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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Technical Forums > Suspension | Brakes | Chassis > XI Factory front strut and rear shock internals



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      06-13-2015, 12:41 PM   #1
PhaseP
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XI Factory front strut and rear shock internals

Internals of my cut and taken apart old front strut and rear shock.

Interesting is the internal coil spring on the front strut. It limits and resists the full extension of the piston rod. This gives trouble with spring compression when assembling/disassembling. Either have to fully compress the springs, or somehow extend and hold the piston rod against the force of that inner spring. It may be an XI feature only. My replacement Billstein B4's didn't have these, they were fully extending by themselves.

Also there is no difference (with the exception of that inner spring) between the struts and the shocks, other than substantially thicker outer tube wall, and thicker piston rod of the strut.

I didn't notice any wear on the shock internals, and they had not leaked yet. But they were a lot easy to move in and out than the replacements. I think they had started loosing the gas fill, and oil in them may be foaming during use due to this. They were extending rather slowly when let go.

I was hearing a lot of hissing sound over bumps. Replacement Billstein B4's so far free of this sound.
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      06-13-2015, 04:05 PM   #2
ajsalida
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Wow that is super interesting, thanks for putting this up.
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      06-20-2015, 03:32 PM   #3
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nice!!! thank you
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      06-21-2015, 05:36 PM   #4
PhaseP
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Just in case the picture gives that impression, that inner spring only works against extension of the piston rod, and at the top part of the extension. It doesn't resist compression of the piston rod at all.
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      06-21-2015, 06:15 PM   #5
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Quote:
Originally Posted by PhaseP View Post
Just in case the picture gives that impression, that inner spring only works against extension of the piston rod, and at the top part of the extension. It doesn't resist compression of the piston rod at all.
Do you have any idea how strong that internal spring is? If it is not present in non-XI's this might explain some of why the front tends to bottom so often on XI.
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      06-22-2015, 10:02 AM   #6
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It is a very strong spring.

But because it only acts at towards the end of full extension, it may not be reason for those hard bottoming out on pot holes. It is like a bump stop for extension. The real bump stops are for compression, that sit on top of the strut

I had a 2006 and 2014 front struts, later from accident repair, they changed only one side. Didn't want to argue since I was going sooner or later replace them.

The one pictured is 2014.

The 2006 had even a stronger and longer spring.

I don't know what exactly is the purpose, may be trying to protect the CV joints on the front axles.

Last edited by PhaseP; 06-22-2015 at 10:18 AM. Reason: Corrected, it is like reverse bump stop
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      06-22-2015, 11:23 AM   #7
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Quote:
Originally Posted by PhaseP View Post
It is a very strong spring.

But because it only acts at towards the end of full extension, it may not be reason for those hard bottoming out on pot holes. It is like a bump stop for extension. The real bump stops are for compression, that sit on top of the strut

I had a 2006 and 2014 front struts, later from accident repair, they changed only one side. Didn't want to argue since I was going sooner or later replace them.

The one pictured is 2014.

The 2006 had even a stronger and longer spring.

I don't know what exactly is the purpose, may be trying to protect the CV joints on the front axles.
I understand that, my point was since it resists extension, the wheel will not follow road contours as well going into a hole, ergo have less compression travel available than otherwise to absorb a bump once it hits. Plus it is a very long spring so appears to be engaged nearly all the time to a greater or lesser extent.

Once that extra internal spring compressed (in extension of the suspension) it is also going to mess up compression behavior. Essentially having a one directional stiffer restoring spring force that might overwhelm a conventional shock's compression damping force, pulling the wheel back up perhaps too quickly. Again interfering with tire following the road optimally. One sense I got from the car with stock suspension was shock behavior was very strange. Maybe it was due to something else like what you've discovered.

Basically this spring will cause some odd effects. At full extension of the suspension the wheels will have an extra force making them want to snap back like being on rubber bands. I am sure they had good reasons, like you mentioned, but maybe did not think it through all the way. XI stock suspension is the worst I've ever experienced in a BMW.

It was always a mystery to me that with yellow Konis, big sway bars, and lowering springs my car bottoms out less (like never) than with stock suspension (constantly) even after ditching the run flats for Conti DWS tires. The extra spring in there probably has something to do with it.

Last edited by ajsalida; 06-22-2015 at 11:31 AM.
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      06-22-2015, 11:55 AM   #8
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Another way to look at this is, the two springs combined, work against each other and in opposite directions. The combined effective spring would be strongly progressive. In fact it may have an effective negative spring rate (overall restoring force) at full extension. This may have been done to counteract the extra weight of the front drive shafts and hubs etc. and prevent a hard top-out at full extensions.

Regardless it is going to cause havoc on the lower quality OEM conventional shock absorber element, and I can't begin to think about how you'd spec damping for something like that.
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      06-23-2015, 12:59 AM   #9
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Quote:
and prevent a hard top-out at full extensions
That is probably major reason. The front differential on these cars is bolted on to the engine oil pan. I don't think it is a structurally strong place.

On regular FWD drive cars, the CV axles go into the transaxle, which I believe is directly bolted on to the engine block.

Mercedes 4-matic has similar design, also has the front axle going through the engine oil pan. I wonder if those 4-matic has those explosive pot hole hits.
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      06-23-2015, 02:09 PM   #10
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Quote:
Originally Posted by PhaseP View Post
That is probably major reason. The front differential on these cars is bolted on to the engine oil pan. I don't think it is a structurally strong place.

On regular FWD drive cars, the CV axles go into the transaxle, which I believe is directly bolted on to the engine block.

Mercedes 4-matic has similar design, also has the front axle going through the engine oil pan. I wonder if those 4-matic has those explosive pot hole hits.
The R34 GTR had the same design as well. There's only so much room to work with under a longitudinally mounted inline engine.
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      06-23-2015, 07:21 PM   #11
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Well quite a few people throw away the stock suspension and go with aftermarket shocks/coil overs that I assume do not have similar internal spring, without issues. It may be that the XI springs are so stiff and tall that topping out is more of a real concern with them than with lowered aftermarket suspensions.
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      06-26-2015, 11:34 AM   #12
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Ride quality?

OP, how's the ride quality on the new B4s? Also, are you running the stock springs, or did you swap those as well?

Toying with replacing stock XI dampers and springs on for B4s and Eibach Pro Kit at some point, but want to make sure the ride is still relatively family-friendly, with either just the B4s, or with a combo of those and springs.

Thanks.
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      06-26-2015, 01:29 PM   #13
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I just received a set of B4 to replace the oem ones on my XI. Will let you know how this ride in a week or two.
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      06-27-2015, 04:04 PM   #14
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Stock springs on B4's no problems no complaints so far.
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      06-28-2015, 10:43 PM   #15
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Sounds good. Thanks for the reply.

woodpecka, love to hear your impressions too.
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      06-30-2015, 08:16 PM   #16
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New front suspension is ready to be installed tomorrow!

-shocks B4
-Bushing & ball joints
-Bump stop
-hardwares

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      07-01-2015, 07:10 PM   #17
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Results: I love the B4 and brand new suspension setup. I don't remember exactly how the car was driving when I got it but I can confirm it now drive like a real bimmer again.

For the price, this is a low brainer.
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      07-06-2015, 02:59 PM   #18
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I installed the Bilstein B6 HD shocks. Major improvement in the way the car rides and handles. The stock shocks were constantly bottoming out. I could easily push the oem front strut piston with one finger. Not very durable for a strut with ~40k miles.
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