Leveling the Playing Field
ZF Sachs’ self-leveling monotube shock is a proven and inexpensive way to improve vehicle stability.
America’s love affair with SUVs has not only raised the ire of environmentalists nationwide but also caught the attention of the National Highway Traffic Safety Administration which is preparing tests to determine what needs to be done to keep them from rolling over. But the fact remains that while many SUV owners use their big trucks as daily commuters, they also need them to haul boats, campers and the gear that goes along with them.
This creates a dilemma for chassis engineers who need to design a system that is optimal in both loaded and unloaded conditions. “The compromise or tradeoff with SUVs is loaded handling and stability,” says David Hunt, director of engineering for ZF Sachs.
Hunt says that as the un-laden ride is softened for consumer comfort, when you load the vehicle the stability decreases. By increasing stiffness and damping to the rear to accommodate a larger load, un-laden ride and handling suffers.
ZF Sachs Nivomat shock absorber system reduces the trade-off when tuning the suspension. With Nivomat you can tune for the best optimal ride in the un-laden condition knowing that the system will optimize the vehicle in the fully loaded state.
Nivomat (from the French Niveau, meaning of a higher class or different position and mat, as in automatic) is monotube designed shock absorber that provides a mechanical self-leveling feature utilizing the energy that is generated by the relative movement of the axle and body when driving.
“Nivomat maintains a level suspension, front and rear, while trailering or at a fully loaded condition,” says Uwe Grasse, product engineering manager.
Nivomat is a semi-supporting system working in combination with a mechanical spring. The Nivomat shocks are only mounted on the rear where most of the additional weight is located.
Nivomat looks like a standard shock absorber with a piston rod with damping valves at the end, an outer tube and a cylinder tube. Several components are added to provide the leveling function. Two reservoirs are contained in the outer tube, an oil reservoir (or low pressure reservoir) and a high pressure chamber. Inside the piston rod is the pump chamber (with inlet and outlet valve) and the pump rod, which serves as a height sensor or regulator and a release bore which releases the pressure after the vehicle has reached level.
A load initially causes static compression of the vehicle’s suspension. Once the vehicle begins to move, the pump is activated by the relative movement of the body.
Extension of the piston rod causes oil to be drawn through the inlet valve into the pump. Compression then pushes the oil through the outlet valve into the high pressure chamber. The pressure in the oil reservoir decreases as the pressure in the high pressure chamber increases. The increasing pressure acts on the piston rod and raises the vehicle at a continuous rate.
Once the vehicle has reached optimum height, oil is no longer drawn in. The height regulator opens a bypass between the high pressure chamber and the pump chamber preventing oil from flowing out of the oil reservoir.
When the vehicle is unloaded the vehicle begins to rise. The height regulator opens the release bore. Oil flows out of the high pressure chamber into the oil reservoir, the pressure drops in the high pressure chamber and the vehicle lowers to the initial height.
Since Nivomat is mechanical, the vehicle needs to be moving before the pump starts to work and it takes about a mile to a mile-anda- half of travel before the vehicle reaches its optimal level point.
“It takes very little input to actuate the pump,” says Hunt, “about plus or minus a millimeter is all it needs, so even on smooth roads the Nivomat pumps up quite quickly.”
ZF Sachs is currently developing a Nivomat with a small electric pump attached to the tube. Once the vehicle is started the electric pump will level it and the mechanical pump will take over once the vehicle is moving.
But the Nivomat system doesn’t just level the vehicle under load. As the load increases, the pressure inside the shock increases oil is displaced from the reservoir to the inside of the unit, compressing the gas volume. This creates a progressive increase in spring rate and damping with little or no change to ride frequency.
“At a high GVW or max load, the ride frequency would be almost the same as curb position, but would never go as low as a conventional suspension,” says Grasse.
“Rear air leveling is the direct competitor to this product,” says Hunt. “But with either a conventional suspension or air-leveling suspension you don’t get a significant increase in spring rate with load like you do with Nivomat.”
He also points out that a standard air compression system is made up of 14 parts, many exposed to the environment, adding some 30 pounds to the vehicle. The Nivomat system has only two parts weighing half as much, which reduces assembly plant inventory and logistics problems.
When DaimlerChrysler installed Nivomat its minivans it found the vehicles were able to reduce the workforce by six or seven people and eliminate an entire operation on the line, having the Nivomat shocks installed the same workers who were doing the regular suspension installation.
Air compression systems also rank high on warranty issues lists.
Grasse adds that Nivomat offers significantly improved roll stability at gross vehicle weight without a penalty to curb weight. On a Chevrolet Suburban the Nivomat system adds 442 Nm/deg roll rate.
“A very noticeable improvement in roll stability,” says Grasse.
ZF Sachs currently supplies Nivomat shocks to nine OEMs in Europe and North America, including DaimlerChrysler, Ford, Fiat, General Motors, Jaguar, Kia, Mitsubishi, Opel, Saab and Volvo. The system is available on the GM Suburban and Tahoe as part of a towing package. Hunt says that if there is a downside to the system it’s that Nivomat is rather stealth to the user. There is no compressor sound and dealers don’t often tout the systems capabilities.
“It seems,” adds Hunt, “that people aren’t realizing what they’re getting.”