The future of Hydrolastic suspension

[nileseh]

Hmm. Much to consider.

External valving occured to me as well. Theoretically removal of internal restrictions and the addition of external would deal with dampening. To match the factory performance, there would have to be two paths, inflow and outflow, because measurements suggest there is a difference between bounce and rebound dampening. I haven't quite reasoned out the impact of full fluid flow on the spring. I think we tend to discuss the "hydro" element of the system more that the "lastic" bit. The springing on the car is flexure of the rubber donut just as in the dry cars. The donut is deflected in response to suspension travel operating thru the in-compressible fluid. The valving is a restriction to flow that provides dampening (as in shock absorber). External valving should do the same, its just a matter of where the flow is restricted. It would require a fair amount of plumbing to get the same action as provided by the rather simple internal valves.

One could perhaps achieve the same end by removing the internal valving and adding telescoping shock absorbers as in the dry cars. Make them adjustable (are they available with bounce and rebound adjustments separate?). If the fluid flow is impeded by dampening the motion of the suspension, it seems that the same effect is achieved as impeding fluid flow thru an aperture. But, as I think I noted previously, what's the fun in that?

The test car is interesting. The controls appear to be push-pull cables attached to rotary valves in the line from the front displacers and presumably elsewhere. There are three rows of controls, 2 in each row with one cable at the bottom. I think the 1st and 3rd rows are ganged together to be operated simultaneously, and placarding as I make it out is "front springs" and "rear springs". The left control in the 2nd row is placarded "fore-aft OUT". Can't make out the other side. Can't make out the bottom control either. The yellow tape on the side marks the un-laden and laden "CofG" (center of gravity). Another thing I find of interest is what appears to be a yellow pressure vessel under the front right wing. Do you suppose it's possible that this is a test bed for the development of "hydrogas" suspension rather than "hydrolastic"? Recall that the methods are similar, with the rubber donut replaced by compressible nitrogen to support the car. Ganging the front and rear controls could have been to level the car fore-aft with adjustable ride height afforded by gas suspensions, and the front-rear fluid transfer is the same.

Anyone know where this car is now?

[Glacier white]

I have been through the whole thread, and was surspised to see that the role of internal valves is not really understood. Only nileseh in his last message seems to address the problem that an external in-line valve will only damp the front-rear interconnection, but not the direct bump-rebound action of each wheel.
The placement of the valves in the port plate before the rubber spring, manages to damp the action of the spring. An equivalent valve after the spring, will allow the spring to deflect without control, so would be unsuitable.

By the way, has anyone's car failed the MOT or any equivalent roadworthiness test due to deteriorated displacer damping valves?

[Peter Laidler]

Mmmmmmmmm...... Glacier, you haven't given me/us (Spider, Nilseh, Mab) enough to get to the point but it seems that you're thinking of the hydro system as being too complicated. I/we seem to think that it's made to be AND sound complicated when really, it's just fluid moving between two inflatable bags. And you DO want to control the front to rear connection - that's the whole idea! I'll come back after giving a bit of thought but like all things on the forum, it's all our own opinions of course. Some based on mechanical and hydraulic facts, others based on fuzzy logic. But of itself, the hydro/liquid system is simple. It's the internal mechanicals that complicate things and it's these internal mechanicals that cause the problems...... apart from leaky bags of course which are another matter entirely.

Need to sit down with a glass of Scotch after Sunday lunch. Join me for a tot Spider and Niles?

[nileseh]

Hmmmmm indeed.

I think I agree with Glacier on the placement of the valve on the suspension side of the rubber spring. In this location the velocity of the fluid is mitigated before the spring, or parallel to the spring as is the case with a shock absorber and the total fluid flow to the opposite displacer is also regulated.. On the other hand, since the fluid is non-compressible, it theoretically makes no difference where the flow is impeded. I don't know, it makes my head hurt. I'll just stick with Mr. Moulton's configuration.

I am making headway. The aluminum bits for resealing via a circumferential clamp are fabricated and out for anodize. I designed a valve body of plastic, and printed (3D printer, FDM process from ABS) a trial. I think it will work fine. I'll reuse the rubber valve bits, as well as steel valve restricter bridge. Unfortunately I was unable to get any useful dimensions off the rusted out displacer I was sent so I'll have to extrapolate the valve dimensions from what I have. I have to seal the large diameter standard relief holes and drill the smaller diameter Cooper S holes then I'll have the displacer bodys cadmium plated for corrosion resistance. I (I tested it on an unrecoverable unit and the plating process does not affect the rubber). I have a new hose material and I can crimp the connection to the chassis tubing, but I'll have to use hose clamps on the displacer nipple because I cant get a crimp tool in the cavity. So in a few weeks I should be able to assemble 4 of these things and start pressure testing. When I tested the prototype I took it to 300 psi with the factory service unit and left it connected for 4-5 days without any loss in pressure. Seem to work fine.

Once they are assembled I'll try to post a picture.

Then move on the the Scotch.........

[Spider]

Wonderful work once again there Niles. Let's see how it all goes.

Need to sit down with a glass of Scotch after Sunday lunch. Join me for a tot Spider and Niles?

Yr on buddy

[Peter Laidler]

Welcome to the discussion Glacier and thanks for your thought provoking input. Join Me, Spider and Niles with a large glass of scotch while I try to take the matter on a bit further along the simplicity route. I'm not a familiar with fluid transfer situations beyond large field gun recoil and recuperation systems which, believe it or not, are quite similar to Mr Moultons practice. What is different is that in field guns, we want the recoil to he short and well buffered but the recuperation (or run-out) has got to be a lot more gentle. Look at it in slow time next time you see an anti-tank gun or 25 pounder in action. That's the only system I can think of where it's necessary to have UNequal flow/transfer of fluid (there'll be others but I can't think of any because....., anyway). That's all I'm going to say.

In the mini system, has to be the same BOTH ways...... the transfer of fluid front to rear and rear to front MUST be the same. Here's an example why. Take the EMER or workshop manual, Chapter H, page 9, illustrative figures 10 and 11. When the front wheels ascent the hump in the road, the fluid transfers from the front and lifts the rear of the car via the hydro unit bags, struts and radius arms, keeping it level. To achieve all this and at the same time, the rear radius or swinging arm rotates downwards, stretching the helper spring. We've already got TWO facts of physics that we can't defeat.
a) you can't compress a liquid - it's moved from front bag to back bag (and yes, it's stretched the bags a bit too......)
b) a spring is simply a means of storing energy - and in this case, it's a strong spring storing a lot of energy.

Car moves along a bit, so what do you think is going to happen? Yes, the helpers immediately reassert themselves and pull the radius arms upwards. Yep....., immediately transferring the fluid forwards. Once again leveling the car out. Ready for the REAR wheels to repeat the same process, but in reverse........ The REAR wheels are going to lift and transfer the fluid FORWARDS to repeat the process

Just think of the mayhem if the IN-flow and OUT-flow of the valves were different. In this simple Fig H10 situation the front wheels would descent the bump and instead of the immediate transfer of fluid back to the front, it'd transfer slower. To achieve what?
Or think about it in the opposite direction, where it transferred SLOWLY to the rear and FASTER to the front. Nope, wouldn't work that way either.

The fact that both front and rear hydro units are identical just emphasises this point. I believe that the transfer of fluid to achieve the same effect can be achieved by in-line restrictors. The car is held level by the strength of the helper springs and the shock absorbing is the restricted flow of the fluid through the valves or restrictors (which in fluid applications are just valves).

Now where did I put that almost empty glass of Scotch. Want a top-up Spider? What about you Niles? Top-up Glacier.....? Cheers

[Dr S]

Do the helper springs not also act to counter the difference in leverage ration from the front to the rear of the car? If not then a bump on the front would have an excessive reaction at the rear?

[smithyrc30]

I thought on a hydro car the lever ratios front to rear were the same at 5:1.

As far as I can see all the helper springs do is prevent the front pulling down because most of the cars unladen weight is on the front units.

I don't see why the valving cannot be different front to rear, there is a big rubber 'spring' in there and while fluid is incompressible, the rubber is not. So and equalising of the bounce and jounce would be done in the rubber, with the fluid catching up a little later.

[Peter Laidler]

Well, in my opinion until I think about it a bit and try to make my wooden classroom model a tad more realistic, I'd say no, they don't because all things being equal, such as rear weight = front weight, i'd say you could do without the helpers as the car would naturally sit level. But that's just speaking/thinking on my feet. The helpers pull the rear DOWN to help balance the car or equate the weight artificially - if that's the right word. I had both my helpers unhooked a few weeks ago and it just wallowed front down like a stuck pig of course. It's thought provoking Doc........ Come back with your thoughts because between us I think that we've got this pretty well cracked. Another Scotch on the verandah Niles, Spider?

You're right Smithy, but don't forget that once again, the rubber diaphragm is just another 'spring' in another form that will reassert itself PDQ

[Spider]

Another Scotch on the verandah Niles, Spider?

Respectfully, can we end this thread?

It's turning me in to an alcoholic

[Glacier white]

Gentlemen, a belated thanks for the welcome and the glass of scotch!

In order to make more clear my introductory ramblings a few weeks ago, i would suggest you to think of the function of an isolated displacer and avoid the complications of interconnections and helper springs.

The fact is, that an isolated displacer is a self contained spring and damper unit, a different interpretation of a coilover if you like.
The rubber takes the role of the spring, and the valves on the port plate are restricting the fluid movement, that transfers the movement of the piston to the spring, and the opposite. In this way the spring (rubber) movement is damped. If these valves fail, then we are left with an uncontroled spring, a sort of a coilover with a dead damper.
However, due to the interconnection, and the fact that displacers work in pairs, we don't have the same dramatic effect as with a failed coilover. Part of the piston movement in one displacer is transfered through the fluid movement to the other displacer, deflecting its own rubber spring (this fluid motion is damped through the washer in the front displacer connection ) and pushing the piston through (hopefully effective) valves. What actually happens is that both displacers contribute to the damping of one side.

I hope that it is a little more clear now that we cannot substitute the internal valves with simply restricting the interconnection.

Cheers!

[Seamist Green 1100]

I hope that it is a little more clear now that we cannot substitute the internal valves with simply restricting the interconnection.

Exactly. I don't know how people can seriously think that they can externally alter the damping and rebound of a displacer. All it is doing is altering the interconnection and upsetting the smooth ride qualities.

[Peter Laidler]

Nope........ Put even simpler, it's all about controlling the flow of a fluid between two points. You CANNOT think of a mini hydrolastic system as 4 separate units. Well you can, but if you do......., think those 70's big round orange space-hopper things kids played with. Fill it with water and THEN see what happens. Time to get my old physics teachers hat on again.

[nileseh]

Hmmm. After all the discussion, I think the conclusion is that hydrolastic displacers are an interesting suspension concept, but it's not entirely clear how they are intended to work. In the course of designing a rebuild capability I've measured a number of elements of the system and data doesn't provide any clarity. While I wait for the rebuild components to get plated I'll throw some numbers to see if anyone else can make any sense of it.

1. spring rate: the rubber cone has a spring rate of 3556 lbs/in deflection. That is quite a lot, in testing we actually only deflected the cone about .400" and extrapolated to the stand units for rate. The rate is pretty much straight line force to deflection. The interesting thing here is that at static loading the springs don't move much. The suspension geometry ratio is 4.08 front and 4.7 rear, the weight distribution is about 828 lbs front and 466 lbs rear. When you run the numbers, static deflection at the spring is only .116" front and .066" rear and at the suspension arm .475" front and .308" rear. The suspension barely moves when you lower it off a jack.

2. damping rate: Its hard to make sense of the intended damping. Damping is provided by restricting the motion of the fluid from the diaphragm chamber side of the valve to the spring chamber. Its hard to get an absolute measure of the flow cross section thru the valve because it's simply a block of rubber about 3/8" thick that flexes up to a limiting stop, but I found that the more restrictive valve is the one on the inside of the diaphragm chamber. That means that by valving, the fluid moving out of one displacer is restricted more at the other end of the line flowing in to the other displacer. I think that means that a deflection of, for instance the front suspension, is supported by movement of both spring units (fluid is prevented from leaving the spring chambers by the restricted inlet to the diaphragm chamber so the springs have to deflect).

3. front - rear differences: The original Coopers had the same displacers front and rear, part number 21A2008. At least I saw that on some sort of documention. The lift (max valve deflection) on this displacer is .074" inner (inflow to the diaphragm chamber from the spring chamber) and .114" outer (outflow from the diaphragm chamber top the spring chamber) Cooper S went to different part numbers front and rear. I have good data from the rear part number 21A2014: .059" inner and .099" outer. Not so lucky on the front part number 21A2012, but I got some information and sort of extrapolated to a reasonable guess: .069" inner and .109" outer. I think these are the settings I'm going to use on assembly. Assuming that more restriction means a "stiffer" suspension, this puts the Cooper S stiffer than the Cooper, which makes sense, and rear Cooper S stiffer than the front, which I'm not sure makes sense. If anyone has any thoughts on this, I appreciate hearing them. I would have expected the heavier front to need more dampening that the rear. But then again, since the rear inlet valve is most restrictive, the effect may be opposite of what I expect.

On the project: the anodizing is complete, I abandoned the 3D printed valve plate. I wasn't convinced by the test piece, so I'll water jet a plate and build a retaining ring to hold it in place. The steel parts are out for plating, should be back next week. I should be able to assemble next week or so.

On to Lagavulin.

Niles

[Spider]

Hi Niles,

Good to see you've been hard at it while we've been drinking Peter's Scotch.

1. spring rate: the rubber cone has a spring rate of 3556 lbs/in deflection. That is quite a lot, in testing we actually only deflected the cone about .400" and extrapolated to the stand units for rate. The rate is pretty much straight line force to deflection. The interesting thing here is that at static loading the springs don't move much. The suspension geometry ratio is 4.08 front and 4.7 rear, the weight distribution is about 828 lbs front and 466 lbs rear. When you run the numbers, static deflection at the spring is only .116" front and .066" rear and at the suspension arm .475" front and .308" rear. The suspension barely moves when you lower it off a jack.

Just to be clear here, you've measured the spring rate of the dry suspension rubber cone? Off hand, that rate looks close, but I'm not sure if you did this with or without a trumpet? The rate I found of the cine on it's own to be near linear. Also, I found I needed to cycle the cone quite a bit (over 50) before it settled to the point where I could get repeatable numbers.

The Trumpet, or rather it's flange diameter and shape has a very profound effect on the rate and the shape of the rate curve.

Also, I think the suspension arm ratio's you've measured & quoted here are wet types? I measured the dry fronts to be (off hand) 4.43 : 1 and the rears to be smack on 5.0 : 1.

2. damping rate: Its hard to make sense of the intended damping.

It would be a very difficult task to come up with damping rates with the hydro system and likewise is also difficult on the dry, though a little easier.

Rubber, when used dynamically as a spring has a lot of self damping properties, and it changes as it ages. Add to that fixed valving as used in the wet system, rebound & compression and you can see that there's a few sets of numbers here that need to come together in a meaningful way.

I do have some charts for damping rates of some rubbers, how relevant they'd be here,,,,,, ????

On to Lagavulin.

Niles

Quite.

Keep up the good work (and Pete, my glass is getting empty mate),,,,,,,,

[Glacier white]

1. spring rate: the rubber cone has a spring rate of 3556 lbs/in deflection. That is quite a lot, in testing we actually only deflected the cone about .400" and extrapolated to the stand units for rate. The rate is pretty much straight line force to deflection. The interesting thing here is that at static loading the springs don't move much.
Niles

Niles, it's even more complicated than you think. I take it that you have tested the rubber cone from a used displacer, right?
In my experience displacers that are in constant use for 40-50 years tend to have their rubbers settled to a halfway permanent deflection. Just by observation, it is obvious that the top of the rubber cone in a well used and tired displacer sits considerably higher in comparison with a fresh one. When empty, the top of the rubber cone of old displacers look more or less flat, while new ones were concave. And when i say new, i don't necessarily mean out of the box, the comparison is mostly done with displacers after 10-15 years of service.
What i actually mean is that after 40-50 years of service the rubber springs just don't have have the same properties. Just as old rubber cones from a dry suspension get permanently compressed giving a stiff ride and almost no suspension travel, you should expect the same to happen to the rubber springs of hydrolastic displacers. This is why you have measured such a high spring rate, and hydrolastic cars just don't ride like they used to.

[nileseh]

Oh dear. Another variable. However not having a new displacer to compare I may have to ignore the potential hardening of the rubber. However I can note and clarify a couple of things:
1. I checked the rubber cone (actually donut) on a disassembled wet unit. The trumpet does not come into play. There is a steel cup on the inside of the rubber which is contained by the upper part of the housing. The end of the cup is the outlet to the hose and line to the other end of the car. The rubber is in shear to a certain extent, but I think a fair amount of radial compression as the cup is moved upwards. There is a cross-section somewhere above where the geometry can be seen and the rubber movement anticipated. For a lack of the means to apply pressure evenly over the bottom of the cup-donut surface as would be the case with load transmitted by fluid, I simply placed an arbor in the cup with the housing on the base of an Instron load testing machine and recorded the force and distance. it was a straight line to 1500 lbs at 10.5 mm deflection, about .400", that worked out to 3556 lb/in deflection. Not exactly duplicating the displacer in service, but something.
2. All of the parts I have are quite old, but I'm not sure on how long they were actually in service. The upper visible surface of the rubber is concave with the outlet nipple in the center. I didn't measure the concavity, but it appears to be about the same for all of them, 8 or 9 parts. I have never seen a new one so I assumed that the degree of concavity is normal. I'll have to look for some pictures of a new displacer to compare.
3. Yup, suspension bits from the wet car, sitting on the workbench waiting for installation. easy to measure. However, I thought the wet and dry suspension parts were the same. I have a factory parts book, I can check part numbers.

Niles

[nileseh]

Okay, I was able to check a couple of things this morning:
1. Good call by Spider. Hydrolastic suspension arms do indeed have a different part number than dry cars. So the ratios are different.
2. I measured the concavity in the hydrolastic upper rubber at about .520", certainly not close to flat. There doesn't appear to be any permanent deformation of the rubber and they all are about the same geometry. I also found an illustration of a displacer that has a visually similar concavity. So I don't know. I don't suppose anyone has come across original specifications that would provide a durometer specification for the rubber compound?

Niles

[Peter Laidler]

Are you there Niles, Spider and Glacier? Come and join me for a small glass of Auchentoshan single malt with a colonial flavour,. The bottle says it's stored in American Oak barrels. Or do you want a can of Fosters Spider?

I fear that we're going off at a red-herring tangent a bit when we talk about the rubber compound especially when comparing the hydro system rubber compound with the dry equivalent. What the basic hydro system is, is fluid transfer which of itself is simple, working on volumes and areas and some sums and physics thrown in for good measure. That's how our brakes and JCB/CAT plant work - instantly (if you ignore mechanical linkages etc). It's my opinion that the rubber mix is not really relevant EXCEPT for the fact that
a) it must be substantial enough to stand the stresses and strains
b) not leak
c) hold the whole lot together inside the open ended steel housing
d) be pliable enough to act as a shock absorber during the sometimes harsh and violent transfer of fluid from one displacer unit to the other - and vice verca.
That's why the ends of the system are made from rubber. It is the obvious material so we've got to use it by dafault!

Can you imagine a system working if there wasn't some absorbing feature? It'd have to be via pistons (recall the anti tank gun recoil and recuperation systems) because when the front of the car goes over that lump of wood, the front wheels WILL bound or lift and the rear wheels WILL drop to compensate. Without the absorbing features of the rubber bags, it'd be absolute and instantaneous movement...... bang bang. Which is the last thing a suspension unit(s) system needs. This instantaneous movement (or transfer of fluid) is good for a braking system or a JCB/CAT back hoe bucket but not a car suspension I say. Silly statement now, but read on. Fluid is solid..... Don't believe me....? Just do a belly flop and see how solid it is until it dislaces.

I say that the rubber hydro units act as inbuilt shock absorbers to absorb the initial instantaneous over-loading of the hydro system. And STILL parts of the system can and do split. You can't compress the fluid but you CAN allow the pressurised fluid to slightly increase the area it is contained in by inflating them. 'Them' being the rubber hydrolastic units .

Another point that Niles raised of course is that insofar as the rubber compound is concerned is that regardless of anything else, we've got to work with what we've got. Ain't nobody goin' to make any more rubber inserts that's for sure.

Open to good or bad comments and even ridicule of course. Incidentally, can anyone come up with anything that Moulton invented that has stood the test of time? I'm sure that his hydrolastic system was another of those ideas of the time that were an expensive frill if not a direct fraud. A bit like the 'principle' of metal to metal adhesion - or Blish locks. Anyway, back to my old Uni physics books

[Seamist Green 1100]

I fear that we're going off at a red-herring tangent a bit when we talk about the rubber compound especially when comparing the hydro system rubber compound with the dry equivalent. What the basic hydro system is, is fluid transfer which of itself is simple, working on volumes and areas and some sums and physics thrown in for good measure.

That is so very wrong.
What happens when the front and rear hit a bump at the same time, just where does the fluid go?
The rubber spring is the essential part of the displacer. The interconnection is just a neat way of getting a smoother ride. The suspension units will still work even with the hoses capped off.