Jerry Feather

At this point I am convinced that the recoil of a spring in these struts is a total non issue. I am focused on a spring, notwithstanding that Jeff might be right that there is no possible spring that would meet the 5 mph or even 2.5 mph manufacturer's requirement. I am also certain that there is no requirement that applies to any of the rest of the world that prevents us from putting anything we want into the bumper strut holes and bolting the bumper to them. So, any spring that is reasonably stout and fits the space for it is going to be quite suitable.

The other consideration is the economics of the project, as pointed out by Greg and a couple of others. My thought at this point is that even if "good used ones" are available for $75, they are still good for only one time and must be replaced. If anyone could convert any of them to a spring for say $150 each, that might very well be a bargain since they will last forever.

The economics are however much more complex than that. So far what I have discovered is that in one out of two instances of strut collapse the bottom seal ruptures and the fluid, probably over some time, completely dissipates. In the other case however the seal is not ruptured and there is still some, but not all of the fluid inside. That suggests to me that there is a very strong chance that either the fluid naturally dissipates over time without regard to collapse or that the cylinder was never full of fluid in the first place.

That last thought leads me to consider the question that has kind of been brought up before and that is why BOGE has printed on these "DO NOT OPEN"? At this point I can think of only two good reasons for that. Both are based on the fact that it is obvious that when one of these is filled with fluid, which I think it must be to be effective for what it is designed to do, and there is an impact, some of the fluid that cannot be compressed must go out. So, if fluid has not gone out there must be air space inside of the strut either from when it was made or over time with natural dissipation through leakage. If there is air space inside and the strut is collapsed then there is the danger of compressed air and/or fluid under pressure escaping causing harm to anyone opening one of them. The other thought is that BOGE simply does not want anyone to discover that they are not filling the struts with fluid, but leaving an air space in them for the expansion and which therefore reduces the effect of the strut. Can anyone else think of any other reason for the warning, now based on my having opened two of them without any danger?

So, if there is air space left inside when these are made, even a new one recently manufactured, it is defective. If the air space develops over time then any one of them that has not been collapsed is still not a "good used strut" worth $75. For me I think I am going to end up replacing just about all of mine with spring loaded struts.

Mrmerlin

if you could fit a zerk fitting and fill them grease that would drive the shock to full extension after a compression,
the trick where to mount a grease fitting.
so the shock could be fixed in place unless it needs to be mechanically unlocked from a wedge on the bore.
In that case removing the shock results in the same amount of work as putting in a used / new part

Like I said they use this idea to load the front wheels on bull dozers to keep the tracks in tension,
so its a valid way to investigate.
KISS

Jerry Feather

Stan, I see that you have mentioned your idea about grease now a couple of times. I gave it some thought the first time and I think I concluded that it would not work, but now I don't recall exactly why. I do seem to recall that my first analysis was based on a mere guess about how the struts work inside since I don't think I had the first one completely open then. Now I have two of them completely open and have a better analysis for you. About the grease, my first conclusion is that it cannot be used as a force to put one of these fully collapsed struts back into it original 70mm position. The reason for that is kind of involved and requires a detailed description of the inner structure of the struts.

The hydraulic mechanism in these struts is basically and totally inside of the inner shell of the strut which is the part that bolts to the bumper. That is with the exception of the connecting rod which has the piston at its top end and the bottom end connected to the center plug at the bottom of the outer shell, the part that bolts to the car. Inside the inner shell is the hydraulic cylinder and it is separate from the inner shell. There is about a 3/32 inch space around the cylinder and between it and the inner shell. The cylinder is the full length of the inner shell and is lodged at the bottom with a seal between it and the bottom plug of the inner shell. The seal also seals, kind of, the inner shell and the bottom plug, although it is not really between them but just up next to both of them. Then it pretty much does the same kind of seal around the connecting rod. At the upper end of the cylinder is a plastic plug and it is firmly captured in the head of the inner shell. The cylinder has some significant holes in it around where the piston rests at full extension (not collapsed), both above and below it, and then the 6 tiny holes above the piston that provide the resistance to piston travel upon impact with something.

So, upon full collapse of the strut the piston is at the top of the cylinder "impact chamber" with all but maybe one of the holes in the cylinder below the piston travel. The only place to access the cylinder chamber inside the inner shell will be to drill a hole in the upper end right next to where the thing mounts to the bumper and into the top of either the impact chamber or just outside the cylinder wall. Tap for a zerk. When you pump grease into the impact chamber it is going to either move the piston a tiny bit or simply escape thru the tiny hole near the top of the chamber, depending on where it is in relation to the piston. If you get some movement with that that is all the movement you are going to get because the grease will simply squirt out the hole and into the rest of the cylinder chamber in the inner sleeve. You will be able to fill the entire cylinder chamber inside and outside the cylinder, depending on the capture of air in the process, but you will not restore the strut to its fully extended condition.

The solution to this is to first forcefully in some way pull the strut back to its extended 70mm position and then do the grease-fill exercise. If you are successful in getting the cylinder chamber full of grease with no air (and I don't know how you will know) you will have a fully functional strut except that it is going to function totally differently from the way it was designed; and I think your biggest problem is now where is the surplus grease going to go since it is going to be much more resistant to rupturing the bottom seal to escape. I suspect that the strut is now going to be essentially solid with almost total resistance to any collapse. Then all of this kind of assumes that there is no other kind of fluid in the cylinder.

I have pointed this out before, but I think many have not quite grasped the physics of this strut function, and the deeper I get into it the more I think that BOGE may not have either. The situation is that with the strut fully extended (70mm) the piston is at the bottom of its throw. The space above the piston is equal to the diameter of the piston (about an inch) and the length of its travel which is 1.625 inches. So when the piston travels its full length it must displace that volume of fluid. That fluid has to go somewhere else in the fully sealed cylinder chamber and that is outside the cylinder and inside the cylinder behind the piston where the rod is. And, that is the problem, because the space behind the piston is not the same volume as the space above the piston, for a given stroke of the piston, and it is in fact just about a third of a cubic inch less because that much of the connecting rod has now moved into the cylinder chamber from outside. So where does the one third cubic inch of fluid go because it cannot be compressed?

My now minimal experience with opening a couple of these up says that the extra fluid must rupture the bottom seal. Or it may be that the cylinder chamber is not in fact full of fluid, but rather has some air space in it. Then the question is where did the air come from? Was it leakage over time or was it left out at the beginning because BOGE didn't realize the problem themselves, which I am beginning to think is the case, mainly because there is simply no meaningful solution incorporated in the strut for the excess fluid. Maybe they thought "so, lets just leave some of the fluid out and put a warning on the strut 'do not open'."

Jerry Feather

I had cut the second strut open the other day and pulled the insides out of it, but the outer shell and inner shell were firmly stuck together. I kind of assumed that there must be more to the tight spot locking them together than I had originally thought or found. However, this morning early I decided to take them apart. I had already hammered on it a bit with no movement, so this time I put a one inch aluminum rod about twice as long as the strut into the opening down to the bottom and began to hammer on that. Then I noticed some movement, so I turned it upside down and started striking the rod on the concrete floor. With that I got a lot more movement. Finally I just used the rod as a ram and kind of hammered it by hand into the opening and the strut finally came apart. I found that it was not locked together but rather stuck together by rust and maybe some other kind of gunk.

I am now firmly convinced that the hydraulic fluid in these is water based and most likely just water with some antifreeze in it. I cleaned up the outside of the inner shell nicely in the lathe with a file and sandpaper; so I need to figure out a good way to do the same with the inside of the outer shell. I do find that there may very well be a bit of the tight spot on the bottom end of the inner shell that will have to be removed so that these pieces can be painted and slipped back together when I put the spring inside and reassemble any of them..

What an interesting project. Now I need to go out and fiddle with the grill stanchions a while.

Mrmerlin

Jerry thanks for taking the time to type out what you found ,
I have a much clearer picture of the workings of the strut.
The grease idea wont work

Jerry Feather

I think I have discovered some more revelations about these BOGE struts. Aside from the rust in between the outer and inner shells that I found was sticking them together in the second one of these I have opened up, I find that the inside of the inner shell is also heavily coated with rust. The cylinders themselves and the pistons and connecting rods appear to me to be stainless steel. So, today I thought I ought to inspect the bottom seal of the cylinder chamber to be sure that it did not in fact fail when the strut was collapsed. I couldn't find any evidence of a rupture as with the first strut I opened, but I do notice that the rim of the seal is also coated with a layer of rust.

So with the hydraulic fluid being either water or something water based it is clear that the rust inside the cylinder chamber is natural. However, the rust around the outer edge of the bottom seal says to me that it is not sealing very well. Then, of course, all the rust in between the inner and outer shells, both of which are carbon steel and apparently not painted or treated against rust, is caused by exposure to water. The only place for that water to come from is inside. The only other place would be from the outside and that would have to be from around the rubber seal of the front of the strut, but those appear pretty tight.

The struts are fully painted on the outside and there is almost no sign of rust from the elements on the outside even though the struts live pretty much outside at least most of their lives. However I notice that on the two struts I have cut apart, and now also on all three of the struts Stan sent me, there is a very significant amount of ruts on the bases of all of them, and except for a little on one side of the outside rim of 4 of them that is the only rust on them. Too, the bottoms are also painted. If they were rusting from the outside elements why is it only on the base plugs?

That is telling me that these struts have all leaked and the leakage is escaping to the outside around the bottom plug in the outer shell and pretty much staying there causing rust. This tells me that if one were to inspect any strut, whether 70mm or less, if there is rust on the base plug the strut has been leaking. That of course still does not say that if there is no rust there that the strut has not leaked or that the strut was ever full of fluid. Some of that may still remain to be determined, if ever.

Jerry Feather

The answer to the original question I am sure is that yes we can put a spring in these 928 bumper struts. With a spring I think they will function adequately whether or not they might pass the tests required of an original car manufacturer. I suppose that raises the next question which was mentioned earlier in this thread and that is, should we? In the course of this investigation I have had the unique opportunity to evaluate the design and function of the original struts. The results of that endeavor lends itself directly the the new question.

In the course of this thread I have tentatively reached some conclusions that I now think are just wrong. The main one of those is that I thought it would have been a huge error for BOGE to have put these struts together with any air in them in the cylinder chamber. Now I am certain that just the opposite is true. Here is why I think that:

First I thought it would be helpful to study the CFR that Jeff cited for us to see just what it might say about how the bumper mounts/struts should be made or what they should be made of. There is essentially nothing there about them. The only thing is a kind of offhanded mention of failure of the struts which says to the effect that "if" the strut consists of a vessel that contains air or fluid it can be permitted to fail but only if the failure does not result in what I would call shrapnel. The use of the word "if" clearly suggests to me that any other means can be proposed for testing. Something like a spring is not mentioned, but there is a phrase that talks about what many seem to be concerned about with a spring recoiling, and that phrase is written in the context of one of the actual tests. It says "From the point of release of the devise until the onset of rebound. . . ." I take "rebound" to be much like or akin to recoil of a spring.

The tests for manufacturers involves basically swinging a heavy weight equal to the weight of the car into the bumper straight on from an unspecified distance, but presumably calculated to be equal to 2 1/2 mph, and doing that twice at least 30 minutes apart and then doing it again at a 30 degree angle at each corner of the bumper once each and still all at more than 30 minutes in between. That means that each strut must perform successfully three times in kind of short order. I think the 30 minute minimum is likely allowed to give the strut some time to reset for the next test. Allowing the failure of the strut is not specified when, but it seems obvious that that can occur only on the last of the three tests. This time for reset is what has given me the clue to the original design and fabrication of the BOGE struts.

My basic conclusion about the structure of these struts is that they are designed and built to contain all the fluid put in them with no opportunity for escape. The structure of them is very solid and the seal is pretty robust with no indication of intentional failure such as rupture or leakage. Based on that, if the struts are filled full with the fluid used they will have in fact NO ability to collapse, AT ALL. A fluid cannot be compressed, but with more of the connecting rod entering the fluid chamber there is no space for it or the fluid hoped to be squeezed out of the piston chamber. It will be SOLID. Therefore, the ONLY way for this to work as intended is to leave some air space in the fluid chamber. The amount of air must be at least equal to the volume of rod entering the chamber and probably slightly more, maybe even up to twice that.

What happens then upon an impact is that the piston moves forward in its chamber squeezing fluid out through the 6 tiny holes made for that pretty-much-controlled squeeze, and when that fluid is displaced it compresses the air in the overall chamber. No leakage required. The now compressed air is what gives the strut the ability and force to pretty much reset itself. Simple as that. However, I think that the potential for not resetting itself fully and having a residual of compressed air in it is the reason for the warning DO NOT OPEN. None of this however explains for me why the struts are obviously designed to lock into the fully collapsed position when pushed that far.

Back to the spring . . . Tim is working on a K factor and with that, since we pretty much have the dimensions of the spring, we can see just how much spring it will take to meet or even come kind of close to the spring needed for 2 1/2 mph or even something maybe close.

Jerry Feather

Now I think I have figured out the answer to the question about why the struts lock in the fully compressed position. So, now, contrary to what I think is/was popular belief, these struts are designed and intended and actually required to absorb the 2 1/2 mph impact THREE TIMES in at least 30 minute succession. That means that they must be designed with the ability to reset themselves after such an impact. With my description given just above it is obvious that the compressed air resulting from an impact is the force that must reset them. (Any kind of spring will do the same thing and in much less time.)

The way the compressed air works is that when created from an impact, presumably from an impact that does not fully collapse the strut, the compressed air now pressurizes the fluid in the chamber. That pressure is in all directions. The pressure toward the top is against a circle an inch in diameter, but the pressure against the bottom is against an inch in diameter MINUS the cross section of the connecting rod through the bottom. However, in addition, the pressure on top of the piston, if it is exposed, which it should be in a partial collapse, is equal to the pressure on the top and is added to the pressure downward. That pressure difference resets the strut. I think of it actually as much like a rocket engine, but that could take some explanation. I think that the only thing that will cause the strut not to fully reset is either some friction that can resist the residual air pressure which goes down as the strut resets, and/or the loss of some of the fluid leaving a larger air space than original resulting in lower resulting pressure upon impact - which probably happens over time. That may very well explain the three struts that Stan sent me that are only partially collapsed.

Now to the lock. The answer is that if the strut is fully collapsed upon an impact it looses its ability to reset. That is because, as can be seen and measured, the top of the piston is now firmly up against the top of the piston chamber, or otherwise isolated from the chamber pressure, so there will be no pressure on the top of it to reset with and the overall residual pressure (against the underside of the piston in particular) will be holding it collapsed. I think BOGE deemed that as a total failure point and therefore designed in the tight spot to lock it there for all time. Then, because of the theoretical air pressure in the chamber they posted on the strut DO NOT OPEN.

I don't think BOGE ever accounted for the potential of leakage of the fluid from these struts over time as I see now is quite common.

Jerry Feather

Let me explain this a slightly different and perhaps much briefer way as follows:

Upon an impact the air pressure created pressurizes basically two chambers. One is the chamber above the piston which has up and down pressure equal to the full surface area of the top of the piston and the top of that chamber. The other one is under the piston which has pressure up and down equal only to the up and down surface areas each of which is the area of a one inch circle MINUS the cross section of the connecting rod which is half inch in diameter. The top pressure wins out and resets the strut.

However when the strut is fully collapsed then the chamber just above the piston basically goes away because the piston it right a the top of that chamber. The top chamber is gone. The bottom chamber wins out holding the piston from underneath it right at the top and the strut will not reset.

GregBBRD

Jerry:

In my experience, the Germans only stamp "Do not open" when there is a presurized fluid involved (like in a nitrogen filled shock absorber) or when there is very high pressures involved (like in a hydraulic accumulator.)

Is it possible that instead of an air space inside these bumper shocks, that there was originally high pressure nitrogen, which has escaped over the years?

Jerry Feather

Hi Greg. I had given that a thought for about a minute, but the main problem with the idea is that there is simply no way to have assembled these struts with any air or gas under pressure at the outset; and that is aside from there not being any need for it. The only purpose the air pocket performs is space for the fluid displaced by the piston that isn't otherwise available because of the extra connecting rod entering the fluid chamber upon compression. The air has essentially nothing to do with the absorption of shock upon impact. That is all taken care of by the resistance involved in having the piston push the fluid out of the piston chamber through the 6 very small holes and having to do it very quickly. There is no need for any kind of gas pressure to assist in or resist the fluid displacement. Edit: Although I can see that as the air space is "used up" by the displaced fluid there is some pressure being built up as the sir space gets smaller. Still I think that is still only incidental even though it would play some role in the displacement of the fluid. Nevertheless I still think the buildup of air pressure is not an intended part of the impact absorption.

However, when the strut is fully collapsed and locked there the air pocket has been reduced significantly and is now under very high pressure, but only as a result of the fluid taking up the air space that was provided for it. Hence, as you point out with the Germans, DO NOT OPEN.

Jerry Feather

This thread took a little bit of a turn, at least for me, in trying to figure out just how these struts were made and how they were intended to function, notwithstanding that that further investigation actually helped me to justify in my own mind that a spring in these is perfectly logical. So, the answer to the original question is YES, we can put a spring in these struts. The answer to the other question, about "should we" is also YES. Then the final question is still a bit open, but very close to a positive answer and that is what should the spring consist of?

I have turned the spring question over to Tim. He is an ME and has about 20 years of ME experience. As it turns out in his former job he spent about 16 years designing conveyor systems for the packaging industry and in those things they utilized all kinds of mechanical, pneumatic and hydraulic mechanisms. In the course of his design work he frequently had the task of doing just what is needed here and that is to design a spring to meet a particular need and then to find one or more of them. The result is that he is an expert in doing just what is needed here; and he is in fact on the cusp of that. He has already calculated the size of a spring or springs to absorb the 2 1/2 mph impact and is now searching for a good source. It turns out that a spring much like the one I posted as my intuitive spring is going to fill the bill.

Jerry Feather

It turns out that given the nature of the Test the government required for these struts and basic logic suggests that the needed force from the frontal impact can be absorbed by two springs, that calls for a particular spring. However, when the impact is from the front at a 30 degree angle, in the corner impact test, that spring must be 86 percent of the total needed force. That is the spring we are designing for.

Aside from the springs, I am thinking thru the process of the actual conversion of the struts. I have listed some steps in the process before, but some refinement is perhaps useful. Some of that refinement has to do with the flutes inside the inner sleeve and inside the cylinder chamber, the flutes that pretty much capture the top or head of the cylinder itself. They must be removed. I originally thought that they are an integral part of the inner sleeve, but on a bit closer inspection, and even that is not conclusive, I think that the flutes and a slight narrowing of the sleeve below the flutes may in fact be a separate component that is likely pressed in. I'm still going to try to ream them/it out and I now think that that is going to leave a very thin layer of metal that I hope can be simply displaced and pulled out leaving the inside of the cylinder chamber smooth all the way to the front/top.

By measurement I find that the inside diameter of the basic inner sleeve is 1.415 inches. The closest reamer I can find to that is 1 3/8 inch or 1.375 inches. I have ordered that. With it I hope to be able to set it up in my mill and ream most of that inner component out and then somehow displace what is left of it, about .020 in thick, and then fish it out. We will see when I get the reamer.

Gary Knox

Seems like you need a 36mm reamer. Probably available from anywhere in the EU (or Canada?).

Jerry Feather

Right on, Gary. I didn't think of doing the math. I'll do a search.