The objective of this project was to develop and test an apparatus and procedure for determining natural frequency and damping ratio for heavy vehicle suspension systems.
A test has been developed that involves supporting an axle group on collapsible platforms located under the vehicle's wheels. These platforms are released simultaneously and drop a short distance, exciting a transient response. The displacement of each axle relative to the vehicle's chassis is measured using string potentiometers. The natural frequency is then estimated as the location of the peak magnitude of the displacement signal in the frequency domain. Also, the damping ratio may be estimated using a type of logarithmic decrement method in the time domain.
Our test is based on the EC drop test which specifies that, to qualify as road friendly, the transient signal should have a natural frequency of less than 2 Hertz and a damping ratio greater than 20 percent (Council of the European Communities 1992).
Testing initially resulted in data that were hard to interpret and had poor repeatability. The main reason for this was that the platforms were not releasing simultaneously which caused the roll mode of the vehicle to be excited. After some modifications to the dropping mechanism and careful tuning the test was shown to provide repeatable estimates of natural frequency (±7%) and damping ration (±18%).
Keywords: damping ratio, heavy vehicle suspension systems, natural frequency, pavement wear, road-friendly