Vehicle Suspension Measurements: Evaluation of the Benefits of Dynamic over Quasistatic Kinematics and Compliance Testing3
Vehicle dynamics packages can be used to simulate handling or braking maneuvers that would otherwise need to be performed outdoors. Many of the parameters for these types of vehicle models are determined through kinematics and compliance (K&C) measurements. Machines that perform these measurements apply various forces or moments and measure the response of the vehicle. The rate that these forces or moments are applied can be quasistatic or dynamic. Machines that are capable of performing dynamic tests are more expensive due to the need for inertia compensation, sensors that can acquire data at higher rates, and larger actuators or hydraulic power supplies. However, it is possible that measurements of dynamic vehicle response may increase the fidelity of parameter identification, compared to the sole use of quasistatic tests. One reason that parameter identification may be more accurate is the rate of force and moment application in dynamic tests is more like those in the actual maneuvers that are desirable to simulate. A study was performed to determine where the advantages of performing dynamic K&C testing lie. Quasistatic K&C tests, along with dynamic tests performed at several frequencies up to 3.0 Hz, were performed on the front axle of a front-wheel drive compact sedan, using an MTS Systems High-Rate K&C Machine. Assessment of any advantages of dynamic K&C testing has been made through correlations of the vehicle response between dynamic and quasistatic tests.Abstract
MTS Systems K&C. The vehicle pictured was not the one used in this study. Copyright © 2007 MTS Systems.
Dynamic test inputs. In the vertical tests, the height of the ground under each tire was raised and lowered. When the vertical ground position was zero and the suspension was at rest, the vertical load on that tire was equal to curb weight conditions. in the dynamic tests, the shapes of the inputs were sinusoidal. The amplitudes of the inputs for the 0.5, 1.5, and 3.0 Hz tests were kept the same, while the 0.5 Hz long tests had larger amplitudes.
Camber vs jounce curves for the left front wheel in both quasistatic (QS) and dynamic roll tests (left). A close-up of the dynamic test data, shown in the dashed box on the left) is shown on the right. The amplitude of the input (jounce) was smaller in dynamic tests. Since the camber angle change is almost identical, regardless of the speed the suspension is compressed, the data from the different tests lie right on top of each other.
Frequency content of the left front camber data from the 1.5 Hz roll test. Fourier transforms of the dynamic roll tests reveal that the major frequency content consists of the test input frequency (1.5 Hz), along with the vibration (around 20 Hz) that can readily be seen in the dynamic test data in Fig. 3.
Lateral wheel movement vs test input (vertical ground height) for the quasistatic (QS) and 1.5 Hz vertical tests is shown on the left. The correlation of those responses is shown on the right.
Lateral wheel position response to steering tests. (Left) As a result of differences in boundary conditions during the steering tests, the lateral response of the suspension appears to be very different during the dynamic tests, as compared to the quasistatic one. (Right) Correlation analysis of the lateral response between the quasistatic and 0.5 Hz steering tests confirms the difference.
Phase lags in steer angle response. The tests were performed at different frequencies, so the time scales shown have been normalized. Even though there is no phase difference between the test inputs (left), there is a 33° phase lag in the road wheel steer angle response between the 1.5 Hz roll test and the quasistatic one (right). This phase lag is the cause of low correlations between the quasistatic and dynamic test responses.
Hysteresis in the vertical force data from the roll tests. The forces generated by the suspension dampers increase the amount of hysteresis seen. A close-up of the dynamic test data (boxed area in the top plot) is shown in the bottom plot.