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MG MGF Technical - Hydragas Temeratur compensator

A question to all the engineering types, would it be possible to devise some kind of temperature compensation device for the Hyadragas system. I think we could solve a lot of suspension setting problems if we could find a way of keeping the ride height constant. I have in mind some kind of device that will increase the fluid pressure when the temperature lowers thus maintaining a constant height.

Does anyone know what the effect of using a different fluid that does not change volume quite as much with temperature. I'm sure there must be something better than water / antifreeze mix. I assume Rover investigated this when designing the Hydragas system, and had good reasons for using this fluid.
Tony Smith


I think the temperature difference has more effect on the gas in the upper sphere than on the the fluids in the lower sphere - so this would require a gas rsevoir of very high pressure to compensate for differences. I am sure that some engineers could come up with a solution but at what cost?

Ted Newman

Wow, Tony, what idea :)
I see a very yery long thread coming up :)
Caaaarllll, where are you ?
Just thinking bended (as ever) looking at Teds message and yours.

What is the target ?
IMO target is to keep the ride hight constant !!!

How can it be done... is the last question :). So the soution at last IMO not at first.

What has influence on the ride hight ???
Brainstorming not required, because we only have to collect our knowledge.
1- Temperature of the fluid
2- temperature of the metal parts covering the fluid and the Nitrogen and pipes
3- temperature of the nitrogen Gas
4- related physical effects:
... covered volumes of gas and fluid

What can be influenced ?
- volume
- pressure
- temperature

How can it be influenced ?
- pump

We had some physical explorations on temperature terms as 'isobar', 'isothermal' system in our german board in the end of 99.
I'll look in our archive for the already calculated values(guesses) of our friend Ludger H.

go on guessing everybody, it helps to sort the brain contents.



a chap I know had planned to build to 2 small pumps to be connected to the Hydragas valves and to be operated from the cockpit, but he gave up. (he will soon sell his F)
There's another solution to have influence on the ride height : to change the gas temperature in the sphere with a closed loop system with thermostat and a heat resistors. That would IMO be very tricky to implement and expensive.

If someone finds an easy solution for a constant ride height, I'll offer him some beers !


You owe me a beer!

Throw the Hydrogas units away and fit leaf springs!

Ted Newman

you've no chance on that belgium beer, because leaf springs do not provide constant ride height... *g*

So lets add to
What has influence on the ride hight ???

1- Temperature of the fluid
2- temperature of the metal parts covering the fluid and the Nitrogen and pipes
3- temperature of the nitrogen Gas
4- related physical effects:

5 - weight change of the car (driver/co-driver, red whine from france, beer from belgium)

... covered volumes of gas and fluid

serious, but ROFL


>>Throw the Hydrogas units away and fit leaf springs!

You could throw the hydragas units away and fit convential shock absorbers.

I understand that this idea was floated in the MGF Cup series after the first season of racing. The teams were moaning because the cars were difficult to set up at the track.

However, fitting convention suspension to the race cars while keeping hyragas on the road cars would be admitting that the hyrdgas system was fitted because it is cheap and not because it is good.

So, unsurprisingly, the idea came to nought.


Please correct me if I am wrong, but the primary eliment prone to expansion on heating is the gaseous component of the hydragas suspension medium. All over factors are ofcourse contributary but are overwhelmed by the effect of gaseous expansion.

QED- I agree with Ted! Not so sure about the leaf spring idea though ol' chum... ;o) LOL

A temperature/pressure compensation device is an interesting idea Tony- but difficult to engineer as one would have to vent superheated gas from the hydragas sphere is maintain pressures at a constant level and then return the gas when the system returns to cold. A simple valve system might cope- but then how would it cope with suspension movement/ hydragas compression? Very tricky. Computer control?

Maintaining hydragas sphere temperature might be another method- but would require a cooling and/or heating system. Heavy and expensive.

Another possibility would be to replace the gaseous medium with something else. However, gaseous compression is an essential springing eliment in the sphere, so the whole sphere would need to be redesigned.

The best solution is as Paul suggested IMO, that is to junk the hydragas altogether and fit convestional spring over damper units. I am not sure, but I such a kit may (have)exist(ed) for the Metro, and marketed by Avonbar Racing...

Just some ideas for the pot. ;o)

Rob Bell

Rob, your on the right track with the coil over conversion, but you will find that Avonbar no longer do anything like this, at least thats what they said last week.

Conversion to a coil over would be technically feasible, but would require some body alteration to create a suspension turret. For anyone seriously interested in what this may look like then get a copy of the March issue of CCC mag. (You should have it anyway for the K series tuning feature!) Pages 38 to 40 has an interesting tarmac rallying Rover 100 (Metro) with such a conversion and piccies.

Roger Parker

I see nobody likes to play with me :)
So, no failure mode analysis.. :)
Anyway I'll post my measurement results of pressures and volumes later.

Back to the 'leaf springs'...

Paul, what you heard of the Cup Cars exists obviously already!
Not in the UK but here. Have a look at the archive and find the thread were I reported from that german guy who let the car lower by german tuners.....

Keyword is 'jacky' ... ;-) in November 99.
I state what I found in my Email archive:

That guy got special made Koni Dampers !! non standard.
The piston is 5mm thicker then 'standard' and the piston is 60mm shorter then 'standard. (whatever standard means)

We all had ROFLs because he 'overtuned' it with fitting 9j x 16 ET15 rims , 22mm 'distance plates' ? (Spurverbreiterung). result was 8mm offset. And the tuner company fitted no lowering knuckles.

He wrote anything what I did not understand because I'm no expert. That related to the direction (up or down) where the Konis work as spring and in the opposite they work as damper. Sorry I can't explain things which I do not understand...

And what I found realy impossible was the ride hight. He was not able to measure it from the middle of the axle because that rim is so damned deap ;-)
So I sent him as reference a JPG and told him to measure the chassis hight at the gap of the frontbumper to the wheelhouse.
He replied the measre and I compared it with my MGFs hight at that point.

Result was 288 mm !?!?!?!
I told him that any activity of the hydragasspring at this hight is switched off and it can't have any spring behavior at such low pressure. (guess on future subframe damadges)

But now, while reading your above message... I get some daughts wether that action was silly.
I'll see wether I can get reactivate contact to him. He disappeared and seamed to be a diappointed because of equal 'jacky' reactions of the friends in the german board.

The cars unsharp pictures are still online on

PS, Rog I would be very interested in a copy of that CCC mag pages... :) Rob, can you manage that for me ?

Dieter, Ted etc.

Actually if it is the gas causing the problem that still does not stop you from compensating with the green stuff. You trigger this by either gas pressure or temperature. I assume the only thing seperating gas and liquid is a the diaphram in the unit so it still should be possible.

In a more expensive design this would be one of the first problems to be solved. It's a pitty Rover have had to do things on a shoe string for so many years.

Well it started a bit of a debate anyway :-)
Tony Smith

Tony I just see while reading your message that I diddd only write 'static' constant things.
Indeed the long term componet of
Why does it get low ? is not included in my far above thoughts.
IMO your right. Not the fluid leaves the system, buut surely the gas.
No evidence on that from my side ... recently.
But from my knowledge of 'gas' media its nearly impossible to 'seal' it in any rubbermaterial for a 'endless' time.
Directly without looking up the physical name.. Its called gas diffusion ?

I'll think about it.... Hmmm, shall I cut the old unit in the cellar ? I wanted to do some pressure experiments... lets see.

Anyway, we know about the temperatur effect. That is much more relevant then the longterm loss of the gas.

And we found by raugh calculation in last year that the expansion of the fluid volume versus the expension of the gas volume is app 10 times larger.

>It's a pitty Rover have had to do things on a shoe string for so many years.

Tony I wrote about 'blockers' for car improvements on another thread. I'm sure they can't because of cost... and I fear they will have no more chance.

PS. next I'll have a look at the fluid volumes/mm hight... I had anywhere made a webside... does someone remember on what account I blew that ;-)

This thread was discussed between 11/02/2000 and 12/02/2000

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