Tech : Recharging Hydragas spheres
The main problem with Hydragas is that, over time, the sealed units would lose gas and become less effective.
Alexander Boucke describes the steps involved in getting your Hydragas suspended car back into rude health
Update: More than 11 years after the initial work, the first units that were fitted to a Maxi in autumn 2003 got a quick check-over – see at the bottom.
From 1977/78, Maxis leaving Cowley no longer used the Hydrolastic suspension system, having been upgraded to the more advanced Hydragas system already introduced in Allegro and Princess before. The new system should offer improved ride quality and handling due to the possibility to separate the damping rates of the suspension units from the damping rate of the fore-aft-interconnection. This means losing more of the “bounciness” of the original Hydrolastic setup and gaining an increase in comfort on bad roads due to using a compressible gas-filled cushion as spring-medium instead of rubber. Sadly, after a slow decline in ride quality, one day I found both my Maxis so firm that I could not enjoy driving them anymore. A visit to the late Dr. Alex Moulton helped in finding a solution to prolong the life of Hydragas suspension units and getting some ride comfort back into the cars.
Typical Problems with Hydragas Units
There are three basic problems which occur with Hydragas suspension systems, by now mostly age-related:
Loss of fluid: The most common failure, occurring at either the suspension units themselves or somewhere along the pipes. I am not going further into this topic in this article, apart from saying that there are thoughts about renewing all rubber components in the units, but this will be a much more involved process than described below.
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 escaping.
Loss of gas (2): A very common problem on older cars is the firm or harsh suspension. The usual cause is the slow loss of gas due to slow evaporation 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 the use of the cars this is probably not noticeable for the owner in the first 10-15 years, but should be dominant amongst most Hydragas cars by now.
Owners will have noticed that resetting the ride-height to normal will help in the beginning, but later on the car will get harsher the higher it sit’s and doesn’t seem to have any movement in suspension at all.
The third problem was the one affecting my Maxis so much that I hardly used them since it was so uncomfortable to drive in them. This is when I approached Dr. A. Moulton in search for a solution. I am very glad he offered me to meet and discuss the topic in summer 2003, leading to the solution described in the following sections.
About ride-height and gas-pressure
Looking at a Hydragas displacer, the steel sphere sitting on the rear of the unit is most prominent. This does contain gas (Nitrogen) at the outer end and Hydrolastic-fluid (basically water and alcohol) at the inner end separated by a flexible diaphragm.
If there is a big part of the sphere filled with gas, the car will ride softly, since there is a big cushion that can be compressed, meaning a longer wheel travel. However, at the other extreme, if there is nearly no Nitrogen left, then the gas-cushion is very shallow and not much wheel-travel is possible resulting in a firm ride.
The second component affecting the volume of the gas-cushion inside the unit is the line-pressure of the suspension fluid. Due to the shape of the pistons acting on the Hydragas displacers (a cone), the pressure needs to get over-proportional higher if the ride height is set higher. Since the cone-shaped pistons mean that less volume needs to be displaced when the wheel hits a bump, the higher pressure should result in a softer spring rate. However, as in the case of old suspension units with low residual gas-pressure, if the gas-cushion becomes too small due to the high fluid pressure, the car will appear to be very harsh and will get softer again if the car is lowered a little.
There is a significant risk of damaging the internal diaphragm when using the car with very low amount of gas remaining. The rivet on the top sphere, which covers the original opening from filling the gas spring in the factory, does protrude into the sphere. When the gas spring becomes very weak and the gas cushion very shallow, the internal diaphragm can chafe on the end of the rivet and start to leak – which will render the unit unserviceable currently.
On the other side a car sitting low due to low fluid-pressure may be dangerous since the diaphragm inside the sphere may expand to fill the complete sphere off the Hydragas displacer. In this case there will be no springing at all unless a bump raises pressure enough to overcome the gas pressure and compress the gas spring! This is the reason why simply lowering a Hydragas car by reducing the amount of fluid can be a very bad idea!
Recharging Hydragas springs
To get old units back to proper working order, Nitrogen needs to the refilled to the spheres through the fitting of valves. The picture below shows three units ready to go into a car again.
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 socket where it can be screwed in. This socket needs to be of steel, so that it can be welded into place.
Next you need to look out for a solution on how to fill the units with Nitrogen, once everything is put together. A tyre fitting place might sell ‘tyre-gas’ or whatever they call it – this is usually Nitrogen. Many of these are able to supply up to just under 10bar pressure. More convenient, but also more expensive, is to buy a pressure regulator with a regulating range of up to 20bar and some hoses and a filler adaptor. Using this equipment I was able to rent a (small) bottle of Nitrogen and do the pressurizing of the gas-spheres at home. Costs may vary between £50 and £100 for the equipment.
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’ll be amazed how much muck will be coming out there!
Now widen the drilled opening in the sphere so much that the socket for the valve will fit nicely into or above it. Remember that it has to be welded into place later.
For welding the adaptors for the valves onto the units I would recommend using a WIG welder, since this keeps the welding spot relativly cool and helps to avoid problems with warping metal and heat. It is also adviseable to cool the rest of the sphere with wet rags so that the danger of damaging the diaphragm with heat is lessened.
The valves can then be screwed in. We sealed the threads additionally. Each unit can now be tested by putting some air-pressure on it with a tyre filler. Examine the area around the valve for leaks in particular. If there is enough time, leave the units pressurized for a few days to have the chance to detect very slight leaks before everything is going in the car again.
If the car is run at standard ride height the fluid line pressure will always exceed 20 bar. A Nitrogen charge between 1/2 and 3/4 of the fluid pressure is recommended – the exact value does not matter too much, as the spring rate is mostly determined by the cone shape in the displacer. With a higher volume charged the time to the next re-charge will be longer.
Clearance Issues on the Maxi
On the Maxi there is enough space around all displacers to be fitted with a valve as described above. But for later servicing (the unit might need to be recharged every few years) it is adviseable to use not to high sockets at the front. It is then possible to fill Nitrogen with the displacers in the car. On the rear end the problem is different. On the RH side only the spare tyre needs to be removed to access the valve. But on the LH side the tank hides the access to the valve so either the tank or the displacer will have to be removed.
To get around this problem one of my cars received the valve sitting on a piece of bent pipe, so that it is accessible. The parts were sourced from the Hydragas suspension pipes of an Rover Metro, but there might be other ways to achieve that.
The first car we used to test modified Hydragas units according to the above given description was Alexander Boucke’s blue Maxi, which went on test in October 2003. Initially, we were only able to fill the gas spheres to a relatively low pressure of 10 bar, because our only source of Nitrogen at the time was a local tyre fitter. The suspension was set a little lower – and thus firmer – than standard, so the car ran well on good roads, if a little firm on potholed ones. As London roads proved to be too bad for comfortable driving in this setting, the car was ‘uprated’ to a softer setting in the summer of 2007 – the accessible front spheres were charged to 20 bar and the car pumped to standard ride height afterwards. Later, in Autumn 2012, the ride height was re-set one more time. By then the car had sunk noticeably on the front, but slowly and evenly over the years, indicating all was in good health – the ride height was restored to correct trim level.
However, at the end of April 2015, 11 1/2 years after the original re-charge took place, the suspension system was drained and the tank removed to access the rear units – which had been un-touched since the low charge of 10 bar in autumn 2003. The car was still on even keel, but had again lost a little height and felt quite firm at the rear end. A test with a simple, hand-held electronic tyre pressure gauge showed a remaining pressure of 4.3-4.4 bar in both rear units. The front units were not properly accessible with the gauge as it was not possible to set the gauge onto the valve without Nitrogen hissing past the gauge. Such a reading trial ended showing nearly 6 bar, indicating that the front units still contained a significantly higher remaining pressure – as would be expected considering the intermediate upping of Nitrogen in 2007.
There are three more cars where the units have been re-charged between 2003 and 2007. We know of one unit that started to leak fluid, but this has been returned so that we are able to re-assess the gas fill. Apart from that particular unit, these cars also sit level and seem to ride well according to the owners. Going from that experience it seems safe to say, that the new charge of Nitrogen does help to significantly prolong the useful life of Hydragas suspension systems. We can also assume that checks of the gas pressure will not be needed at too frequent intervals – depending on access of the units on the car a check and top-up at 7 to 10 year intervals seems reasonable.
Now the blue Maxi is back on properly charged Nitrogen spheres (at 18 bar) and standard ride height – ready for another 10 years of motoring!
If you have questions regarding the recharging of Hydragas units, please contact me by email at firstname.lastname@example.org.