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PostPosted: Thu Nov 16, 2017 11:23 pm 
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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! :D


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PostPosted: Fri Nov 17, 2017 6:25 am 
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Glacier white wrote:
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.


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PostPosted: Fri Nov 17, 2017 10:23 am 
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Location: Abingdon Oxfordshire
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.


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PostPosted: Mon Nov 20, 2017 2:14 am 
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Location: Eugene, Oregon USA
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


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PostPosted: Mon Nov 20, 2017 6:26 am 
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Hi Niles,

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

nileseh wrote:

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.

nileseh wrote:
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,,,,,, ????

nileseh wrote:
On to Lagavulin.

Niles


Quite.

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


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PostPosted: Mon Nov 20, 2017 9:08 am 
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nileseh wrote:
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.


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PostPosted: Mon Nov 20, 2017 6:18 pm 
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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


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PostPosted: Mon Nov 20, 2017 9:14 pm 
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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


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PostPosted: Tue Nov 21, 2017 12:48 pm 
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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


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PostPosted: Wed Nov 22, 2017 5:13 am 
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Peter Laidler wrote:
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.


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