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Semi-active shock absorbersMaximum performance – no compromises
Shock absorbers have to balance two opposing demands: maximum driving stability and a high level of comfort. Ole Götz explains how thyssenkrupp BILSTEIN masters this balancing act and which technologies will shape the future of damping.
The solution: semi-active suspension systems
This conflict can be resolved with so-called semi-active suspension systems, as they allow the damping force to be adjusted depending on the driving situation. This applies both to rebound, when the shock absorber extends, and compression, when it is compressed. Ole Götz explains: “We can continuously adjust between the minimum and maximum characteristic curves by varying the valves in the shock absorber. There are different approaches to this. For example, we can adjust the flow cross-sections or use pressure relief valves to change the resistance to the oil flowing through the damper, which determines the shock absorber’s characteristic curve.”
Damping force adjustments within milliseconds
This is exactly how the most advanced system currently in series production works: BILSTEIN DampTronic X, which is equipped with a very fast-adjusting pressure relief valve. Its high adjustment speed allows the required damping forces to be adapted within just a few milliseconds to the damping demand calculated by the central control unit. Based on sensor data such as wheel and body movement, vehicle speed or changes in steering angle, the control system calculates and adjusts the optimum damping force for each individual wheel several hundred times per second, depending on the driving situation. The result: agility, ride comfort and safety can all be improved. BILSTEIN DampTronic Sky technology, on the other hand, places a clear focus on ride comfort. This system uses two valves that control rebound and compression independently.
Semi-active systems are on the rise
Today, semi-active suspension systems are already standard in the premium segment of the automotive industry. However, these systems have also long since made their way into mid-range and compact vehicles, where they are generally offered as optional equipment. Will they soon replace passive systems altogether?
“The market share of these systems will definitely increase as customer expectations continue to grow. However, I assume that even in 30 to 50 years, there will still be cars on the road equipped with passive systems.”
New areas of development and challenges
Electromobility, on the other hand, is no longer a vision of the future – it is an area that the shock absorber specialists at BILSTEIN have been working intensively on for years. It comes with a number of very specific challenges. The biggest is the high weight of electric vehicles, as their considerable mass requires high damping forces to keep the vehicle body under control. Energy efficiency also plays an important role.
“Energy consumption is a very important aspect of our systems. They therefore need to be as energy-efficient as possible. After all, every kilometer of range counts in an electric vehicle.”
Another challenge posed by electric vehicles is noise inside the cabin. Because electrically powered vehicles are extremely quiet, unwanted noises are perceived much more clearly. Even the slightest suspension noises, which were previously masked by the sound of the combustion engine, suddenly become a real issue. “We positioned ourselves very early on in the field of Noise, Vibration and Harshness – the area concerned with audible or perceptible vibrations in vehicles. It is a topic we pay a great deal of attention to,” says Ole Götz.
Connected vehicles enable predictive damping
Autonomous driving and increasing connectivity are also key areas of development and challenges that BILSTEIN engineers are addressing today. Connected vehicles in particular open up opportunities for highly innovative features: Cloud-based data on road surface conditions or information provided through car-to-car communication could take driving to an entirely new level. Based on real-time data, the vehicle could respond by proactively adapting the shock absorbers’ characteristic curves to the condition of the road. This could not only reduce vehicle body movements but also bring shock absorber developers a step closer to their dream of a “flying carpet” ride.
What may sound like a tale from One Thousand and One Nights is an exciting reality for the suspension experts at BILSTEIN. They are already working today on the shock absorber of the future.
Conflict between comfort and driving safety
Driving safety always has the highest priority. This is confirmed by Ole Götz, Head of Technology & Innovation at thyssenkrupp BILSTEIN, one of the world’s leading original equipment manufacturers of shock absorbers: “We have to ensure that the tire maintains safe contact with the road in every situation.” The second priority is achieving the best possible balance between ride comfort and sporty handling as desired by customers – while keeping costs reasonable.
Soft damping versus a secure road holding
According to Ole Götz, shock absorber developers therefore face a conflict between comfort, driving safety and agility. “From a comfort perspective, of course, we want road inputs to be prevented from reaching the vehicle body in the first place, so that the occupants do not notice them at all,” says Götz. “That calls for low damping forces. However, if the setup is too soft, the wheel could lose contact with the road or the vehicle body could begin to oscillate.”
Passive shock absorbers remain a compromise
This presents a challenge for conventional passive shock absorbers. They have only one damping force characteristic, which describes the damper’s resistance depending on the compression and rebound velocity. Their setup can therefore only ever be a compromise that has to cover all driving situations, road inputs and vehicle load conditions. Depending on the priorities of the respective application, the characteristic can be tuned either for sportier handling or greater comfort.