Since 1954, top-spec Citroën models have famously ridden on what felt like a cushy wave of fluid—because, well, they actually rode on a cushy fluid. The French automaker’s innovative hydropneumatic suspension system used pump-driven hydraulic fluid and small nitrogen-filled bladders at each wheel to absorb road impacts like no metal spring and gas-charged damper setup could, while also offering load-leveling and, eventually, mitigating body roll. That the hydropneumatic system has survived to today is a testament to its fundamental effectiveness, but alas, pressure to reduce weight and complexity has finally done it in. According to Automotive News Europe, Citroën won’t continue to develop its hydropneumatics, the company’s CEO stating that “it is an old technology.”
It is true that Citroën’s hydropneumatic suspension is at its core old-school and wonderfully mechanical, and modern electronics and manufacturing have probably brought the technology to its maturity. When it debuted on all four corners of the groundbreaking Citroën DS in 1955 (it first arrived with fewer features in 1954 on the Traction Avant’s rear axle), the system wowed with its adjustable ride height, load-leveling, and pillowy ride quality. The original system worked by filling a cylinder at each wheel—capped by a nitrogen-filled bladder—with incompressible hydraulic fluid; an engine-driven pump kept the system pressurized, and could route more fluid to the cylinders as needed to offset heavy loads and raise the car. The nitrogen gas in the bladders served as the “spring” in the system, as it was compressible. On the crappy roads France had at the time, the hydropneumatic suspension was a revelation, and while early cars were plagued by hydroscopic hydraulic fluid that could absorb water—and thus rot the system from the inside out—later cars gained improved fluid that didn’t do that.
While the setup still works, and is sold on the Citroën C5 luxury sedan, its weight and power-sapping hydraulic pump make meeting today’s increasingly stringent fuel-economy standards harder. Given how hydropneumatics are Citroën’s thing, and as the company’s CEO points out, ride quality is a hallmark of the brand, new solutions that live up to that promise are in the works. Citroën isn’t saying what those solutions are, but it’d be difficult to imagine that they’re anything but the less-expensive air suspension designs and electronically adaptive dampers ubiquitous in most luxury vehicles today. When the current C5 is replaced, Citroën’s hydropneumatic cleverness will die with it. Pour out some hydraulic fluid for a dead homie, gearheads.
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