OK now, thanks to the very welcome provocation from @cRiPpLe_rOoStEr, I think I now have enough posts accumulated for me to post links.
So let me tell you about my car, which (AFAIK) was never imported to the countries that I presume most of you live. It's a 1997 Xantia X1 (pre-facelift) VSX, hatchback. The same body shape as the following, but in dark blue:
https://www.citroenorigins.co.uk/en/cars/xantia
Not as aerodynamic as its bigger brother XM, but with a very aerodynamically dirty underbelly, I believe it has potential.
The highlight of this car is its suspension. There were 3 types of suspensions on Xantias, one was the regular hydropneumatic suspension, second was Hydractive II (my car), and third was the Activa, which still is the top car on the famous Moose Test.
Hydropneumatic Citroens have steel spheres filled with nitrogen. They don't have steel springs and conventional dampers. Both springing and damping occur in the spheres, nitrogen gas acting as the spring and holes in the sphere "neck" acting as damper elements. The green thing is an example:
https://en.wikipedia.org/wiki/Hydrop...bol_Xantia.jpg
Hydropneumatic Xantias have 5 or 6 spheres, depending on version. 4 of these spheres are responsible for ride, assigned to each wheel.
My Hydractive II Xantia has 8 spheres (yeah, I know, my car has balls. 8 of them
). In Hydractive II system, there are 3 spheres responsible for ride on each axle. On "normal" mode, the car is suspended on 6 spheres, 3 front and 3 rear. Spheres on each axle are interconnected hydraulically.
An ECU tracks various sensors, such as speed, steering wheel angle, steering wheel angle change rate, brake pressure etc. When a sudden change in direction, or a high speed curve is detected, the middle spheres are disconnected from the system. This also cuts the connection between right and left wheels. So each wheel is now suspended by its own sphere.
The spheres for each wheel, also called "corner spheres" are stiffer than those on regular hydropneumatics (in other words, normal soft ride is provided by the middle spheres and the hydraulic interconnections). This decreases body roll. I believe maximum roll angle is around 2.5 degrees in this mode.
So, does this help with fuel economy? It does when traveling downhill, or when you want a little more rapid progress. I had this mountain pass in the city I lived before; I would just let the car go downhill, my foot off the accelerator. Most other cars, including modern and upmarket ones, would brake on every curve. I, on the other hand, would get to keep momentum.
This also goes when you're in a hurry. Again, where most "hot" drivers would accelerate until the curve and brake hard, I would just hold an average speed, with minimal if any braking on curves, and still keep up with them.
The power loss for the hydraulic pump is negligible.
Suspension is height adjustable, though only one out of four positions is intended for normal driving. The lowest and the highest positions are maintenance modes, and the high mode is only for driving over obstacles slowly.
But the "normal" driving position can be lowered or raised by tinkering with the height correctors on each axle. This no doubt would compromise comfort, but I don't think it would risk any damage to the car when done sensibly.