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Inside a strut for self levelling rear suspension

Started by oscar, 27 April 2008, 11:51 PM

oscar

Quote from: koan on 28 April 2008, 06:56 AM
There's one thing that had me  puzzled, and this applies to both systems.

When the strut is compressed it forces oil out into the sphere, while this happening the volume below the piston is increasing, where does the oil come from to fill the increasing volume? if it just passes through the piston valve then what goes out to the sphere?

But looking at the diagrams of the full hydro setup the 6.9 piston rod looks much bigger in than in oscar's pictures.

The volume change in the upper compartment is much greater than that in the lower compartment because of the size of the piston rod. On compression some fluid passes through the piston but some goes to the sphere, a lot more in 6.9 case because of the bigger piston rod.

koan



When I first read that last week I didn't realise you answered your own question about fliud movement out of the strut on compression due to the displacement of the piston shaft.  The thing is, how much goes out?

Now that I've had a chance to measure the diameter of the piston shaft of the SLS strut (it would be interesting to know how much thicker a 6.9's is), I googled a few geometry formulas from a maths site for 9 year olds ( ::) the things you forget) and found -


Diameter of piston shaft = 24mm so cross sectional Area, A = Pi x r2 = Pi x 122 = 452.39mm2 = 4.52cm2

1ml = 1 cubic centimeter
In terms of volume, V = A x distance moved, V = 4.52cm2 x 1 cm 
So for every 1cm of movement, 4.52ml is displaced.
Not much really  :-\

Measuring shaft travel at over 12cm from fully extended to fully compressed I'd assume that travelling over a decent bump might only cause movement of around 5cm. 
5cm of compression causes around 22ml's to be injected into the sphere.  

However, because of the partial occlusion of the holes in the piston head and the restriction of flow of oil it causes between the upper and lower part of the strut, I wouldn't be surprised if a sudden 5cm compression of the strut causes a momentary void or vacuum to form below the piston head. 

So assuming for that moment of instant 5cm compression that the piston head has practically let no fluid flow through it and in effect is virtually and only momentarily non permeable.  Using the same formulas from above, that sudden 5cm compression on my strut which has an internal diameter of 45mm would cause a maximum of 79.5ml to flow into the sphere.  Now we're talking some decent fluid displacement.  22ml v's 79.5ml.  What do you think?


Another burning question re struts particularly a 6.9's.  We know the piston head is porous in both setups and the height of the vehicle is not set by pressure above the piston like what I used to think, and assuming the lower shaft seals aren't leaking, why do some 6.9's sag?  Have some people unnecessarily replaced a strut or two because of a supposed internal leak?  I hope not since there's really no such thing because of the permeable piston head.  So what causes the loss of presure particularly if there's no visible leaks?  Is it the levelling valves? 

Quote from: WGB on 28 April 2008, 09:32 AM
The 6.9 strut is also threaded together so is theoretically serviceable.
I think so too with SLS struts
Slightly different matter to a 6.9's sag but in the same vane of wrong advice about struts.
I don't know how many times my dad (previous owner) of my 350 was told "need new struts" when the ride became bouncy and kept getting quotes for $900 a side to fix when the problem was only flat spheres.   It's still got the same struts although with rubber boots that are becoming crusty, but there's nothing faulty about their function.  From the looks of this SLS strut, I would assume it is also perhaps threaded.  I wouldn't think a press fit would do the job of keeping the pressure inside but I've no way to undo the seal (unless I keep cutting).  Replacing the lower shaft seal, rubber boot and that ball joint are possibly the only things that will fail with an SLS strut and if no one does service them, they bloody well should.
1973 350SE, my first & fave

Big_Richard

Bravo oscar, you always post pure gold ;)


Quote from: oscar on 05 May 2008, 12:04 AM
Replacing the lower shaft seal, rubber boot and that ball joint are possibly the only things that will fail with an SLS strut and if no one does service them, they bloody well should.

Perhaps theres a business oportunity there for you?  ;D


oscar

Cheers Pat, glad it made sense.  I can only imagine a sudden compression of a 6.9 strut will make a theoritcal maximum displacement greater than 79.5ml.  Who wants to sacrifice a 6.9 strut and measure it up ;D  And as far as a strut refurb business opportunity is concerned, I've had so many ideas, I'll add it to the list.  Clearly though a 280 wrecking business isn't my calling :D

A further note to add regarding those equations.  No mention is made of the effect of the levelling valve and hydro pump pressure.  But since I'm talking about a sudden 5cm compression, the flow to the sphere is instant at that moment compared to the flow from valve to strut which isn't instant at all.  Especially as the SLS valves seem to take approx 20-30deg of movement before they open.  Anyway, you can see how a sudden compression brings the spheres into play.  The faster the compression over a given length of travel, the more fluid goes into the sphere and the dampening occurs as the nitrogen bladder forces it back out at a much slower rate past the slotted disc in the sphere. 

You can also imagine why flattened spheres which are full of hydro fluid have no give.   In this scenario, it fits my observation that when my 350's spheres were flat, pushing the rear of the car down whilst stationary produced some give as the compression is realtively slow and the fluid moves from upper to lower strut and into the system relatively easily.  Hit a speed bump at speed however and there'd be no give at all, the rear would feel like it was catapulted upwards.  During the stationary press-down-on-the-rear test, the struts ability to dampen somewhat come into effect here as if nothing major was wrong and it's why I got confused as to whether I needed spheres at all and it's why I asked the question a couple of months ago about struts. 

1973 350SE, my first & fave

Big_Richard

when i got my 6.9 the only sphere that wasnt flat, was the central one.

all 4 struts were hydraulically locked, no movement was possible at all, either stationary, or while driving. The self leveling function still worked too, in that it still was able to control ride height, and the positions on the pull knob made the car higher and standard again.

The only suspension at the time, was offered by the tires bouncing on the road. It made for fun cornering, IE, don't even have to slow down, and steering like it was on train tracks, but it was very, very uncomfortable.

koan

I was thinking about the "niclas" post on area and pressure when it occurred to me the same reasoning applied to area and volume.

Your calculations look correct oscar but I haven't worked through them. What they've made me realise though is if the strut stays full of fluid and no voids form (see later) the volume of fluid pushed out to the sphere on a bump is the same as the volume of piston rod pushed into the strut.

I've not seen the valves on the piston so not sure how they operate, or what you mean by permeable oscar. Are they (all, some?) closed and forced open on bump and rebound or are they always open and close off under bump and rebound? I've assumed it to be the former, closed and forced open. EDIT:  It must be the former because a piston with holes won't produce any force.

Not sure if a vacuum or void could form under the piston. The bit in the book on testing spheres talks about "opening pressure", the pressure at which the sphere starts to accept fluid. For a 6.9 the pressures are 900 psi front and 675psi rear (converted from bar). These are the minimums for used spheres, for new its about 225 psi higher. The pressures in the struts must be at least these figures or nothing would flow into the spheres. I reckon shock loads are going to open the valves because fluids are incompressible. Any bump or rebound will open the valves and allow fluid to pass through the piston in a controlled (damping) manner and the sphere accepts or supplies fluid as a result of the change in volume of fluid of both chambers in the strut.

In an SLS system the springs and pressure support the car, in the 6.9 set up it is only pressure. In both systems the average pressure stays about the same (a different pressure for SLS and 6.9 though). As the vehicle goes along hitting bumps and dips the pressure goes up and down but the average doesn't change because that's determined by what load is in the boot, with a few bags of concrete in the back and the average pressure will be higher. In a 6.9 when the height knob is pulled more fluid is forced into the strut until the fluid is shut off but the pressure stays the same because the weight of the car hasn't changed

What you say PB about having dead flat spheres, no suspension movement and a hydraulic lock but the ride height knob worked must mean that fluid was able to pass from the the rod side of the piston through the valves to free the lock and let the car rise

I replaced my front struts without touching anything else, spheres or valves and they certainly fixed my sagging frontend.  I believe the holes and valves in the piston head are supposed to be closed under static conditions, any seepage through them or around the piston is going to let the strut compress, just like a bike pump with a stuffed washer.

Sorry if the above is a bit rambling but the brain is still working on it.

"Oscar's Strut Works" has a nice sound to it.

koan
Boogity, Boogity, Boogity, Amen!

oscar

Quote from: koan on 05 May 2008, 03:59 AM
I've not seen the valves on the piston so not sure how they operate, or what you mean by permeable oscar. Are they (all, some?) closed and forced open on bump and rebound or are they always open and close off under bump and rebound? I've assumed it to be the former, closed and forced open. EDIT:  It must be the former because a piston with holes won't produce any force.


By "permeable" I'm just referring to the ability of oil to travel through the piston.  Although valve like, I've realised that an SLS piston does have a permeable feature as well.  I should really mail the assembly to you so you can see for yourself because after closer inspection, the holes aren't completely occluded by the washers.  The six holes on the underside are occluded, but on the topside, have a look and compare the two photos here and here.

-In the first photo, a stripped view of the top of the piston, three holes allow oil from the lower part to the upper part of the strut.  Notice a little notch extending outwards from each hole. 
-The second photo shows the washers in situ.  Notice in the foreground one of the notches is especially visible but all three have the same clearance.  Although small in size, these notches allow a very slow transfer of oil between the upper and lower sections if there is any kind of pressure difference regardless how small. 

I was surprised to read that in your 6.9, front struts alone cured your sagging front end.  This is important as this appears to be what may be distinctly different between the 6.9 strut piston and the SLS one.  A static SLS strut allows pressure equalization above and below the piston whereas the 6.9 strut doesn't.  There's neither benefit nor shortcoming in either.  It just highlights how the two are uniquely designed for separate applications.

Further to this, the six holes on the underside are the same size (earlier I stated they were smaller (just an optical illusion)).  Each of the underside holes have these notches too in the same pattern but are occluded.  Given a forceful compression and the high pressures you quoted I can see how these valves that I initially assumed to be too stiff do actually open even if only a little.  Under this condition, it wouldn't take too long for the lower part of the strut to equalise in pressure to that of the top though it is still delayed.  But why notches in the underside holes if they are completely occluded when static - I think they assist with opening the underside valve initially and uniformly directing oil flow.

BTW, I was thinking "Oscar's Strut n Stuff" ;D  but now you guys have me thinking how these buggers come apart further.  Tomorrow I'll grind away again to see if there is a thread or not.  If they can be serviced, how do they come apart.  Pics tomorrow, same time, same thread.  8)
1973 350SE, my first & fave

oscar

Ok, I'm done I think.  Rather than cut the base open I thought I'd try and see how one would go removing the piston complete.  For SLS struts there is no threaded nature.  The bottom seals and collet that houses them are held in place by a simple split O ring.  Getting that O ring out is another story.

Anyway, here's where we left off, just imagine half the strut isn't missing, click any pic for a full size view.



First thing I did was remove the clips to the boot.  Notice though how there's three holes towards the ball joint end.  Allowance has been made for seepage.


Ball joint unscrews by using two 19mm spanners.  On the shaft side, the nut is thin so I had to grind down a cheap 19mm spanner to fit.  Once removed, the boot slides off.


Removing the split O ring was difficult.  Without the right type of pick tool I had to grind into the side of the strut and tear the ring out with a pair of pliers. 



Once the split O ring is removed, the collet with lower seals can simply be pulled out.  Please note!!!! if the strut hadn't been cut up yet, the shaft would not have come out though as pictured.  If the strut was intact there'd be the piston head and washer arrangement which I removed previously.  There is a second split O ring further up the interior of the strut that prevents the collet from moving further into the strut.  Consequently it also stops the piston head from being removed from the strut.  See second pic below


Second split O ring as discussed above viewed from the bottom of the strut.  The groove for the first split O ring is visible in the foreground


So now with collet removed, there's an outer rubber O ring.  On the inside, a stiff stepped ring is backed by a round section rubber O ring.  The collet is one piece, though the inside contains a second metal cylinder that houses the inner O rings.  Judging by the three crimpings on top of the collet, they didn't want this section to come apart ever.  Not that it's needed.  I replaced those rings without too much difficulty after I removed them.



Yeah, so, basically that's it.  If you could find a way to remove those two big split O rings without scoring surfaces with sharp metallic tools an SLS strut is serviceable.  Whether the parts or rings are available is another story.


EDITS; just fixing me gramma n spellin ;D
1973 350SE, my first & fave

koan


So the bottom of the strut is just the end of the bore/tube with the collet  held in by 0-rings in grooves and the lot then covered up with the flexible bellows.

Is that correct?

koan
Boogity, Boogity, Boogity, Amen!

oscar

Correct, the casing of the strut is just an open ended cylinder.  There's two metal O rings, one on either side of the collet.  Remove the lower ring, the collet comes out.  Remove the upper second ring, the whole lot comes out.
1973 350SE, my first & fave

Big_Richard

wow, thats stupidly simple to rebuild. I wonder if the 6.9 ones are so easy

TJ 450

That's great, there's really not a lot that could go wrong internally aside from excessive wear by the look of things and perhaps those o-rings which should be straight forward to replace.

Tim
1976 450SEL 6.9 1432
1969 300SEL 6.3 1394
2003 ML500