Shock absorbers

MacPherson strut : hydraulic shock absorber and spring plate

The shock absorber is a safety-relevant component in chassis , which allows the vibrations of the sprung mass to subside quickly and dampens the vibrations of the unsprung mass on the tire spring . By converting the vibration energy into heat through special measures, a significantly dampened vibration is generated. Without these measures, the oscillation would decay too slowly.

The term " vibration damper " would be correct , because it is not the impact that is influenced, but its effect. The yielding of the vehicle body against jerky movements caused by uneven road surfaces on the wheels is brought about by resilient wheel suspensions . The yielding should not lead to a long lasting swing.

Importance of shock absorbers in motor vehicles

Hydraulic shock absorber on the crank arm axle of a VW Beetle , here in a Formula Vee racing car

When the wheels swing, the force of the vehicle against the road varies. If this force is too small, the wheels can slip or lift off, which, especially when cornering and braking, reduces the controllability of the vehicle and can lead to accidents. The swing must therefore subside as quickly as possible. Since the swaying desired for driving comfort and constant high ground contact desired for driving safety are mutually exclusive, the constructive coordination between suspension and shock absorption is always a compromise that can be changed in different directions by retrospectively retrofitting with kits from the accessory trade. An improvement in ground contact is associated with a loss in driving comfort.

Shock absorbers can become less effective, which is why constant observation is necessary.

Detection of defective shock absorbers on cars

Decreasing damping is often unconsciously compensated for by a change in driving behavior on the part of the driver. There are some signs of weakening shock absorbers, whereby the occurring effects do not occur suddenly, but are associated with increasing wear of the damper:

Multiple post-oscillation when the vehicle is vibrated by hand near the wheel (simple function test, the behavior is particularly evident with dampers that have become completely inoperable)
The vehicle swings after unevenness
Rumbling noises on bad roads at low speed (30 zone)
Uneven tire wear and increased tire wear
Fluttering steering or multiple interrupted brake tracks after an emergency stop due to jumping wheels
spongy cornering behavior, on an undulating road the vehicle drifts outward depending on the excitation of the vertical vibrations

Completely defective dampers can also be recognized by significant amounts of oil escaping from the piston rods of the dampers. Conversely, however, proper functioning cannot be derived from a completely tight shock absorber.

Physical principles of damping

Hydraulic dampers

Nowadays, conventional shock absorbers in passenger cars are mainly manufactured as hydraulic telescopic shock absorbers in one and two-tube designs. Their principle is based on the fact that the resistance against the flow of displaced liquid ( shock absorber oil ) depends on the flow speed. It is not linear but progressive, that is, it increases with the flow velocity.

Friction damper

Before the development of hydraulic shock absorbers, vehicles were equipped with mechanical friction dampers. The disadvantage is that the static friction that acts at rest is greater than the sliding friction, which is almost independent of speed . This greater breakaway force causes the suspension to respond poorly.

Friction dampers consist of several friction disks stacked on top of one another and axially pressed against one another by a spring. These disks alternately form two groups that can twist against each other. One of them is connected to the chassis, the other to the part whose vibration is to be dampened. Such a friction damper works in the same way as a multi-plate clutch .

Types of shock absorbers on cars

Basically, a distinction is made between an axle damper , i.e. a vibration damper installed on its own, a spring-damper unit ("strut"), in which the spring and damper are combined into one assembly, and the MacPherson strut , which in addition to both the wheel lengthways - and cross direction.

Lever shock absorber

The usual rotating friction shock absorbers are called lever shock absorbers. An egg. d. As a rule, linear movement to be damped requires the interposition of a lever that rotates around the center of the damper.

The Houdaille shock absorber is a hydraulic shock absorber with swivel pistons in a subdivided cylindrical housing, which is also rotated via a lever.

As lever shock absorbers, however, designs are also mentioned in which a linearly moved damper acts on a lever rotating elsewhere on the chassis. Such a lever is usually a link in the wheel suspension . This also includes the older toggle lever shock absorber , in which a reciprocating piston in the cylinder is actuated from the outside via a toggle lever.

Friction shock absorber with linkage

Friction shock absorber on the crank arm

Houdaille lever shock absorbers

Linear motion shock absorbers (telescopic shock absorbers)

Monotube damper (gas pressure damper)

The single-tube shock absorber is subdivided into the working chamber (oil chamber) and the counter-pressure chamber (gas chamber). The actual damper work is carried out in the oil chamber, i.e. the damping valves on the piston oppose the oil flowing through the piston. This creates a pressure difference which opposes the piston rod moving relative to the container with a damping force. The gas chamber compensates for changes in volume when the piston rods extend and retract and due to temperature fluctuations. A monotube damper usually has a base internal pressure of approx. 20–30 bar. This preload is required so that the oil column in the upper working chamber (chamber above the piston) does not tear off during compression and gas bubbles form in the oil (risk of cavitation ). This would have a detrimental effect on the force characteristics of the damper. Due to the gas pressure, the shock absorber is also a weak gas pressure spring .

Twin-tube damper

In addition to the cylinder tube in which the piston, which is attached to the piston rod and equipped with further valve parts, moves axially, the twin-tube damper has a further coaxially arranged container tube. The piston divides the inner oil chamber into an upper and lower working chamber. In the pressure stage, the piston rod retracts and part of the oil flows from the lower working chamber through the piston valve into the upper working chamber. The oil volume corresponding to the plunging piston rod is pressed through a bottom valve located at the lower end of the cylinder tube into the so-called compensation space between the cylinder and container tubes. A pressure difference relevant for damping is also generated by the bottom valve. When the piston rod extends (rebound), the piston valve takes over the damping, while the oil volume corresponding to the extending piston rod flows back largely unhindered through the bottom valve.

Structure and function of a hydraulic telescopic shock absorber

Shock absorber movement model

The movement model of the (twin-tube) shock absorber shows how the oil level in the shock absorber rises and falls with the inward and outward movement of the piston rod. The movement of the oil level is shown greatly exaggerated. The stroke of the oil level is greater than the stroke of the piston rod. This corresponds neither to the dimensions of the model nor to the proportions in a real car damper. For the movement of the oil level, the following applies: the volume of the retracting piston rod is equal to the volume of the oil rise in the ring surface between the tubes, i.e.:

{\ displaystyle A_ {S} \ cdot h = A_ {R} \ cdot H}

or

{\ displaystyle H = h \ cdot A_ {S} / A_ {R}}

With

{\ displaystyle A_ {S}}

= Cross-sectional area of the piston rod

{\ displaystyle A_ {R}}

= Annular area between the outer and inner tube

{\ displaystyle d}

= Diameter of the piston rod

{\ displaystyle D_ {a}}

= Inner diameter of the outer pipe (container pipe)

{\ displaystyle D_ {i}}

= Outer diameter of the inner tube (cylinder tube)

{\ displaystyle h}

= Stroke of the piston rod

{\ displaystyle H}

= Lift of the oil level

With real damper dimensions (d = 11: Da = 36; Di = 29.4) the result is ${\ displaystyle H = h \ cdot 0 {,} 28}$

The stroke of the oil level is only 0.28 times the stroke of the piston rod. The movement model should also show this realistic value. This can be done simply by changing the oil level lift. Ideally, of course, with additional adjustments to the dimensions d, Da and Di in the movement model, so that the calculation and appearance match exactly.

Oil shock absorber with compensating volume (twin-tube damper)

Hydraulic shock absorbers essentially consist of an oil-filled cylinder and a piston rod guided in it . When the piston rod (and thus the piston) moves axially in relation to the cylinder, the oil must flow through narrow channels and valves in the piston. The resistance that the oil is presented with creates pressure differences that generate the damping forces via the active surfaces. The resulting damping work is converted into heating of the oil. The viscosity and thus the damping effect of the oil is also temperature-dependent. In order to limit the temperature rise of the damper to a level that is tolerable for the components involved, the damper must be able to give off sufficient heat to the ambient air.

The volume of the sinking piston rod must be equalized within the damper. There can be no such thing as a pure oil damper, because like all liquids, oil is almost incompressible. Compensation can be achieved using a gas cushion made of nitrogen or air under high pressure (~ 30 bar), which is separated from the oil volume by a movable piston (single-tube damper). By moving the separating piston, the gas cushion takes over the volume compensation when the piston rod is retracted. The gas acts like an additional spring, so that the effect of the suspension is supported.

Rebound and compression

A directly hinged hydraulic shock absorber is subjected to tension during rebound and compression during compression. That is why the damping is called rebound during rebound and as compression during compression.

In order to improve the "springing down" when approaching individual ramp-shaped obstacles, the rebound stage is usually made harder than the compression stage. Another reason for this design is a harmonious structure of the roll angle during quick evasive maneuvers.

Other forms

A special type of design used in racing such as Formula 1 is the external rotary shock absorber . A new development are the air spring dampers , which are installed in both commercial vehicles and passenger cars. In addition to the suspension and damping, you can also regulate the level. Motorcycles and bicycles are also equipped with air spring dampers, in which the medium air takes on both spring and damper tasks.

The development of an electromechanical damper system for road vehicles is being researched. The advantage here is that instead of heat, primarily electrical energy is generated that can be used directly in the vehicle.

literature

Peter Heuslinger: Modern Mechanics in Automotive Technology , Liebentreu-Haslinger Verlag, Ulm 2002
Peter Causemann: Motor vehicle shock absorbers , Verlag Moderne Industrie, Landsberg / Lech 2001

Web links

Commons : Vibration Dampers - Collection of images, videos, and audio files

Wiktionary: shock absorber - explanations of meanings, word origins, synonyms, translations

Individual evidence

↑ Braess / Seiffert: Handbook of Motor Vehicle Technology , 3rd edition 2003, ISBN 3-528-23114-9 .
↑ Rolf Isermann (Ed.): Driving dynamics control . Modeling, driver assistance systems, mechatronics. 1st edition. Friedr. Vieweg & Sohn Verlag, GWV Fachverlage GmbH, Wiesbaden 2006, ISBN 978-3-8348-0109-8 , 12.5 air spring damping system.

[1] Braess / Seiffert: Handbook of Motor Vehicle Technology , 3rd edition 2003, ISBN 3-528-23114-9 .

[2] Rolf Isermann (Ed.): Driving dynamics control . Modeling, driver assistance systems, mechatronics. 1st edition. Friedr. Vieweg & Sohn Verlag, GWV Fachverlage GmbH, Wiesbaden 2006, ISBN 978-3-8348-0109-8 , 12.5 air spring damping system.