If you want better handling, why not upgrade to the Charger SRT-8 shocks and springs and keep the stock ride height? Actually the '08 Charger shocks will be even better with Bilsteins all-around, I believe.
Lowering a 120" wheelbase car as a daily driver makes for a lot of scraping underneath...
You didn't miss anything. I said that earlier, but some people don't want to hear it. They'll put the springs on and then the rear will still be up in the air...
The Automatic Level Control System with Spring Function and Damping Function
Dr.-Ing. Dieter Eulenbach
Mannesmann Sachs AG
Nivomat Product Division, Eitorf
Lecture given as part of the course
"Springing and damping systems for road and rail vehicles"
at the Technical Academy of Esslingen
on 11 Oct. 2000 __________________________________________________ _______________________________ __________________________________________________ _______ Nivomat Product Division 3
Mannesmann Sachs AG Chassis Division 1. General Description
The Mannesmann-Sachs Nivomat is a compact device for vehicle level control,
containing all necessary system elements (supporting element, pump,
accumulator, reservoir, regulator, etc.) in one housing. The Nivomat is installed
instead of a conventional shock absorber, spring shock absorber or spring strut
and automatically establishes the optimum vehicle level under all load
conditions. In general, the Nivomat also takes over the spring and damping
function. The installation of the Nivomat is usually carried out at the rear axle of a
vehicle, thus level control with the Nivomat is also carried out there.
The specific characteristic of the Nivomat level control system lies in the fact that
the energy necessary for adjusting the optimum height level is generated from
the relative movements between the axle and the vehicle body arising from road
irregularities while driving. This means that - in contrast to other systems - the
Nivomat operates without any pollution since it does not need any external
energy supply. 2. Principle of Operation
The principle of operation of the level control element is illustrated by
Fig. 1. The
figure shows diagrammatically the major function elements of the Nivomat in two
different operating states. The following elements are shown: low-pressure
reservoir, high-pressure accumulator, pump with inlet and outlet valves, height
regulator and supporting element. The working media oil and gas are identified.
Height regulator, supporting element and the push rod of the pump are rigidly
connected with the piston rod. Fig. 1: Nivomat Schematic function Fig. 1
shows the state "loaded and uncontrolled", which comes about, for
example, when the stationary vehicle is loaded. When the vehicle moves off, the
-High Pressure Reservoir Low Pressure
-
Reservoir
Strut Hight Sensor Pump Piston Rod Release Port Bypass High Pressure - Accumulator Low Pressure- Reservoir Strut Hight Sensor Pump Piston Rod Release Port Bypass Loaded and Unleveled Loaded and Leveled __________________________________________________ _______________________________ Nivomat Product Division 4
Mannesmann Sachs AG Chassis Division
relative movements between the axle and the body result in the oil being
pumped from the low-pressure reservoir against the gas cushion in the highpressure
accumulator. During the outwards movement of the piston rod, the oil is
sucked into the pump through the inlet valve; during the inwards movement, the
oil is pressed into the high-pressure accumulator through the outlet valve. The
pressure in the low-pressure accumulator decreases continuously, and the
pressure in the high-pressure accumulator increases continuously.
Also shown is the operating state "loaded and controlled", which comes about
when the Nivomat has adjusted the optimum vehicle level position. The
increased pressure in the high-pressure accumulator, which acts on the
supporting element at the same time, has increased the piston rod extension
force and has lifted the vehicle body. Further pumping does generally not lead to
a further pressure increase because the height regulator opens a bypass
between the working chamber and the pump chamber, which prevents further oil
supply from the low-pressure reservoir. 3. Structural Design
The major design elements of a Nivomat are illustrated in
Fig. 2. The piston rod
is hollow and guides a so-called control sleeve which, along with the fixed pump
rod and the inlet and outlet valves, makes up the pump. The damping piston with
its valve discs is attached at the inner end of the piston rod and moves in a
cylinder tube. Gas and oil are separated on the high-pressure side by a
diaphragm. 4. Spectrum of Capabilities 4.1 Spring Function
The Nivomat is generally used as a partially loaded element on the rear axle of
the vehicle. In this case, the greater part of the dead weight of the vehicle (rear)
is supported by a mechanical spring (spiral or leaf spring), which is installed
parallel to the Nivomat. Here, the Nivomat's function is to support the major part
of the payload. When deploying the fully loaded Nivomat system, the Nivomat
supports and cushions the entire vehicle weight, including the payload.
When deploying a partially loaded Nivomat system, three spring elements are of
importance. These elements are the mechanical supporting spring, the gas
spring (due to the enclosed gas volume in the high-pressure accumulator of the
Nivomat) and a pressure bump stop. The mechanical spring is designed to be
weaker than a conventional shock absorber application as the Nivomat already
provides part of the spring force. The pressure bump stop becomes effective with
increasing compression and limits the compression travel. __________________________________________________ _______________________________ Nivomat Product Division 5
Mannesmann Sachs AG Chassis Division Fig. 2: Nivomat structural design High pressure (HP) gas accumulator Pump rod Pump chamber Piston rod Oil reservoir (LP) Level position Release bore NIVOMAT - Main elements __________________________________________________ _______________________________ Nivomat Product Division 6
Mannesmann Sachs AG Chassis Division
The spring characteristics of the Nivomat result from the overlapping of the three
spring elements, as illustrated in
Fig. 3 by a comparison with a conventional
chassis suspension. The linear characteristic of the mechanical spring passes
through point A* in case of the dead vehicle weight and through point B* in case
of maximum payload and static compression. Fig. 3: Nivomat spring characteristics
In case of the Nivomat application, a dynamic level position is determined
together with the vehicle manufacturer. The level of the unloaded, stationary
vehicle when the Nivomat is used (point A) can be set at the same point or lower
as compared to the conventional suspension springing. However, the static
compression in the case of maximum payload (point B) should correspond
exactly to the conventional deflection under full load (point B) so that the vehicle
has the same ground clearance in this case. While being driven, the vehicle will
then be lifted to the predefined "dynamic" level (point C). This requires a driven
distance of 500 m to 1500 m, depending on the road conditions.
The characteristics diagram clearly shows the increase of the spring rates with
increasing payload, caused by the increasing compression of the gas cushion in
the Nivomat. For reasons of comfort and security, the vehicle manufacturers'
objective is to reach an oscillation frequency of the vehicle body as constant as
possible over the entire payload range. With conventionally suspended axles the
oscillation frequency generally varies clearly between the dead weight and the
full payload (e.g. 1.47 dead / 1.01 full), whereas it is almost constant with a __________________________________________________ _______________________________ Nivomat Product Division 7
Mannesmann Sachs AG Chassis Division
Nivomat system (e.g. 1.38 empty / 1.48 full) (
Fig. 3). Thus, Nivomat applications
are usually less hard in the empty state and less soft in the fully loaded state.
A further advantage of the Nivomat-suspended axle results from the possibility to
decrease the overall spring travel while still obtaining the same or even a larger
dynamic spring travel (
Fig. 4). This is often used especially in lowered vehicles.
Fig. 4: Nivomat stroke in comparison with conventional damper
4.2 Level Control
The level control with the Nivomat is usually carried out at the rear axle and can
only be performed while driving because the internal pump is operated by the
relative movements between the body and the axle caused by road irregularities.
However, the Nivomat does not drop immediately as soon as the vehicle stops
but, due to its internal tightness, it can maintain the level reached for a longer
period.
The Nivomat pump is operated by the piston rod. When the piston rod is moved
out (pull), the pump chamber is expanded. Oil is sucked from the low-pressure
reservoir into the pump chamber through the suction tube, the hollow pump rod
and the open inlet valve. When the piston rod is moved in (push), the pump
chamber is made smaller, the inlet valve closes and the outlet valve opens. Oil is
pressed into the working chamber between the exterior side of the control sleeve
and the interior side of the piston rod. At the same time, oil is displaced into the
high-pressure accumulator through the open side of the cylinder tube. The highpressure
gas cushion is increasingly compressed during pumping. NIVOMAT suspension conventional suspension
Nivomat Product Division 8
Mannesmann Sachs AG Chassis Division
When approaching the intended vehicle level, a spiral groove, located on the
pump rod and until then covered by the control sleeve, is opened. The opened
groove forms a bypass between the pump chamber and the high-pressure
accumulator. Thus, no more oil is sucked out of the low-pressure reservoir; oil is
only moved between the pump chamber and the working chamber.
When the vehicle is being unloaded while stationary, the piston rod first moves
out further since the balance between the Nivomat extension force and the load
on the Nivomat is disturbed. This further extension of the piston rod causes a
relief bore on the pump rod to be opened. At the level position, this relief bore is
covered by the control sleeve. It allows an oil flow from the high-pressure
accumulator into the low-pressure reservoir, which results in a corresponding
pressure reduction.
When driving on bumpy roads the Nivomat is excited more than normal. In this
case, the Nivomat adjusts to a higher level (15 - 20 mm). This results in the
vehicle reaching a greater ground clearance, depending on the ratio of
movement between the Nivomat and the wheel. Fig. 5: Nivomat pump diagram Fig. 5
shows a typical Nivomat pump diagram as recorded during a functional
test. In the lower section, the basic characteristic of the device at a base
pressure (20 - 50 bar) is recorded. Then the device is pumped up to the
supported load (90 - 130 bar) in the area of the pump (bypass closed) by
constant strokes. During this, the increase of the spring rate can clearly be seen.
Then the relief function is activated by the extension of the piston rod, and the
pressure in the Nivomat drops to base pressure. In case of dynamic pressure
application, pressures of up to 350 bar may occur in the device; sealing and
guidance on the piston side are therefore of special significance.
Function
max. load
basic load
extended length compressed length
characteristics at basic load
level position release length pump stroke
begin of
bypass
-
pump cycles __________________________________________________ _______________________________ Nivomat Product Division 9
Mannesmann Sachs AG Chassis Division 4.3 Damping
The damping of the Nivomat is characterized by a speed-dependent basic
damping and a load-sensitive additional damping.
The basic damping results from a single-tube design, as with conventional
vibration dampers. When the piston moves in the damping liquid, the liquid flows
through the piston valves and the resulting energy of flow is converted to heat.
The damping curves (
Fig. 6) can be influenced by the design of the piston and
the valves. Factors influencing damping include especially the shape and size of
the constant passage (CP) and the number, size and thickness of the valve discs
(spring leaves). Fig. 6: Nivomat damping curves
A newly-developed piston system (comfort piston) leads to manifold possibilities
of designing the damping curves individually.
Fig. 7 shows some of the curves
that can be realized with this system. The independent determination of the CP
values in the tension and compression strokes and the development of
degressive curves should be emphasized.
Kraft Weg (Hub) Diagramm Kraft Geschwindigkeits Diagramm
Damping Force travel (stroke) diagram Speed diagram Force
Push
Pull __________________________________________________ _______________________________ Nivomat Product Division 10
Mannesmann Sachs AG Chassis Division Fig. 7: Design of characteristic curves with comfort piston
The load-sensitive additional damping results from the pumping work by the
Nivomat. It always acts in pull direction and increases with increasing load
supported.
Nivomat Product Division 11
Mannesmann Sachs AG Chassis Division 5. Dimensioning
The use of the Nivomat control system in a vehicle requires some marginal
design conditions.
Firstly, the Nivomat requires more installation space than a normal shock
absorber. The standard outer tube diameters are 54 mm for separating piston
devices and 60, 63, 68 and 72 mm for diaphgram devices today. However, the
outer tube can be adapted - within certain limits - to the specific conditions in the
vehicle's wheelhouse. This, however, generally also causes higher costs.
Secondly, the movement ratio of the Nivomat and the wheel must be considered.
If the ratio is small, the pump in the Nivomat is excited less, and vice versa. This
is of significance especially for dimensioning of the pump (pump rod diameter 10
or 8 mm). High ratios and thus small pump rod diameters can advantage
passenger comfort.
The attachment points on the vehicle must be dimensioned adequately for the
Nivomat application. The attachments have to transmit greater forces than the
damper since the damping force and bump force are supplemented by the
Nivomat spring force component.
The mechanical spring must be dimensioned weaker than a damper solution, as
indicated above, since the Nivomat takes over a portion of the spring force.
When combined with the Nivomat, the bump stop must be dimensioned
separately. Since the mechanical spring, the Nivomat and the pressure bump
stop comprise one system, individual elements must not be modified
independently from each other during vehicle design.
If a level control system with the Nivomat is planned for a new vehicle, we would
recommend participation of the SACHS Design Department in the design
process as early as possible for the above reasons. 6. Applications
The Nivomat can be implemented as a conventional shock absorber, spring
shock absorber or spring strut design (
Fig. 9). In principle, the Nivomat can be
installed with the piston rod pointing upwards or downwards. The attachments to
the vehicle are generally customer-specific and can be a pin-type or eye-type
joint. __________________________________________________ _______________________________ Nivomat Product Division 12
Mannesmann Sachs AG Chassis Division Fig. 9: Nivomat design versions
Especially suited for level control systems are vehicles carrying heavy loads,
passenger cars with high comfort and safety requirements, lowered vehicles and
vehicles intended for trailer operation.
Today, typical Nivomat vehicles are estate cars, MPVs, SUVs, saloons and
various special vehicles (ADAC, ambulance cars, etc.). Applications for
lightweight vehicles and pick-ups are increasingly designed now. A Nivomat for
motorcycles also exists.
The table in
Fig. 10 shows some of the recent applications currently in series
production.
Damper Spring damper Strut Strut module module __________________________________________________ _______________________________ Nivomat Product Division 13
Mannesmann Sachs AG Chassis Division Fig. 10: Most important NIVOMAT applications (OE/OES) 7. Production
At present, Nivomats are produced by Mannesmann Sachs AG in two production
plants. The plant in Munich (Germany) produces ca. 300,000 units annually for
the European and Asian markets. The plant in Florence (Kentucky, USA)
produces around 750,000 units annually for the American market. In total, about
95 different types are currently produced, which are delivered to 14 different
customers. Both plants are certified according to QS 9000 / VDA 6.1 / KBA.
Due to the Nivomat's principle of operation the requirements regarding the
cleanliness of the assembly processes and the individual parts used must be
very high. The usual general conditions for shock absorber production are not
adequate here. All purchased and in-house produced parts must be subjected to
special cleaning processes. After the final assembly, every Nivomat is subjected
to a 100% function and damping test. 8. System Comparison
To allow the comparison of the quality of different level control systems, a
catalogue of characteristics was laid down by the Chassis Development
Department of Mannesmann Sachs AG.
Fig. 11 shows the characteristics with
an evaluation for the air spring, hydro-pneumatic suspension (suspension
cylinder) and Nivomat systems. The level control systems were each assumed
as being of the partially loaded, single type with all accessory components,
designed for single-axle control (acting on the rear axle).
Fig. 12 shows the result
of the evaluation in a graphic way. __________________________________________________ _______________________________ Nivomat Product Division 14
Main elements of the
NG Nivomat __________________________________________________ _______________________________ Nivomat Product Division 16
Mannesmann Sachs AG Chassis Division
The fundamental difference in the design of the devices as compared to the
present Nivomat generation is that the enclosed volumes required are no longer
arranged radially but lengthwise. This arrangement makes it possible to achieve
the required separation between gas and oil through a separating piston
arranged in the high-pressure accumulator. The minimum diameter of the outer
tube can thus be reduced to 54 mm. Should the required space in the lengthwise
direction of the device not be available, the necessary volume can also be
accommodated in an external separating-piston accumulator, which is mounted
in a suitable position on the device. 9.2 Static levelling
An additional pump can be adapted to the Nivomat for special application
purposes. This electrically driven pump is of a piston pump design and makes it
possible to adjust the optimum level whilst the vehicle is still stationary. The
auxiliary pump can be used for separating piston and diaphragm devices. It is
activiated for a short time, for example when starting the car, and needs approx.
60 seconds to bring the fully loaded vehicle to the required level.
This does not require an addional height sensor, since one is already
incorporated into the Nivomat design. It prevents further pumping when the level
has been reached in the same manner as in the standard device. Fig. 14: Nivomat with static levelling __________________________________________________ _______________________________ Nivomat Product Division 17
Mannesmann Sachs AG Chassis Division 9.3 Nivomat with CDC
The Nivomat can be integrated into a vehicles SACHS CDC (Continuous
Damping Control) system. In this case, only an adapted external CDC valve is to
be used, as the interior is required for the pump components.
Damping on the tension and pressure sides only occurs in the CDC valve. To
ensure a unidirectional oil flow in the valve during tension and compression
strokes, an oil guide pipe and a check valve are additionally installed on the
piston and in the bottom of the cylinder tube. The CDC valve used is a special
variant that has been adapted to the high pressures involved in the Nivomat
system. Fig. 15: Nivomat with CDC valve 9.4 Lightweight design
The chassis design of so-called lightweight vehicles, i.e. vehicles which often
exhibit extreme payload ratios, poses a special problem since the necessary
compromise between driving comfort and driving safety is usually most
noticeable here. The realization of acceptable handling characteristics with the
weight being low is hardly possible with conventional spring-and-shock absorber
systems. Here, the Nivomat with its inherent load-dependent damping behaviour, __________________________________________________ _______________________________ Nivomat Product Division 18
Mannesmann Sachs AG Chassis Division
designed as a fully loaded level control element, has special advantages. Special
lightweight versions have been developed for the application in such vehicles.
The weight per unit can be reduced to approx. 1 kg by the use of typical
lightweight materials. 10. Summary
The Nivomat is currently the most common passenger car level control system in
Europe.
The Nivomat owes this to its special characteristics:
Compactness (pump, regulator, accumulator, reservoir and damping in one
housing)
Additional load-sensitive damping
No hydraulic lines or electric cables
No adjustment of an additional height level regulator is necessary
No electric energy or motor is required
(Required energy is generated from kinetic energy of the wheels)
Environmentally friendly since no energy in the form of fuel is
consumed
Easy to install and to retrofit
Lighter and less expensive as compared to other level control systems.
So far there is a variety of vehicle applications especially for medium-size cars,
estates, vans and saloons, which give evidence for the device's wide range of
applications.
The new Nivomat generation, which offers even more favourable conditions with
regard to size and weight, will make level control also attractive for vehicles of
the smaller classes. Since their kerb weight-to-payload ratios are usually even
more unfavourable than those of the higher classes, it is also possible to achieve
considerable comfort and driving security improvements for these vehicles by
using a Nivomat-controlled axle.
The previous post is not complete. The illustrations did not show up on the cut and paste. The Nivomat system is a FIXED height hydraulic system that is not suited to lowering coils. I tried to find the ZF PDF link and could not. I hope this helps.
Coilovers (or a matched spring/shock set), if properly engineered, are superior to lowering springs.
When you install stiffer springs, you generally need less compression and more rebound on the shocks. If the shocks and springs are mismatched, handling and ride quality will suffer.
People often confuse reduced sway with better handling. Reduced sway is just one piece of the puzzle. Wheel control is at least as important, and the right spring rates and shock valving are critical for wheel control.
Well stated Tim. People often forget that the rubber bushes on a vehicle are an active part of the suspension as are the sway bars. When you alter one element of the equation and not others you may create subtraction by addition. The net gain can become a net loss. Suspension components must complement each other to be successful. Higher rate coils effectively create lower rate dampers. Dampers higher on the critical damping scale cause the spring to 'act' as though it is higher in rate. Bigger sways will reduce lean but will do nothing for bump-steer...
AutoForums.com is the premier network of enthusiast-owned
enthusiast-operated automotive communities.
We operate more than 100 automotive forums where our users consult peers for shopping information and advice, and share
experiences and opinions as a community.
Linear Mode
