LAD / SLS Suspension Refresh and Rear Strut Seals

[moroza]

A 2000psi pressure gauge arrived in my mail recently, will soon get around to fitting it to the circuit and getting a reading. Meanwhile I reset my ride height following a diff reseal; it ended up at ~535-540mm (compared to 523 at minimum pressure, 528 where it previously was set) and IMO this is about the perfect amount of damping, maybe a hair too low still.

I'm looking further into both lowering the rear while keeping adequate damping, and resealing as a preventative measure. One interesting piece of information from one of shogun's links

My car is a 1991 E32 730i (remark by Sean750: in some countries LAD was an expensive factory option and could be ordered on a standard i model. Not in the US, where only iL models are equipped with LAD). I firstly rebuilt the spare struts I had found and once I had exchanged struts I rebuilt the original units. I found a number of differences, some incidental and some very pertinent the rebuilding process.
I do not know exactly what model my spare set of struts came from. The spare strut cylinders were longer as were the lower spring retainers ( by about 10mm).

Now I want to track down exactly which struts had these longer spring retainers, and see if they can be swapped onto the shorter struts to lower another 10mm.

As a side note, the spring shop that compressed and heat-treated my springs, twice with perfect results, advised me that they can make custom coils, to the tune of $360 for a pair.

[m60power]

That is interesting info, I would love to find LAD strut lower spring perches that are longer. That would be a great way to lower the rear without using shorter springs, assuming the lower spring hats are universal fit between the various LAD struts.

I had posted a thread some time ago asking about the physical and performance differences in e32-specific LAD struts; here's some info from my post about the various strut part numbers:

there are a few versions of the rear LAD struts (at least for later model year e32's)

37121137671, 37121137672 - What's on my US 740il, and post 9/91 735's, 740's, and 750's, Euro and US.

37121138921, 37121138922 - Euro E32's with low-slung 'Sporty Suspension', 735, and 730 / 740

37121138929, 37121138930 - Euro E32's with low-slung 'Sport Suspension', only 750's

37122226393, 37122226394 - E32's with 'M-Sport Chassis', 735's and 750's, Euro and US

Does anyone know what the deal is with these various LAD struts? Are there actually any functional / performance differences between them all, or is it just differences in the upper and lower mounting hardware or main piston length? Maybe firmer dampening in the LAD Module on the struts? Also, I don't think I knew there was an M-sport package for the E32, I've heard of the 'low-slung m-tech' suspension bits, but it looks like there are actually different part numbers for whatever the 'M-sport' chassis thing is.

And this info too: I've found that the E32 models with the 'M-Sport Chassis' rear LAD struts (listed above in my first post) use rear springs that are the same part number as the E34 M5 self-levelling rear springs. This configuration on E32s also specifies the shorter bump stops and dust bellows of the E34 M5, as well as the E34 M5 rear spring pads and strut mounts. All of these ancillary parts are different part numbers than those same parts on my US E32 740iL. Maybe because E34 and E32 rear LAD springs are different overall diameter? I was hoping I could use these E32 M-Sport rear springs on my car, but based on the fact all the other components are different part numbers, they probably wouldn't fit properly to the rest of my standard 740iL rear strut assembly parts...

I haven't messed around yet with adjusting the rear ride height to find the optimal damping on my car. I wanted to first get a baseline feel with the new springs I installed with the module unmodified from stock then go from there. I really want to try hooking up a pressure gauge rearward of the valve and set the ride height (and therefore pressure) until I feel the damping is ideal, like you suggested. From there it should be easy to modify the rebound and compression springs preload of the module at minimum pressure to mimic that damping rate from the measured pressure where it was 'optimal'. I'm interested to know what pressure your struts are at with your preferred damping (at the 540mm ride height)

I want to lower the rear to ~510mm with slightly firmer spring rate and damping. I wonder if the best path to that result is compressing my stock rear springs at that shop you've been using. I never really thought about custom springs, but that may be the best option at this point to get exactly what I want. $360 for a pair seems pretty damn reasonable!

[m60power]

Ok, so as a fun exercise - if I pursue the path of having custom rear springs I made, here's a hypothetical specification for them using Silicon-Chromium steel spring wire (and limited by typically available standard spring wire diameters). The goal is to increase rear spring rate by 15-20% and lower the rear by maximum of 0.75" from factory spec. in the unpressurized state, with the intent to increase nominal LAD pressure to achieve about 0.5" ride height reduction overall:

Custom rear LAD springs

• Spring overall length = 310.0mm
• Spring OD = 112.5mm
• Spring wire diameter = 12.70mm
• Number of coils = 8.5
• Number of active coils, loaded = 5.8

A 310mm overall spring length may be encroaching on the limits of having the spring 'loose' with the strut assembly fully extended, so I'll need to be careful about measuring the strut rod extension limit and stack-up with strut mount / spring pads / washer discs, etc. The new 750iL sport springs that H&R sent me didn't need much compressing at all to install the top hats and strut rod nut. I suppose at some point rather than shortening the spring, I would need physically shorter LAD struts to lower the car more....

And for reference, here are the (slightly updated) specs of the other springs I have, after re-measuring them, and the bold text is even newer edits after 6/24/22 post:

OE US 740iL LAD springs (pn '33531134552')

• Spring overall length = 344.5mm
• Spring OD = 112.5mm
• Spring wire diameter = 12.60mm
• Number of coils = 8.9
• Number of active coils, loaded = 6.9 -6.5-
• Spring rate = 205.55 lbf/in
• Ride height, 15" wheel, min pressure = 531mm

H&R 740iL LAD sport springs, from Kit '29656-2' (Rear pn 'T/W 7 87/2 HA')

• Spring overall length = 344.5mm
• Spring OD = 113.5mm
• Spring wire diameter = 13.22mm
• Number of coils = 9.5
• Number of active coils, loaded = 7.1 -6.7-
• Spring rate = 239.33 lbf/in
• Ride height, 15" wheel, min pressure = 567mm

H&R 750iL LAD sport springs, from Kit 29666, (Rear pn 'C/I 29666 HA (R)')

• Spring overall length = 322.0mm
• Spring OD = 114.0mm
• Spring wire diameter = 13.20mm
• Number of coils = 8.75
• Number of active coils, loaded = 6.5 -6.25-
• Spring rate = 255.85 lbf/in
• Ride height, 15" wheel, min pressure = 551mm

Maybe someone could sanity check my math to arrive at my custom spring dimensions for lowering the desired ~0.75". I'm assuming the rear sprung corner weight is about 930lbs (with full tank, no cargo), but I would definitely need to weigh the car beforehand to verify that assumption.

Also, without rear strut LAD fluid pressure values yet, the difference between final desired ride height and unpressurized ride height of the rear spring is a currently a guess regarding the additional fluid pressure for achieving what would be my desired strut damping at the desired ride height (hopefully that makes sense)

[moroza]

Ok, so as a fun exercise - if I pursue the path of having custom rear springs I made, here's a hypothetical specification for them using Silicon-Chromium steel spring wire (and limited by typically available standard spring wire diameters). The goal is to increase rear spring rate by 15-20% and lower the rear by maximum of 0.75" from factory spec. in the unpressurized state, with the intent to increase nominal LAD pressure to achieve about 0.5" ride height reduction overall:

Custom rear LAD springs

• Spring overall length = 310.0mm
• Spring OD = 112.5mm
• Spring wire diameter = 12.70mm
• Number of coils = 8.5
• Number of active coils, loaded = 5.8

Two things don't look right to me. First, your active coil count seems too low. For most E32/34 rear springs that I've measured, the numbers square at about 1.0 dead coils, up to a maximum of 1.15. Your 2.7 dead coils seems too high... however, this depends on the spring pads and the angles of the end coils. Accepting 5.8 live coils, the resulting rate is about 43 N/mm (245 lb/in), coincidentally about 43% higher than stock at ~30 N/mm (171 lb/in). Resulting ride height assuming the same weight as my E34T comes to 533mm depressurized, 545 at minimum pressure.

Second, even with 1.0 dead coils, you end up with a rate of 33 N/mm and rim gap of 525mm at minimum pressure, 15mm higher than target.

A 310mm overall spring length may be encroaching on the limits of having the spring 'loose' with the strut assembly fully extended, so I'll need to be careful about measuring the strut rod extension limit and stack-up with strut mount / spring pads / washer discs, etc. The new 750iL sport springs that H&R sent me didn't need much compressing at all to install the top hats and strut rod nut. I suppose at some point rather than shortening the spring, I would need physically shorter LAD struts to lower the car more....

My modified 735 coils are 314mm long and still needed a spring compressor.

And for reference, here are the (slightly updated) specs of the other springs I have, after re-measuring them:

OE US 740iL LAD springs (pn '33531134552')

• Spring overall length = 344.5mm
• Spring OD = 112.5mm
• Spring wire diameter = 12.60mm
• Number of coils = 8.9
• Number of active coils, loaded = 6.5

H&R 740iL LAD sport springs, from Kit '29656-2' (Rear pn 'T/W 7 87/2 HA')

• Spring overall length = 344.5mm
• Spring OD = 113.5mm
• Spring wire diameter = 13.22mm
• Number of coils = 9.5
• Number of active coils, loaded = 6.7

H&R 750iL LAD sport springs, from Kit 29666, (Rear pn 'C/I 29666 HA (R)')

• Spring overall length = 322.0mm
• Spring OD = 114.0mm
• Spring wire diameter = 13.20mm
• Number of coils = 8.75
• Number of active coils, loaded = 6.25

Again, I think your live coil count is underestimated. Given your numbers apart from that one, and assuming congruence with the E34T, rim gaps for the above springs compute to 522/553, 537/564, and 522/547 (zero pressure/minimum pressure). How do those square with reality?

Maybe someone could sanity check my math to arrive at my custom spring dimensions for lowering the desired ~0.75". I'm assuming the rear sprung corner weight is about 930lbs (with full tank, no cargo), but I would definitely need to weigh the car beforehand to verify that assumption.

Yes, you should verify that weight, even if it can be derived from other measurements. In my chart, I took the DOT-certified rear axle weight of my car, divided by two, subtracted known and estimated weights of wheels, brakes, hubs, half of the control arms, and half of the axleshafts, and arrived at 381.8kg per corner amounting to a spring load of 3744N. That's 840lb, which is pretty close to your 930 after unsprung weight is taken into account.

Also, without rear strut LAD fluid pressure values yet, the difference between final desired ride height and unpressurized ride height of the rear spring is a currently a guess regarding the additional fluid pressure for achieving what would be my desired strut damping at the desired ride height (hopefully that makes sense)

Well, we have -some- numbers...

On my car, the ride height gain from the system at minimum pressure, across three or four datapoints, extrapolates to about 900N exerted by the struts. This is consistent with a 22mm strut piston diameter and minimum pressure of 25 bar. Note that pressures over the static charge in the accumulators (23 bar) compress the accumulated gas volume and raise spring rate. At 25 bar, 30N/mm rear spring rate becomes about 31.5. Derived pressure at the current point of near-perfect damping comes to 1.6 times minimum.

The 22mm piston diameter was derived from other numbers, and seems plausible. Can anyone who's taken one apart verify?

Cutting coils is normally a bad idea, but the numbers suggest that cutting 0.5 coils off my compressed springs will raise their rate to 32.6 and reduce their free length to about 297, resulting in an unpressurized rim gap of 482 and a pressurized one of 511mm.

Send me your email and I'll send you my springs spreadsheet with built-in calculators. I'm unable to upload it as an attachment, even embedded in a DOC or ZIP.

[Johnnygees]

m60power, I have just joined this forum and I have a question. Do you still have two of the lower seals in the 32mm nut that you would be willing to part with, or alternately, a vendor to provide those? I am in the process of rebuilding my rear struts and one is really trashed and the other is marginal. I appreciate any assistance.

[Johnnygees]

m60power, Is there any chance that you still have two of those seals that you had made available. I can get in touch with Gallagher Fluid, but I don't know what name you purchased them in. I will gladly par you for the seals. Do you still have the CAD drawing, and could I get a copy of the CAD file? I would rather just pay you for the seals, but I know that it is a lot of hassle for you to order more just to sell them to me. I am rebuilding the rear struts for my 740iL. My email is jgeus@comcast.net Thanks a bunch.

[m60power]

Thank you for the input on this!

Two things don't look right to me. First, your active coil count seems too low. For most E32/34 rear springs that I've measured, the numbers square at about 1.0 dead coils, up to a maximum of 1.15. Your 2.7 dead coils seems too high... however, this depends on the spring pads and the angles of the end coils. Accepting 5.8 live coils, the resulting rate is about 43 N/mm (245 lb/in), coincidentally about 43% higher than stock at ~30 N/mm (171 lb/in). Resulting ride height assuming the same weight as my E34T comes to 533mm depressurized, 545 at minimum pressure.

Second, even with 1.0 dead coils, you end up with a rate of 33 N/mm and rim gap of 525mm at minimum pressure, 15mm higher than target.

~245 lb/in was what I was shooting for compared to the two other H&R springs (with my original incorrect assumptions of live coil count). Yeah, 525mm at min pressure is still too high, although weights and strut / body dimensions may not be 1 for 1 to the e34 touring.

My active coil count may likely be incorrectly estimated, it's the one thing I'm not confident in consistently determining between the different springs. However, the rear springs definitely have their end coils gradually plane-off to a much flatter angle at the ends, and with the design of the upper and lower spring pads it seems that they would have 2.0 (total) dead coils at a minimum (assuming the springs are preloaded a little bit). That being said, I think I'm still slightly underestimating the live coil counts. I grabbed a pic of the 750iL H&R springs with no cargo load, and you can see at least 1 full coil is 'dead' on top and bottom, the gradient after that is sort of a gray area to me. It seems these springs are somewhat progressive as the live coil count would easily decrease under any compression.

20220624_145409.jpg

My modified 735 coils are 314mm long and still needed a spring compressor.

That is good to know!

Again, I think your live coil count is underestimated. Given your numbers apart from that one, and assuming congruence with the E34T, rim gaps for the above springs compute to 522/553, 537/564, and 522/547 (zero pressure/minimum pressure). How do those square with reality?

Yes, you should verify that weight, even if it can be derived from other measurements. In my chart, I took the DOT-certified rear axle weight of my car, divided by two, subtracted known and estimated weights of wheels, brakes, hubs, half of the control arms, and half of the axleshafts, and arrived at 381.8kg per corner amounting to a spring load of 3744N. That's 840lb, which is pretty close to your 930 after unsprung weight is taken into account.

I'm going to edit my above post with new estimated live coil counts, real ride height measurements, and I'll add my new estimated spring rates. I agree on the weight thing; and the unsprung hardware weights will have to be a best-effort estimate.

Well, we have -some- numbers...

On my car, the ride height gain from the system at minimum pressure, across three or four datapoints, extrapolates to about 900N exerted by the struts. This is consistent with a 22mm strut piston diameter and minimum pressure of 25 bar. Note that pressures over the static charge in the accumulators (23 bar) compress the accumulated gas volume and raise spring rate. At 25 bar, 30N/mm rear spring rate becomes about 31.5. Derived pressure at the current point of near-perfect damping comes to 1.6 times minimum.

The 22mm piston diameter was derived from other numbers, and seems plausible. Can anyone who's taken one apart verify?

Cutting coils is normally a bad idea, but the numbers suggest that cutting 0.5 coils off my compressed springs will raise their rate to 32.6 and reduce their free length to about 297, resulting in an unpressurized rim gap of 482 and a pressurized one of 511mm.

Send me your email and I'll send you my springs spreadsheet with built-in calculators. I'm unable to upload it as an attachment, even embedded in a DOC or ZIP.

This is great info. Also, I'll PM you my email for the spring spreadsheet.

Also, I haven't taken pressure measurements at the rear yet, but I do have the LAD valve arm disconnected and I've been driving around trying different ride heights (fluid pressure, and therefore strut damping) to see what 'feels' best to me. I can definitely say I much prefer higher damping than what the damping is for the minimum pressure ride height with these 750 H&R springs, but damping with the rear raised to the max feels a little too stiff over the bumps. As for spring rate - I think the first set of H&R springs were close to perfect for the rear end (but obviously not the height aspect), I wouldn't want them any firmer. And I would also prefer that these new 750iL H&R springs were just a touch less stiff, but otherwise I really love the cornering feel / stiffness of the rear end; it's not too harsh to ruin the ride comfort on bumps.

EDIT: I've updated my previous post with new live coil counts and actual ride heights at minimum pressure for each spring. I'm still not 100% sure I'm estimating the total dead coils correctly though. Maybe I should be using the live coil counts with the spring completely unloaded in the spring pads? Also - something must be wrong about my spring measurements or rear weight estimate because the ride heights of the stock springs and the 2nd H&R springs (750iL) should be almost identical at min pressure, but real ride height measurement shows the stock springs sit much lower... I suppose it's possible that 29 years of metal creep (spring sag) could account for some of the difference, but that would mean the stock springs would have put the rear end a lot higher than the 522mm factory spec when they were brand new.

[m60power]

m60power, I have just joined this forum and I have a question. Do you still have two of the lower seals in the 32mm nut that you would be willing to part with, or alternately, a vendor to provide those? I am in the process of rebuilding my rear struts and one is really trashed and the other is marginal. I appreciate any assistance.

m60power, Is there any chance that you still have two of those seals that you had made available. I can get in touch with Gallagher Fluid, but I don't know what name you purchased them in. I will gladly par you for the seals. Do you still have the CAD drawing, and could I get a copy of the CAD file? I would rather just pay you for the seals, but I know that it is a lot of hassle for you to order more just to sell them to me. I am rebuilding the rear struts for my 740iL. My email is -deleted- Thanks a bunch.

Short answer is no, I don't currently have extras of those seals for sale.

Yes the vendor that I had machine them was Gallagher Fluid Seals. I recently had them quote another run of 16 seals, give me some time and I'll dig up the PO information / specs and I can forward you the details and your email address to their sales rep. so they know exactly what you're trying to order if you want to purchase them directly from Gallagher.

Also, I've been doing a bunch more driving lately on these struts and I'm happy to report the Gallagher seals in both struts have still been holding up perfectly without any leakage!

[Johnnygees]

I am in the process of rebuilding the rear struts in my E32 740iL. I am in need of two lower seals in the 32mm nut. I see where you have procured them in the past. Is it possible to purchase four of the seals?? I am brand new to this forum and don't know if this is the proper way to ask or contact a member. I appreciate any help you can give me. I see that Gallagher is still on the web, but I wouldn't know how to order from them.
Thanks
John Geus

[shogun]

Johnnygees : I removed your email address from above post, otherwise you will get too much scam and spam email, internet spiders are crawling the net for such info. You will soon be able to send PM = private messages after you got 5 posts, such details send by PM to m60power

[Johnnygees]

m60power, I did not realize that the latest answers went at the bottom of the thread. I apologize for asking so many times. Any way that works for you to get me four of the seals would be great. It would be great if you could forward the details and my email to the sales rep. Whatever there minimum is acceptable to me. Thanks a lot.
John Geus

- - - Updated - - -

Thanks Shogun, I have never been a member of a forum like this, and don't know about those things. Again, Thanks
Johnnygees

[m60power]

m60power, I did not realize that the latest answers went at the bottom of the thread. I apologize for asking so many times. Any way that works for you to get me four of the seals would be great. It would be great if you could forward the details and my email to the sales rep. Whatever there minimum is acceptable to me. Thanks a lot.
John Geus

- - - Updated - - -

Thanks Shogun, I have never been a member of a forum like this, and don't know about those things. Again, Thanks
Johnnygees

When you have enough posts, send me a PM with your email address. I didn't get a chance to write it down beforehand; Shogun has a good point to not publicly post your email. Just an FYI, the seals are pretty expensive from Gallagher (about $22 a piece); they get much cheaper per unit with higher quantity orders, but if I remember correctly their minimum production is 10 per lot, so it will be at least $220. Not exactly cost-effective but they work great, and other members will likely want some too, so you could sell your extras to recuperate some of the cost.

EDIT: Crap, I just realized my quote of your first post has your email in it, I'll edit my post to delete it there too.

[Johnnygees]

m60power. I would split the cost of your entire run of seals 50/50 and maybe pay extra to expedite them to get these struts back together. Yes, the extras would be able to be advertised and sold, but I am only interested in four (two to use and two to keep in case there are other problems).

[m60power]

m60power. I would split the cost of your entire run of seals 50/50 and maybe pay extra to expedite them to get these struts back together. Yes, the extras would be able to be advertised and sold, but I am only interested in four (two to use and two to keep in case there are other problems).

Johnnygees, I'm not currently planning on ordering another production run of the seals (I don't need more seals at the moment). The reason I had asked them for another quote a couple months ago was because another forum member was also potentially interested in having some more produced by Gallagher, but that fell through; he decided to hold off on ordering them.

[moroza]

FWIW, I'd buy another four. Johnny, I'm somewhat local to you, as are a bunch of other E34 Touring owners.

Dead coil count was the variable derived from others, and varies with both ride height (yes, making these springs progressive) and spring brand/model. Two interesting outliers in this family of springs are E34 Eibach...


(Yes, it says it's a stock photo, but I can confirm they actually look like that)

...and E34 M5T with SLS (second from left; note the tapered wire)

[Sansouci]

Sign me up for a pair of seals. My SLS was converted to shocks/springs but if I can find a set of repairable units, I am going to do it.

Sent from my SM-G955U using Tapatalk

[Johnnygees]

I sent you a PM
Johnnygees

[moroza]

I finally got around to overhauling my damper modules, and running into an obstacle of confusion. First, some numbers:

Upper inner spring (compression): 22.6 free length, 8.5 coil OD, 1.53 wire OD, ~9.4 coils, ~8.3 active. Rate 18.8N/mm or 107lb/in.

Lower inner spring (rebound): 12.6 free, 8.1 coil, 1.24 wire, 5.5 coils, ~4.5 active. Rate 15.7N/mm or 90lb/in.

Outer springs (identical): 21.6 free, 23.7 coil, 4.41 wire, ~3.4 coils, ~2.4 active. Rate 211.3N/mm individually, 105.65N/mm together.

Stock nut preload compresses the assembly, measured from the lowermost and uppermost edges of the outer springs, from 56.0 to 53.8, of which 12.0 is the center disk. In this state, the inner compression spring is loose, rebound spring slightly preloaded. Rebound spring base is 9.65mm above lower outer spring base.

Valve shaft diameter 5.0, area 19.63mm^2. At 25 bar = 2500000N/m2, the force on that shaft and everything attached to it is 49.1N. At 130 bar, that force is 255.3N.

The nut preloads the assembly by 2.2mm, which with stacked outer springs and ignoring the inner springs, amounts to a preload of 232.43N.

That can't be right; it implies there's no additional loading of the inner springs until system pressure is nearly at maximum, which of course is not the case. Can anyone point out what the error is here?

(As an aside, I solved my ride height problems by cutting the lower spring perches and welding in a length of steel pipe, such that the lower spring seat is 20mm lower than before. There remains plenty of clearance with 225/50R17 on Style 21 and even more with 215/60R16 on E38 Style 5. However, that's the case at normal ride height. Towards the end of this job, since I've got the system depressurized, I will compress the rear corners with assembled struts, but no springs or hydraulic pressure, to make sure clearance remains if the suspension is held up by the coils alone. Bump stops and spacers will be added as needed.)

[m60power]

Let me digest this some more and I'll try and reply with more thoughts, but for now the first thing that sticks out is the valve shaft diameter - 5.0mm seems too small, mine is 12mm at the seal (where the fluid pressure will ultimately act on that area to compress the spring stack). Also, if I remember correctly (I'll need to look at my notes to confirm), my LAD module compression spring was the one slightly preloaded at the 'stock' nut position, and the rebound spring was just ever so slightly loose.

[moroza]

Aha! I hadn't taken apart the shaft seal yet. Sure enough, 12.0mm.

I understand the upper inner spring holds the dampener plates against compression, and the lower inner is for rebound, yes?

I've got one of the modules apart, took measurements, crunched some numbers, arrived at an answer for how much to shim the inner springs. Now waiting to hear back from Pentosin regarding any chemical incompatibility with zinc electroplating, since that's what I have laying around in the correct size.

The valve diameter is indeed 12.0mm, which at 25 bar amounts to 282.7N force. At 130 bar, it's 1470N.

Testdriving suggested that optimum damping was with a rear rim gap of about 545mm, which computed (I'll post the details on request) to a system pressure of about 1.6 times minimum, or 40 bar.

The problem is best understood as stepwise linear functions, whose slope is the spring rate, which varies discretely as loose springs become engaged.

Upper and lower outer rates 211.3N/mm. Upper inner rate 18.8, lower inner 15.7.

Upper outer compression (additional / total)

Upper outer load (additional / total)

Upper inner compression (additional / total)

Upper inner load (additional / total)

Upper combined rate

Upper combined load (additional / total)

Lower outer compression (additional / total)

Lower outer load (additional / total)

Lower inner compression (additional / total)

Lower inner load (additional / total)

Lower combined rate

Lower combined load (additional / total)

Whole stack rate

Whole stack load (additional / total)

Whole stack compression (additional / total)

Stock, before nut preload

0

0

-1.0 (loose)

0

211.3

0

0

0

0

0

211.3+15.7= 227

0

(211.3)(227)/(211.3+227)= 109.4

0

0

Stock, preloaded 1.3mm

0.67 / 0.67

142.2 / 142.2

0.67 / -0.33 (still loose)

0

211.3

142.2 /142.2

0.63 / 0.63

211.3*0.63= 133.2 / 133.2

142.2/227= 0.63 / 0.63

15.7*0.63= 9.9 / 9.9

227

142.2 / 142.2

109.4

1.3*109.4= 142.2 / 142.2

1.3 / 1.3

Add 25 bar, to point where upper inner engages

0.33 / 1.00

211.3*0.33= 69.7 / 211.9

0.33 / 0

0

230.1

211.3*0.33= 69.7 / 211.9

69.7/227= 0.31 / 0.94

211.3*0.31= 65.5 / 198.7

69.7/227= 0.31 / 0.94

15.7*0.31= 4.9 / 14.8

227

69.7 / 211.9

(227*230.1)/(227+230.1)= 114.3

69.7 / 211.9

69.7/114.3= 0.61 / 1.91

Keep going to full 25 bar

0.31 / 1.31

211.3*0.31= 65.5 / 277.4

70.8/230.1= 0.31 / 0.31

18.8*0.31= 5.8 / 5.8

230.1

70.8 / 282.7

70.8/227= 0.31 / 1.25

211.3*0.31= 65.5 / 264.2

70.8/227= 0.31 / 1.25

15.7*0.31= 4.9 / 19.7

227

70.8 / 282.7

114.3

282.7-142.2-69.7= 70.8 / 282.7

70.8/114.3= 0.62 / 2.53

Keep going to 40 bar

0.74 / 2.05

211.3*0.74= 156.4 / 433.8

170.6/230.1= 0.74 / 1.05

18.8*0.74= 13.9 / 19.7

230.1

170.6 / 452.3

170.6/227= 0.75 / 2.00

211.3*0.75= 158.5 / 422.7

170.6/227= 0.75 / 2.00

15.7*0.75= 11.8 / 31.5

227

170.6 / 452.3

114.3

452.3-282.7= 170.6 / 452.3

170.6/114.3= 1.49 / 4.02

0.75mm shims on both inners, before nut preload

0

0

-0.25 (loose)

0

211.3

0

-0.75 (loose)

0

0

0

15.7

0

211.3*15.7/(211.3+15.7)= 14.6

0

0

Take up lower stack slack

11.8/211.3= 0.06 / 0.06

11.8 / 11.8

0.06 / -0.19 (still loose)

0

211.3

11.8 / 11.8

0.75 / 0

0

0.75 / 0.75

15.7*0.75= 11.8 / 11.8

227

11.8 / 11.8

109.4

11.8 / 11.8

11.8/14.6= 0.81 / 0.81

Take up upper stack slack

0.19 / 0.25

211.3*0.19= 40.1 / 51.9

0.19 / 0

0

230.1

40.1 / 51.9

0.18 / 0.18

211.3*0.18= 38.0 / 38.0

40.1/227= 0.18 / 0.93

15.7*0.19= 2.8 / 14.6

227

40.1 / 51.9

114.3

40.1 / 51.9

40.1/109.4= 0.37 / 1.18

Finish 1.3mm preload*

0.06 / 0.31

211.3*0.06= 12.7 / 64.6

13.7/230.1= 0.06 / 0.25

18.8*0.06= 1.1 / 1.1

230.1

13.7 / 65.6

0.06 / 0.24

211.3*0.06= 12.7 / 50.7

13.7/227= 0.06 / 0.99

15.7*0.06= 0.9 / 15.5

227

13.7 / 65.6

114.3

114.3*0.12= 13.7 / 65.6

1.30-1.18= 0.12 / 1.30

Keep going to full 25 bar

0.94 / 1.25

211.3*0.94= 198.6 / 263.2

217.1/230.1= 0.94 / 1.19

18.8*0.94= 17.7 / 18.8

230.1

217.1 / 282.7

0.96 / 1.20

211.3*0.96= 202.8 / 253.5

217.1/227= 0.96 / 1.95

15.7*0.96= 15.1 / 30.6

227

217.1 / 282.7

114.3

282.7-65.6= 217.1 / 282.7

217.1/114.3= 1.90 / 3.20

*Here I've assumed that the preload starting point is where no spring is preloaded. This is not the same position between the stock setup (where the starting point has both outers and the lower inner at zero and the upper inner is loose) and the shimmed setup (where the upper outer and lower inner are at zero, the upper inner and lower outer are loose, and the whole stack height is 0.75mm taller).

[m60power]

I understand the upper inner spring holds the dampener plates against compression, and the lower inner is for rebound, yes?

Yup, upper inner spring controls compression damping, lower inner spring controls rebound damping.

So if I understand your approach correctly, you are determining the stock upper and inner spring loads, with the stock nut preload, and at the 40 bar system pressure (where damping was "ideal" for you), then working to hit those same inner spring loads but at a minimum system pressure (25 bar) using an appropriate shim thickness? And it also looks like you're accounting for the slight additional affects of the inner spring forces when determining total compression (displacement) of the upper and lower portions of the stack?

Are your rear spring rates/heights set up so that your desired rear ride height of the car unloaded is achieved when the system pressure is at 25 bar? Have you gotten a pressure measurement at the struts when your car sits at the desired rear ride height to confirm?

I haven't cross-checked all your calculations yet, so far they make sense as you incrementally 'step' to the next pressure and displacement in the table, but one thing I noticed off the bat is your total stock stack displacement of 3.02 at 40 bar is less than the modified total stack displacement of 3.20 at only 25 bar? Not sure if I understand why that is?

*Here I've assumed that the preload starting point is where no spring is preloaded. This is not the same position between the stock setup (where the starting point has both outers and the lower inner at zero and the upper inner is loose) and the shimmed setup (where the upper outer and lower inner are at zero, the upper inner and lower outer are loose, and the whole stack height is 0.75mm taller).

Not sure if I can visualize how the lower outer could be loose at a modified nut preload of the stack? Does increasing the stack height of the lower inner with the shim make it tall enough so that big outer spring floats between the bottom plate and the orifice plate?

Good call on checking the materials compatibility.

EDIT: One other thing I just thought of - be sure to measure the module stack bore depth of the strut itself; IIRC for my struts, the bore depth was such that it may have just barely compressed the spring stack more than the stock nut preload does when the module is out of the strut. There's a machined lip at the bottom of the bore which the top disc of the spring stack stops against. Therefore you would need to compare how much the minimum pressure 25 bar will compress the stack vs. the physical pre-compression of the stack from the bore depth (hopefully that makes sense).

[moroza]

So if I understand your approach correctly, you are determining the stock upper and inner spring loads, with the stock nut preload, and at the 40 bar system pressure (where damping was "ideal" for you), then working to hit those same inner spring loads but at a minimum system pressure (25 bar) using an appropriate shim thickness?

Yes.

And it also looks like you're accounting for the slight additional affects of the inner spring forces when determining total compression (displacement) of the upper and lower portions of the stack?

Yes.

Are your rear spring rates/heights set up so that your desired rear ride height of the car unloaded is achieved when the system pressure is at 25 bar?

Yes, though only after welding extension tubes to the lower spring seats.

Have you gotten a pressure measurement at the struts when your car sits at the desired rear ride height to confirm?

No. I have a gauge but no adapter fitting.

I haven't cross-checked all your calculations yet, so far they make sense as you incrementally 'step' to the next pressure and displacement in the table, but one thing I noticed off the bat is your total stock stack displacement of 3.02 at 40 bar is less than the modified total stack displacement of 3.20 at only 25 bar? Not sure if I understand why that is?

Good catch. 2.53 + 1.49 should be 4.02, not 3.02.

At 25 bar, stock displacement is 2.53 while modified is 3.20 despite being shimmed to be stiffer. This discrepancy is because the modified stack needs 0.75mm of displacement to get to stock baseline height. Displacement from that baseline, stock and modified, is 2.53 and 2.45, respectively, which squares with the whole stack being slightly stiffer due to more inner spring preload.

Does increasing the stack height of the lower inner with the shim make it tall enough so that big outer spring floats between the bottom plate and the orifice plate?

Yes. The lower springs end up at the same height, stock and unloaded. Shim the inner, and the outer becomes loose by the same amount.

EDIT: One other thing I just thought of - be sure to measure the module stack bore depth of the strut itself; IIRC for my struts, the bore depth was such that it may have just barely compressed the spring stack more than the stock nut preload does when the module is out of the strut. There's a machined lip at the bottom of the bore which the top disc of the spring stack stops against. Therefore you would need to compare how much the minimum pressure 25 bar will compress the stack vs. the physical pre-compression of the stack from the bore depth (hopefully that makes sense).

Good call; I had not thought of that.

[m60power]

Good stuff! That all makes sense. It's interesting your compression damping spring is higher rate than the rebound damping spring; on my struts it was the opposite. Also, my rebound spring is the one slightly loose in the module stack-up with stock nut preload. Must be due to differences in the LAD strut part numbers.

Yes, though only after welding extension tubes to the lower spring seats.

This is an excellent solution. It would be cool to somehow make that spring perch tube threaded for adjustable height of the rear end, independent of the LAD valve / fluid pressure adjustment...

[moroza]

Yes, it's a higher rate but less preloaded. Sticking different numbers into the spreadsheet (which I'm now working on making automatic, like my Springs.xls) shows another interesting bit: the ratio of compression to rebound starts at around 1:3.5 at minimum pressure and steadily lowers from there, with 1:2 at around 38 bar, and basically 1:1 (compression actually slightly higher than rebound) by 130 bar. I've been simultaneously wading into the vast sea of shock tuning, where one of the few discernable landmasses is that that ratio is typically 1:2.5 to 1:2.0.

From what I've seen, 80% of the infomation on damping rates is written for and by motorcycle mechanics, mostly related to dirtbikes. The shop that compressed my springs specializes in dirtbikes as well. I guess having to design for routinely going airborne (and more to the point, landing) makes the subject rather more crucial than finetuning the pea under some fussy princess' ass. Regardless, this princess perseveres.

The cavity in my struts for the damper module are deeper than the module heights. In any case, the above method is required only until all springs are engaged. Once that happens, their preload stays constant and the three stacks (upper, lower, and whole) can be treated as linear equations.

These are 525iT struts I'm modifying (albeit with 735iL springs). I also have 735iL struts to take apart and compare. I was once told by Bilstein's tech support that E32 dampers are softer than E34, though I'm not sure what to make of that information quite yet.

As if the monster didn't already have enough heads, I'm also playing Musical Bushings with my rear strut mounts, which are essentially the Moosehead design and use generic rubber donuts. I've got four polyurethane Energy Suspension, four soft and dinky Isuzu Pup, and two each of three different Mercedes bushings that I'm shuffling around - currently on iteration No.6 IIRC, not including trying different top nut preloads - to finetune the rear ride. Oh, and I've gone through three different tires while at it.

I've yet to hear back from Pentosin, and meanwhile observed with 90%+ certainty that the preload nut is electroplated zinc. It doesn't matter for the shims - revised to 0.65 bottom and 0.38 top - because I'm using chopped up feeler gauges, but this allowed me to use a jamnut setup on the preload nut, rather than expending a nylock that I don't stock anyway (and living an hour out of town with gas over $5.50, I'm not about to "pop down to the corner hardware store" to buy more).

[blckstrm]

I sent you a PM
Johnnygees

I was the one who inquired about making another set of seals, but my wife dailies our E32 so I just used the hydraulic seal trick to get it back on the road.

This other side failed this summer while we were on vacation, so I did the same thing to the other side.

Whoever winds up making them, I'm definitely in for a set. The hydraulic o- ring trick works, but doesn't last from what I've read. I'd like to get a set in- hand so I can replace both sides at once as soon as the first to go starts leaking again.

I assume the o-rings will begin leaking gradually rather than failing all at once like the original material.

m60power - if no one else has stepped up I'd be glad to front the cost to get these started.