Basic Know-How

Interaction between springs and dampers in suspension is complex. In this section, we explain how they work.

Basic Know-How

Suspension: road grip – traction – comfort

The suspension is the link between the road and the car body. The job of the suspension is to reduce – as far as possible in a controlled fashion – the vibrations from the wheel and the car body caused by an uneven road or changes of direction. Every attempt must be made to prevent these vibrations from being transmitted to the car body in order to reduce rocking, pitching and diving, avoid swaying, and ensure optimum road contact and traction with minimum slip. The suspension system comprises a range of components, including strut support bearings, springs, shock absorbers, connecting rods (coupling rods), stabilisers, axle supports/wheel supports, wheel bearings, suspension arms (control arms and pull rods), wheel brakes, rims, tyres, final drives and steering.

Springs and how they work

A spring cushions the effects of road unevenness and impacts from the road, turning these into vibrations. The spring forms an important link between the individual suspension components, connecting up the sprung and unsprung masses in the vehicle. The unsprung masses include the vehicle components situated between the road and the spring, i.e. the wheels, brake and parts of the wheel suspension and steering. All other vehicle components are classed as sprung masses and include the car body, drive train and the remaining parts of the wheel suspension and steering. In terms of a car’s suspension comfort, the basic rule is that the smaller the ratio of unsprung to sprung mass, the greater the comfort. The spring works in conjunction with the stabiliser, the tyres and the seats.

Shock absorbers and how they work

The shock absorbers reduce and slow down the vibrations from the springs, which is why technically they are correctly referred to as vibration dampers. Vibration dampers convert kinetic energy into thermal energy through fluid friction. This involves the flow of oil being slowed down by the valve passages inside the damper. The valve passages in the shock absorber are specifically designed to ensure that the vibrations transmitted by the spring are reduced right from the start. The shock absorbers can heat up to between 100 and 120oC in the process.

Interaction between spring and damper

When a car passes over an obstacle, this first has an impact on the spring, which must not be hampered by excessive damping performance on the part of the shock absorber. When a car passes over a bump in the road, for example, the obstacle forces the wheel up into the wheel housing. In the process, the spring is compressed. The shock absorber is now in its compression stage. Once the spring has levelled out the obstacle, the shock absorber has to slow down the movement of the spring as it releases its tension with great force. The shock absorber is now in its rebound stage. Compression stage (compressing of the springs and the damper, e.g. when driving over bumps in the road) = usually approx. 25% of the damping force. Rebound stage (when the spring pulls the damper apart) = usually approx. 75% of the damping force.

Conclusion: A spring with a higher spring rate (sport or lowered spring) will only work at its best in conjunction with the appropriate high-performance or sports shock absorber.

Hydraulic shock absorbers

Nowadays, hydraulic shock absorbers are rarely used in today’s automotive industry. From a technical point of view, this type of damper is a poor solution, because the oil contains approximately 10% air. Under load, the air and oil molecules separate (foaming/cavitation), resulting in a noticeable drop in the damping force – on longer journeys on country roads and motorways, a loss of up to 35% damping force can be measured. That means the wheels provide poorer road holding; only after a break does the damper regain its full force and the vehicle its original driving performance.

Gas pressure shock absorbers

With gas pressure shock absorbers, this loss of damping force does not occur. The use of nitrogen ensures that the shock absorber oil is permanently under pressure, thereby preventing foaming, even under load. As a result, the driving performance of a car fitted with gas pressure shock absorbers can be precisely defined and remains reliably stable, whatever the situation; this constitutes a substantial bonus in terms of safety. Gas pressure shock absorbers are available with either mono- or twin-tube technology. The proven twin-tube system represents top-class technology at an attractive price. The mono-tube system, on the other hand, is the technology of choice in motor sports and on standard sports cars. The comparatively larger effective area of the working piston ensures greater damping force and better handling. Optimum discharge of thermal energy into the atmosphere means consistent high performance can be achieved and the sturdy design provides maximum safety reserves in flexible fitting conditions, e.g. inverted technology.

In general, the following applies:

If the vehicle is fitted with hydraulic shock absorbers as standard, switching to gas pressure shock absorbers (BILSTEIN B4) will optimise the driving stability, road holding, comfort and safety of any normal vehicle. Vehicles originally fitted with standard gas pressure shock absorbers only retain their driving performance if replacement gas pressure shock absorbers are fitted when the shocks are exchanged. Switching to mono-tube gas pressure shock absorbers (such as BILSTEIN B6) improves the vehicle’s overall driving performance and provides greater damping force, thereby improving safety and fun at the wheel, even under heavy load, for example, with high superstructures, gas units or when used with trailers.


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