We have to start with a short video.
By now you’ve deduced that the hydropneumatic suspension system on the Mercedes 6.9 is a little different than the suspension system on most cars. For starters, most cars don’t drop to the ground after sitting for a few days, but this one does. The system has no traditional steel springs, instead using a combination of high pressure nitrogen acting as a progressive spring and sealed hydraulic struts pressurized by an engine-driven pump. A reservoir maintains pressure after the engine is turned off, but only for a few days, after which oil pressure drops and the car lowers down. Upon engine startup, after a minute or so the pressure builds and the car returns to ride height.
When driving, the suspension provides a comfortable ride that is never harsh, no matter the speed. On the highway, the car feels planted and body roll is minimal. The system uses sealed charges of nitrogen in each of the five suspension reservoirs (1a in the illustration), and controls ride height via the amount of oil in each strut (1b). The system is self-leveling, with independent sensors for the front and rear axles. If the system detects an uneven load, for example five people in the car, it will direct more oil to the lower corners in order to level the system.
The progressive spring rates delivered by the nitrogen (2,100 – 2,900 psi) are soft at the beginning of suspension compression, but more compression only makes it even harder to compress further. The lower spring rates are offset with stiffer anti-roll bars in the 6.9.
At slow speeds, for example driving like a normal person in a parking lot, the car has lots of body roll. But that body roll is magically gone at speed. We’re still trying to understand how it all really works. In the meantime, we know it works and we know it’s going to be expensive when it doesn’t work. The pressure reservoirs tend to last 10-12 years, after which you’re in for about $750 of parts just for those bits. But we’re not going to think about that right now.
Ordinarily, the lower initial spring rates can make brake dive and rear squat a problem. To combat brake dive, Mercedes touted the anti-dive geometry designed into the W116 chassis on which the 6.9 is based. To control rear squat, the semi trailing arm independent rear suspension of the W116 is upgraded with the addition of a Watts link. Compared to a modern car, the 6.9 still dives under braking and squats under acceleration, but for a car selling the 1970’s this was pretty hot stuff.
Graphics courtesy of www.m-100.co