CHAPTER 8 CHAPTER 8 NUMERICAL MODELING OF WET PAVEMENT SKID RESISTANCE
8.5 Comparing Factors Affecting Skid Resistance
The simulation model can be used to study the effects of tire inflation pressure, wheel load, and water-film thickness on the skid resistance measured in terms of skid number SN.
The ASTM E-524 tire is used for illustration. The ranges covered are as follows: 100 to 250 kPa for tire inflation pressure, 2400 N to 5280 N for wheel load, and 0.1 to 10 mm for water- film thickness. Table 8.3 shows the different cases covered in this study.
The plots that present the effects of wheel load are found in Figures 8.6(a) and (b) for the analysis of skid resistance. The following observations of the effects of wheel load on skid resistance can be made:
(i) In general, all other parameters being constant, wet-pavement skid resistance increases as wheel load becomes larger.
(ii) Figure 8.6(a) shows that as the wheel load is increased from 2400 N to 5280 N, the gain in SN remains more or less constant with tw, the thickness of water film.
The total gain in skid resistance ΔSN is about 10 for the water-film thickness tw
ranging between 0.5 mm to 10 mm.
(iii) Figure 8.6(b) shows the variation of SN with sliding speed as the wheel load is increased from 2400 N to 5280 N. At a low speed of 20 km/h, SN remains more or less constant (i.e. not affected much by changes in wheel load). At wheel sliding speed of 40 and 60 km/h respectively, there are significant increases in skid resistance as the wheel load is raised. However, the rate of increase falls when the wheel sliding speed is increased to 80 km/h. This trend is similar to that exhibited in past research by Sacia (1976).
The mechanism that leads to the changes in the skid number SN is rather complex as can be seen from the following re-written form of Equation (8.3),
z z z
x
v F
Drag Uplift
F F
SN F − +
×
=
×
= ( )
100
100 μ
(8.3)
i.e. ( )
⎥⎦
⎢ ⎤
⎣
⎡ −
+
×
=
z
v F
Uplift
SN 100 μ Drag μ (8.4)
where μ is the coefficient of friction between the tire and the pavement surface. All three variables Fz, drag and uplift forces will vary as the applied wheel load changes. Figure 8.7 shows an example of the computed variations of these variables with wheel load. It is noted that the three variables increase with increasing wheel loads for the same vehicle sliding speed of 64 km/h and tire inflation pressure of 165.5 kPa, but at different rates. The rate of increase is
z
term in Equation (8.4) increases in magnitude with increasing load and hence SN64 increases with increasing wheel load.
8.5.2 Effect of Tire Inflation Pressure on Skid Resistance
The plots that present the effects of tire inflation pressure are found in Figures 8.6(c) and (d) for the analysis of skid resistance. The following observations of the effects of tire inflation pressure on skid resistance can be made:
(i) In general, all other parameters being constant, wet-pavement skid resistance increases marginally as tire inflation pressure becomes larger.
(ii) Figure 8.6(c) shows that as the tire inflation pressure increases from 100 kPa to 250 kPa, the SN remains marginally affected for the different wheel load levels.
(iii) Figure 8.6(d) shows the variation of SN with sliding speed as the tire inflation pressure increases from 100 kPa to 250 kPa. It is noted that SN is not affected much for vehicle speeds up to 60 km/h. This trend is similar to that exhibited in past research by Sacia (1976).
8.5.3 Effect of Water-Film Thickness on Skid Resistance
The plots that present the effects of water-film thickness are found Figures 8.6(e) and (f) for the analysis of skid resistance. The following observations of the effects of water-film thickness on skid resistance can be made:
(i) In general, all other parameters being constant, wet-pavement skid resistance decreases with increasing water-film thickness and tends to level off for water- film thickness larger than 6 mm.
(ii) Figure 8.6(e) shows the variation in SN with wheel loads as the water-film thickness increases from 0.1 mm to 10 mm at a vehicle sliding speed of 60 km/h.
It is observed that the loss in SN is 6 for wheel load P = 2400 N and increases to
thickness increases from 0.1 mm to 10 mm. At a low speed of 20 km/h, SN remains more or less constant. At wheel sliding speed of 40 and 60 km/h respectively, there are significant decreases in skid resistance with an increase in water-film thickness. However, the rate of decrease falls when the wheel sliding speed is increased to 80 km/h. This trend is similar to that exhibited in past research by Sacia (1976) and Rose and Gallaway (1977).
8.5.4 Effect of Vehicle Speed on Skid Resistance
Figures 8.6(g) and (h) present the analysis of the effects of vehicle sliding speed on skid resistance. The following observations of the effects of water-film thickness on skid resistance can be made:
(i) In general, all other parameters being constant, wet-pavement skid resistance decreases with increasing vehicle speed.
(ii) Figure 8.6(g) shows the variation in SN with vehicle speed for different water- film thickness. It is observed that SN decreases from 60 at zero speed to 10 to 20 at hydroplaning (depending on the water-film thickness). At vehicle speeds below 20 km/h, SN remains more or less constant for the different water-film thickness. At higher vehicle sliding speeds, a lower SN is observed for a larger water-film thickness.
(iii) Figure 8.6(h) shows the variation of SN with sliding speed as the wheel load increases from 2400 N to 5280 N. It is observed that SN decreases from 60 at zero speed to about 10 to 15 at hydroplaning (depending on the wheel load). At vehicle speeds below 20 km/h, SN remains more or less constant for the different water-film thickness. At higher vehicle sliding speeds, a lower SN is observed for a smaller wheel load. This behavior is similar to that exhibited in past experimental research (Sacia, 1976; Gallway et al., 1979).
Table 8.4 shows the comparison of the effects of wheel load, tire inflation pressure and water-film thickness on the range of predicted skid numbers. For easy presentation, Figure 8.8 shows the visual representation of the range of SN under the influence of these factors. It can be observed that vehicle speed is the primary factor that influences the skid number, followed by water-film thickness, wheel load and then tire inflation pressure. This is consistent with the experimental findings by Sacia (1976) which indicate that vehicle speed is a key factor in the reduction of skid resistance on wet and flooded pavements followed by water-film thickness, wheel load and tire inflation pressure.