6.9 Handling

[TJ 450]

Not sure the diagonal pitching and lateral roll would involve the leveling valves to any great extent but does use the side to side flow and the damping ports in the struts.

You raise an interesting point there WGB. The correct function of these parts would rely on the viscosity range of fluid specified in the design to work properly, and seeing as my car has obvious issues with damping, there may well be a correlation.

I don't think I would be game to drive with it in the lock setting, but those observations are interesting (for lack of a better word ).

Good point there as well, Cam. There rear end is 100% as far as bushes and mounts go, though. I replaced all of those when the subframe was dropped.

Food for thought.

Tim

[koan]

Do any of you think 6.9 body roll is excessive?

It seems so to me, possibly not excessive but a bit more than it should be.

6.9s have a stiffer roll bar than other 116s, I wonder if this is to compensate for the direct connection of hydraulic lines side to side.

If a front wheel hits a bump the shock pushes fluid into the sphere but it will also push fluid to the other side's sphere and shock, there's nothing to prevent this. The fluid goes through a tee-piece to the other side, not through the control valve. It works against the roll bar, the roll bar transfers some of the force on loaded wheel to the unloaded other side, effectively stiffening up the loaded side spring. The hydraulic connection does the opposite, softening the loaded side by allowing fluid to pass to the unloaded side. Think of two bike pumps connected together.

I had an idea, a restriction in the metal line where it connects to the sphere would limit the side to side flow but still allow the self leveling action. The restriction could be a male to female adaptor with a small hole 0.5mm or so through it. Would need two on each axle to keep the self leveling symmetrical.

EDIT: I've posted over top of replies which have covered side to side flow of
suspension fluid.

I can't see how middle knob position can have much effect on suspension feel and roll as it only isolates each axle's hydraulics from the leveling action in and out fluid flow, each axles pair of wheels are still connected together.

But I did have one occasion where something went amiss with the front leveling valve and the suspension felt very hard. A bit of axial pushing and pulling on leveling valve lever fixed the hardness.

My restrictors would have have the effect of thicker fluid as far as body roll goes, they would slow down the self leveling but that shouldn't be a problem.

koan

[TJ 450]

I agree that it's tending towards excessive.

The restrictor sounds reasonable, Koan. Also, check out this quote from Wikipedia, regarding the Citroen system;

The legendary comfort of Citroens is also due to another factor, which is a specific choice in the set up of the system: Citroen chose to hydraulically interconnect each wheel at the same axis. Thus the two front wheels are connected with each other. And so are the rear wheels. This has a very specific result for body control: when one of the two wheels meets, say, a bump, that wheel will tend to compress and absorb the bump. In the degree not the entire height of the bump can be absorbed, the car's body will lift at that side. In the same time, through the anti roll bar, this wheel will exert force at the other side wheel and tend to also lift it in the same direction (compress the suspension). This is where the hydraulic interconnection of these two comes to play. In an ordinary steel-sprung car, this anti roll bar effect would mean that the car's body at the bump-absorbing side would lift, while it would tend to drop at the other side. This amplifies the bump's effect of destabilising the car and creating discomfort. In the Citroen system, the anti roll bar will also tend to do the same, but it is stopped by the hydraulic oil that is sent from the bump-side wheel at the moment it is pushed upwards to absorb the bump. Thus the suspension at the other side will receive a force opposing that of the anti roll bar, and of a degree of definition equal to the size of the bump. This in effect translates into a much reduced overall feeling of impact from any bump, as the shock is effectively automatically shared with the wheel at the other side of each axis. In practice this feels as if the whole front or the whole rear is slightly lifting, rather than the car receiving an unpleasant shock from one side.

Tim

[s class]

Do any of you think 6.9 body roll is excessive?

Yes and no.  

Yes : when I first experienced a 6.9 I thought it was horrendous

No : After a while one gets used to it, and is able to push harder in corners.

My trusty rusty rolls a lot more than the red car.  There are three differences, all of which probably contribute :

a) trusty rusty has new, standard struts.  The red car has original struts, and I *think* they may just be the sports version.
b) trusty rusty's bushes are poorer than the red car's ones
c) trusty rusty currently is set with a higher ride height than the red car.  

Ride height shouldn't affect the hydraulic aspects, but it does affect the behaviour of the mechanical aspects due to a change in geometry.  

[wbrian63]

Could the "diagonal" action of the car be also called "corkscrewing"? By this I mean instead of going up and down, it also goes "round and round"?

If this is the case, I suspect the issue (while not discounting how worn bushes and ball-joints can dramatically effect how a suspension performs) is unequal pressure in the spheres from side to side.

If the pressures in the spheres aren't the same or close to it, the way the suspension dives and rebounds will be different on opposite sides of the car, and that is not a good thing. As one side compresses more easily than the other, that can actually create a lateral movement in the car as differing pressures act on the various pieces of the suspension - the suspension was designed to have equal pressure side-to-side in the spheres. Think about how a car with normal springs and shocks will behave when one shock is heavily worn as opposed to it's across-the-car partner, and I think you'll see where I'm coming from.

I just got a vintage MB catalog for the 1979 model year, and after reading it from cover-to-cover, one of the consistent topics discussed for all models, from the "lowly" (if an MB could ever fairly receive such a label) 240D to the flagship 6.9, was the "zero offset" front suspension. The way it was described was the geometry of the system is such that no matter how far a front tire extends or retracts in response to road conditions, no "steering" input is created. Fully extended or fully compressed, the tire points the same way.

That being said, and coupled with the fact that (as far as I know) the only difference between a 450SEL and a 450SEL6.9 suspension is the use of the hydro-pneumatic struts and spheres in place of springs - ignoring the well-known difference in the steering knuckle casting. I checked EPC, and the steering knuckle and the upper control arm do have different part #'s 116032 vs 116036, but I suspect it's not geometry, but casting size that's the difference. The lower control arms show the same part #, regardless of vehicle. As such, other than ride quality, there should be no functional difference in how a 6.9 should handle an undulating road vs a regular-sprung W116.

Insofar as the movement of hydraulic fluid from one side of the car to the other, it's definitely the reason for the stiffer roll bars, but the effect should actually be beneficial, as the increased suspension pressure on the uncompressed side of the suspension should serve to plant that tire tighter to the road. Knowing the diameter of the lines and the volume of fluid involved, I suspect there's a fair amount of natural restriction to rapid movement of fluid from one side of the car to the other.

We must remember that spheres are a product that actually ages while sitting on the shelf. When not installed in a car, there's high-pressure nitrogen on one side of the bladder, and 14.7psi on the other side. The "willingness" of the nitrogen to migrate thru the bladder in this situation is far higher than when the sphere is installed on the car and there's hydraulic fluid on the other side of the bladder, and the pressures are equal.

Most quality spheres have a manufactured date stamped on the bell of the sphere. It pays to check to make sure the ones you're buying haven't been sitting on a shelf somewhere for a few years...

All this being said, the front end of my 6.9 is completely shot and will soon be rebuilt. The amount of unintended steering input I receive over undulating roads is frankly frightening. I can't wait to get it all back as-new to see how a 6.9 is really supposed to drive.

[koan]

I agree that it's tending towards excessive.

The restrictor sounds reasonable, Koan. Also, check out this quote from Wikipedia, regarding the Citroen system;

QUOTE DELETED

Interesting read, some Hydrolastic suspension cars, Minis and 1100s were connected front to back with similar logic to the Citroen quote.

Sounds like it's compromise between ride with little restriction of side to side flow or limiting body roll with limited side to side flow.

Yes : when I first experienced a 6.9 I thought it was horrendous
No : After a while one gets used to it, and is able to push harder in corners.

Agreed, I think I've learned to live with it but on the occasions in the past  where I've encountered the steering correction and over corrected the result I think were roll induced, and it wasn't pleasant.

Insofar as the movement of hydraulic fluid from one side of the car to the other, it's definitely the reason for the stiffer roll bars, but the effect should actually be beneficial, as the increased suspension pressure on the uncompressed side of the suspension should serve to plant that tire tighter to the road. Knowing the diameter of the lines and the volume of fluid involved, I suspect there's a fair amount of natural restriction to rapid movement of fluid from one side of the car to the other.

Agree with most of what you say except the quoted part above. Roll bars are to limit body roll, the stiffer they are the less body roll (better for going round corners).

The part where you say "increased suspension pressure on the uncompressed side of the suspension should serve to plant that tire tighter to the road" has the effect of lifting the already unloaded side of the car higher.

Because of the cornering G force the centre of gravity shifts to one side of the car which becomes effectively heavier on one side than the other, this is what causes the roll. Any force applied to the unloaded side tyre can't push it into the road surface, it only serves to jack the already high side of the body up higher.

koan

[wbrian63]

Because of the cornering G force the centre of gravity shifts to one side of the car which becomes effectively heavier on one side than the other, this is what causes the roll. Any force applied to the unloaded side tyre can't push it into the road surface, it only serves to jack the already high side of the body up higher.

The logic I utilized to make that statement is what you get when you don't get enough sleep...

Agreed - any attempt to plant the inside tire harder to the pavement will further contribute to the roll of the chassis.

As I'm visualizing the role of the roll bar, as the car heaves to one side, that presses up on the end of the link attached to the bar.
Then the upward motion of the link is converted into rotational force, which in turn causes the link on the other end of the bar to rotate in the same direction.
However, as the car rolls, the inside tire wants to drop as the suspension is unloaded - the pressure in the sphere forces the strut downward, and the link arm is pulled downward. This downward action partially counteracts the desire of the outside tire to lift the link arm. The result is a reduction in roll. The battle of the upward vs downward moving suspension is met in the roll bar, which actually twists in response.

So - if I've got it right, that's how a roll bar limits chassis roll.

I stand by my previous statements that "in general" MB's should not experience steering input during chassis movement. The geometry of the front suspension - if in proper nick - guarantees no steering input throughout normal suspension travel.

If the car is wiggling on road dips, one or more of the sphere's is out-of-match with it's across-the-car companion, pressure-wise. If these dips are encountered in a curve, things will get interesting.

I have constructed a device to allow me to test the breakover pressure in the spheres. Total cost was under $100US for everything. Of course, one must remove the sphere to perform the test...

If anyone is interested, I can provide a parts list and method of operation.

Regards

[TJ 450]

That all sounds very logical to me.

If the car is wiggling on road dips, one or more of the sphere's is out-of-match with it's across-the-car companion, pressure-wise. If these dips are encountered in a curve, things will  get interesting.

The above quote explains the issue well.

I remember there was a discrepancy with the dates of the front spheres, them being a good six years apart. I may have to get a new pair if the fluid flush doesn't resolve the issue, I think.

Tim

[koan]

As I'm visualizing the role of the roll bar, as the car heaves to one side, that presses up on the end of the link attached to the bar.
Then the upward motion of the link is converted into rotational force, which in turn causes the link on the other end of the bar to rotate in the same direction.
However, as the car rolls, the inside tire wants to drop as the suspension is unloaded

Good description so far,

- the pressure in the sphere forces the strut downward, and the link arm is pulled downward. This downward action partially counteracts the desire of the outside tire to lift the link arm. The result is a reduction in roll. The battle of the upward vs downward moving suspension is met in the roll bar, which actually twists in response.

I can't follow you here, I think there is an inconsistency in your logic

So - if I've got it right, that's how a roll bar limits chassis roll.

but arrive at the correct conclusion. The Citroen quote gives a good explanation.

Think of what the roll bar does going over speed bumps, forces are equal on each wheel, the roll bar rotates but doesn't transfer any force from side to side - because the forces are equal. Likewise with the struts and spheres, the pressure increase is the same on both sides so there is no transfer of fluid side to side.

Now think of what happens when you hit (or park on for that matter) a speed bump with only one wheel.  One wheel moves up and rotates the roll bar which then compresses the spring on the other side to an extent depending on the stiffness of the roll bar. The suspensions on each wheel are not isolated, they are coupled by the roll bar, the effect is as if the spring on the bumped side was somehow made stiffer. Not only that but the wheel on the non-bumped side moves up as its spring is compressed. But struts and spheres are connected side to side, the bumped wheel increases pressure in its sphere which is transferred to the other side extending that strut and lifting the body. The connected pair of bike pumps is my analogy.

The roll bar and the hydro struts/spheres work against each other.

(I know I've mentioned springs that don't exist, a sphere functions as a spring but would confuse  the explanation)

I agree with your comments regrading asymmetric spheres.

I have constructed a device to allow me to allow me to test the breakover pressure in the spheres. Total cost was under $100US for everything. Of course, one must remove the sphere to perform the test...

Interesting, testing using the MB device is done on the car using supplied pressure. And from my understanding of the process the spheres are still connected together so I'm not too sure how it works.

If anyone is interested, I can provide a parts list and method of operation.

Pictures, parts and a description in the gallery (you can add text to the pics) would make an excellent contribution. This sort of info is often hard to find when just posted to a thread, if you do wish to post to a thread rather than to the galley, start a new topic - with an accurate title!

koan

[wbrian63]

I remember there was a discrepancy with the dates of the front spheres, them being a good six years apart.

I find it very hard to believe that the pressures in spheres manufactured 6 years apart will be anything near similar. I will bet dimes to donuts there's where a lot of your problem is.

[wbrian63]

Interesting, testing using the MB device is done on the car using supplied pressure. And from my understanding of the process the spheres are still connected together so I'm not too sure how it works.

I believe there's a difference between the testing done on the car, which tests the behavior of the system - pump pressure, etc. and testing the individual spheres for their "break-over" pressure.

The device I have uses a hand hydraulic pump like those designed to work with rams and spreaders in auto-body repair. I think it has a 3000psi capacity.

I'll build a picture thread and put out a new topic.

A simple description is as follows:

The system has a hose that's attached to a tee. The connection is at the side of the tee. On the "up" side of the tee is a valve. On top of the valve is a cup I fashioned from a 3" PVC cap. On the bottom of the tee is another tee with a pressure valve in the side, valve, and on the bottom of the tee is another valve with a fitting of the type that attaches to the spheres at the inlet. I use two valves (only one is really required) to keep the fluid spillage/waste to a minimum when switching spheres.

To use the system, you attach the tee/valve assembly to the sphere, and put a plug in the outlet port of the sphere - just loosely insert the plug, don't tighten it yet. Stand the assembly upright and pour a small amount of ATF into the cup (maybe 4oz / 120ml). Open both valves. Open the plug in the sphere outlet enough to allow air to escape - this makes it easier for the ATF to flow into the sphere. When you start seeing fluid come out of the sphere outlet port, close the plug and wrench it tight.

Eventually, bubbles will cease to appear in the cup. You may need to add some more ATF. If you need to add more than 8oz (240ml), the bladder in the sphere has probably ruptured.

Once the bubbles cease, close the valve below the cup.

Close the pressure-release valve on the pump, and start pumping the handle. As you do, you should see the pressure gauge start to rise. It will continue to rise until the pressure in the nitrogen bladder equals the pressure in the ATF and will abruptly stop at that point. There's no missing when this happens. This pressue is the "break-over" pressure. You can add more pressure beyond this point, but that doesn't tell you anything, and it risks rupturing the bladder.

Open the pressure release valve on the pump and allow the pressure to drop to zero.

Close the bottom valve and disconnect the tester from the sphere. There is some fluid spillage, but that can't be helped.

There's a big difference between the desired pressure for a front sphere and a rear sphere. With this rig, you can determine when your front spheres are too tired for their work, but can serve as rear spheres for a while...

As noted, I'll create a new thread with pics and better descriptions. Maybe I can figure out how to add this to the gallery as well.

[TJ 450]

I remember there was a discrepancy with the dates of the front spheres, them being a good six years apart.

I find it very hard to believe that the pressures in spheres manufactured 6 years apart will be anything near similar. I will bet dimes to donuts there's where a lot of your problem is.

Indeed, that's what I first suspected and trying to bounce the front end reveals that it is much firmer than it should be.

I'll be greatly interested in this testing device.

Tim

[koan]

I believe there's a difference between the testing done on the car, which tests the behavior of the system - pump pressure, etc. and testing the individual spheres for their "break-over" pressure.

Pretty sure we're talking about the same thing, my MB suspension book refers to a spheres "opening pressure" which I'm guessing is your "break-over" pressure. The book describes sphere and regulator tests using various bits of plumbing and adaptors from a special kit. Uses the on car pump as a pressure source.

Job 32-520 "Checking pressure reservoir" in the library describes the procedure as well. The section gives the new and OK opening pressures. Most of what's in the suspension book is in the library under jobs 32-5xx and 32-6xx.

I'll build a picture thread and put out a new topic.

A simple description is as follows:

Sounds good. This job is the holy grail of 6.9 suspension work so we're all eager for the pictures.

The gallery is a bit of a challenge at first but once you jump in it makes sense, have a go. I created an album "koan's bits" and within that created further albums, makes finding and managing things a lot easier. There's a "Sidebar" button at the top right, clicking that gets you on the way. I'll help if you encounter difficulties.

koan

[etmerritt33]

This is an interesting topic and I need to go back and carefully read all the posts. I can share my own experience when I first got my 6.9. The car was shipped from Ca. to Va. and looking back I think my struts were damaged by the car being chained down in the closed trailor.

The scary driving characteristic that I was experiencing was on the highway. When going around normal highway sweepers at 60-70 mph the car would begin to teeter on a diagonal and it felt very unnerving and dangerous. I was new to 6.9's and really didn't know the suspension system that well. I was not counting on having to sort it as the car was celebrity owned and well maintained in Ca. I ended up replacing everything except the rear leveling valve. All new spheres, struts, main pressure regulator (rebuilt), new flex hoses, new pressure sensor, new pump, rebuilt front leveling valve, new filters, and fluid. I had the system flushed multiple times as mine as most had not had a filter change even though the fluid looked pretty good when I got the car.

After all of that the car handles better and as it should. But, I think you have to keep in mind that it is a 4400 lb car with pretty small tires and not the original XWX's that came on the cars. I drive the car within the parameters for which it was designed. I don't try to force it out of those parameters. Many will tell you that the design with the nitrogen in the spheres was supposed to stiffen the suspension when it was worked hard by aggressive driving but I have never had any sense that this is the case. I do think it is important to have the suspension height set as per factory specs. That is easily adjustable with the linkage to the leveling valves.

The other factor, to my mind, is the lack of correct tires. There are only a couple tires left with even HR speed ratings and the correct load ratings in 215/70R14. Even those almost certainly do not have the stiff sidewalls of the original XWX's. It is almost sinful that Coker Tire wants $ 500+ ea for these tires today. I am in the middle of doing a euro 6.9 engine transplant project in my U.S. 6.9 and I have a set of 16" Pentas that I am thinking of having restored so I can run Michelin Pilot Sports A/S 225/55ZR16's  in the front and 245/45/16's in the rear. I expect to have at least the factory spec 287 hp and 405 ft-lbs of torque and I want to put that to the ground and not spin the 14" tires as I want the car to haul ass. I have kept the 14's so far as I like the lighter feel but I'm expecting a significant increase in hp and torque and I don't think the 14's are going to be a good choice going forward.

[wbrian63]

Tom - a properly sorted euro 6.9 is no match for modern rubber.

Here's #521 while still owned by the previous owner - 14" standard radials just can't handle the torque:

http://www.youtube.com/watch?v=7otPz_T_ONk

I've got a set of 15" bundts that I'll be restoring and hopefully fitting a little larger and grippier rubber to.