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Damper basics - What shock absorbers are for
Shock absorbers slow down the vibrations generated by wheels, axles and the chassis. Therefore, the technically correct name is vibration damper.
When driving over an uneven surface, the shock absorbers take up the impact of the shock. The damper then tries to transfer the incoming energy. In a short time span, these continuing shocks lead to a vibration. These movements are then transferred to the shock absorber by the piston rod. Thereby, the kinetic energy is transformed into heat by hydraulic resistance in the shock absorber valves. Therefore the vibrations are reduced to a minimum and can hardly be sensed.
An intact shock absorber offers driving comfort and safety and prevents:
- Rocking when driving over continuously uneven surfaces
- Pitching or strong dipping during acceleration and braking
- Spinning wheels
- Swerving out of control while breaking
- Sliding when cornering with minimal traction
In automobiles, vibration dampers are usually used as twin tube or mono tube shock absorbers. In both cases, the piston moves in a cylinder filled with oil. The piston forces the oil though orifices of different sizes, slowing the movement of the rod, and controlling the spring.
Functional principle of twin tube shock absorbers:
With a Twin-Tube shock absorber, the piston works inside of an inner Tube. The space between the inner tube and the casing is used as an equalization chamber. Changes in volume due position of the piston rod change the oil level in the equalization chamber between the outer casing and inner Tube.
When the car swings downwards in the direction to the road, the piston rod is moved downwards. The oil displaced by the piston rod is pressed into the equalization chamber between the Tubes via specially designed orifices in the compression valve at the lower end of the inner Tube. The resistance that is created through this valve creates the actual compression forces. Oil underneath the piston also flows upwards through the piston check valve with low resistance which also influences the damper characteristics.
When the car swings upwards, away from the road, the shock absorber is extended. The piston rod is extending outward from the housing. The rebound phase valve at the lower Tube resists the oil that flows downwards through the holes in the space above the Tube. Therefore the upward motion is slowed down. When the piston rod is pulled out of the case, its volume is balanced out by the oil that flows from the equalization chamber back into the inner tube through the compression check valve.
Functional principle of mono tube shock absorbers:
With a one-piston shock absorber, the piston works directly in the damping case. Both, traction and also compression phase valve are integrated into the piston at the end of the piston rod. Depending on the velocity with which the shock absorber is compressed or extended, the forces rise. A characteristic is the separating piston that separates the oil from the gas space which is under strong pressure. The gas space balances the oil expansion with temperature differences and volume changes while the piston rod retracts. The separating piston thereby moves upwards and downwards. The gas pressure of 25-30 bar in the separating space is necessary in order to support the damping forces in direction of the pressure direction.
When the car swings downwards in the direction to the road, the piston rod is moved downwards. The compression phase valve on the top of the valve resists the oil that streams upwards through holes. Therefore, the downward movement is slowed down. The separating piston is thereby lowered by the same degree as the piston retreats downwards. The gas pressure underneath the separating piston thereby prevents the oils from foaming above the piston.
When the car swings upwards, away from the road, the shock absorber is extended. Thereby, the piston rod is moving out of the damping case. The rebound phase valve at the lower piston resists the oil that flows downwards through the holes in the space above the piston. Therefore the upward motion is slowed down. The separating piston is thereby lifted upwards by the same degree as the piston extends.
KW valve technology
For each vehicle class and for every operational use, KW offers the optimal adjusted suspension, from comfortable, to sporty or performance-oriented.
This demanding objective can only be achieved with the innovative, patented KW technology with an individually adjustable rebound- and compression damping. For this reason, we base our damping developments on twin-tube technology.
With our 2-way pressure valve technology two, basically incompatible characteristics – dynamics & driving comfort – can be uniquely combined.
Our Variant 3 allows adjusting the rebound damping forces completely separately from the compression adjustment. This allows optimal amount of traction to the road if so required.
The determining factor for the overall adjustment is the fact that the compression- and rebound forces have no relation to each other – in other words: an alteration of the compression damping does not necessarily also involve an alteration of the rebound damping.
Valve technology traction stage
Oil flow during closed traction stage
- Bypass duct in the traction stage adjustment is closed
- Declining increase of the characteristic line and the maximally possible traction damping is achieved
- The damping is carried out by the setup-specific preset spring valve at the piston.
A sporty and tense adjustment of the traction stage prevents rolling and pitching while braking and accelerating.
Oil flow during open traction stage
- Bypass duct in the traction stage adjustment is open
- Progressive increase of the characteristic line and the minimally possible traction damping is achieved
- The bypass oil amount (black arrow) is not available anymore for the spring valve at the piston and the damping forces are therefore reduced.
A lower rebound damping improves the driving comfort.
Valve technology compression stage
Oil flow during closed compression stage in the base valve
Spring- loaded bypass valve of the compression damping in a closed state before the compression stage is activated.
Position of the bypass valve with increasing piston rod velocity
- During slow piston rod velocity (black arrows), the oil flows over the bypass valve
- the maximum compression damping (white arrows) is carried out via the spring-loaded piston valve
- the closed compression stage creates a progressive characteristic line of compression damping
A sporty and tense adjustment of the compression stage prevents the vehicle from rolling in curves.
Oil flow during opened compression stage in the base valve
Spring-loaded bypass valve of the compression damping in an opened state before the compression stage is activated
Position of the bypass valve with increasing piston rod velocity
- During slow piston rod velocities (black arrows), the oil flows over the bypass valve
- The maximum compression damping (white arrows) is carried out by the spring-loaded piston valve
- By the opened bypass valve, a declining characteristic line of the compression damping in the low-speed range is achieved
The (safety-relevant) support of the vehicle in the high-speed range of the damping is not influenced during an opened or closed compression stage and is therefore always ensured