The Hydragas Problem - seeking a solution

[Mr_Bo11ox]

There'll always be interest in anything that removes the worry of burst diaphragms

[forddeliveryboy]

The Africar used Moulton displacers, I think they repaired them at the roadside with a bicycle puncture repair outfit, they did carry spares and used most but they do seem very tough indeed from what I read and hear - the Africars were overloaded, not properly engineered in the suspension department and driven on roads no other car would tackle.

 

And it's an amazingly compact suspension system which works superbly well. How many moderns need replacement coils, which have also punctured brake lines, driveshaft gaiters and so on, adding up to bills of many hundreds each time? The more you look at it from that pov, the more £400 or less for all four corners seems quite a fair price.

 

There'll always be interest in anything that removes the worry of burst diaphragms

 

I bet some have failed through fatigue alone even though pressures have still been ok, but aren't they designed to puncture when the pressure gets so low it's risking safety and other component damage/breakage? One guy reckoned the only time they let go even though there's still gas presure is if there was a manufacturing fault from new.

[Noel Tidybeard]

part of the problem for me is not being able to see mgf and any other sphere side by side to see similarities/differences

[tooSavvy]

Would a 'simple' substitiution, should it appear practical, actually be fraught with unknowns.....

 

a 'heavy' car might have a lever fulcrum/compression rate - into the sphere - which is different from the 'less-heavy' car an alternative comes off??

 

A 'light car/sporty' may have a higher compression rate to make the spring rate effectively more agile....

 

 

TS

[Asimo]

^^^^^ This is true. To compare just the spring rate (ignoring damping) it is necessary to know things like the areas of both displacer diaphragms and the exact profile of the thing that pushes against the lower diaphragm, the angle of the outside cone shape below the bottom diaphragm, the volume and pressure of the gas in the top chamber, the leverage ratio of the suspension arms. 

 

And the damping characteristics, even the volume of fluid in the front - rear interconnecting pipe, all these parameters were tuned for each application.

 

So substitutions are possible but they are going to behave and feel different to original. If you modify your own car you can accept these compromises in a way that the maker couldn't, ie you may never carry rear passengers, a full boot and tow a caravan, all of which had to be catered for in the original set up.

Even a Princess that rides like a Mini is better than a broken and undriveable Princess.

[vulgalour]

Digging this one up again.  Chompy and I have been looking at options regardless of cost to see what's the easiest way to resolve the problem in terms of engineering and parts.  We found that you can get self-sealing screw in Schrader valves which are used in some alloy wheels and they're around a tenner each.  You can't fit the rubber ones because you can't push them through from the reverse side as you would on a wheel rim so there's no reliable way to seal them in place.  The screw in ones have another advantage of being fairly short, which is good news for the front displacers where space is quite tight.

 

In theory, remove the closing rivet on the displacers on the gas side, get the hole drilled and tapped out to the relevant size and thread pitch to accept the new Schrader valved and fit that so there's a gas-tight seal made due to the nature of the fitting.  Refill with nitrogen to a suitable pressure - apparently there is a place locally that can do this - then refit to the car and fill up the fluid.

 

So about £40-50 for the valves, however much on top of that for the drilling and tapping of the displacers and a bit more to top up with nitrogen.  It seems more sensible and less dangerous than the £90 method because you're not putting any welding heat at all into the spheres that could damage the rubber displacers.

[dave21478]

The problem there is that you are trying to screw something into a very thin bit of metal - there is only realistically going to be half a turn of a thread in the thickness at the top of the can which I doubt is enough to hold the valve against the internal pressure. Thats why nuts get welded on - to give enough threaded length for the valve to screw into properly.

[vulgalour]

Good point, well made.  Back to expensive option I suppose then.

[danthecapriman]

This is probably a shit suggestion, a crap idea that won't work and too expensive/difficult to execute but I'll say it anyway...

 

What about one of those air ride suspension set ups the yanks seem to like throwing under their cars? You wouldn't need the expensive kits to make a 'low rider' with the control box/remote etc so just the basic air bag system should do the job? It's bound to be a twat to fit mind!

[michael1703]

£90 each but I reckon if a load of owners agreed to get them done in a bulk job it could be done cheaper

[dave21478]

I reckon you would be ok welding them yourself.

 

I looked into this as a "what if...." exercise for the Metro and found a load of pics from some place that does this for folks - didnt look difficult and I wouldnt worry about doing it for myself.

Typically I cant find the site again because I am a disorganised fud.

 

Basically...

-Mark the position for the valve and remove the displacer from the car. Clean it back to bare metal around your point.

-drill a 7mm hole where the valve needs to go, and tap this for an M8 bolt (yeah, I know what I said above, but bear with me)

-get the nut or whatever your valve screws into. This guy was using what looked like thickwall tubing tapped to take the valve.

-Place this over your hole, then use an M8 bolt and a washer screwed down into the hole to securely hold your nut to the displacer. This also plugs up the hole to prevent hot weld spatter going inside.

-weld the join between nut and displacer.

-remove M8 bolt and washer, clean up and screw in the valve.

 

I would suggest being very careful when drilling the hole as if there is pressure in the displacer it will wang the swarf outwards at a fair rate.

 

If you were worried about heat you could submerge most of the unit in a bucket of water for the welding, just leaving the top protruding from the water.

 

 

As for re pressuring, does it have to be nitrogen? Surely any inert gas should do the job. Im thinking either CO2 or Argomix, basically whatever you mig weld with. Its not something I have looked into but my CO2 regulator goes up to about 10 bar or something judging by the gauge, so with a suitable adaptor pipe I should be able to pressurise ok.

[ruffgeezer]

Nitrogen bottles are pretty cheap to get, I seem to recall about £12 a bottle locally (My Father in law has one powering the pneumatic side of his lift)

[Guest Lord Sward]

This is a very good thread.

[rovamota]

It would seem that some people who have had their displacers regassed have still had failures, and worse still, the people who regass them offer no warranty whatsoever. Lets face it, regassing is a bodge, really. A bit like welding a new bolt head onto a rusty old thread.

 

As for replacing Princess displacers with springs and dampers it seems pretty straight forward. All you really need is the damper rates and a bit of elastokinomatic knowledge. But using MGF displacers won't be any good. For a start an F sits a lot lower than the Princess.

 

Koni did a damper swap kit for the Wedge in the early '80s...

[plasticvandan]

Has no one come up with a way of using Citroen suspension? Or is that sillly?

[vulgalour]

Airbags - complicated.  The kits swap in to a car in place of conventional suspension and generally makes use of some of the mounting points.  Even on a scratch build there's conventional points at which suspension is mounted.  The Princess doesn't really have any of this so you need some understanding of how to fit this system to the car and there's not a great deal of free space at the front that's easily accessible for the job.

 

Regas - Best short-term solution but as rovamota points out, not a guaranteed fix.  If there was a stronger guarantee of success with this I'd be more comfortable paying out £90 per unit but there isn't so I'm not.  If I do the job myself for a quarter of the price and it fails I'm less upset by that.  Probably a bit of man-maths there.

 

Citroen - Just as difficult as the airbags solution.  In theory this is the best solution as the Citroen system is robust and competent.  Trouble is you can't just replace the spheres as a straight swap, the Citroen system relies on the pump pressurising the system to a much higher pressure than the Princess.  You also need the struts - or so I'm led to believe - to help control the handling with the Citroen system and that means finding somewhere to mount them on a car that never had them.

 

 

The conclusion we can all draw is that there is no easy solution yet found to this problem and not demand for the wider market to provide a solution.  I wish I had more knowledge about possible solutions so I could work through ideas but this sort of thing has never been something I've been any good at.

[Lacquer Peel]

Would it be easier to use hydropneumatic suspension powered by an electric pump (C5, 405x4)?

The rear suspension on a BX for example is very compact, but I imagine fitting the whole BX axle would need a lot of fabrication work.

 

You could take the shell of a Princess and stick it on a tax exempt* Range Rover chassis for reasons.

[Dead_E23]

Rivnuts can be installed into sheet metal without access to the back, and according to the spec sheet I just Googled take a lot of force to pull them out.

 

You might find a screw in schraeder valve to fit the available rivnuts, else you could tap the rivnuts to match the valve.

[plasticvandan]

I use HD rivnuts at work with the proper ratchet tool, I can confirm that they will take the strain, I can even supply a few of the 8mm thread size for assessment

 

Can gas reconverted to elastic? Or am I being daft again

[dave21478]

I doubt if a rivnut flange would be gas-tight though.

[vulgalour]

Could you apply a sealant with the rivnut, or something like PTFE tape?

 

C5 axle idea is an interesting one, I wonder how close the dimensions are?  Removing the Princess rear axle is a doddle, not sure what you'd do up front though, that bit's always tricky.  BX axle is too narrow by a good margin, unfortunately, so you'd have to find some way to widen it quite a bit.

[vulgalour]

Also, dave said "flange".  So, snigger &c.

[RayMK]

I was wondering whether a very compact transverse torsion bar set up could be engineered for the rear end, or perhaps adapted from something like a Peugeot 205. The front could use Minor/Marina type torsion bar suspension, but the amount of engineering and panel work required to accommodate different suspension and the fine tuning to make it handle acceptably make this quite a problem, as you have indicated.  Major changes to suspension configuration seem to attract attention from the vehicle inspection authorities, so it may be wise to check that side of things beforehand.

[Asimo]

Re-gas for Re-sult.

 

Dave's method, not idiot proof but entirely practical. Assuming a welder with pipe-fitting experiance WCPGW? An infinitely better bet than any alternative spring/damper arrangement.

 

Not a lot to loose either

 

Don't use CO2, it liquifies at quite modest pressures!

 

Use a thread sealant, a Loctite of the correct grade, when screwing the valve unit in place.

[Dead_E23]

I reckon a rubber washer and/or a smear of epoxy might make a gas tight seal round a rivnut.

[vulgalour]

You'll never make the unit entirely gas tight anyway, that's part of the problem as the Nitrogen leaks out over time causing things to go flat.  It's a case of making things as gas tight as possible and treating the re-gas as a service item rather than regarding the displacers as sealed for life.  I'd like to work out how to get to the valves in re-gassed front displacers without removing them from the car to do so, the rears are really easy to get to and, unfortunately, totally unprotected from salted roads and dirt.

[Mr_Bo11ox]

It would seem that some people who have had their displacers regassed have still had failures, and worse still, the people who regass them offer no warranty whatsoever. Lets face it, regassing is a bodge, really. A bit like welding a new bolt head onto a rusty old thread.

 

As for replacing Princess displacers with springs and dampers it seems pretty straight forward. All you really need is the damper rates and a bit of elastokinomatic knowledge. But using MGF displacers won't be any good. For a start an F sits a lot lower than the Princess.

 

Koni did a damper swap kit for the Wedge in the early '80s...

 

Is this a real post or a wind up?

[Bear]

Regassing isn't a bodge, but the techniques to modify a pressure vessel not originally designed for a regas may well be.

 

So. A thought.

At the back of many Mercedes estates, there are two oleopneumatic spheres. They work like Citroën units, with a pump providing pressure, though the car also has springs to prevent sag.

 

They are not packaged into the suspension. They have struts, and then a rigid pipe.

 

For the front of the Princess, does it work like a Landcrab and suit an inboard arrangement?

Also - the reason Citroën need a pressurised system in the way they implemented it was initially because the system provides dynamic load levelling, and also has varying demands for other functions like steering and braking. There is no technical reason that a Citroën sphere should not work with the 400psi hydragas system - it's less than half of the pressure in the Citroën but it is also working with different demands, there's no return flow and so forth. You're simply replacing one ball of nitrogen gas with another.

 

The problem may be that the volume of gas is too great, though.

I'm sure there's a solution if the right thought is put into it.

[fred]

Has no one come up with a way of using Citroen suspension? Or is that sillly?

I have thought about this for ages - planting a BX suspension system into a Princess, be one hell of a build, if successful be bloody epic

[jonny69]

You can't fit the rubber ones because you can't push them through from the reverse side as you would on a wheel rim so there's no reliable way to seal them in place.

 

I can confirm that you can force a rubber valve through from the front with a bit of Vaseline and not too much perseverance. You can get it through a 13mm hole. Stick it in some hot water if you need to soften the rubber a bit. My worry with them is still the pressure because we don't know how high it is and, as I said before, they bulge alarmingly at around 160psi (11-12 Bar).

 

I like the rivnut idea though. I agree that a bit of PTFE or Loctite should guarantee a good seal, then re-tap it to the finer thread on the valve and use more PTFE to seal

 

Disclaimer is of course that I've never seen one of these units before, so I'm speaking purely speculatively.