Rover SD1 suspension, self levelling rear suspension
Rover SD1 and suspension
In the front of the Rover SD1 you will find Mc Phersons with insert struts, we will discuss this part later. Strut insert are still available from Monroe and other suppliers. The rear suspension of the Rover SD1 is something different. 2300 models use standards coil springs with standard shock absorbers, but from the 2600 model on they where supplied with Boge Nivomat self leveling units.
The Nivomat is installed instead of a conventional shock absorber, and automatically establishes the optimum vehicle level under all load conditions. Nowadays these units are no longer available as new parts in the shop and the rear suspension of this car is going to be a difficult item if you have troubles with then, but occasionaly you can find a set on Ebay or other sites.
The Boge company is taken over by Mannesmann-Sachs and the Nivomat system is further developed and find its way into modern cars from today. The basic self leveling system was designed back in the '80's and used in cars like the BMW series, Volvo's and other luxurious cars. Nivomats are also used in the Rover SD1 rear suspension. And here is how Boge controls the level of our car.
But how does it operate?
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.
Principle of Operation
The principle of operation of the level control element is illustrated in the diagram below. 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.
Above figure shows the state "loaded and uncontrolled", which comes about, for example, when the stationary vehicle is loaded. When the vehicle moves off, the 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.
The major design elements of a Nivomat are illustrated in the picture on the left. 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.
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.
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 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.
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 dropimmediately 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.
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 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.
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).
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.
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. Fig. 8 illustrates the influence of the load-sensitive damping.
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.
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.
Customer Type Car version This table shows some of the recent applications currently in series production
Daimler Chrysly Voyager Van Especially suited for level control systems are vehicles carrying
Fiat/Lancia Kappa Wagon heavyloads, passenger cars with high comfort and safety
Fiat/Lancia Lybra Wagon requirements, lowered vehicles and vehicles intended for trailer
Fiat/Alfa 156PW Wagon (std. sport) operation. Today, typical Nivomat vehicles are estate cars, MPVs,
Ford Mondeo Wagon (std.) SUVs, saloons and various special vehicles (ADAC, ambulance cars, etc.).
Ford Galaxy Van
Ford Focus (model 2000) Wagon Applications for lightweight vehicles and pick-ups are increasingly
Mitsubishi Galant 2WD/4WD Sedan/Wagon + sport designed now. A Nivomat for motorcycles also exists.
Opel Vectra Wagon
Saab 95 Sedan (std. + sport)
Saab 95 Wagon
Volvo S80 Sedan (std + sport)
Volvo S70 FWD/AWD Sedan (std. + sport)
Volvo S60 (in 2000) Sedan (std. + sport)
Volvo S40,V40 Sedan (std. + sport)
Jaguar XJ6, XJ 12 Sedan
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.
The story is written by Dr.-Ing. Dieter Eulenbach, Mannesmann Sachs AG and given as lecture. I received permission to publish this online. Further information can be found on the website of Sachs at ZFSACHS.COM