What shock absorbers are for
Shock absorbers slow down vibrations generated by wheels, axles and the chassis. Therefore, the technically correct name is vibration damper. When driving over an uneven surface, absorbers take up all the impact of the road. The damper then tries to transfer the incoming energy. In a short time span, these continuing shocks lead to vibration. The movements are then transferred to the shock absorber by the piston rod. The kinetic energy is transformed into heat by hydraulic resistance in the shock absorber valves. That's how vibration is reduced to the minimum and can hardly be sensed.
A properly operating shock absorber offers driving comfort and safety, and prevents:
In vehicles, 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 oil though orifices of different sizes, slowing the rod movement and controlling the spring.
Functional principle of the twin tube shock absorbers:
With a twin tube shock absorber, the piston works inside of the inner tube. The space between the inner tube and the casing is used as an equalization chamber. Changes in volume caused by the piston rod movement change the oil level in the equalization chamber between the outer casing and inner tube.
Compression phase:
When the car swings downwards in the direction to the road, the piston rod moves downwards too. Oil displaced by the piston rod is pressed into the equalization chamber between the tubes through the specially designed orifices in the compression valve at the lower end of the inner tube. Pressure 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 pressure which influences the damper characteristics.
Rebound phase:
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 presses oil that flows downwards through the holes above the tube. 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 in the damping case. Both, traction and compression valves are integrated into the piston at the end of the piston rod. Depending on the speed the shock absorber is compressed or extended with, the forces rise. The separating piston separates 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. That's how the separating piston 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 the direction of the pressure.
Compression phase:
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 presses oil that streams upwards through the holes. Therefore, the downward movement is slowed down. The separating piston is lowered to the same degree as the piston retreats downwards. The gas pressure underneath the separating piston prevents the oil from foaming above the piston.
Rebound phase:
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 presses 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 lifted upwards to the same degree as the piston extends.
Advantages of a KW coilover spring strut at one glance:
The salt spray test:
The Technical University of Esslingen tested 9 products from 6 different manufacturers in December of 2002. The products were subject to salt spray durability test (DIN 50021 SS) and the goal of this testing was to compare corrosion resistance of various systems in various conditions including salt water. During the test (over 480 hours), the samples were inspected at regular intervals to evaluate corrosion resistance of piston rods and threaded struts, as well as adjustability of aluminum spring collars.
Participants/Products:
KW „Inox-line“ Coilovers Variant 1, 2 and 3
KW Coilover struts Variant 2
5 competitor products with new, galvanized threaded struts
Evaluation:
2x very good KW „inox-line“ Variant 1 and Variant 3
1x good KW „inox-line“ Variant 2;
2x moderately good competitor
1x moderate competitor
2x poor competitor
Test Result FH Esslingen:
KW inox-line coilovers with stainless steel technology are clearly superior to galvanized models with regard to corrosion resistance, and in addition, ensure smooth operation even after 480 hours.
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, they base their damping developments on twin-tube technology.
With their 2-way pressure valve technology two, basically incompatible characteristics – dynamics and driving comfort – can be uniquely combined.
Their 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.
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
•Damping is carried out by the setup-specific preset spring valve at the piston
Result:
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.
Result:
A lower rebound damping improves the driving comfort.
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 begins.
Position of the bypass valve with the increasing piston rod speed
• During slow piston rod movements (black arrows), oil flows over the bypass valve
• Maximum compression damping (white arrows) is carried out by the spring-loaded piston valve
• The closed compression stage creates a progressive compression damping curve
Result:
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 open state before the compression stage begins
Position of the bypass valve with increasing piston rod speed
• During slow piston rod movements (black arrows), oil flows over the bypass valve
• Maximum compression damping (white arrows) is carried out by the spring-loaded piston valve
• By the opened bypass valve, a declining curve of the compression damping in the low-speed range is achieved
Result:
Support of the vehicle in the high-speed range of the damping is not influenced during an open or closed compression stage and therefore is always the same.
KW is the only suspension manufacturer in Germany that operates a 7-post stand that is able to simulate race tracks.
This high-tech test system is used by Formula 1 teams to achieve optimal suspension performance. KW automotive GmbH uses this innovative technology to test prototypes of new suspension kits as well as optimize the existing setups. KW offers all race teams as well as industrial partners to set up their vehicles using the 7-post bench in the KW testing facility in Fichtenberg. The 7-post bench consists of seven hydraulic posts – four main posts for the wheels and three posts to control action of the vehicle superstructure if necessary. The test bench can be used either in the 4 post mode or, as the name suggests, in the 7 post mode.
In the 4 post mode, basic testing can be made. The suspension kit will be pulsed in different frequencies and post strokes. For the race track simulation, the 7 post mode will be used, which offers to simulate brake and speed up applications as well as aerodynamics. This test can be driven with constant and recurrent conditions – much more accurate and fast than on the race track. Abrasion is reduced to the minimum and no transport costs or track fees accrue. The advantage and results of the KW post bench is shown by the success of the KW customers: Manthey got four 24-h-races victories, Raeder placed their Ford GT on pole position several times, and in the WTCC, teams from Engstler and Wiechers are frequently on the podium in the independence trophy. The Alpina B6 and the Gumpert Apollo Speed also use this innovative technology.