Technical : Hydragas explained

Hydragas suspension was one of the quiet achievements pioneered and refined by British Leyland – and yet, it never received the praise it deserved. KEITH ADAMS briefly explains how the system works.

Pioneer that led nowhere…

The Austin Allegro was the first car to feature Hydragas suspension - it wasn't a happy beginning.
The Austin Allegro was the first car to feature Hydragas suspension – it wasn’t a happy beginning.

SINCE the introduction of the Mini in 1959, BL’s predecessor, the British Motor Corporation (BMC), had been right at the edge of cutting technology – its smallest car popularized front wheel drive and miraculous interior packaging, changing the world in the process.

A mere three years later, BMC gave us even more of the same, with the Austin and Morris 1100 – essentially an enlarged Mini with crisp Pininfarina-penned styling and one further innovation: Hydrolastic suspension.

Conceived in the late-Fifties by Dr Alex Moulton, the ingenious Hydrolastic system was the result of his inspired idea to replace conventional suspension units with fluid-filled displacers containing a rubber spring; and then interconnect them with fluid-filled pipes to make it all sit level.

Originally, the Mini was to receive Hydrolastic at its launch in 1959, but Moulton and co-developers, Dunlop, couldn’t produce small enough displacers in time for the launch. However, that setback was undone with the launch of the huge-selling 1100 in 1962 – and with it, we soon became a nation familiar with the Moulton bounce: that phenomenon of BMCs gently rising and falling on undulating roads.

Endearing then, comical now…

BMC refined Hydrolastic during the Sixties, fitting the system to the 1800, 3-litre, Mini and Maxi, before moving on an improved version of the system.

Just as Hydrolastic will always be associated with Sixties BMCs everyone actually wanted, its replacement will be forever associated with that Seventies BL hell child, the Austin Allegro. Yet, the system worked extremely well – after a period of fine-tuning. In principle, it was the same as Hydrolastic – Hydragas cars had displacers, this time partially filled with nitrogen, separated from the fluid by a rubber membrane, to provide the suspension.

Or in simple terms, Hydrolastic cars were suspended on soft lumps of rubber, Hydragas cars made do with balloons…

In effect, the 1970s BLs that used Hydragas rode on a cushion of gas – how cool was that?

From ‘lastic to ‘gas…

Inside a Hydrogas displacer. (Pic:

Like the earlier cars, Hydragas cars were interconnected. The reasoning behind this was devilishly simple: when a wheel hits a bump, the suspension compresses, pushing fluid down the interconnecting pipe to the unit at the rear. In other words – front wheel goes up, rear wheel goes down – and you remain level when you hit a bump.

In early Allegros, Hydragas didn’t work too well, though. Damping wasn’t strong enough, the ride ended up being too choppy on rough roads, and too floaty everywhere else. The boffins at Longbridge soon worked on ironing out the creases, but customers unconvinced by the earliest cars proved impossible to win back – although its suspension was the least of the Allegro’s problems.

Things improved dramatically with the Princess, which did ride and handle very well, but by the time it was launched in 1975, BL was staring down the barrel of bankruptcy – and plans to do anything meaningful with the great Wedge were scuppered by crisis management of disappearing company funds.

Designer of the Rover SD1 and Range Rover, Spen King, was drafted in to create an efficient range of future BLs – and in his opinion, Hydragas was unnecessarily complex and costly to produce. That meant the Allegro/Maxi replacement would lose Hydragas, and gain a carbon copy of the VW Golf’s MacPherson strut/beam axle set-up.

What was this brave new world? The Maestro…

Happily, it wasn’t entirely over for Hydragas: BL still needed a supermini, and didn’t have the funds to create a brand new car. So, bits of Mini and Allegro were thrown together, the pot was stirred, and four years after being first thought of, the Metro appeared.

King didn’t have the resources to engineer Metro to use conventional springs, and kept the Allegro’s system – although to reduce costs and perceived complexity, interconnection was dropped. This denied the Metro of its one major advantage over all its rivals – a level ride. It worked well enough, but no better than any of its more conventional rivals – and as the Eighties wore on, the Metro’s knobby ride became the object of derision for many…

…Except Alex Moulton, who realised the Metro was incredibly compromised when shorn of its extra pipe work. He worked on his own Metro, putting interconnects back in and fiddling with the damping in the Hydragas displacers. And from his workshop in Bradford on Avon, emerged a fine example of automotive alchemy…

Rebirth of the system

In 1987, he lent his homemade Metro to CAR magazine, who realised instantly the significance of this innocuous looking brown W-registered Metro. It rode, handled and cornered like a grown up car. News of this miracle soon spread to the Rover development team in Gaydon, who swiftly borrowed the car, and discovered that Hydragas was a long way from being dead and buried.

Rover was on yet another cost cutting drive at the time, and the replacement for the Metro – a larger and more conventionally engineered car – was in the process of being canned. Moulton’s car came along just at the right time, because it meant the new car’s K-Series engine could be stuffed into the original bodyshell and combined with the sorted version of Hydragas to create a ‘new’ car – thus saving Rover shed loads of cash.

In 1990, the Rover Metro was born – and true to its origins, it was a 1980 car, with a 1990 engine and homemade suspension. And yet, it wasn’t a disaster – it was actually rather good on the road.

Similar thinking brought us the MGF in 1995. Rover needed an MG sports car to compete with the Mazda MX-5, the bosses agreed on a mid-engined design, and so the resourceful engineers at Gaydon took a crateful of Metro engines and front subframes, swapped ends, bolted them together, wrapped them up in smart clothes – and created one of the Nineties’ most successful roadsters.

From this brief resurgence, Hydragas faded from view. The Metro died in 1997, its demise accelerated by poor NCAP crash test results – leaving the MGF the only car left using this innovative suspension system. Five years later, the game was up for the MGF, too… no longer a financially viable system MG Rover killed Hydragas, by replacing the F’s displacers with conventional springs.

Cars that rode on Hydragas

1973: Austin AllegroAlex Moulton’s exciting new suspension system debuted in the Austin Allegro. Early versions were under-damped but offered soft and accommodating ride. In time it was developed successfully, and by the time of the Allegro 3 in 1979, it was very effective indeed.
1978: Austin MaxiIntroduced on Hydrolastic, the Maxi is the only car produced by BL to later be converted to run on Hydragas. Many people consider the Maxi to be one of the more successful Hydragas installations, as it offered great ride quality and very keen handling, too.
1975: 18-22 Series/PrincessBecause it was new and shiny in 1975, the Princess overshadowed the Maxi’s use of the Hydragas suspension system. It was an impressive car with impressive ride quality, and only the curious tyre choice and excess road noise denied it greatness.
1980: Austin MetroDeveloped in double quick time, and overseen by Spen King, the Austin Metro’s Hydragas system was compromised by the lack of proper interconnection – and the earliest cars needed to use supplementary dampers in order to smooth the ride.
1982: Austin AmbassadorMuch more than a Princess with a boot. The Ambassador may have only remained in production for two years, and never made it to left-hand drive, it featured a very highly developed version of Hydragas, which featured new and improved steel valving.
1990: Rover Metro/100Again – much more than a facelifted Metro. The Roverised car sported the exciting new K-Series engine and its Hydragas suspension was significantly revised, with full front to rear interconnection. The difference was night and day – shame it didn’t look it.
1995: MGFAs it was heavily Metro based, it came as no surprise that the MGF used its suspension system. Surpisingly soft for an open-topped sportscar but not lacking in the handling department. When it was replaced by the TF in 2002, the system was dropped.
Keith Adams


  1. I agree about the Allegro 3 having a superb handing / ride compromise. Nearly went for an Allegro 3 1700HLS on a W plate… drove it like I stole it and apart from the vague gearchange… loved it. The chap wouldn’t come down on the price so I bought an Ital instead – FAIL!

  2. Big issue with Hydrogas units leaking nitrogen and so losing ride height. People just added more fluid to raise it back, making the suspension excessively harsh.

  3. Yes, that is the big issue with Hydragas, but usually only if the car is between 15 and 20 years old – so hardly to class as a problem from the manufacturers view. Why MGF seem to last less well than older Allegros or Maxis is not clear to me.

    The solution is not too difficult and on my Maxis already lasting since nearly 10 years wihtout further work: Recharge the Nitrogen using a valve welded onto the unit.

    • The rear suspension went on my Metro after 1 month, it was 11 years and 6 months old and then when I replaced it at 14 years and 7 months. Great suspension, really miss it.

  4. I seem to remember that the first Metro was connected side to side only at the rear. No idea why – the K series version was much better.

    • The Metro was indeed connected side to side at the rear, the reason was safety, Alex Moulton described the principle of the four-legged and three-legged stool effect of Hydragas in his key paper read to the Institute on hydragas suspension, without the interconnection the Metro was likened to a four-legged stool on an uneven surface, ie not stable, with interconnection, the Metro was likened to a three-legged stool, ie stable. I have a copy of the Moulton paper, regret cannot find it to quote the details of publication

  5. Spen King wanted to use only the compact Hydragas units as spring/damper combination on the original Metro, not connecting any of them. If he did not like the interconnection, did not understand it really or if cost was the reason… Surely he preferred more conventional, simple solutions as SD1 and Maestro show. Dr. Alex Moulton insisted on at least one interconnection, as otherwise it would have been easy to deliver a car looking ‘right’ (i.e. standing even) but with dangerous roadholding by pressurising opposing Hydragas units differently in diagonal pairs (e.g. front right and left rear too high pressure, both others too low).

    • Would there have been any functional/cost advantage in keeping the interconnection but removing the damper valves from each displacer and using telescopic dampers all round?

  6. Now then – external dampers.

    I’m 99% sure my 1969 Hydrolastic Mini didn’t have separate shock absorbers. The Hydrolastic unit provided damping. But my Hyragas MGF does have separate shock absorbers. I also had Maxis, Metros and an Ambassador, but can’t remember if any of them had shock absorbers.

    Can anyone tell me when the change happened?

  7. #5 The A plus Metro had a pipe connecting the displacers for the rear wheels. A safety measure, without the pipe, the car would be like a badly-made four-leg stool with a short leg.

    The source of this information, and an elaboration of Hydrolastic, Hydragas development, (also the “Brown Metro” )may be read in ” Bristol to Bradford on Avon” the autobiography of Dr Moulton.

    In his book Dr Moulton implied BL engineers overlooked the important safety feature of that rear connection, imagine the headlines if the car had been launched minus the pipe,

    “Britains New Car a Killer”

    Just like the Audi TT!

  8. @MM – the explanation was already given by me in the answer #6. A correctly pressurised Metro with 4 individual units and no interconnection is no safety risk at all – just like any other car on conventional springs. But there is no possibility for an easy visual check if everything is alright – mostly for later servicing of the car – without the interconnection of the rear units. With this interconnection on the cars, these would lean if any pressures would be uneven – and here the picture of a 3-legged vs. a 4 legged chair are valid.

  9. hello friends,
    i’m enginer for classic cars and i reach to purchase Hydrolastik displacer unit with hose for austin Princess vanden plass 1100
    necessary 2 front and 2 rear
    could you supply or help me
    best regards
    Michel (south of France)

  10. the 2CV suspension and vehicle dynamics was assessed by Alec Issigonis and Alex Moulton in the mid-1950s (according to an interview by Moulton with CAR magazine in the late 1990s); this inspired them to design the Hydrolastic suspension system for the Mini and Austin 1100, to try to keep the benefits of the 2CV system but with added roll stiffness in a simplified design.

  11. Can anyone tell me what the precise difference between the front and rear displacer units on the Rover 100 Metro is ? I can’t see a physical difference in the basic units, is it just the installed pressure of the Nitrogen gas ?

    • There is a significant difference in spring and damping ratio. Much of this is achieved by having a differently shaped ‘cone’ that goes into the displacer, but also the damper and bypass bores on the inside will be different. I do not know about the Nitrogen pressure actually used inside, but as the fluid pressure is the same front and rear, the difference will not be as significant.

  12. The rear interconnecting pipe on early metro’s was a safety issue , if not connected, a pressure leak from one of the displace rs is not evident at the rear because the other unit keeps the car height correct ! The driver only notices when commingl into the next corner at speed when handling of the car can be compromised !

  13. Do any modern(2017) cars have hydragas/hydrolastic suspension systems?I had two Minis in the early 1970s that had hydrolastic suspension-a remarkable suspension system.

  14. By using a Rubber cone as in the mini, in one unit you have a spring and damper at the same time ! saving weight, cost ans space, also by connecting front to rear via fluid, the car during braking tranfer weight from front to back reducing the polar momment of inertia during braking and cornering. I have a Cooper 1275 and years later a Lotus Europa the Mini was a litle better in this aspect.

  15. Learnt to drive mostly on a Metro GTa which was a really nice little car to drive (and my driving instructor was a total nutcase). Just bought a hydrolastic car which is riding a bit low but doesn’t seem to be leaking – just needs pumping up, although it does seem to have dropped a little more on the drivers side. Even riding 25mm low its still as good or better than the modern cars, & much smoother.

    Note: follow the tyre pressure instructions in manuals to the letter. If you have them too high you’ll need a new word for travel sickness.

  16. Update: Have had the suspension pumped up. Thank you Maldon Road Garage, Colchester. Oddly not much different out on the road, but so much better at car park speeds – like suddenly getting PAS. Suspect incorrect suspension geometry pressuring steering.

  17. I had a 74,000 mile Austin Maxi in Australia in 1977. One of the rear hydragas units failed on the gravel road around the north-west of Oz. I had a metal bracket made up to fit in and keep the tyre from rubbing the wheel arch until I could get a replacement unit. Then the rubber bush that let the unit pivot wore so much that the wheel began tilting and the tyre was wearing itself through by rubbing on the inside of the wheel arch. I turned the wheel around, so that it was clear of the arch, and did over 300 miles like that on dirt and tar roads before finding a garage to fit a new bush.

  18. I have a 1989 Metro and was not to sure about replacing the rear interconnecting pipe and individualising the suspension. After reading AOR I am now aware the pipe is a safety feature. It is just proving impossible to find any new old stock, rather than get one made up.

  19. It has been mentioned ADO22 was to receive a revised Hydrolastic suspension system under development, yet was what would ultimately become Hydragas originally conceived for this car or at least conceived during the same period before the ADO22 project was cancelled in favour of ADO67 / Allegro?

    • I wondered if Issigonis was referring to Hydragas when he wrote to Donald Stokes in April 1968. Referring to the 9X, he wrote, “Hydrolastic has had to be abandoned in favour of conventional springing. This is because a simplified version of the Hydrolastic design, which we have been working on for some time, has not yet materialised.” Didn’t I read somewhere that Hydragas was cheaper and easier to produce than Hydrolastic? In any case, the earliest the world got to know about Hydragas was 1970 when the Moulton coach prototype emerged, so the system must have been in development for a good while before that. Some interesting comments on the Moulton coach here:

  20. Years ago Steven Ward wrote of a further development of Hydragas where (if I remember correctly) the system was connected to the engine mounts. Would like to read the article again, but I can’t find it on the website. Any ideas?

  21. Hydrolastic had one big defect – a heavily laden car or one being used for towing adopted a distinctive attitude and the headlamps became useful for spotting low flying aircraft.

    It also ran the driveshaft joints at problematic angles when heavily loaded, just at the time they would be working at high loads.

    No self levelling headlights or brake pressure compensation back then!!

    I gather that there was an Australian attempt to produce an Austin 1800 ‘ute’, I wonder how they planned to deal with the nose in the air problem when carrying a ton of cargo in the back??

    Happily the likes of the Boge Nivomat arrived to help us coil sprung towing types maintain an even keel. Wasn’t something similar used on the Austin 3-litre??

  22. From memory, I think the 1800 Ute used auxiliary torsion bars to mitigate the system’s tendency to sag under a heavy load. The 3-litre used a self-levelling system developed by Armstrong, the shock absorber people. I’ve a suspicion that Bristol might have used this system briefly on the 411.

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