LAD / SLS Suspension Refresh and Rear Strut Seals

[m60power]

This thread will document information related to refresh of the LAD rear suspension in my E32 as well as provide details about the hydraulic seals and o-rings related to the struts. I'd like to try and gather as much info as possible related to seal options for the LAD system as reference for others. My car was off the road for a couple years while I converted it to a manual transmission, and after about ~500 or so miles the rear left LAD strut catastrophically failed dumping all the hydraulic fluid out in less than a minute and I quickly lost power steering. After getting the car home it became clear the fluid was leaking from the bottom of the strut (LAD damper module).

LAD Strut out of the car:

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I ground down a 32mm socket to fit over the large nut cap at the bottom of the strut:

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LAD damper module removed:

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After pulling the damper valve apart it finally became clear to me how this thing actually works. It increases the hydraulic fluid flow resistance (higher strut damping force) with increased hydraulic fluid pressure from the LAD valve. So with more weight in the rear of the car, the LAD system requires more fluid pressure to retain the same ride height, and there is a corresponding increase in damping rate of the rear strut so that the the rear suspension is not underdamped as rear weight increases.

The bottom of that threaded stem is simple rod style valve with hydraulic pressure on one side, atmospheric air on the other side. A dynamic rod/wiper seal keeps the fluid from leaking past the rod and out the bottom of the large 32 mm threaded cap which is open to atmosphere (there is a small plastic dust shield on the bottom of the cap but it does not seal against pressure). The large outer springs of the damper module are sized to resist increased fluid pressure and downward movement of the rod. As fluid pressure increases the rod moves downward and compresses the small inner springs. These small inner springs press against circular plates that close against the orifices of the center plate thing which increases the flow resistance in and out of the strut (increasing damping force).

[m60power]

The valve rod retainer cap at the bottom of the damper module is staked in to the big aluminum nut. It was fairly easy to remove:

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I found that the lower rod/wiper seal is what failed on my strut. Here are some pictures of the seal and aluminum cap housing:

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From what I could tell the seal seems to be made of polyurethane.

[m60power]

I took some measurements of the seal, rod, and features on the aluminum housing, then made a CAD model of the seal showing the key dimensions:

rodseal.JPG

I contacted more than 10 hydraulic and seal companies in the US to try and source a replacement. This proved difficult. Only a few responded and some went radio silent after I provided more performance requirements of the seal, e.g. the application, dimensional info, working fluid, pressure ranges, total travel of the rod, etc. Apparently this style of seal, a hydraulic cylinder dual rod/wiper u-cup seal, is not uncommon however the specific features and dimensions are not common at all. Also, the 1900psi max operating pressure of the LAD system is waaaay above what is typical for a dual rod-wiper seal. Typically at these pressures there is a separate dedicated rod seal to hold the pressure and a second wiper seal to keep debris off the moving rod surface.

One company, Gallagher Fluid Seals Inc. said they could machine a custom polyurethane hydraulic seal for my application. Unfortunately the minimum order was 10 pieces and they were ~$20 a seal. Higher quantity orders would have been much less per unit. I think most of the cost is setup for machining. Here are the new seals:

20201113_190920.jpg

I also had trouble sourcing the correct size o-rings for the bottom of the big aluminum cap. I contacted a bunch of O-ring suppliers and none had an exact size match (some options were very close in dimensions). From more searching I ultimately found an exact fit solution via an AC O-ring from an older model Porsche 911. The dimensions of this O-ring are 23.8mm ID X 2.4mm cross section:

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The O-ring is Porsche part number 99970738240. They are made from Hydrogenated Nitrile Rubber (HNBR), which is better for hydraulic fluid applications than regular Nitrile (NBR, Buna-N) O-rings.

[m60power]

The new rod seal seemed to fit perfectly into the aluminum housing. It fit into place with a satisfying snap and sealed very snugly against the valve rod:

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I staked the metal rod cap back into place with a square driver bit:

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LAD Damper valve reassembled:

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I actually preloaded the valve slightly more than what it was originally set to by tightening the top nut down just a little bit lower than what it was. This will shift the damping force vs. pressure curve slightly higher than original by preloading the inner orifice springs just slightly more for a given fluid pressure. The idea was to increase damping force of the strut a little bit to match the 15% firmer spring rate of the H&R sport springs I installed. This may have been a stupid idea, but I'll find out in time

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.

[m60power]

New rear LAD suspension parts:

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Taking apart the old bits:

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Taking out the old nitrogen accumulators and installing new ones:

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.

[m60power]

Installing new hardware on the LAD struts. I replaced the dust boots, boot retainer rings, spring pads, shock mounts, bump stops, and upper paper gaskets:

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Assembled LAD struts:

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Because the upper strut mount is not a bearing you need to be sure it's clocked correctly relative the the lower strut body before releasing the spring compressors or it will not align correctly in the car. I got the orientation backwards the first time on one side of the car haha. Make sure the spring pads are seated correctly against the ends of the springs too.

Pic from installing an LAD damper module in one of the struts. I had to C-clamp them to boards so they didn't rotate while tightening the module end cap:

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I replaced the rear pitman arms (dogbones) with new Lemforder parts while I was in there:

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Be sure to note the number and location of the washers when replacing the pitman arms; the setup is different between the two sides.

[m60power]

I followed the LAD bleed procedure per the BMW service manual and the Bentley manual. It took quite a lot of fluid to fill the system. The bleed method is to load ~330 pounds of weight into the trunk, make sure the fluid level is correct, start the engine, then turn the steering lock to lock 5 times. Next you jack up the rear end of the car so the wheels are off the ground and the struts are fully extended for 5 minutes or so, then re-check the fluid level. I repeated this procedure 4 times to be sure After bleeding I fully torqued the lower bolts of the struts with the tires on the ground and the rear end loaded with the weight still in the trunk.

I drove the car for about 30 miles and so far there aren't any leaks! I didn't bother to measure the rear ride height until I got back home, and found that the rear is now ~0.75" higher than what it was before (21.30" from bottom of wheel edge to fender lip versus 20.55" per BMW spec. for 15" wheels), which is odd because I didn't do any adjustments to the LAD regulating valve rod that connects to the rear sway bar. I measured the ride height prior to this work and it was at ~20.5". The H&R sport springs I installed are supposed to lower the rear by 1.00" in the rear, but this never made sense to me because the ride height shouldn't change at all unless you adjust it via the regulating valve. No matter what spring you have installed on the car the LAD should try and shoot for the 'zero position' ride height on the valve.

Anyway, I disconnected the rod from the valve arm to re-adjust the ride height (with the required ~440 lbs weight in the rear of the car), and that is where I ran into a problem. Moving the valve to the raise or lower position doesn't change anything. The ride height stays the same regardless of LAD valve actuation. The valve is not leaking externally at all. When I move the arm to the raise position (forward) I can hear a light buzzing sound near the front end of the car, presumably coming from the power steering pump. I'm starting to think I burned out the LAD vane side of the pump when all the Pentosin leaked out after the strut failed. These pumps are a dual vane configuration; one output for LAD, the other output for power steering. The power steering in the car still works perfectly without any weird groaning noises or roughness. The odd thing is the car will not lower at all when the valve is moved to the rear position, so maybe there is an internal failure of the LAD valve?

My other E32 would raise or lower the ride height quite rapidly when moving the valve arm just a little bit fore or aft, and the low and high ride height positions were extreme (like really sagging or really jacked up at each limit respectively). I think the next step is to put a pressure gauge on the output of the pump and see if it is operating within spec. Does anyone have other suggestions to try and troubleshoot this?


EDIT: Note that I did not try and tackle the upper seals and O-rings of the LAD strut rods. Both of my struts did not show any evidence of leaking from the top so I decided not to mess with it. The upper seals apparently start to fail when the nitrogen accumulators have failed and the car is continually driven. When the accumulators fail the ride becomes very harsh in the rear and the hydraulic fluid has nowhere to go but force past the strut seals. My old accumulators had lost pressure compared to the new ones but they still had a tiny bit of movement in the internal diaphragm when sticking a screwdriver into the opening. The new ones were fully pressurized and the diaphragm was right at the top such that I couldn't insert anything far into the accumulator. Shogun has detailed photos on his website of a fully disassembled LAD strut showing the upper seal block of the strut and how to replace those seals.

[Cactuar]

I commend you for documenting this information, m60power. The LAD system is a mysterious black box to me and I never invested the time to understand it fully. I'm glad others are willing to share the knowledge so this system can be preserved.

[whiteghost1]

I commend you for documenting this information, m60power. The LAD system is a mysterious black box to me and I never invested the time to understand it fully. I'm glad others are willing to share the knowledge so this system can be preserved.

+2
Couldn’t have said it better myself.
Thank you m60power!

[robertjlewis1]

An excellent write up and interesting read. Nice to know someone is still delving deep into E32 technology and sharing their knowledge.


Sent from my CLT-L09 using Tapatalk

[rbelton]

Excellent write up and info here, thanks for the effort. I wonder what the H&R spring application is - how do the spring rates compare to the stock springs?

[m60power]

Thanks all! I'm glad the info is helpful!

The H&R springs are application specific to the E32s with self-leveling suspension. Part number for the set is 29656-2. They are the same overall height as the stock LAD spring, but have a different amount of coils, and have a slightly larger diameter and wire diameter.

Stock rear LAD spring:
-Height: 344.5mm
-Outer diameter: 112.50mm
-Wire diameter: 12.55mm
-Number of coils: 9.0 coils (7.0 active coils?)

H&R rear LAD spring:
-Height: 344.5mm
-Outer diameter: 113.50mm
-Wire diameter: 13.25mm
-Number of coils: 9.5 coils (7.25 active coils?)

If you calculate the spring rates between the two, the H&R springs are about ~15% firmer than my stock LAD springs.

EDIT - The H&R rear springs are more like ~18% firmer than stock when recalculating the spring rates with what is probably a better estimation of the 'active' number of coils compared to the last time I estimated this.

[moroza]

This thread is full of win. Thank you for a stellar example of DIY information sharing.

Coil count is critical to spring rate. Can you update your H&R and stock numbers with that information?

A related datapoint to your spring dimensions, copied from a BFc posting somewhere (probably the E32 board): 735i non-SLS rear springs PN 3155351 are 9.5 coils, estimated 8.5 active coils, 12.8mm wire diameter, 112mm outer diameter, 370mm height.

I am tantalizingly close to having figured out an off-the-shelf (and cheaper) solution to the NLA accumulators. Have it on my bench, have yet to install and test. Will hopefully remember to cross-link with this thread once I post results.

UPDATE: Duly cross-linking with this thread. So far, generic accumulators work great.

[rbelton]

Oooo please share the details. There are other nitrogen accumulators for other makes that look very similar to the e32 units for half the cost. I have wondered if they would also work.

[robertjlewis1]

I think (but can't be sure) that I came across a Citroen accumulator that was almost identical when I was looking for a replacement but it was so long ago I could be mistaken.

Sent from my CLT-L09 using Tapatalk

[m60power]

This thread is full of win. Thank you for a stellar example of DIY information sharing.

Coil count is critical to spring rate. Can you update your H&R and stock numbers with that information?

A related datapoint to your spring dimensions, copied from a BFc posting somewhere (probably the E32 board): 735i non-SLS rear springs PN 3155351 are 9.5 coils, estimated 8.5 active coils, 12.8mm wire diameter, 112mm outer diameter, 370mm height.

I am tantalizingly close to having figured out an off-the-shelf (and cheaper) solution to the NLA accumulators. Have it on my bench, have yet to install and test. Will hopefully remember to cross-link with this thread once I post results.

Thanks moroza! I updated my post above with the number of coils and active coils. My judgment of 'active' coils may not be exact, I made a best guess based on what would probably be 'dead' coils when the spring is loaded.

I look forward to what you find out regarding the alternative cheaper solution to the accumulators. Are the stock ones NLA now? I just bought new ones directly from BMW this July; I think they are made by Febi.

[m60power]

I did some more tinkering to at least find out why the car won't lower down any further when the regulating valve is moved to the rear (lower) position. I moved the lever to the lower position with the engine off this time and let it sit overnight without the weight in the trunk. The idea was to relieve the hydraulic fluid pressure and let the fluid cycle back through the return line to the front of the car. The rear didn't seem to lower any more. To rule out the regulating valve I cracked open one of the LAD hydraulic lines rear of the valve to fully relieve the pressure. I also loosened the bottom strut bolts so the strut connection point at the trailing arm could rotate freely. Well it turns out this is as low as the rear will go, the force of the H&R springs are holding it up.

Based on the fact the new H&R springs are the same total length as the stock springs, and their spring rate is higher than stock, it makes sense the car won't go as low as it could with the stock springs. This is quite frustrating because the rear sits too high on the lowest setting. The car has a forward 'rake' to it and I still have the stock front springs in. I measured the height again without the weight in the trunk and the rear is 22.25" (it was ~21.30" at full low position when I had the weight in the trunk, and ~20.5" with weight in the rear on the old springs). For comparison I also measured the rear ride height on my old E32 with the engine running and it sits at 21.0" without weight in the trunk, which is close to what it should be (would probably be close to the specified 20.55" with the 400 lbs in the trunk).

I may need to re-think my plan going forward in regards to the lowering springs. The rear now sits 0.75" - 1.00" higher than stock with these new springs. I think the first step is to contact H&R directly and see if they have any suggestions. Has anyone here installed the H&R sport lowering springs (self-leveling version) on their E32 and had issues with the rear ride height?

Does anyone know if there is an online system for the BMW spring tables? I know there were 'sport', 'm-technic', and 'm-sport' versions of the Euro E32 suspension; I wonder if there were various rear LAD springs available for these cars? I'd like to find out if there is an OEM LAD spring option that is lower and firmer than my stock LAD springs...

[moroza]

Weird about the ride height. For consistency, can you measure 1. the diameter of the rim, and 2. the distance from the bottommost part of the rim to the lip of the fender directly above it? The latter method removes tire variables and is how BMW specifies it. For 15" Basketweaves, expected "rim gap" is in the 500-540mm range.

Based on your numbers, expected ride height with these springs should indeed be roughly 20mm higher.

Perhaps you could change the lower spring perch and upper mount to use E34 springs?

There were multiple SLS options for the E34, so I imagine also for the E32. I've found online catalogs that list spring part numbers based on options, but they're all in Russian. If you don't read Russian or have the patience to translate pages, post or PM me your VIN and I'll dig up that info.

Stock accumulators are still in production by Febi, but a recent change to maritime shipping law (I'm told) classifies charged accumulators as explosive devices, requiring impractical amounts of paperwork to ship. Aftermarket Febi are all dried up in the US AFAIK, and I've been unable to get a European seller to ship any to me, but it appears that OE BMW are still available in the US. My solution involves US-made accumulators and adapting hardware.

I looked into Mercedes accumulators, and found that their pipes are smaller - M12 vs M14. They might work anyway, but I decided not to chance that.

[m60power]

The measurements I posted are from the very bottom lip of the 15" rim to the bottom edge of the fender. My old e32 has 16" wheels so I subtracted a 1/2" from the actual measurement on that car for comparison. The measured lip to lip diameter of the 15" basketweave wheels is 16.3", and the 16" wheels are 17.3".

[moroza]

Oh, so those are already correct measurements. Looks like pretty much the same specification as E34. 520ish loaded and 530-some unloaded is about right, but 565mm unloaded? That's enormous. Something's wrong.

[m60power]

Did more messing around this evening with the LAD. The regulating valve definitely works, but unfortunately my pump may be shot. I don't have a pressure gauge or the right fittings yet so TBD on actual pressure numbers.

I still have the sway bar linkage disconnected from the LAD valve actuator arm so I tried adjusting it this time without any weight in the car (engine running at idle). When moving the arm forward (raise) position the car does raise, but not willingly haha. It raises very very slowly accompanied by the buzzing sound, which is definitely coming from the pump. After about 45 seconds it reaches what I assume is the highest point it can get to. The car lowers down rapidly when moving the actuator to the rear, and the height stays locked in place with the arm in the zero position at any ride height. I moved it up and down a few times and it works as it should. I'm guessing the LAD side of the pump just isn't making enough pressure to raise the car with the car loaded up. The full lower position is still too high - at 22.3" from the bottom lip of the wheel to the bottom edge of the fender (engine running, no weight in the car).

I spoke to H&R today and described the issue in detail. They confirmed I have the correct kit part # for the 740iL with self-leveling suspension. I'm going to be sending them pictures and they said they'll have their technical team try and figure out what's going on.

[rbelton]

Interesting info on the accumulators and shipping laws. Hopefully they are available again soon.

Here's the rebuild kit for the power steering pump: https://www.fcpeuro.com/products/bmw...it-32411134836

BMW number 32411134836

[moroza]

It increases the hydraulic fluid flow resistance (higher strut damping force) with increased hydraulic fluid pressure from the LAD valve. So with more weight in the rear of the car, the LAD system requires more fluid pressure to retain the same ride height, and there is a corresponding increase in damping rate of the rear strut so that the the rear suspension is not underdamped as rear weight increases.

Does this mean that damping rate depends on what springs are used, and what the ride height is set to per a given spring? Suppose one car, one set of hardware, only the springs vary such that A sits at 540mm without cargo, B at 480mm.
1. Car has springs A and no cargo. If you set ride height to 540mm or less, and drive without cargo, then no pressure goes to the struts and damping is at a minimum.
2. Same thing except now there's a quarter ton of beluga whale dung in the trunk. Ride height is still 540mm but there's significant damping.
3. Car now has springs B and no cargo. If you set ride height to 480mm (very low), damping is the same minimum as in 1.
4. But if you set ride height with B springs to 540mm, you have a lot more damping than in 1, even without cargo. Add the whale dung, and you're still at 540mm, but with more damping than in 1 or 2.

The monster grows even more heads when you take into account that ride height is a function of both spring rate and free length. Let's introduce spring C which is way softer than A, but very tall, so that it also sits at 540mm without cargo but sags much more with any load increase. Driving around without cargo, it's all soft and floaty, but once the whale has relieved itself in your trunk, you've got the same soft spring rate and even more damping.

The concrete scenario this has implications for is mine: my ride is at 540mm with zero pressure, and I want it at 515. If I install springs that sit at 515, great, but if I install springs that sit at 480 - rough estimate based on photos of E34 Tourings with leaky hydraulics - and pump up to 515, my damping will be stiffer. And furthermore, it matters whether the lowering is due to a softer rate or lower length. If a softer spring rate of the coils, then adding cargo will increase damping rate by a greater amount than the same cargo with firmer coils. I can imagine how this could lead to oversprung/underdamped vs. undersprung/overdamped ride.

Or is there yet another head to be grown, that increasing pressure not only increases damping rate, via the module on the strut, but spring rate, via the compression of the accumulator gas? This could mean that softer coils might result in a higher overall rate once hydraulics are taken into account.

headache++;

[m60power]

Does this mean that damping rate depends on what springs are used, and what the ride height is set to per a given spring?

Yeah that is the case. I pondered this exact thing when putting in the H&R springs, because they are the same height as stock but they are firmer than stock, so less hydraulic pressure would be needed to hold my car at the same ride height and therefore less damping than with stock springs.

Suppose one car, one set of hardware, only the springs vary such that A sits at 540mm without cargo, B at 480mm.

For this comparison, I'm assuming these are the hypothetical heights the car sits at riding only on the springs A or B (SLS on the minimum pressure at full lower on the valve)?

1. Car has springs A and no cargo. If you set ride height to 540mm or less, and drive without cargo, then no pressure goes to the struts and damping is at a minimum.

Correct. (well, there is a minimum pressure that the system always delivers to the struts, it's not 0 psi)

2. Same thing except now there's a quarter ton of beluga whale dung in the trunk. Ride height is still 540mm but there's significant damping.

HAHAHA! But yes, that would be the case.

3. Car now has springs B and no cargo. If you set ride height to 480mm (very low), damping is the same minimum as in 1.

Correct.

4. But if you set ride height with B springs to 540mm, you have a lot more damping than in 1, even without cargo. Add the whale dung, and you're still at 540mm, but with more damping than in 1 or 2.

Correct on the first point, and correct on the second point assuming you aren't reaching the maximum pressure capability of the LAD pump when using springs 'B' and all the whale poop in the trunk (the damping will max out if the system needs the full 130 bar pressure to raise the car to the set ride height).

The monster grows even more heads when you take into account that ride height is a function of both spring rate and free length. Let's introduce spring C which is way softer than A, but very tall, so that it also sits at 540mm without cargo but sags much more with any load increase. Driving around without cargo, it's all soft and floaty, but once the whale has relieved itself in your trunk, you've got the same soft spring rate and even more damping.

Correct. There are many variables in this to get the damping rate and spring rate you want for a given nominal ride height you choose.

The concrete scenario this has implications for is mine: my ride is at 540mm with zero pressure, and I want it at 515. If I install springs that sit at 515, great, but if I install springs that sit at 480 - rough estimate based on photos of E34 Tourings with leaky hydraulics - and pump up to 515, my damping will be stiffer. And furthermore, it matters whether the lowering is due to a softer rate or lower length. If a softer spring rate of the coils, then adding cargo will increase damping rate by a greater amount than the same cargo with firmer coils. I can imagine how this could lead to oversprung/underdamped vs. undersprung/overdamped ride.

Based on the OEM design, I would assume if you want the ride height to be 515mm, you'll want a spring that is short enough, or soft enough (or a combination of both) to have the rear sit significantly lower than 515mm when there is no (or minimum) hydraulic pressure assisting the springs. You are correct, it does matter for the specific damping and spring rate performance whether the lowering is due to softer spring rate or shorter overall length. With my E32 740iL as an example, I want to lower the stock height rear between 0.75" - 1.00" AND have slightly firmer overall spring rate, which means I'll need a rear spring that is short enough to have the rear well below my desired ride height (when there is minimum hydraulic pressure), then find a better way to increase the fluid flow resistance in the LAD strut module to shift the overall damping curve up for a given hydraulic pressure since the spring is firmer. You can see in my above post I preloaded the LAD valve module before installing them back in the struts, but in hindsight this isn't the correct way to do it. I think it only increases damping at minimum pressure because it is also preloading the outer compression springs in the module in addition to the inner orifice plate springs. The correct thing to do would be inserting more washer spacers at the ends of the small inner orifice plate springs so only those springs are preloaded more.

Or is there yet another head to be grown, that increasing pressure not only increases damping rate, via the module on the strut, but spring rate, via the compression of the accumulator gas? This could mean that softer coils might result in a higher overall rate once hydraulics are taken into account.

headache++;

Correct, increased required pressure to maintain a given ride height would mean more overall spring rate for the rear. As the pressure increases the gas in the accumulator is compressed more by the accumulator diaphragm, so there is less gas volume left in the reservoir. So for a given amount of compression stroke distance, the remaining gas volume in the accumulator would decrease faster (as an overall percentage) than if the system was at minimum pressure and there was a lot more volume to compress for a given stroke distance. Remember the ideal gas law, PV=nRT and Boyle's law. Although I imagine BMW sized the accumulator for the full range of pressure of the LAD system, so that at the full 130 Bar the spring rate isn't too high. I guess it's possible that super soft spring coils such that the ride height is maintained by the maximum hydraulic pressure of the system could lead to a higher overall spring rate than a firm spring mostly supporting the rear and low hydraulic pressure. I guess it really depends on accumulator size?

All that being said, I have not gone deep enough into this to calculate the ideal combo of spring dimensions/rate for the existing constraints of the LAD system pressure range and the strut damping module (and accumulator specs).

EDIT: Thinking about this some more - I believe the overall takeaway is if you want to lower the car to a given ride height, a properly sized shorter spring of the same spring rate as stock springs would keep all performance parameters of the SLS system pretty much the same as designed by BMW. The question becomes, how to increase/decrease damping rate and spring rate of the rear at an appropriate ratio to each other? Assuming the accumulators stay the same, would it be correct to use a softer spring than stock to achieve a higher hydraulic pressure for that lowered ride height (which will increase damping, but who knows what happens to OVERALL spring rate), or would it be better to use a shorter and firmer spring in combination with modifying the LAD strut damping module?

[moroza]

So many variables to wrap my head around... I'm working on it.

Do we know the minimum pressure of the system?

Do you have any measurements of ride height with SLS depressurized?