Ride quality evaluation methods for off-road vehicles

Abstract

Analyses were made on the FED-Alpha vehicle model using the ADAMS Car program to observe the effect of road characteristics on ride quality. The vehicle was developed by Ricardo Inc. as a fuel-efficient all-terrain military vehicle. The FED-Alpha is a 4x4 wheeled vehicle and the front and rear axles have an independent double-wishbone suspension system. The suspension system has air bellows and titanium helical springs and dampers. The dampers give different damping characteristics at low and high frequencies. In the ride quality evaluation standards, the constraints of analysis or test have been determined. The constraints mentioned were discussed and analysis studies were performed with these constraints. For example, speed deviation, which is one of the absorbed power method constraints, is examined and the effect of this value on ride quality is examined by exceeding the restricted value. Analyses were made by collecting data on three axes at three different vehicle locations. Waviness, phase angle, wavenumber, wavelength and roughness were chosen as the road parameters to examine ride quality effects. Synthetic roads were created by changing the road parameter to be examined. Synthetic roadways were modeled in a format that could be analyzed with ADAMS Car. Absorbed power, vibration dose value, and frequency weighted rms acceleration methods were used for ride quality evaluation in the scope of this thesis. Absorbed power was calculated with the transfer function and the FFT method, and the different results between the two methods were examined. Since the difference in results was negligible, the calculations continued with the transfer function. MATLAB and SIMULINK software were used simultaneously in the ride quality evaluation method calculations. The acceleration data was collected from the driver seat base, passenger seat base, and middle point of the rear passenger seat base. Acceleration data were exported from the ADAMS Car. With the help of MATLAB code, the tab files were imported to SIMULINK and the acceleration data was passed through transfer functions. Obtained results are converted into graphics with the help of MATLAB. Analyses were made between the speed of 8,05 km/h and 24 km/h to determine vehicle 6-Watt speed in the z-axis on synthetic roads. First, the found absorbed power values are plotted in the speed domain. Then, 6-Watt speed was determined by fitting a second-degree polynomial to the plotted graph in the speed domain. Finally, using the fitted second-degree polynomial, 6-Watt speed was calculated. Using the same second-degree polynomial calculation model, the vibration dose value and frequency weighted rms acceleration values corresponding to the 6-watt speed were calculated. The purpose of comparing these values is to determine how other methods evaluate the 6-watt ride quality limit defined in the absorbed power method. A higher correlation rate was detected with frequency weighted rms acceleration and a slightly lower correlation with vibration dose value. Ride quality result graphs were plotted in three axes by analyzing four rms roughness values. As the road rms roughness value increases, all vibration evaluation values in the z-axis increase. This result means that ride quality deteriorated in the z-axis due to roughness. It has been determined that the ride quality decreases depending on the increasing speed. It has been observed that the absorbed power values are most affected by the change of vehicle speed in the z-axis. It has been determined that with the increase of the road rms roughness, the ride quality decreases in the x and y axes. But the change is not continuously increasing depending on the speed. Analyses were made at three different waviness values and ride quality graphs were plotted for each axis. As the waviness increase, the ride quality increase in the z-axis. The ride quality decrease depending on the speed increase. No change was detected in the ride quality in the y-axis depending on the waviness change. The decrease in the waviness led to a decrease in the ride quality in the x-axis. Still, no correlation could be observed in the change depending on the speed in the x-axis. Analyses were made on roads with four different wavenumber bandwidths. It is observed that the ride quality change as the wavenumber bandwidth change in the z-axis. The ride quality increase on wider bandwidth at low wavenumber range. The ride quality increase on narrow bandwidth at high wavenumber range at low speed. The ride quality increase on narrow bandwidth at high wavenumber range at high speed. As the speed increases, the ride quality in the z-axis decrease, but it has been observed that the slope of the speed-related increases differs on the roads with different wavenumber bandwidths. It was determined that one of the parameters affecting the ride quality in the x and y axes is the wavenumber bandwidth, but no correlation could be detected. Analyzes were made on the road with four different phase angles. It has been determined that as the phase angle increases, the ride quality in the z-axis increase. As the speed increases, the ride quality values in the z-axis decrease. As the phase angle increases, the ride quality on the y-axis decrease. It is evaluated that the ride quality decrease is the roll motion caused by the opposite movement of the right and left wheel. It was observed that the phase angle change in the x-axis did not have much effect on the ride quality. Analyzes were made at different speed profiles on the 5,08 cm rms roughness road. As a result, it has been determined that even if the speed deviation factor specified in the standard is exceeded and the average speed is maintained and driven for approximately 300 meters, the absorbed power values in the z-axis will not change. However, this result needs to be supported by the results of the analysis to be made in different vehicles. When the ride quality evaluation methods are compared, it has been determined that the absorbed power will give the most sensitive results changing by vehicle speed. It is considered that the absorbed power, vibration dose value and frequency weighted rms acceleration methods will be sufficient for the evaluation of the ride quality in the z-axis. However, it is thought that it is necessary to include the suspension and wheel parameters in the x and y-axis evaluations.