The history of Hydrolastic and Hydragas suspensionHydragasHydragas Register
The history of Hydrolastic and Hydragas suspension and how they work

Stephen Moulton was the man responsible for bringing vulcanised rubber to the England from Goodyear in the USA, and later went on to start a business in a disused Woolen Mill in Bradford-on-Avon, Wiltshire, producing rubber for industrial applications. This developed into a specialisation in rubber suspension for railway coaches. In 1891, eleven years after Moulton's death, his company amalgamated with that of George Spencer in London to form George Spencer Moulton & Co. Ltd. Rubber is an excellent spring as it can store more potential spring energy per volume and weight than any other known substance.

After the Second World War, Stephen Moulton's grandson, Alex, started to point the company in the direction of automotive suspension. One of them was Flexitor, as used on the Austin Gipsy, a form of rubber torsion bar, in which a rubber cylinder attached to the trailing arm was bonded to an outer metal cylinder attached to the chassis of the vehicle. The rubber twisted and provided the spring, and was complemented by separate hydraulic dampers. Various experiments followed in the 1950s, including a rubber-suspended Morris Minor covering 1000 miles on the pave at MIRA. Inspired by the mechanically interconnected suspension on his Citroën 2CV van, Dr Moulton soon realised that by creating a space between two rubber cones and filling it with fluid, the suspension units can be inter-linked to reduce pitch. The first prototypes featured two 7" diameter rubber springs on each wheel, mounted one above the other and connected by 5/8" bore pipes.

Ex-Morris Engineer Alec Issigonis, working for Alvis on the TA350 luxury saloon car, noticed these with interest. Issigonis and Moulton had met socially before they worked together, and had worked together on the Morris Minor's all-independant suspension. Sadly however, the TA350 project was cancelled, and Issigonis and Moulton moved to BMC. There they further developed the interconnected design into Hydrolastic suspension, introduced on the Morris 1100 in 1962.

Vanden Plas 1300

The ADO16 was the first car to use Hydrolastic suspension, shown here is the Vanden Plas Princess 1300

Hydrolastic suspension featured a metal housing with a rubber spring mounted in the top. The rubber spring had a hollow in the centre in which was the fluid, and below this a larger fluid chamber. Between these two chambers was a damper valve, which controlled the fluid flow and therefore deleted the need for separate damper units. The bottom of the lower chamber was formed by a Nylon reinforced rubber diaphragm (this is known as the lower diaphragm), and below this was a tapered piston which was mounted on the top suspension arm. In the top of the unit a flexible hose, similar to a brake hose, connected the unit to the one at the other end of the car via a metal pipe running along the floorpan. When the front wheel hits a bump, the piston pushes up on the lower diaphragm which forces a quantity of fluid through the damper valve, and acts on the rubber spring. This absorbs most of the shock, but a portion of the fluid passes down the pipes to the rear Hydrolastic unit and jacks up the rear of the car, keeping the car level. The very first Hydrolastic units shared a common fluid chamber called a 'cheese' which sat under the front seats, but due to the noise it was changed to the separate displacer system, initially remotely and later on the suspension arms (as on production cars).

If you imagine an interconnected system with no springs riding over a bump of height hcm, the front unit will rise h/2cm, and the rear will push the wheel down; i.e. the body up h/2cm, so the bump is halved and the car stays level. Now introduce the spring and damper effect and you can see how the remaining bump is dealt with. Note that the Hydrolastic units are connected front to rear on each side, and not across the car. This means that when the vehicle goes round a corner, the body leans out on both units, there is no fluid flow, and the only suspension is the rubber spring and the damper valve. This gives the very useful effect of soft suspension over bumps that stiffens up when cornering, giving a degree of anti-roll without the need of an anti-roll bar which can compromise the ride quality (note that some setups incorporate anti-roll bars).

Hydragas Moke

An experimental Moke with dash-mounted adjustment and gauges for the Hydragas suspension

Hydragas, introduced in 1973 on the Allegro, is exactly the same interconnected system except Nitrogen gas is used as the springing medium instead of rubber. Nitrogen gas does not expand excessively with temperature, and its behaviour is much more predictable than air. It gives a progressive springing action and the lack of Oxygen prevents the units rusting from the inside out. In a Hydragas unit, the fluid chambers are in the lower section, with a damper valve as in Hydrolastic, but the interconnection pipe is below the damper and there is a gas chamber on top, separated by the upper diaphragm. The gas is sealed for life and cannot be pumped up without modification to the Hydragas unit, although the fluid can be pumped up as with Hydrolastic. The reason for the change to Hydragas was to reduce size and weight, and it was potentially cheaper to build. Contrary to popular belief, Hydragas was a cost effective system when mass produced, the Allegro's Hydragas arrangement costing 4% less than the Volkswagen Golf's coil spring-based suspension. It is also lighter, at 13.5kg compared to the Golf's 14.4Kg. The Allegro did not use subframes because Leyland engineers claimed that as the body needs to be strengthened at the subframe mounting points, why bother with a subframe at all?

An interesting Hydragas concept is Alex Moulton's safety coach. Featuring eight small wheels, with the Hydragas interconnected on each bogie, the coach also featured space frame construction. The reason that the coach was only connected on each bogie and not all together was that the very forward engine mounting and luggage boot at the rear, together with the front bogie having a lower bounce frequency than the rear, meant that the primary ride was sufficiently flat. The interconnection within the bogies also gave excellent adhesion, also enhanced by the small wheels with low unsprung mass.

The tapered pistons in the base of the displacer are very carefully designed as the volume of fluid displaced depends on the shape of the piston. Over time the suspension may sink, and to rectify this a suspension pump is required (or when carrying out repair work). The trim height is measured from the centre of the wheel to the lower edge of the wheel arch, and then adjusted using the pump. The pumps are connected to the pump-up points by schrader valves, but to save leakage a low-loss connector is used on the pump end, which has a T-shaped handle to manually open the valve once the connector is screwed on. The original pumps used by BMC/BL dealerships were the Churchill models, tool number 18G 703, called 'Dalek' type pumps by enthusiasts owing to their appearance, but were later replaced by the Liquid Levers Hydravac pump. Professional pumps such as those mentioned feature a vacuum facility, which is required to expel all air from the units before pumping in fluid- anyone who has done air conditioning servicing will know the procedure. If you just pump fluid into an empty unit, you will never get the air out of the unit, so first the pump vacuums out all air (the same procedure is used for brake bleeding on car assembly lines and car windscreen repairs). After pumping up, it is necessary to take the car for a short drive and re-check as they tend to settle after just being pumped up.

Spen King liked the compact suspension units but did not understand the interconnection. This is why the Austin Metro of 1980 was not connected front to rear, although Dr Moulton did persuade the design team to link the suspension across the rear, to give better stability. However Dr Moulton bought a Metro and connected the suspension, as well as fiddling with the spring and damping rates. The result was an excellent ride and handling quality, and Rover fitted this system to the new Mk3 Metro with the K-series engine. The system lived on in the Rover 100 (updated Metro) until 1997, and in the Metro-based MGF sports car until 2002, when the coil-sprung MGTF replaced it. Many people agree that the coil sprung TF and Hydragas MGF have simillar handling but the ride is a lot better with Hydragas.

Gold Hydrolastic Trophy

The one millionth Hydrolastic unit was gold plated and presented to Dr Moulton

As for now, all we can do is save as many Displacers as we can to ensure parts are available for many more years.

Alex Moulton died on 9th December 2012. You can read his obituary here or buy his autobiography here.

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