Bilstein EDC-compatible struts for E9X M3

MidLifeM3 wrote

yep count me in too as being interested. Besides checking the spring perch height, is there any other physical difference (mounting locations, EDC connector etc) between the two or is there difference in the shaft length. For one this would help to conclusively answer the oft-asked question "what makes ZCP 10mm lower" ie perch height or springs.
Considering the persistent and extending backorder on all things Bilstein I'm now leaning toward just a fresh set of OEM shocks. Wondering if there's any value to replacing with ZCP shocks on my 08 originally non-ZCP. If I do go the OEM route I'd pair with Dinan or Eibach springs to get a ZCP-like ride height (not any lower since there are already some challenging driveway entrances on my daily commute). If the ZCP shock is identical except with 1/2" shorter shaft then it could be ideal for my case.

I had seen mention in the forum that ZCP EDC incorporated some additional sensors into the EDC control algorithm. But studying the parts diagrams I only see the same wheel sensors and same EDC control module with same p/n between ZCP and non-ZCP EDC cars. I'm still researching that one.

MidLifeM3 wrote

At this age of our M3s, many E9x owners will be looking for replacements of worn out stock shocks and for those of us who like to keep our EDC functionality I'm sure this thread and the related ones on B16 coilovers will be visited a lot. Although prices on OEM replacements seem to be coming down, still there will be interest in these Bilstein as an alternative to OEM. Certainly for me I was drawn to these because of Bilstein reputation as a quality shock and to address what I felt was a bit loose rebound damping on the stock EDC shocks. So all well and good ....

one question that comes up over and over an over in threads about the B6 is "will it raise my car?", followed by "how much?" Since I just went through my own analysis to try to quantify that effect, I thought for public service to the community I would share so others could have an idea what to expect.

For many on this forum the following may be obviously apparent but I'll start with the basics. All shock abosrbers, at least when new, are pressurized to suppress foaming and cavitation of the shock oil. Twin tube shocks like the OEM ones are commonly pressurized to about 100psi, while monotube shocks like the Bilsteins use a higher pressure around 300psi. As you'd expect intuitively, that pressure tends to force the shaft out of the shock body, and when installed that force will push up on the car tending to lift it slightly. But how much is that force? We need to know the relevant area to convert the psi to absolute force.

if you consider the inside of the shock, the piston (with the shaft attached to it) is sitting there inside this pressurized tube. But, the pressure is hydrostatic and because the piston has holes through it the pressure is the sameabove and below the piston. That pressure on both sides almost balances.... except for where the shaft attaches to the piston. Below the piston, the pressure pushes up on the piston surface everywhere, but above the piston the pressure can only push down on the exposed parts of the piston that are not covered by the shaft. The net force exerted on the shaft is simply the pressure in the shock times the cross-sectional area of the shaft.

For the Bilstein "inverted monotube" design note that the huge-looking cylinder sticking out of the yellow housing is not the shaft. That's the main tube, the shaft sticks out the bottom of that and is hidden inside the housing. On the stock OEM shocks (and all twin-tube shocks) in the conventional "right side up" configuration the shaft sticks out the top.

Though I haven't received my B6 yet (that pesky back order) I believe the shaft on these is 14mm. So the cross sectional area is about 0.24 sq in. Then the force exerted by 300psi is 300*0.24 = 71 lbs.

For the OEM twin-tube shocks assuming a typical 100psi pressure, the force exerted would be about 1/3 as much, roughly 24 lbs.

And for all those of us with 90k miles on our shocks, chances are good that some or most of that 100psi has leaked out by now so the force might be pretty limp. If when you remove your old shocks the shafts don't extend (even slowly) then you didn't have much pressure left inside....

On the front axle the shock and spring both have a motion ratio close to 1, and the stock front springs are about 160 lb/inch rate. compared to a fresh stock setup, the Bilstein will be pushing up with 71-24 = 46lbs more force.
46lbs / 160 lb/in = 0.3 inches, about 7mm

Compared to worn out OEM shocks with little to no pressure remaining, the delta will be larger, around 0.4" or 10-11mm.

In the rear its slightly more complex because the motion ratios are far from 1 but this is the reason why users report less lift at the rear than at the front when installing Bilsteins. The force exerted on the shock shaft is still the same, that's just determined by shaft diameter and pressure. So 70lbs or so on Bilstein, 24lbs or so on fresh OEM. The spring rate is 550lb/in, with a motion ratio of about 0.58 relative to the wheel. The shock is not concentric with the spring and has a different motion ratio - roughly 0.8 (though I admit I could get this more precise with more careful tape measure work - but close enough for this purpose). Converting both to wheel rate makes the math more intuitive
550lb/in * (0.58)^2 = 185 lbs / inch spring wheel rate
46lbs * 0.8 = 37lbs more wheel-equivalent force from the shock

Then the 37lbs wheel-equivalent more force exerted on the shaft of Bilstein vs fresh OEM results in 37/185 = 0.2" higher ride height at the rear. If your reference point is fully deflated shocks then the relative rise will be up to ~0.3" higher than your very tired worn out shocks. In either case only a little more than half as much effect at the rear as at the front.

Since Lutfy has kindly provided a rather precise description of his case we can use his experience to assess and validate these calculations. He started with a non-ZCP car and added Bilstein B6 plus Eibach springs and ended up at nearly identical to ZCP height. ZCP is 10mm or 0.4" lower than non-ZCP. the Eibachs are supposed to give 0.8" drop in front and 0.6" in rear. The B6 would counter that with 0.3" rise in front and 0.2" in rear. Net result, 0.5" drop front and 0.4" drop rear, spot on to a ZCP.

For those who use B6 with stiffer springs like the Swift, the effect of the "bilstein lift" will be a little less in proportion to the additional spring rate. The bilstein will push up by the same 70 lbs, but your sport springs with higher rate will extend a little less in response.

All of this only matters with the non-adjustable spring perches, since the coilover or sleeve-over guys can just dial up or down to whatever static ride height they want. Since unfortunately the B6 is not compatible with the sleeve-overs that trick won't work for those of us considering these shocks. If we don't like the impact on ride height our only option is to change to different springs. Or to go for the B16 coilover kit. This is purely a cosmetics question, those few 1/10's inch change in ride height will have no deleterious effect on handling, but replacing floaty worn out shocks with fresh ones will certainly be a positive improvement.

I've also seen questions of whether the high pressure shock increases the spring rate. Technically yes but the effect is smaller than the impact on ride height. The effective spring rate is due to the additional volume displaced by the shaft as it moves down into the tube, which further compresses the gas raising its pressure. The increase in pressure per inch of shaft insertion, multiplied by the 0.24 sq in of shaft cross section, is the effective spring rate due to the shock. To calculate it exactly we'd need to know the actual volume of the gas chamber which I do not have easily on hand. But considering the shaft cross-section is pretty small relative to the piston or tube cross-section this added spring rate should be fairly insignificant.

Hope this is helpful to all those out there searching the threads and wondering about the effect on ride height.

For my case, my plan is to go for the B6 along with Dinan springs (0.5" drop) and recover back to about my current ride height on well-worn OEM shocks. Otherwise the B6 look like a good match to any of the mild lowering springs out there like Eibach.


I have not seen any quantitative data on the length of the internal bumpstop of the B6 (my inquiries to Eibach have not been responded to). Nor have I seen anyone publish shock dyno results on these or OEM. I guess the guys who dyno their shocks are all focused on race shocks. But it would sure be nice for the engineering-minded to have some mroe quantitative comparisons of these aftermarket parts so we'd know ahead of time what we're getting into. Though the Bilsteins are slightly easier on the wallet than OEM, you're still looking at close to $1800 for the set of four plus labor (or an afternoon of your own time if youre the DIY type) so its not an insignificant investment to "try out and see if you like it". And the qualitative reviews are great but one guy's plush ride is another's buckboard; one guy's taut control is another's flabby boat. Pretty hard to translate to our individual expectations. If anyone out there has measurements of the bump stops, or shock dyno data on these B6, I would love to see it!

I also wonder if anyone has weighed the unsprung weight of B6 vs OEM. The B6 steel shock overall is much heavier, but since its the inverted design some of the extra weight will be sprung weight and in increased unsprung weight might not be so much worse than stock.

SYT_Shadow wrote

lutfy I hope you realize you aren't going to heaven anymore due to not tracking the M3... just sayin'

lutfy wrote

I sold every single bit of mod in my car with the EXCEPTION of these Bilsteins.

I have been thinking of buying Eibach springs back and putting it with this setup as the car was absolutely amazing with that setup... those who track has some data point that the suspension was awesome on an otherwise stock car

MidLifeM3 wrote

well, dang it. originally Lutfy's reviews got me started down the path of considering the Billys. With the long backorder situation from Bilstein combined with dropping prices on OEM (plus that cool aluminum body construction) I had almost convinced myself to just get a fresh set of OEM shocks. Taken in isolation, they're actually pretty good, at least compared to much of the lower-budget stuff in my past. (lets not even mention the lever shocks on the Triumph TR6 I used to own ... fortunately, thanks to the tall skinny tubed bias-ply tires and the twist-o-matic ladder frame chassis, you hardly noticed that the shocks were more like slight friction enhancers than anything exerting any actual control over the springs.)
I was driving around this week convincing myself that the stock shocks work well and all I really needed was fresh ones. But now Lutfy comes back with these comments and I realize I just have to put in my order and wait until Bilstein works their way to my position on the list some many months from now.... The one advantage to being of the age where I'm half gray and slightly balding is that I can have patience ...

I ran across a thread in an E46 forum where someone called it "Vitamin B12" as a good recipe for health and happiness of your M3

MidLifeM3 wrote

Lutfy, do you still have the front bar on your DD or did you ditch that too along with the other mods?

z3papa wrote

I just installed the rears. My ZCP EDC rears were really dead. You can definitely feel the difference but I'm bird dogging getting the fronts, and have several vendors looking to how a pair of fronts. For anyone who watches the install video, my advice is to definitely disconnect the rear lower control arm so you can install the shock or you will have a bear of a time getting it in.

SYT_Shadow wrote

How many miles on your zcp struts?

z3papa wrote

Just scored the EDC B6 fronts. They should be in the next week. Definitely looking forward to whole package.

SYT_Shadow wrote

Please post your impressions

SYT_Shadow wrote

Please post your impressions

z3papa wrote

SYT_Shadow wrote

Please post your impressions

OK -- here is my initial thoughts. I installed the rears last weekend, and had the fronts installed today as something came up that got in my way timewise. I feel the rears are a huge improvement over the underdamped OEM units, especially when the are worn. I can definitely tell you the rear was far more planted on irregularly pothole worn surfaces even when pushed, and I'd definitely say they beat the OEM ZCP EDC rears, but are admittedly a bit of a PITA to install. The online video does not show you needing to drop the lower control arm to get the shock in but that's the ticket.

As for the fronts which went in today, they really raised ride height probably 7-8/10" over the worn OEM fronts. I was aware of this as it's been discussed here but this is more than I expected. The ride/handling over the worn OE fronts is much better but i'm mildly concerned about change of rake placing more weight on the rears, the lost of front negative camber from raising up, etc. For those who are looking for aesthetics, I get the disappointment. That's not a concern for me and in fact will probably keep my front bumper cover from scraping as I run 18" wheels. I understand the OE units start with 100 psi and go downhill from there, and the B6's start with 300 psi to prevent initial cavitation. I suspect they will settle some (not so much springs but the stuts pressure) but this may be a learning process. For those concerned about looks, I could see running OE fronts with B6 EDC rears.