Tech : Recharging Maxi spheres
August 5, 2011
· 
Keith Adams · 
8 Comments
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
From 1977/78, Maxis leaving Cowley no longer used the Hydrolastic suspension system, having been upgraded to the more advanced Hydragas system. 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.
Typical Problems with Hydragas-Units
There are 3 basic problems occuring with Hydragas suspension systems.
Loss of fluid: the most common failure, occuring at either the suspension units themselves or somewhere along the pipes. I am not going further into this topic.
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 Hydragas Maxis by now.
Owners will have noticed that resetting the ride-height to normal will help in the beginning, but later the car will get harsher the higher it sit’s and doesn’t seem to have any movement in suspension at all.
The 3rd 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. And I am very glad he offered me to meet and discuss the topic, 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. On 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-proporional higher if the ride height is set higher. This means that the gas-cushion itself shrinks when the car is pumped up to a higher pressure, so it may get firmer. This is not neccessarily so, since the cone-shaped pistons mean that less volume needs to be displaced when the wheel hits a bump. But — as in the case of old suspension units with low residual gas-pressure — if the gas-cushion becomes to small the car will appear to be very harsh and will get softer again if the car is lowered.
On the other side a car sitting low due to low fluid-pressure may be dangerous since the diaphragm inside the sphere may cover the openings of the damper-unit inside the Hydragas displacer. This happens when the gas cushion becomes too large and will leave you with a car with hardly predictable behaviour!
Repairing Hydragas Displacers
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 3 units ready to go into a car again.
What is evident is all four units will need to be taken out of the car.
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 socket where it can be screwed in. This socket needs to be of steel, so that it can be welded. I found zink-plated reduction pieces from 1/4in outer to 1/8in inner thread. After sawing of the 1/4in outer thread they where well suited for the job.
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 it 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 pounds 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 additionaly. Each unit can now be tested by putting some air-pressure on it with a tyre filler. It is no high pressure needed, so 2 or 3 bar would be sufficient. 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.
Now the displacers can be charged with Nitrogen to the final pressure setting. I did try 2 different settings which lead to pleasing, yet slightly different results:
* Firm car: 9.5 bar Nitrogen and a line pressure of around 18bar on the fluid later.
* Soft car: 13.5 bar Nitrogen and fluid to the recommended setting of at least 20 bar or higher.
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.
Contact
If you have questions regarding the recharging of Hydragas units, please contact me by email:
Alexander Boucke send e-mail
Due to good contacts to a garage for mechanical work, I can offer to undertake this kind of overhaul. Work can be done on either a full car, or the displacers only, and this can be done here in Aachen, Germany. Charges would be around £500 for the delivery of a complete car and include getting the displacers from the car, recharge them and put everything together again. This naturally depends on the condition of the car. Fitting only the displacers with valves and getting them pressurized will be £60 each.
But I would be happy to help anybody out who will be trying to do the work on their own.
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8 Responses
Dear Mr. Alexandre
Many thanks for all the information. But I would appreciate, more details and ir I coul repair mu Hydragas suspension of my wife’s Rover SL 111 from 1995
Best regards
João Rodrigues
my austin maxi o/f displacer unit is leaking there is one for sale on e bay are the units the same for the front and rear or are the units sided many thanks andy
details required for repairing displacer units on austin princess 2.2 auto 1979 model
The process for the Princess is exactly the same. We have done some by now and they respond very well to recharged displacers. A charge between 15 and 19 bar will give optimal results, go as high as you can up to 19bar for longer lifetime of the charge.
Hi Alex
Have you had any experience with the Hydragas units in MGF’s? What pressures would you recommend and I presume that you prefill the units with Nitrogen prior to reinstalling them and that the pressure (without the fluid on the otherside of the diaphram)is not sufficent to damage the diaphram.
Also is the problem of lost gas usually due to the diaphram rupturing or is it more likely to be porosity?
Thanks Mike Todd
Hi Mike,
Personally I have not yet recharged MGF units. But I know it has been done already.
The gas gets lost mainly by diffusing through the walls and rubber diaphragm – just like any tyre will lose some air over time.
As in the factory the gas will be charged with the unit out of the car and no pressure on the fluid side.
You should fill the unit with 1/2 to 2/3 of the pressure you run on the fluid side.
The main problem on the MGF is that there is no space for the valves without modifications to valve position or bodywork.
Regards,
Alexander
Hi There
Firstly, My name is Jason Birmingham & thank you for such an informative article
I live in Australia & have a MK1 Metro. The Metro was never sold here in Australia & is quite rare, thus information here is very limited . I have just bought this low mileage vehicle & the Nitrogen has leaked out of the spheres. I have removed them & TIG welded bosses in to the top of the spheres & now wish to recharge them. I see you have provided the Nitrogen pressures for the front & rear displacers (Thank you), so is it just a matter of charging the nitrogen to that pressure, & them pumping the fluid up to the correct ride height?
Also, I wish to convert it to the later interconnected type suspension. I have organised 4 Rover 100 displacers & interconnection pipes that are in transit at the moment. The question I have is do you know what the nitrogen pressures are for the interconnected suspension type spheres? & again, is it simply a matter of evacuating the suspension & then pumping the fluid up to the correct ride height?
Thank you Very much
Regards…Jason Birmingham
Hi Jason,
the process on the Metro is the same as on all other cars – be it interconnected or not. Nitrogen needs to be filled before pressure is applied to the fluid side. If you know the pressure of the fluid for the car at your preferred ride height, the pressure of the new Nitrogen charge should be between 1/2 and 2/3 of that of the fluid. Never put in a highre pressure gas charge than the fluid pressure you intend to use – the result will be a car with no springs and hardly any damping! Obviously the pressure will be the same on all four units of an interconnected car. With the recharged displacers fitted to the car, the process is as given in the manuals (i.e. evacuate and then fill to the desired ride height).
Regards,
Alexander
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