## 4 Quarter Car Simulation
### Quarter Car Simulation Result  
Using the configured 1/4 car simulator, the 60,000 km of driving was reproduced in the simulation mode described above. The damper's damping force was measured every 1500 km for the first 6000 km and then every 6000 km after that to confirm the change. After 60,000 km, the rebound hard damping force decreased at an excitation speed of 0.3 m/s to approximately 32 kgf, a 9.7% decrease compared to the initial value. This value satisfies the target durability test spec's judgment criterion: \"The damping force change after the end of the test should be within 20%\". However, after 1500 km of driving in the initial stage of the test, a damping force decrease of 24 kgf, equivalent to 75% of the total damping force decrease, was measured. Therefore, it was analyzed that the overall damping force decrease occurred at the beginning of the driving, and the damping force decrease after that  
Figure 11: RLDA: (**a**) combination of country/highway/city/unpaved; (**b**) country road; (**c**) highway; (**d**) city road (general case); (**e**) city road (construction area); (**f**) unpaved road.  
showed a considerably gradual trend. This initial damping force reduction phenomenon appears to be due to the oil leakage and cavitation effect inside the damper.  
Oil leaks inevitably occur through the piston rod due to damper operation. When the durability of the oil seal deteriorates, cavitation usually occurs when the internal pressure of the damper momentarily drops below atmospheric pressure, causing air to be generated inside the oil, which increases the compressibility of the oil and reduces the pressure generation performance. Nitrogen gas pressure is generally used on the reservoir side, where the internal pressure of the damper is the lowest, to set it high compared to atmospheric pressure. However, as the number of damper operations increases, the gas component is absorbed into the oil, causing the reservoir reference pressure to drop to the same as the atmosphere pressure. Therefore, the possibility of cavitation inside the damper increases when a transient damper displacement occurs. When cavitation occurs, air is generated inside the oil, which increases the oil compressibility and is disadvantageous for damping force generation.  
That is one of the potential failure modes from the FMEA results. In addition, the rebound soft damping force reduction was measured to be 5 kgf, a 7% decrease from the initial value, and the compression hard and soft damping forces showed almost no change as measured in the test results. The damping force reduction before and after the simulation test was mainly focused on the rebound hard damping force, which had a relatively large damping force, and the change in the compression damping force was insignificant. The damping force change by driving distance in the simulation test is shown in Table 4. The damping force change by excitation speed in each mode is shown in Figure 12.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Damping Force** & \multicolumn{2}{c}{**Rebound Damping Force**} & \multicolumn{2}{c}{**Compression Damping Force**} \\ \cline{2-5} /**Driving Mileage** & & **(kgf)** & & **(kgf)** \\ \hline  & **Hard** & **Soft** & **Hard** & **Soft** \\ \hline
0 & 330 & 72 & 96 & 33 \\
1500 km & 306 & 67 & 97 & 33 \\
3000 km & 305 & 69 & 97 & 34 \\
4500 km & 308 & 67 & 96 & 33 \\
6000 km & 310 & 68 & 96 & 33 \\
12,000 km & 303 & 70 & 96 & 33 \\
18,000 km & 309 & 67 & 98 & 33 \\
24,000 km & 301 & 68 & 98 & 32 \\
30,000 km & 305 & 68 & 97 & 34 \\
36,000 km & 303 & 67 & 98 & 34 \\
42,000 km & 298 & 66 & 95 & 32 \\
48,000 km & 298 & 68 & 96 & 33 \\
54,000 km & 299 & 67 & 94 & 32 \\
60,000 km & 298 & 67 & 97 & 32 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Damping force variation at 0.3 m/s with respect to driving mileage.  
Figure 12: _Cont._