MGF Hydragas Suspension

English language Original Source (closed down unfortunaltely
German language edited Source
Experimental characterization and mathematical modeling of the Moulton Hydragas automobile suspension Author: Geoff Rideout 1998

Summary of Research

The Hydragas suspension system (see below) from a Rover Group MGF sports car was statically tested to determine the static spring properties. A stiffness matrix was generated that included terms due to individual unit and interconnection effects. Dynamic tests were performed at various sinusoidal amplitudes and frequencies to calculate the damping matrices. One unit was held at its datum position while the other was moved. The damping force vs. velocity diagrams indicated the presence of inertial effects and dynamic stiffness due to rubber hysteresis.

Three models were developed to predict forces transmitted through the units. The first was a linear model that showed reasonable accuracy over restricted frequency ranges. The second model used bilinear spring and damping constants, and was accurate for predicting force at both the front and rear units for frequencies from 1 to 10 Hz. The third model treated the inertial and compressibility effects by inserting a mass into the spring-damper system. The magnitude of the mass was estimated based on the areas of the damper force vs. velocity diagrams. The inertial model did not predict peak forces as well as the bilinear spring/bilinear damper model, but was better able to predict phase lags between the forces at the dynamically excited unit and at the stationary unit.

The Hydragas System

Designers have considered for decades the interconnection of the front and rear wheels as a means by which to reduce unpleasant pitch behaviour of small cars. In an interconnected, or "equalising" suspension, the front and rear wheels on each of the driver and passenger sides are interconnected, forming two independent systems wherein an input at one wheel will cause a force to be generated at the other.

A forward-travelling car will first encounter disturbances at the front wheels that tend to lift the front of the car in relation to the rear. Interconnection has striven to transmit some the upward vertical motion of the front wheel into a downward force exerted by the sprung mass on the rear wheel, through the rear suspension. Reducing the differential between front and rear forces on the car body induces more of a bounce mode of vibration than a pitching mode.

Pitching motion, which causes fore-and-aft movement of the occupants, is generally regarded as being more objectionable than bounce motion [1]. Smaller cars are particularly susceptible to being set into a pitching mode compared to large cars, and it is this "choppy" quality to which many people attribute the inferior ride of small cars.

The Hydragas suspension was developed by Dr. Alexander Moulton of Great Britain to improve the ride quality of small cars. The system uses nitrogen gas as the springing medium and hydraulic fluid pressure drop as the damping mechanism. The damping fluid chambers of the front and rear wheels on each side of the car are connected via a hydraulic hose, such that an input at the front wheel pumps fluid through the pipe to the rear wheel. The increase in fluid pressure at the rear unit creates an upward force on the sprung mass, thus reducing the differential between suspension forces on the front and rear of the car body.

Figure 1 [2] depicts a later-generation Hydragas unit. Each unit contains both a gas spring and a damper spring

Figure 1 - Hydragas Suspension Unit Cutaway [2]

A butyl rubber separator, in concert with metal sealing rings and overlapping case pressings, seals the nitrogen gas from the fluid. The lower diaphragm is composed of a sealing rubber layer atop a structural rubber layer. Upward vertical movement of the tapered aluminium piston increases the area of the diaphragm against which fluid pressure acts, and vice versa. Sufficient pressure differential between the upper and lower chambers compresses either the bump or rebound compression block, increasing fluid flow as a function of pressure difference. The Hydragas units used in the present research project were from the new Rover MGF sports car, and featured a smaller-volume gas chamber and a damper valve of different construction. The function of the compression blocks was performed by normally-closed metal leaf-spring flaps.

Figure 2a [2] shows the general response of an interconnected Hydragas system to pitch-inducing motions in which the front wheel is lifted in relation to the car body, while the rear wheel is lowered. The upward motion of the front wheel in pitch displaces fluid to the rear unit, causing the rear wheel to exert an upward force on the car body. Simultaneous upward front wheel and downward rear wheel motion not only positively displaces but also draws fluid from the front to the rear unit. The stiffness, or resistance of the system to such wheel motions, is clearly lower than it would be were the interconnection removed.

The variable-area pistons stabilize the pitch oscillations. After the wheel motion of Figure 2a occurs and pressure stabilizes in the system, the larger area of the front piston causes a higher downward force on the front wheel than does the fluid pressure acting on the smaller piston area of the rear unit. The piston area variation thus produces a restoring moment about the pitch axis.

Figure 2b [2] depicts response in pure bounce or roll in which both wheels on one side of the car move the same distance in relation to the body. No flow occurs from one unit to the other, with flow occurring only between the upper and lower chambers of individual units through the damper valves. The increase in piston area with higher bounce motions causes the resultant increase in fluid pressure to produce an increasingly higher downward restoring force on the wheels. The system thus has a progressive rate characteristic. The combination of higher fluid pressures and increasing piston areas give the system a higher stiffness in bounce and roll than in pitch,
possibly obviating the need to fit a front anti-roll bar [2].


Figure 2 - Hydragas Suspension System Excitation Modes [2]


1. Gillespie, T.D. (1992) Fundamentals of Vehicle Dynamics. Warrendale, PA: Society of Automotive Engineers.
2. Moulton, A.E., and Best, A. (1979) "Hydragas Suspension." SAE Paper SAE-790374.
Other Interconnected Suspension References
1. Moulton, A.E. (1962) "Hydrolastic Springing." Automobile Engineer, Sept. 1962, pp.328-336.
2. Moulton, A.E., and Best, A. (1979) "From Hydrolastic to Hydragas Suspension." Proc. Council of the I Mech. E v.193, no.9, pp.15-25.
3. Moulton, A.E., and Turner, P.W. (1956) "Rubber Springs for Vehicle Suspension." Proc. Auto. Div. Inst'n Mech. Engrs 1956-7, no.1, pp.17-41.
4. Pevsner, J.M. (1957) "Equalizing Types of Suspension." Automobile Engineer, Jan. 1957, pp.10-16.

Rideout, D.G., and Anderson , R.J. (2003) "Experimental Testing and Mathematical Modeling of the Interconnected Hydragas Suspension System." SAE Journal of Passenger Cars – Mechanical Systems

#More stuff from Geoff (2000 on)
File #1
/ File #2 / File #3 / File #4

"JH Gillson " post to the BBS in 2002

Anyway, for all you Moulton fans out there here's an article on the great man that I have cheekily culled from the pages of "MiniWorld." He has some interesting things to say about the B*W 116, and you might also be intrigued to learn of Ford's response to Hydrolastic after its launch on excellent 1100 of 1962. Remember, at this time Ford's state of the art small car was the angular Anglia; Ford's answer to the Morris Minor and only 11 years late.

Suspension engineer Dr Alex Moulton was a fundamental part of the design team that worked on the original Mini back in the late 1950s. He gave us some insights into the development of the Car of the 20th Century - and his opinion on the BMW Mini of the 21st century

After the Second World War I was determined to move my family firm — Spencer-Moulton based at Bradford on Avon — from the manufacture of rubber suspension for railway coaches, and into automotive suspension. I had heard of Alec Issigonis by reputation and we met socially before we worked together. Issigonis was working at Alvis in Coventry at that time, on a new large car to replace their (famous but dated) “Grey Lady” model. He was given a free hand to do what he liked, more or less. He had heard of an experimental rubber-suspended Morris Minor that Jack Daniels and I had built at Cowley. It had been driven over 1000 miles on the pave at MIRA, an unimaginably tough test that the standard car would not have been able to withstand.
Following my experimental developments of the mid ‘50s. Issigonis was determined that he should have interconnected fluid and rubber suspension on his Alvis. Interconnecting front and rear suspension units with fluid in pipes meant that the pitch mode was separated from bounce and roll. As a result, the car moved smoothly over a bump in the road, giving a much higher quality ride.’ The Alvis was an interesting technical exercise, but it never reached the production line.
“Leonard Lord and George Harriman had formed BMC in tbe mid ‘50s by merging Austin and Morris. Lord was determined to make a range of new and innovative cars, and he plucked Alec Issigonis back from Alvis to design them. The Suez crisis occurred in 1957 and petrol rationing meant that tiny bubble cars, mostly of German manufacture and including some BMW-Isetta models, began to flood the roads. Lord wanted to sweep them away and so Issigonis got the order to build the smallest of his new cars first, a proper miniature four-seater. Issigonis told Lord that he wanted me to be involved with my suspensions and so I went to ‘the Kremlin’ at Longbridge to meet him. He looked at my portfolio of experimental work and said, ‘If you’ve done this, you’ll do more.’ He was very gruff but I got the OK.
“I formed Moulton Developments to work on the Mini suspension and other projects. BMC took a shareholding and paid all the costs of development. This left me free to work on the things I wanted to do. We actually developed the principles of the Hydrolastic system during the ‘5Os but it wasn’t ready for production in time for the Mini launch in 1959. I had previously designed the rubber cone suspension system — it was a specially shaped rubber and metal unit that gave a smoothly rising spring rate. This meant that, when the car cornered or went over a bump the suspension stiffened up as it was deflected. When practically undeflected (ie: the car was at rest or on a smooth road) the rate was low and so the ride was soft. We fitted this system to the original Mini of 1959. It was a parody of the interconnected hydraulic system but that simply wasn’t ready in time.
“Issigonis took Jack Daniels from Cowley and got together a small team, no more than six people. Jack had been running an experimental transverseengined, front-wheel drive Morris Minor on the road over the snowy winter of 1956/7. It performed particularly well in the wintry conditions and Leonard Lord and George Harriman were well aware of the benefits of the transverse set-up as Jack parked it outside ‘the Kremlin’ every day where he knew they could see it. Issigonis’ team designed and developed the Mini from start to production in 27 months —a fantastically short time. They had total authority from Lord: the whole of BMC was at their disposal plus all the component suppliers. The whole of the Midlands was committed to the new project.
“It is unfair to say that the Mini was undeveloped when it was introduced. OK, there were some small problems, such as the floorpan sealing, but when you think of all the technical innovation in that one small vehicle, it was amazingly right, and look how little fundamental change it then needed in the following 40 yearsl
The next suspension innovation for the Mini was the Hydrolastic system introduced to the saloon models in 1964. Hydrolastic suspension involved similar rubber cones to the “Dry” set-up, but they formed a displacer unit with an integral diaphragm and damper valves. They were filled with a water and antifreeze mixture and interconnected front to rear via pipes under the car. This gave the car a rising and falling motion rather than the sudden pitching of the rubber cone system. Alex Moulton takes up the story once more...
The Morris 1100 was introduced in 1962 with the first Hydrolastic installation. It was a great success, and Ford in particular were very upset by it. Dunlop and Moulton Developments took some time to make a unit small enough to fit in a Mini, but it was done by the early 1960s. The majority of the Mini’s racing and rallying success was achieved with Hydrolastic cars. The whole Mini racing thing was started by John Cooper. We never thought about racing during the design phase; we were worried about safety, particularly when the car was overloaded with lots of students aboard or such like.”
The final phase of the Moulton suspension system evolution was the Hydragas system. In this later version of the interconnection principle, the rubber cone was replaced by using a gas under pressure. Hydragas was introduced with the unloved Austin Allegro in the 1970s and is still in use on roads today fitted to the MGF. Hydragas was never fitted to a production Mini, but Alex Moulton has some fascinating prototypes in his stable: a 1966 Cooper S fitted with Hydragas suspension and a 1980 Metro fitted with a more developed, fully interconnected and refined version of the system. We were able to take a short trip in both of these cars, and the sophistication of ride and handling has to be experienced to be believed.
“The prototype Metro installation I have is the ultimate expression of interconnection for a small car. I showed the Mini Cooper S with Hydragas to BMW during the early development of their new Mini and I am certain they were impressed. We then worked with Rover on the Minky 2, a one-off based on extended Mini sheetmetal that was essentially a test rig for a potential suspension set-up for their new car. However, there was a major personnel change at board level. Bernd Pischetsrieder, with whom I had had encouraging discussions left and the decision was taken that the new Mini should be wholly conventional in its suspension.”
Alex Moulton has some forthright views to share on the BMW Mini: “It’s enormous. The (original] Mini was the best-packaged car of all time this is an example of how not to do it. The interior space is not much bigger than the old Mini, but it’s huge on the outside and weighs the same as the Austin Maxi! The crash protection has been taken too far. I mean, what do you want... an armoured car? Princess Diana was killed in a two-tonne Mercedes: you can have a fatal accident in anything if you drive fast enough.
With the original Mini, we set out to prevent any accidents by having excellent handling, not by cushioning people from the consequences of their own folly The old Mini was the absolute apogee of this philosophy of built-in safety via the handling —people avoided accidents by driving around them. The suspension of the [BMW] Mini Cooper is set far too stiff, giving a most uncomfortable ride. To he honest, it’s an irrelevance in so far that it has no part in the Mini story.
We were also able to get a second opinion on the BMW Mini’s handling from Doug Milliken, an American suspension engineer who happened to be visiting. After his first drive in the BMW Cooper he confirmed Alex Moulton’s opinion that the suspension was too hard for comfort and that it was overdamped. His conclusion was basically that he would not want to drive it on anything other than dead-smooth tarmac.
What is the future for Dr Moulton’s innovative interconnected suspension systems which were first demonstrated in the Mini and other BMC cars of its generation? I can do no better than to paraphrase his published thoughts:
“There will he a new breed of small cars, sufficient in size for the typical [urban] journey, but with the convenience in driving on crowded roads for the longest journey, provided they have big car ride comfort and absolute security of handling. I suggest that these cars will have the Issigonis layout of transverse engine and front wheel drive. Their suspension could comprise fore and aft fluid interconnection to separate pitch from bounce and roll. These cars could have uncompromised damping which, with the wheel at each corner layout. gives the [ultimate] handling capability.” (Moulton Suspension. Past and Future — Institution of Mechanical Engineers. Automobile Division, 2000)
Amazingly. 42 years on from the production of the most enduring of his and Sir Alec Issigonis collaborations — the Mini, of course — Dr Alex Moulton is still thinking about the development of suspension for small cars. No doubt the Mini’s eventual successors, whatever they may be, will benefit from his highly original thinking.

back or to my MGF home site

© 1999 - 2009 with permission of Doctor Anderson (CA)
Dieter Koennecke