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In the 21st century, the automotive scene has seen companies such as Audi, Buick, Ford, GMC, Jeep, Nissan, Scania AB, Subaru, Tesla, and Volvo embrace air suspension by equipping their top models with electro-mechanical self-levelling devices. Collectively, these systems are often referred to as “adaptive (or passive) suspensions.” This technology is not limited to luxury cars; it is also making an impact in public transport by improving accessibility for people with disabilities through “kneeling buses.”

Citroën maintained its hydropneumatic primacy with the C5 (2001) and the C6 (2006) featuring the latest Hydractive developments. In 2010, the Nissan Patrol, followed by the Infiniti QX, adopted a Hydraulic Body Motion Control (HBMC) system. A year later, McLaren introduced its ProActive Chassis Control (PCC) in the 12C, a no-roll suspension system inspired by both the Xantia ACTIVA and the Hydrolastic/Hygragas principles, but taking it a step further. While the 2017 Bollinger B1 electric truck generated interest with its hydropneumatic suspension, it unfortunately remained a prototype.

Around 2017, Lexus jumped on board with its AVS (Adaptive Variable Suspension). The realm of adaptive and active suspension systems has seen names evolve into a complex and creative lexicon, with terms like Adaptive Solenoid and Adaptive Magneto-Rheological suspensions. Magnetic ride dampers use iron particles that can be magnetised, allowing for changes in fluid viscosity and adjustments to damping behaviour. On the other hand, Mercedes has been at the forefront of hydropneumatic technology, unveiling the Magic Body Control pre-scan suspension system in 2007, followed by the Active Curve Tilting feature in 2014.

In 2019, the Mercedes GLE 450 4MATIC became the first AWD vehicle to feature the latest version of ABC hydropneumatic suspension. Around the same time, Rolls-Royce introduced an important advancement with their Planar suspension system, where engineers pushed the boundaries of innovation by creating a simple yet brilliant solution that actually dampens… damping (see picture on the right). By 2023, the Chinese BYD Yangwang SUV and U9 sports car unveiled another active hydropneumatic suspension system, whimsically dubbed DiSus or…”pneumohydraulic.” This system is marketed as allowing the car to be driven on three wheels. NIO also joined the ranks with their SkyRide Fully Active hydropneumatic suspension, enabling their new ET9 model to be driven with two of its tyres blown. It’s almost ahocking to realise that both these safety concepts were first explored (and advertised) by the Citroën DS nearly 70 years ago!

Citroën, a pioneer in developing hydropneumatic systems, after having commercialised more than 10 million hypcars over six decades, made the tough decision to stop using them in 2017. This was largely due to the high production costs associated with hydropneumatic components. Furthermore, the rise of electronically controlled systems, which often deliver similar comfort and handling, combined with the adoption of hydropneumatic suspensions by select high-end brands, made these costs difficult to justify.

While Citroën may have stepped back from the hypcar scene, many modern suspension technologies, including Citroën’s Progressive Hydraulic Cushions Technology, still pay homage to Pascal’s fluid law and Magès’ groundbreaking hydropneumatic invention. And if you still wonder why, it is simply because gas has a clear advantage over steel in terms of flexibility, smoothing out road imperfections, while hydraulics excel in their ability to enable self-levelling and ground clearance adjustment without compromising flexibility, something gas suspensions struggle with. J.P. Chassin’s book “Why Citroën” reveals that cars equipped with hydropneumatic suspensions can travel 35% faster than their conventional counterparts on rough and uneven surfaces, thanks to their superior ability to maintain trajectory and handle road irregularities with ease. It’s no wonder we are seeing a resurgence of hydropneumatic systems lately from manufacturers like Mercedes, BYD, NIO, and others. Let’s not forget that almost all commercial jet airliners have relied on hydropneumatic struts for their landing gear for many years!