Hydropneumatic Suspension - Advantages

Advantages

Hydropneumatics have a number of natural advantages over steel springs that are poorly understood, leading to general public perception that hydropneumatics are merely "good for comfort". They actually also have great advantages related to car handling and control efficiency, solving a number of problems inherent with using steel springs that suspension designers have always dreamt they could eliminate.

• Hydropneumatic is naturally a progressive spring-rate suspension; i.e., the more it is compressed, the harder it becomes. This results in the suspension being extremely soft around its initial course (softer than a steel spring) but getting harder and harder as compressed (more than a steel spring). This is because of the properties of gas: halve its volume, and its pressure doubles. When the suspension operates, the ram is pushing oil into the sphere altering its gas volume (and therefore the pressure). This natural principle of hydropneumatics has not been met so far by any other type of suspension. The nearest is steel springs with a softer course and a harder course (two different spring rates, while hydropneumatics offer an infinite number of rates). Usually steel-sprung cars are either too soft ("comfortable"), or too stiff ("sporty"), or some intermediate compromise, while hydropneumatics offer "two cars in one".
• This advantage pays off in a spectacular way when slaloming (otherwise known as the 'moose test'): the swinging speeds and acceleration patterns of the body of a hydropneumatic car offer ideal body control, and "load" the tyres in an ideal linear-like manner, helping to get the most out of them. A steel-sprung car acts more like a violently-swinging pendulum, "crashing" on its tyres (and abusing them) when leaning from side to side.
• The same natural law governing gases also ensures that the suspension's spring-rate (hardness) is continuously adapted to the weight it has to carry, and to infinite positions. For example, when the car is standing empty, the pressure within its spheres is in balance. If one passenger enters the car, this pressure becomes higher by the value of his weight (the gas in the spheres compressed to an equal degree, i.e. has now become "harder"). The car will have lost some height, so the self-leveling system immediately reacts and brings the car up to the predetermined ride height. The result is that the spring rate is kept constant, regardless of the load of the car. I.e., a car with 4 passengers and full payload will be equally well controlled as a car with just one passenger (bar the tyres, which of course remain at the same pressure.). With a steel-spring car, either the car would be set up to be comfortable with 1-2 passengers but getting too soft as more weight is added (becoming uncontrollable under full payload), or it would be too stiff with 1-2 passengers and okay on full payload.
• This effect is especially pronounced at the rear axle, where the designer of a steel-sprung car has to make the greatest compromise: the rear suspension has to be able to deal satisfactorily with a large range of load. Because of the above property of hydropneumatics, Citroën vehicles can have a rear that is set very soft; one can easily push the empty car down with his hand. When load is added, it stiffens as much as necessary. Steel-sprung cars need to have rear springs much stiffer than necessary for average daily driving.
• The self-levelling system makes it such that there's always and at any time an equal travel available for suspension compression and extension, no matter the car's load. Citroën have calculated that the ideal suspension should have at least about 18 cm of motion range, i.e. 9 cm each way, for achieving effective continuous contact between pavement and tyres (by absorbing any road unevenness). With a "height corrector" for each axle, the car suspension always remains at its ideal middle position, providing a steady compression and extension course, no matter the car's load. As you load a steel-sprung car, its bump absorption capability becomes totally asymmetrical (too small a compression margin and too much of an extension course available, and the suspension moves far from its ideal operating angles, reducing lateral/longitudinal grip, etc.).
• Very importantly, the continuous self-levelling function also rids suspension design of a number of unwanted compromises that commonly designers of steel-sprung cars have to incorporate: as the suspension is always functioning around one predetermined position, no matter the car's load, the various suspension-geometry issues become a much simpler equation to solve. A hydropneumatic suspension operates from its ideal angles at all times and conditions.
• The suspension being self-levelling, the possibility opens for dynamic height control. This has actually been implemented in Citroëns from the C5 I and onwards: the cars are programmed to lower by about 1 cm above a specified speed, thus reducing aerodynamic resistance, improving fuel economy and increasing high-speed stability.
• Ride height is manually adjustable (in all hydropneumatic and Hydractive Citroëns) in 4 positions: "low", "drive", "mid-high", "highest". "Low" is only for service's purpose and should never be used in normal driving. "Mid-high" or "Highest" may be selected to tackle some road obstruction (flooding, pavements, off-road, etc.) at very low speed .
• Because the suspension doesn't need to be set stiff to overcome all sorts of restrictions imposed by the steel spring, the ride comfort is excellent (the ride is described as floating above the road surface), with the difference that the suspension never 'wallows' uncontrollably like an equally soft car on springs would do. This preserves precise handling and road-holding (like a sports car). Orthopaedic doctors advise that patients with spinal injury or disk problems can only drive Citroëns with hydropneumatic suspension. The legendary Rolls-Royce comfort is partly due to this system equally fitted to millions of Citroëns sold to this day since 1955 with the DS. The possibility of having such a soft suspension but NOT the uncontrolled wallowing (e.g., like American cars) is due to another natural property of nitrogen gas: it inherently has much less endogenous friction than steel. In other words, it is relatively much more neutral, more inactive. Think of a leaf-spring being compressed: leave it free and it will pop and spend some time vibrating around its centre position till it comes to rest. This is due to its internal mechanical friction (due to the material), and this is why cars need shock absorbing dampers. A coil or torsion bar steel spring performs much better, (which is why it has long replaced leaf-springs in cars). Gas however behaves even better than the spring, in fact it represents a leap in effectiveness: compress a gas and release it: the gas will want to only return to its initial volume, not much more. Thus damping can be also reduced, resulting in an unearthly softness.
• This inertia of the gas is also the reason why on a hydropneumatic Citroën the driver will many times not even realise the event of a blown tyre—if not for the added noise—which can cause dynamic unrest and prove fatal on a steel-sprung car.
• The low-frequency wallowing characteristic of older Citroëns (prior to the BX) is due to the gas inertia as described above, and a soft damping. Commonly manufacturers believe that the optimal undulating frequency for a car's suspension is the one of human walking, i.e. about 0.2 cycles per second (0.2 Hz), thus many cars have this "rubbery", abrupt suspension feeling, which tends to increase as speed increases. Citroën tuned their cars to undulate at 1.6 Hz early on (DS, CX). Gradually they have brought this frequency down, with the Hydractive XM being set at 0.6 Hz (in the "soft" mode; in "hard", the car is sprung and damped like a sports car). The gas gives infinite possibilities: the operational frequency is just adjusted with the dampers (which look like disks the size of a large coin).
• The legendary comfort of Citroëns is also due to another factor, which is a specific choice in the set up of the system: Citroën chose to hydraulically interconnect each wheel at the same axis. Thus the two front wheels are connected with each other. And so are the rear wheels. This has a very specific result for body control: when one of the two wheels meets, say, a bump, that wheel will tend to compress and absorb the bump. In the degree not the entire height of the bump can be absorbed, the car's body will lift at that side. In the same time, through the anti roll bar, this wheel will exert force at the other side wheel and tend to also lift it in the same direction (compress the suspension). This is where the hydraulic interconnection of these two comes to play. In an ordinary steel-sprung car, this anti roll bar effect would mean that the car's body at the bump-absorbing side would lift, while it would tend to drop at the other side. This amplifies the bump's effect of destabilising the car and creating discomfort. In the Citroën system, the anti roll bar will also tend to do the same, but it is stopped by the hydraulic oil that is sent from the bump-side wheel at the moment it is pushed upwards to absorb the bump. Thus the suspension at the other side will receive a force opposing that of the anti roll bar, and of a degree of definition equal to the size of the bump. This in effect translates into a much reduced overall feeling of impact from any bump, as the shock is effectively automatically shared with the wheel at the other side of each axis. In practice this feels as if the whole front or the whole rear is slightly lifting, rather than the car receiving an unpleasant shock from one side. Another solution, as e.g. implemented in the Austin Princess, (and pioneered with mechanical interconnection on the 1948 Citroen 2cv), is to interconnect the front and the rear wheel of each side. This has also beneficial results comfort-wise (the front and the rear share the impact forces) but also unwanted counter-effects on braking and accelerating: in braking the car tends to nose-dive (all hydraulic oil is pushed to the rear). In the Citroën set-up the adverse effect is that the car tends to roll a lot (the oil is pushed from one side to the other) however this is filtered by the anti roll bar (there's no equivalent item on the Princess to filter front-rear balance, though this issue was addressed on later versions of that suspension). This effect in Citroëns, perceived by many as the only potential disadvantage of the hydropneumatic suspension (although it has not real effects on the lateral accelerations the car can achieve), has been totally put under control with the advent of the Hydractive systems, from the XM onwards.
• The master cylinder ("brake doseur valve") on Citroëns includes specific solutions that take advantage of the hydropneumatic suspension, e.g. the pressure for rear braking is taken directly from the rear suspension (they are hydraulically connected). This means that the braking force of the rear axle is continuously adjusted to the weight it carries (as the pressure within the rear suspension is equal to the weight it carries). So the rear brakes will come in significantly harder the more the rear is loaded.
• It is a matter of minutes for the home mechanic to replace an old sphere—which includes the "spring" and "damper" in one—using a simple tool that can be home-made. Also, one is free to try various sphere combinations on his car, selecting from the several varieties available for the various Citroën models, thus going for a more "comfortable" or more "sporty" set up.
• Compact suspension design, lies horizontally under the rear of the car avoiding suspension turrets taking up luggage space
• Maintenance, for a do-it-yourselfer, is relatively easy, once you know what you are doing. It doesn't require any specific tools.
• Replacement of a suspension sphere is much easier and safer than replacing a conventional spring and shock-absorber arrangement.
• Inexpensive in mass production; for vehicles that would otherwise have a conventional power steering pump, hydropneumatic suspension adds no new equipment and in many cases results in a lower unsprung mass. Also, the same system pressure produced from one central pump is used for braking (up to and including the Xantia).
• Upon body roll, the pressure exerted between the tires of the same axle is not subject to the same differential as on some other cars. The pressure in one suspension strut equals the pressure in the other through Pascal's law, potentially giving the 'light' tire more footprint pressure.
• Can be conveniently interconnected in the roll plane to improve roll stiffness and thus roll stability limit, especially for heavy vehicles.
• Can be connected in the pitch plane to improve braking dive and traction squat.
• If they are interconnected in the three-dimensional full car model, the interconnected hydro-pneumatic suspension could realize enhanced roll and pitch control during excitations arising from steering, braking/traction, road input and crosswind, as with the Hydractive arrangement
• Flexibility in the suspension strut design in the interconnected suspension system to realize desirable vertical, roll and pitch properties for different types of vehicles.
• Horizontal orientation of the rear suspension cylinders below the boot floor makes the full width of the boot available for cargo.
• Mechanical steel spring suspension systems that try to replicate only some of the inherent advantages of hydropneumatic suspension (electronically adjustable shock absorbers) end up being lesser solutions and more complex to build and maintain than the straightforward hydropneumatic layout.
• People who are prepared to carry out simple maintenance can acquire a used luxury car for a fraction of the cost, as hydropneumatic suspension scares potential buyers and dealers despite more complex and maintenance-intensive systems on other cars. Most of the components are not repairable by a DIY mechanic, but they are easily exchanged for new or re-conditioned units. Pumps, height correctors, accumulators (including suspension "spheres"), steering units, etc. can all be reconditioned, and simply interchanged with the use of ordinary automotive mechanics' tools. Hydraulic fluid is drained and refilled with fresh, much like changing engine oil. Later Citroën automatic transmissions are conventional modern units similar to those of other makes.