCSU variable-speed friction tester
The variable-speed friction tester (VST) is sim-
ilar to the BSRT except that it has a locked-wheel
smooth rubber tire at the bottom of the pendu-
lum. It can also be used in the laboratory and in
Skid Trailer Number, SN
the field. One advantage of this device is that it
Figure 22. Correlation between variable-speed number
can be used to simulate variable vehicle speed,
and skid trailer number at 40 mph.