Why not Hydragas?

[ACourtney]

Justin, your Cortez had new rear units installed in Autumn 2009 after one of the rusty originals sprung a leak and lost all the fluid while on holiday in France. The air (nitrogen?) pressure in them is therefore as it should be. .

i know mate, thats why im a bit puzzled regarding the bouncyness of the rear end, any longer than 10 mins behind the wheel and i feel sea sick

I suspect that the mismatch between the old units at the front and the new ones at the rear is at the root of the problem.

In a properly operating set up, when the front wheel hits a bump, most of the wheel travel is taken up by the gas spring compressing, but some of it is taken up by the fluid being pushed to the back.
This is the classic drawing that explains how hydragas works showing a Metro, Allegro, or whatever with the front wheel going up a bump and the rear rising to keep the car level - oh look this ones an MGF!



The flow of fluid to the rear is restricted, to ensure that the front spring does some work and also to provide some damping to prevent the car rocking backwards and forwards.

I suspect that in your case the front gas springs have lost most of their Nitrogen and so have become very stiff, as Hans explained above. Therefore, when you hit a bump most of the travel is provided by fluid being displaced rearwards. This is will be the reason why you are finding the ride uncomfortable.

Unfortunately, new front hydragas units are pretty rare now. Rear ones are easier to find and I have some in stock, but that is of little use to you. The alternative is to recharge them, which is the Schrader valve mod also mentioned above.

The problems with the units leaking fluid is different to them losing Nitrogen gas. The Nitrogen loss is due to osmosis: the Nitrogen gas is held under pressure so it is natural for the molecules to attempt to escape.
The top part of the unit is steel so as long as it hasn't rusted through the gas finds it very hard to penetrate - I could go on about rusted hydragas units, but it does make me wonder when I look under people's cars and see rust encrusted hydragas units.
Below the gas is the rubber separator diaphragm. In this environment, with Nitrogen above and an anti-freeze style fluid below, the rubber won't perish, but it still a natural polymer and over time the Nitrogen molecules will slowly penetrate - we are talking about 15 to 20 years here. The Nitrogen just becomes bubbles in the fluid below and the car sinks down lower.

Below the rubber separator is the fluid section. This also has a rubber diaphragm at the other end that is moved by the suspension knuckle. This diaphragm is much more vulnerable to damage. At the bottom of the unit the should be a rubber cover to keep dirt and grit out. If this is missing then the rubber diaphragm will be vulnerable to damage. This is particularly important at the rear, where the hydragas unit is laid horizontally. Any thing that finds its way in, like bits of grit will just sit there abrading the rubber diaphragm.
So long as the correct grade of fluid is used the units won't rust from the inside. Similarly the rubber won't fail on its own, only if some foreign matter has been allowed to get in and wear through it. So clean the rust off the hydragas units and give them a lick of paint and make sure the rubber skirt is in place when you put them back on.

On the whole I have had good experiences with Hydragas sprung cars and if anyone doubts that I would invite them to come for a ride in my Excelsior. David Johnson took up the offer at MWAD and was quite impressed with the way it went around Castle Combe. I have driven and ridden in Golds with full hydragas and half hydragas and can say that I have experienced good and bad examples of both. My view is that they need to be fairly soft at the back and well damped at the front. I have yet to ride in a good one which didn't have additional front dampers.

[PerS]

I find it strange that converting to full coilovers on a Midas is regarded as a step forward, when it comes to cars that are mainly used for ordinary road work.
If you use your car on the track most of the time, ok, you won´t have to cope with speed bumps or lesser surfaces as a rule.

The first point: the cost for converting the spheres with a valve on the top, must surely be less than the money and work put in to converting to coilovers.

The second point: with a hydragas system in good order, the ride ought to be more pleasant and enjoyable comfort wise.

The work done by some of the Metro Cup drivers with hydragas suspension is really inspiring to read about.
Per

[Rich]

I find it strange that converting to full coilovers on a Midas is regarded as a step forward, when it comes to cars that are mainly used for ordinary road work.
If you use your car on the track most of the time, ok, you won´t have to cope with speed bumps or lesser surfaces as a rule.

The first point: the cost for converting the spheres with a valve on the top, must surely be less than the money and work put in to converting to coilovers.

The second point: with a hydragas system in good order, the ride ought to be more pleasant and enjoyable comfort wise.

The work done by some of the Metro Cup drivers with hydragas suspension is really inspiring to read about.
Per

Hi Per,

I converted for several reasons, the first being the difficulty in finding decent units, by converting to springs I have future-proofed my car and in turn provided it with a more comfortable, yet just as controlled ride, that won't dump its guts all over the road when the already considerable corrosion takes its toll on the displacers. I also wanted to fit the rear undertray which was never going to happen with the rear hydros in place.

With regards to the K series set up, I may be weird, but I don't actually want self levelling suspension, I want each wheel to reflect exactly what is going on between the road and my car.

That's just me and I suspect others will have different reasons, but isn't that the whole point of kit cars?

Rich

[streetsy]

One of my front displacer units had a corroded valve seat and there fore kept leaking, I could not screw it out due to the rust so I had to drill out the valve and cut threads to fit a high pressure line and high pressure grease nipple, I have filled my unit with grease, it is only a temporary solution as I will be swapping to coil springs but having done a thousand or so miles I can say that the grease is a world better than the fluid was.

[PerS]

H Rich,

Don´t get me wrong. You have done the right thing!

You have made the convertion and it works they way you want. That´s perfect.

I have the spheres in front and coilovers rear, as the original set up.
My front spheres are hard and I´ve found spheres and had them rebuilt and are now hoping this will make the ride more pleasant.

My thoughts on the convertion as an "only way" just stems from the many problems that seems to plague many owners.
This is why I wonder, nothing else.
I have read that many many owners are having problem with their suspension after the change, I have read on Mgf sites owners trying the special kit produced
for a convertion on the Mgf, that has failed.

I am just curious how we succeed with our projects.

It really is the point in having a kit car!

Per

[ACourtney]

Like Per, I too have read bad reviews of a certain conversion available for the MGF (best not to mention the name to avoid legal issues).

I don't know specifics, but those conversions were a bolt on replacement for the Hydragas unit and I suspect that the limited space available is at the root of the problems.

By comparison, the coilover systems that add a new top mount from the subframe and then bolt on to the top arm, in the conventional damper mounting position, should provide adequate spring and damper travel. Then it is just a case of making sure that the spring doesn't foul on the wheel and selecting the correct spring and damper rates. I think that it would be the latter that would be hardest to get right. You couldn't use any existing Metro damper as they are designed to work in parallel with the damper already inside the hydragas unit, so you would need to seek the advice of a specialist damper supplier like Pro-tech or Ohlins, which will probably make the conversion as expensive as tracking down new Hydragas units.

In respect to Rich's comment:

With regards to the K series set up, I may be weird, but I don't actually want self levelling suspension, I want each wheel to reflect exactly what is going on between the road and my car.

Although the diagram shows the suspension as self levelling, that really isn't the major benefit. Rather it is the additional stability afforded by the extra damping effect under braking and acceleration, as anyone who has ridden with me around Castle Combe will attest. I remember reading an interview with Alex Moulton (inventor of Hydragas) a few years ago, where he gave his views on the installation in the different Metros and the MGF. I was hoping to find it somewhere on the net, so that I could quote directly, without success. His views were that the original installation in the A-series Metro was a waste of time "for the want of a few pennies worth of connecting pipe you threw away the main benefits" or something to that effect. When MG-Rover were designing the MGF they tried both Hydragas and conventional prototypes and found the Hydragas version to be much more stable, something especially important in a mid-engined car. The MG TF of 2002 then did away with Hydragas, I don't know whether this was another Phoenix group false economy (like plastic dowel pegs for K-series heads), but it was met with many less favourable reviews having lost the good ride and stability that had endeared the MGF to the motoring press.

Having said that I couldn't agree more with Rich's last comment:

That's just me and I suspect others will have different reasons, but isn't that the whole point of kit cars?

Yes indeed, that is the point of kit cars: to allow you tailor a car to meet your own requirements and add whatever features that you want.

[manifold]

As the initiator of the Watson spring conversion in a Midas I found the ride difference to be very significantly better compared to hydrogas. It might be worth asking one of the guys who have done the conversion to give you a test drive to prove this. It isn't that difficult a job to do really either and is a fit it and forget (almost) conversion. David.