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Common problems: Hydragas de-pressurisation low ride height and poor ride and handling
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| Figure 1 Hydragas suspension in an MGF – upward movement of the front suspension forces fluid up towards the springing medium (the nitrogen egg) and also towards the rear suspension, as each side is interconnected front to rear. The system reduces pitch in a short wheel base car, and also resists roll well without recourse to thick anti-roll bars. |
What is Hydragas?
Hydragas suspension was one of the technical high-lights of the MGF from launch in 1995. Derived from the Rover Metro/ Rover 100, the MGF utilized this incredibly compact suspension system that offers incredible pitch and roll damping characteristics to provide a sports car package that was also capable of handling the long-distance Gran Tourismo role with ease.
Hydragas consists of front to rear hydraulically interconnected suspension units using nitrogen gas as their springing medium. With a Hydragas unit at each wheel, suspension movement is transmitted to a displacer that moves incompressible fluid both in the direction of the nitrogen ‘egg’ – a capsule of gas encased within the alloy hydragas sphere and a rubber diaphragm – and to the interconnected suspension unit at the other end of the car (figure 1).
Nitrogen, being a gas, is compressible – and this is what provides the springing
of the Hydragas unit. Damping is achieved through the fluid valve dividing the
displacer sphere from the fluid/gas divided nitrogen egg above it (figure 2).
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Figure 2 The Hydragas sphere explained: it consists of two spheres – the top containing the nitrogen ‘egg’ – which performs the springing function, and a lower fluid containing chamber (the displacer chamber) where the hydragas fluid is either displaced through a 'damper' valve between the bottom and top champers or to the other linked Hydragas sphere at the opposite end of the car (as in figure 1). |
The main problem with Hydragas is that over time, the sealed units lose gas and become less effective.
The analogy here is with the helium balloon you might buy for your children. When new, it is fully inflated and under pressure. But a day or two down the line, the balloon appears to deflate until one day, it is found lying limply in a corner completely flat.
The balloon hasn’t punctured – and indeed you could potentially re-inflate it if you were suitably inclined. The problem is that the helium gas has simply diffused through the skin of the balloon.
A similar process is at work with the Hydragas sphere – but in this case, nitrogen is the gas, and it takes in excess of 15 years for a significant (i.e. noticeable) amount of it to escape through diffusion from the well-designed Hydragas nitrogen containment egg.
Alexander Boucke pioneered a method for recharging the Hydragas sphere – and with Alexander’s permission and technical input from Dr Alex Moulton I have up-dated his original article with particular reference to the MGF.
Typical Problems with Hydragas-Units
There are 3 basic problems occurring with the Hydragas suspension system that leads to depressurization and loss of suspension/ride height.
Unfortunately, the most common is loss of the Hydragas fluid. Unfortunate because the commonest mode of failure is due to rupture of the rubber outer membrane for which there is no known repair and replacement is the necessary recourse. It is easy to spot though: you’ll find a trickle of fluorescent green fluid dripping from the suspect suspension unit. Failure of Hydragas units on the MGF typically affect the rear units – and appears more common in parts of the world where there are higher ambient temperatures and harsher road conditions than are typically found in Northern Europe. A much rarer cause of loss of Hydragas fluid is through failure of the interconnection pipe work, but is worth investigating a sudden drop in suspension height affecting one side of the car. It isn’t unheard of for the careless to place a jack under the interconnection pipe causing it to split and fail… You can read more about the fluid loss problems on Dieter’s web page http://mgfcar.de/hydragas/blownhydra.html
The other two causes relate to loss of nitrogen gas pressure.
Loss of gas (1): the diaphragm separating the gas from the fluid is defective. Due to the gas and the liquid mixing there is a (sudden?) drop in pressure, but no fluid visibly escaping. Repair is not possible and replacement the only recourse.
Loss of gas (2): a very common problem on older cars is the onset of very firm or harsh suspension. The usual cause is the slow loss of gas due to slow diffusion of nitrogen through different parts of the Hydragas displacer. This cannot be avoided and shows in a car settling at a lower ride height evenly over the years. Depending on how the car is used, these symptoms may not become noticeable to the owner in the first 10-15 years, but will become the dominant mode of hydragas failure in MGFs in the coming years, if the experience of owners with older Hydragas-equipped cars (Allegro, Princess, Ambassador, Maxi, Metro) is replicated in our MGs.
Nitrogen diffusion is insidious
You don’t go out one day and think: “Crikey! My MGF’s Hydragas has lost all of its nitrogen!” Nitrogen loss is a slow and insidious process. Every few years, owners will find that their car’s ride height has become a bit low and the ride isn’t as good as it once was. On taking the car to a garage, the charge pressure will have dropped considerably below the original 400psi (probably more in the region of 300psi by the time the ride height [measured between the wheel centre and the wheel arch lip vertically above] drops below 340mm). Initially, resetting the ride-height to normal (measured at the front wheel: 368mm+/-10mm at an ambient temperature of 17C) will help, but later, as nitrogen depletion becomes more advanced, the car’s ride will get harsher the higher the vehicle sits. When ride quality starts to become a problem, there will be practically no nitrogen left in the upper sphere; the nitrogen volume now replaced with hydragas fluid, and the Hydragas sphere will effectively have become hydraulically locked with practically no suspension movement at all.
The latter problem was the affecting Alexander’s Hydragas-suspended Austin Maxis so much, that he hardly used them; they had become too uncomfortable to drive. This is when Alexander approached Dr. Moulton in search for a solution. Between the two of them, they discussed the problem, leading to the solution described in the following sections.
Ride-height and gas-pressure
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| Figure 3 Progressive depletion of nitrogen gas means less available suspension movement due to loss of the springing medium – meaning that for a given amount of suspension movement, the suspension runs out of available ‘travel’ much sooner (B) when compared to a suspension with the full compliment of gas (A). |
Looking at a Hydragas displacer the steel sphere sitting on the top of the unit is most prominent. This contains the gas (Nitrogen) at the top end (the “nitrogen egg”) and Hydrolastic-fluid (basically water and alcohol) at the lower end separated by a flexible, rubber diaphragm.
We can regard the nitrogen egg as a nice, large nitrogen-filled cushion, which
absorbs the movement of the suspension in much the same way as a metal coil
spring. If there is plenty of nitrogen filling the sphere, the car will ride
softly, the large compressible cushion permitting plenty of wheel travel. On the
other extreme, if there is virtually no Nitrogen left, then the gas-cushion is
very shallow and not much wheel-travel is possible. The result of this is a
harsh ride – essentially the suspension media is ‘bottoming out’ over even very
modest road undulations (figure 3).
The second component affecting the volume of the gas-cushion inside the unit is
the line-pressure of the suspension fluid. Due to the conical shape of the
suspension displacement pistons acting on the Hydragas displacers, the pressure
exerted by suspension movement increases proportionately with linear deflection.
The higher the Hydragas line pressure (which will manifest by a higher ride
height), the greater the resistance to suspension movement – and hence a firmer
ride will result.
The pressure of the nitrogen gas cushion and that in the hydragas fluid line are equal – and thus if hydragas fluid volume is increased to achieve a particular pressure, the nitrogen gas cushion decreases proportionately, which in turn may mean less available suspension movement. If hydragas fluid has to be pumped into the system to compensate for lost nitrogen gas pressure and volume, nitrogen gas pressure can be restored – but at the expense of the size (volume) of the nitrogen gas cushion – and thus suspension movement. As the units become older, with more advanced stages of nitrogen depletion, the amount of available suspension increases until there is practically no movement left – at which point the units are essentially hydraulically locked solid, and the car’s ride consequently will feel extremely harsh. Interestingly, if the pressure is backed off a little, the ride will feel a little softer, but with the problem that there will be little in the way of suspension resistance to movement, and the suspension will tend to bottom out.
As an aside, dropping an MGF with essentially normal nitrogen gas volume by letting out Hydragas fluid for cosmetic reasons may be dangerous. Dropping ride height in this way means reducing the pressure of the Hydragas circuit through the removal fluid volume, and that in turn causes the nitrogen pocket to balloon under the reduced pressure in the system, potentially blocking the openings of the damper-unit inside the Hydragas displacer. Obstructing the damper valve in this way can leave the car with somewhat unpredictable behaviour! If you want to lower the ride height of an MGF, the preferred method remains the same – shorten the length of the suspension knuckle.
How to repair/service Hydragas Displacers
To get old units back to proper working order Nitrogen cushion needs recharging. The way that this can be achieved is through the fitment of a valve to the nitrogen egg. The picture below shows 3 Austin Maxi Hydragas units ready to go into a car again having been so modified (notice that their size and proportions are somewhat different to those used on the smaller MGF and Metro).
What is evident is all four units will need to be taken out of the car. [Picture,
opposite, three Austin Maxi Hydragas unuts - image
credit: Alexander Boucke]
Parts needed
For each displacer a Schrader-valve, just like the ones fitted to the fluid-lines of Hydrolastic- and Hydragas-cars, is needed, together with a boss into which it can be screwed. This boss needs to be of steel, so that it can be welded onto the top of the nitrogen egg. Alexander found zink-plated reduction pieces from 1/4in outer to 1/8in inner thread. After sawing off the 1/4in outer thread they where well suited for the job.
Next you need to look out for a solution on how to re-charge the units with Nitrogen, once everything is put together. As originally manufactured, Hydragas units were charged with a 99% nitrogen, 1% SF6 (by volume) gas mixture. This won’t be readily available to the home mechanic. However a viable alternative is available from the tyre trade; many tyre fitting specialists sell 'tyre-gas' (or whatever they call it) – and this is usually nitrogen. Nitrogen is ideal for high-performance tyres owing to its thermal/volume stability – the same reason why it is used in a suspension application! Many of these tyre specialists are able to supply nitrogen at up to just under 10bar pressure. However, we’ll need nearer 16-17bar – which might pose a problem.
For the more committed with the available workshop space, a potentially more convenient, albeit more expensive alternative is it to buy a pressure regulator with a regulating range of up to 20bar, some high-pressure hose and a filler adaptor. Using this equipment Alexander rented a (small) bottle of Nitrogen (available from BOC etc) and undertook the pressurizing of the gas-spheres at home. Costs may vary between 50 and 100 pounds for the equipment if you choose to go down this route.
Working on the Displacers
First the old gas needs to be released from the displacer. Carefully bore out the small rivet on the end of the displacer. This is also the place where the valve is going to be fitted. It would be a good time now to flush the fluid-side of the displacer with fresh water thoroughly. You will be amazed how much muck will be coming out there!
Now widen the drilled opening in the sphere so that the boss for the valve will fit nicely into or above it. Remember that it has to be welded into place later.
For welding the bosses for the valves onto the units, Alexander recommends using a TIG welder, since this keeps the welding spot relatively cool and helps to avoid problems with warping metal and heat. It is also advisable to cool the rest of the sphere with wet rags so that the danger of heat damaging the internal rubber diaphragms is lessened.
The valves can then be screwed in. Alexander sealed the threads with PTFE tape. Each unit can now be tested by putting some air-pressure on it using a tyre-pump. Very high pressure is not required; 2 – 3 bar is sufficient. Examine the area around the valve for leaks in particular. If there is time, leave the units pressurized for a few days to have the chance to detect very slight leaks before reassembling the modified units back into the car.
Now the displacers can be charged with Nitrogen to the final pressure setting. For the MGF, the original figures supplied by Dr Moulton from the original Dunlop data are 16.55bar +/-1.5 bar at 20 degrees Celcius at an ambient atmospheric pressure of 1 bar (giving a charge volume of 491cm3) front and rear.
Future servicing of modified Hydragas spheres
At the time of writing, I have yet to undertake Alexander’s modification on the MGF Hydragas sphere, so the optimal positioning/specification/dimensions of the Schrader valves have not been worked out, but an important implication is consideration of future servicing. Depending on the rate of future nitrogen gas diffusion (original factory specifications state that gas leakage should not exceed 1.8cm3/sec – a figure I include purely for interest, as I can’t see how this can be accurately measured at home), the units may require recharging every few years – and complete suspension disassembly would be somewhat inconvenient! It may be desirable to make some additional access holes in the inner wings to aid access to the Schrader valves – or perhaps mount the Schrader valves on an extension. More details on this once I undertake the modification!
