Issues in assessing the condition of shock absorbers on an EUSAMA test stand

Abstract: The paper concerns the diagnosis of damping elements used in passenger car suspensions. Its main objective was to illustrate several problems that may occur during the assessment of the condition of shock absorbers mounted in a vehicle. The relationship between the wear of the damping element and the value of the diagnostic parameter in one of the EUSAMA-type testers was analyzed. The disturbing influence of dry friction in the suspension and the radial properties of the tire on the results of non-disassembly shock absorber inspection were described. Exemplary errors that may occur in the information provided by diagnostic devices were also presented.1. INTRODUCTION
Assessing the condition of shock absorbers mounted in a vehicle is problematic in many respects. Stands implementing commonly known diagnostic tests (“Boge” and
EUSAMA) [1, 5, 11, 12, 14, 15] are not measuring devices, but merely inspection devices.
It is futile to expect a test result in the form of a characteristic or damping coefficient. The result in this case is a parameter whose value approximately determines the degree of shock absorber wear. There is also no information about the measurement error. The specificity of both methods lies in the fact that the stand has no direct connection with the tested element. The only contact of the car with the device takes place through the road wheels. The disadvantage of such a solution is the disturbing influence of many vehicle features on the test result [3, 6, 10, 13, 16]. Literature sources contain modest information about disturbances caused, among other things, by dry friction in the vehicle suspension. Sporadic analyses of this issue have so far been conducted only on the basis of simulations [2, 4, 6, 7, 8, 9, 17].
The above facts may be a premise for conducting experimental trials. It is also worth illustrating the scale of this phenomenon against the background of a much more prosaic factor, which is the air pressure in the tires.
2. EUSAMA METHOD
One of the newer ways to assess the condition of shock absorbers mounted in a vehicle is the EUSAMA method [1, 5, 11, 12, 14, 15]. It consists of kinematically exciting the road wheel to vertical vibrations with a constant amplitude of 3 mm and a frequency decreasing from over 24 Hz to zero. (…)

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