PHANTOM DEFECTS IN SOUND AIR SUSPENSIONS

BILSTEIN points out potential problems when using test benches

Due to numerous enquiries, BILSTEIN Customer Service would like to explicitly point out that conventional suspension test benches are normally not suitable for vehicles with air suspensions. Unlike in Germany, they are used in both Austria and Switzerland even for regular general inspections, which in the past has repeatedly led to false diagnoses: significant defects were regularly determined despite the air suspensions being sound. It is exclusively the active managing suspension system that is affected by the issue – this is independent of whether installed with original equipment or OE replacement parts.

Mustafa Yavuz, who is responsible for worldwide BILSTEIN customer service, explained: “As the vehicle is in fact standing during the test bench simulation, the systems generally nestle into the best comfort mode, which is solely designed for low speeds.”

As the vehicle is not moving during the testing procedure, the control unit of the active suspension control system is missing important input variables, such as speed & yaw rate. These parameters flow into the active control system of the suspension, which adjusts this dynamically to the changing driving conditions. As the control unit is working on the basis of the conditions of a stationary vehicle “in maximum comfort,” yet the test is simulating drive mode, the wheels lift off more quickly. This results in the incorrect assumption that the suspension is defective.

Currently this problem only arises during the general inspection and only in the countries in which a corresponding shock absorber test bench is used. At this point, it is currently only a visual inspection that is required in Germany. However, it cannot be entirely excluded that individual inspectors would use a test bench on their own voluntary basis. In addition, the use during the general inspection within the framework of the forthcoming reforms could become obligatory.

For those who would like to take their air suspension into a garage on a voluntary basis to have it checked over, will possibly be affected by the test bench problem. Due to this reason, an experienced specialist company which is familiar with air suspension systems should be sought out. A regular suspension check is recommended for all drivers. BILSTEIN also suggests that the suspension is carefully examined at every inspection and every wheel change. Because the best tires and brakes aren’t worth much if the suspension is worn or defective.