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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: Carbibles.com)

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

About the Author:

Created www.austin-rover.co.uk in 2001 and built it to become the world's foremost reference source for all things BMC, Leyland and Rover Group, before renaming it AROnline in 2007. Is the Editor of the Parkers website and price guide, formerly editor of Classic Car Weekly, and launch editor/creator of Modern Clsssics magazine. Has contributed to various motoring titles including Octane, Practical Classics, Evo, Honest John, CAR magazine, Autocar, Pistonheads, Diesel Car, Practical Performance Car, Performance French Car, Car Mechanics, Jaguar World Monthly, MG Enthusiast, Modern MINI, Practical Classics, Fifth Gear Website, Radio 4, and the the Motoring Independent... Likes 'conditionally challenged' motors and taking them on unfeasable adventures all across Europe.

16 Comments on "Technical : Hydragas explained"

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  1. kevin says:

    where could i buy hydroelastic please for an austin alegro

  2. Mike Humble Mike Humble says:

    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!

  3. Chrisk says:

    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.

  4. 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.

  5. Steve L says:

    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.

  6. 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).

  7. Steve L says:

    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?

  8. After all, Moulton was right: Mercedes now uses a system using the same type of interconnection with great success in formula 1 – named FRIC (front and rear inter-connected). Have a read here: http://www.formula1.com/news/technical/2013/894/1050.html

  9. MM says:

    #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!

  10. @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.

  11. Prudhomme marseille says:

    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)

  12. david says:

    Of course the Citroen system was more complicated as it used a pump but the pressure was used for other functions .http://en.wikipedia.org/wiki/Citro%C3%ABn_DS Even the humble 2cv has an interconnect front wheel to rear by the way of horizontal springs in steel tybes.

  13. david says:

    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.

  14. Phil Sutcliffe says:

    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.

  15. Dave Sharp says:

    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 !

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