Semi-active shock absorbers: Maximum performance – no compromises
Developers and engineers face a special challenge when tuning shock absorbers:
On the one hand, the shock absorbers should always ensure that the wheels grip the road and calm the body movements, for which greater damping forces are required. On the other hand, customers expect ever-increasing drive comfort, for which a rather soft tuning is helpful. Ole Götz reveals how shock absorber specialist thyssenkrupp Bilstein resolves this conflict of interests – and what the future of the shock absorber will look like.
The shock absorbers – or more precisely in physical terms: the vibration dampers – of a vehicle are the central link between the body and the wheels. They have the task of absorbing vibrations caused by unevenness in the road, in order to ensure wheel grip. They are also designed to control the movements of the body, which are also excited by the road, as well as by dynamic driving maneuvers such as steering or braking. They are consequently highly safety-relevant components that also contribute significantly to the driving characteristics and comfort of a vehicle.
Trade-off between comfort and driving safety
Road 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 make sure that the wheel safely remains in contact with the road in all situations." In second place comes the best possible distribution between ride comfort and the sporty driving desired by the customer – and at a reasonable cost.
According to Ole Götz, this results in a conflict of interests for shock absorber developers between comfort on the one hand and driving safety and agility on the other. "From a comfort point of view, of course, we don't want road stimuli to be conducted into the body in the first place, so that the occupants won't notice them at all," says Götz. "This speaks in favor of low damping forces. However, if the tuning is too soft, the wheel will lose contact with the road or the body will vibrate."
This leads to a problem with passive shock absorbers of the classic design. This is because they only have a damping force characteristic curve that describes the resistance of the damper as a function of the spring compression and rebound speeds. As a result, their tuning can only ever be a compromise, which must cover all driving situations, road stimuli and loading conditions of the vehicle. Depending on where the priorities lie in the respective application, the characteristic curve can be designed to have either a more sporty or more comfortable tendency.
Situational adjustment of the damping force
This conflict of interests can be resolved with so-called semi-active suspension systems. This is because semi-active systems allow the damping force to be adjusted depending on the situation. And this applies both during the rebound stage, when the damper is extended, as well as in the compression stage, when the damper is pressed together. Götz: "In doing so, we can continuously adjust between the minimum and maximum characteristic curve by making the valves in the shock absorber variable. There are various ways of doing this. For example, we can adjust the flow cross-sections or even work with pressure relief valves to change the resistance to the flow of oil, which determines the characteristic curve of the shock absorber."
This is the case with the most modern system currently in series production: Bilstein DampTronic X, which is equipped with a pressure relief valve with very high-speed adjustment. Its highly dynamic adjustment capability enables the required damping forces to be adapted to the damping requirements calculated by the central control unit, within just a few milliseconds. The control concept calculates and adjusts the best damping force several hundred times per second on the basis of sensor data such as wheel and body movement, driving speed or the change in steering angle, depending on thë driving situation and selectively for each wheel. The result: Agility as well as ride comfort and safety can be increased. Bilstein's DampTronic Sky technology places a clear focus on ride comfort. This system uses two valves that individually control the rebound and compression stages.
Today, semi-active suspension systems are already standard in the premium segment of car manufacturers. However, the systems have long since started to be used in the mid-size and compact car segments as well – although then usually as optional extras. Will they soon replace passive systems altogether? Ole Götz does not think so: "The market share for these systems will definitely increase as customer demands continue to grow. But I expect that cars with passive systems will still be on the roads 30 to 50 years from now."
New development fields and challenges
Electric mobility, on the other hand, is not a dream of the future, and the shock absorber specialists at Bilstein have already been working intensively on this for years. It is a field that presents very special challenges. The biggest one is the heavy weight of e-cars. This is because the very significant vehicle weights require high damping forces to control the vehicle body. Energy efficiency also plays an important role here, as Ole Götz also emphasizes: "The energy consumption of our systems is a very important aspect. They must therefore be as energy-efficient as possible. After all, every kilometer of range counts when it comes to electric vehicles."
Another challenge that electric cars bring with them is interior noise. Because electrically driven vehicles are extremely quiet, disturbing noises are perceived much more strongly. Even the slightest suspension noise, which used to be masked by the noise of the internal combustion engine, is now suddenly a real issue. Ole Götz: "We positioned ourselves very early on in the field of noise vibration harshness, i.e. the area that deals with audible or perceptible vibrations in motor vehicles. We are devoting a lot of attention to this issue."
Today, the Bilstein engineers are also facing challenges in the developing fields of autonomous driving and increasing connectivity. Particularly innovative features are possible in networked vehicles, for example: Data obtained from a cloud about the condition of the road surface or even information supplied as part of car-to-car communication could take driving to a whole new level: Based on real-time data, the vehicle can react by adjusting the characteristic curves of the shock absorbers to the quality of the road ahead. In this way, not only could the movements of the vehicle body be reduced, but the damper developer's dream of a "flying carpet" could also come a little closer.
What sounds like a fairy tale from A Thousand and One Nights is exciting reality for the suspension experts at Bilstein. Because here they are already working on the future of the shock absorber.