# In Situ Rubber-Wheel Contact on Road Surface Using Ultraviolet-Induced Fluorescence Method
## 3 Results and Discussion
### Contact Area in Dynamic Condition  
The dynamic contact images shown in Figure 10 illustrate the condition between a rubber-wheel and road surface replica moving at 40 mm/s under a normal load of 40.2 N. The high-speed camera recorded the moving images along the path using 100 fps. This means it took 0.75 s to cross the camera capture area of 30 mm. Figure 10 only shows 9 out of 75 images, which sufficiently show the movement of rubber-wheel contact from one side to the other. Each position of rubber-wheel contact revealed a divergent contact footprint pattern, displaying different amounts of the real contact area. Figure 11 illustrated the amount of real contact area between rubber-wheel and road surface replica for rubber contact of 0.31-0.63 s. The 5 selected contact images displayed the entire rubber-wheel real contact area. Due to the irregular pattern of the road surface, the real contact area of the rubber-wheel in these images ranged from 11 to 15 mm\({}^{2}\). It could vary according to surface roughness and pattern.  
Figure 10: The images of rubber-wheel contact while running at moving speed 40 mm/s under 40.2 N of the normal load.  
Figure 9: Rubber-wheel real contact area due to applied load for static condition.  
Figure 12 shows the images of moving speed influences on the contact area between a 40.2 N loaded rubber-wheel and road surface replica in the same position. The images display the contact area of the rubber-wheel that decreased as the moving speed increased. The actual amount of contact area of rubber-wheel is shown in Figure 12. The grip force is produced by the molecular adhesion interaction between rubber-wheel and road surface. Consequently, the bond breaks and forms repeatedly along the movement of rubber-wheel. The stretching adhesion bond gets weaker as the rubber-wheel moves at a certain speed. Therefore, the contact area of a stationary rubber-wheel was always higher than in the dynamic condition. The faster-moving rubber-wheel weakened the adhesion interaction. Similar results were reported by [PERSON] and co-workers in their article titled \"Adhesion Characteristic of Tire-Asphalt Pavement Interface Based on a Proposed Tire Hydroplaning Model\" [28]. Consequently, the rubber-wheel contact area was smaller as the moving speed increased. These phenomena were present in all the 3 applied loads in the negative logarithmic trendline. The only different thing was that a higher applied load increased the wider contact area. A higher applied load makes the penetration of road surface roughness into the rubber-wheel to be deeper. Consequently, it reaches a wide range of road surfaces. The real contact area of the rubber-wheel observed through fluorescence microscopy widened as the applied load increased.  
There are filled marks representing the dynamic condition of the rubber-wheel contact area. Also, unfilled marks are regarded as static conditions of rubber-wheel contact, as shown in Figure 13. The static condition of the contact area was used as a comparison with the dynamic contact. It also helps determine the trendline of rubber contact at a very slow moving-speed. As a result, it can be predicted the reduction of rubber-wheel contact was very significant at a moving speed below 20 mm/s. On the contrary, it reduced slightly to an almost constant line at a moving speed above 20 mm/s. This shows that the contact area of the rubber-wheel is highly affected by the moving speed of vehicles. According to the Micheline report, the grip coefficient of tire reduced as vehicle speed increased. Hence, there is a significant relationship between the contact area and the grip performance of the rubber tire. This was seen when they both decreased as the moving speed increased.  
Figure 11: The images of rubber-wheel real contact area for some position along track path.