thump from rear Bilstein HDs

[thehackmechanic]

Ok. It's official. This is driving me ****ing nuts.

As per a bunch of recent posts I went from:
--Scoring staggered Alpina 16s
--Cutting the front springs to improve the stance
--Cutting the back springs to level it out
--Removing the too-stiff KYBs that have been in the back for 15 years and replacing them with Bilstein HDs

The car was quiet and well-behaved with the cut springs and the KYBs.

Then I found a pair of 15 year old inverted Bilsteins for short money (stamped F4-B46-0813-H001). I bought them, made sure that they felt ok (they do; when compressed by hand, they feel slightly less stiff than the KYBs), and installed them. And started hearing a thump in the back over any kind of bump. Seems to be coming from both sides. I can't reproduce it bouncing the back of the car. I've stood in the trunk and thrust up and down and hear nothing, then I drive the car, and thump city.

Thinking that the cut springs were most likely, I looked at them closely. The cut end is at the top. I convinced myself that it was possible that they're brushing against the metal part that sits in the middle of them, so I took them out and put back in the original springs.

No change.

I took the Bilsteins out and bounced the car up and down.

Quiet.

I put the KYBs back in and drove it.

Quiet.

I put the Bilsteins back in and drove it.

Bump.

Again, these are used, 15 year old, inverted Bilsteins. I looked at them closely against the KYBs. They're perhaps 3/4" shorter than the KYBs, which is curious. When I installed them, I tightened down on the nuts on the washers against the rubber bushings, and made sure there's lots of compression. So it's not like the bushings aren't being squeezed; they are.

It seems like there are four possiblities:

1) The shocks themselves are bad.

2) The bumping is coming from the eye at the bottom. This is unlikely; I installed extra washers on both sides of the eye and tightened it with an impact wrench, and it made no difference.

3) The bumping is coming from the top. As I said above, sure looks to me like those bushings are being squeezed tight; don't know what would be bumping against what.

4) The bumping is coming from the shock being slightly too short.

I'd buy a new set of HDs (they're on sale at bavauto for $89), but I'm scared that somehow the problem will be with the length of the shock, though I can't explain it.

Any ideas?

The good news is that I've become wicked fast at getting shocks and springs into and out of the car.

At this point I need to get ready for V@V. I'm tempted to just put the KYBs and the cut springs back in there and be done with it.


--Rob

 

[bmw art car]

hmmm

did you call bilstein? 800-747-4636
that's what i would do...
you don't move the shocks fast enough to replicate that thumping
imagine it as if you would pull on your seat belt. it will release.
but if you pull vey sharply it will lock.
same with your shocks...
you jumping in the trunk moves the shocks in a linear way
the sudden shock on the road will hit the shock differently

why dont you use koni? i have a very good set of red koni.
they are adjustable and especially for the cs... i totally dig them

 

[bengal taiga]

The car was quiet and well-behaved with the cut springs and the KYBs.

Then I found a pair of 15 year old inverted Bilsteins for short money (stamped F4-B46-0813-H001).

Started hearing a thump in the back over any kind of bump. Seems to be coming from both sides. I can't reproduce it bouncing the back of the car. I've stood in the trunk and thrust up and down and hear nothing, then I drive the car, and thump city.

I took the Bilsteins out and bounced the car up and down.

Quiet.

I have no experience with the inverted bilsteins, so my thoughts are gratuitous conjecture.

You mention that the blue shocks are shorter than the KYBs. This makes me wonder about the width of the shock cylinders at or near the upper attachment points. Could it be that at some part of the suspension travel you have contact between the shock body and the car body? Any obvious wear marks?

How about the upper attachment points and rubber bushings? Are they the same size (thickness) as the KYB's? (Even too much lateral movement between the mounting stud and the receiving hole should be filled by the rubber bushes, but maybe not?) I mention this since I remember someone struggling with noise in an E3 after replacing troublesome Nivomats. From memory, the exhaust was pulled and hangars changed before discovering that the upper shock bushes were inadequate to completely isolate shock movement.

If the issue/noise is not obviously related to the adequacy of the shock bushes or actual shock-body contact, my attention would be directed towards the shocks themselves. You said the shocks were old. You didn't describe their condition. One would expect those shocks to have a long shelf life, but even new shocks can occasionally fail. :wink:

Good luck.

 

[pamp]

My Vote

I would say you have insufficient shock travel. Curious, but what brand of spring compressor are you using? As you seem to have the drill down...how much does the spring need to be compressed over what the weight of the car gives you, if at all?

 

[thehackmechanic]

still can't figure it out

This is so damned strange.

Attached is a photo of the top of the KYB and the Bilstein side by side. The rears of both shocks are aligned against the wall. The Bilstein is about 23" tip-to-tip. The KYB is about 3/4" longer.

There also is a subtle difference in the shape of the top of the shock. In addition to the length, the KYB has a little spacer that's welded in at the base of the stud, effectively creating space between the rubber bushing and the top of the shock. I tried to ape this on the Bilstein by putting two washers about the same height on the stud and then sliding the fat washer and the bushing onto it, but it made no difference; the thumping noise is still the same.

I like the idea of not enough shock travel, but it's strange. The springs that are in it right now are the original full-length springs, but I tried the same shocks with the other set of springs that I cut a coil (about an inch of height), which you'd think would increase shock travel, and it made no difference.

There are, candidly, unforgivable issues with the way my shock towers were repaired by the previous owner, but I don't know why that would make one set of shocks noisy and the others quiet, especially when I can't see where it's banging, if it's banging.

Part of me wants to put the KYBs back and just be done with it, but part of me HAS TO KNOW THE ANSWER, and so I may have to buy the new non-inverted HDs and put them in. If they bang too, I can probably sell them for close to purchase price.

Thanks for the input guys.

 

[HB Chris]

What I find interesting is that you can compress both shocks and take a picture? Sounds like they are both worn out if they're that easy to compress. I also think that the Bilstein might be too short. You might also have too much play in the upper mount bushings with the HD for some reason. My coupe once had those same Konis on it and that shoulder on the post allowed a washer to rattle and not tighten down which drove me nuts until I discovered why and replaced the Konis with HDs which are great shocks. Also, why refer to them as inverted, I thought that meant they pointed down and not up?

Chris

 

[bmw art car]

koni koni 80 c 2092

go with something that works and is designed for your car
thats what i have in mine...they are adjustable , red koni...
so they are not as stiff as yellow and they are cheap $110.25 bucks each new.
the guy who worked on it put them in...and he must know, he was peter greggs crew chief
this is what my koni looks like

(i payed 250,- each for my 8 series just some days ago)
http://sciontist.com/Product.aspx?c=859&m=BMW

 

[bmw art car]

i was just thinking...
maybe your pieces on top of the shock are worn more from using the other
shock and have some play when you put the bilstein in...just a thought

 

[tochi]

Assuming the same springs are installed, does the stance of the vehicle vary with the two shock sets?

Over the years I have tried Boge, KYB and Bilstein. The KYB's made the rear sit appreciably higher than the Bilsteins. The Boges, I recall were not nitrogen filled shocks so they had no "lifting" effect.

I do not think the shock length is your problem unless you are at the suspension travel outer-limits when you hear the noise. Instead, I would focus more on the upper connections to the body or toward the shock innards. You already mentioned previous repairs in that area. Those shorter shocks may be placing a different load on that portion of the body than the KYBs. The flexing (weaker) portion of body is merely relaying the message.

As HBChris noted, compressing both gas shock brands by hand should require much effort. Maybe both sets have "issues."

 

[thehackmechanic]

Solved... blissfully quiet

I took the plunge, bought new rear HDs from Bavarian ($89 on sale), installed them, and... blissfully quiet.

A few notes:

--A few people asked what I meant by "inverted" shocks. These are the old style of Bilsteins in the photo with the blue corrugated bellows at the bottom. They're next to the new shock. The new shock has the housing at the bottom and the piston at the top, like most shocks. The older inverted style has the housing at the top and the piston at the bottom.

--Someone said I shouldn't be able to compress a pneumatic shock by hand, and if I could, it probably was bad. Not true. Of the KYBs, the old Bilsteins, and the new Bilsteins, the stiffness was in that order -- KYBs the most stiff, then the old Bilsteins, then the new Bilsteins. Although the new Bilsteins were the softest, they also rebounded (when compressed by hand) the fastest. Once they were installed in the car, their correctness was abundantly clear. Previously, with either the KYBs or the old Bilsteins, the back of the car was so stiff I could barely budget it, like my '99 M Coupe. Now it feels much closer to my 2002 (which has Bilstein HDs as well).

--I attached some photos of the bushings. The used Bilsteins that I bought (that either are bad or too short) have the one-piece rubber bushing on them like in the real oem photo bengal kindly posted below. This is for the original hole in the shock tower, which is a big hole, about 1 1/4". The KYBs that were in the car had a split rubber bushing, where each half of the bushing had a lip about 5/8" that inset into a hole. I now understand that my car has had plates welded at the tops of the shock towers that don't have the big 1 1/4" hole, they have the smaller 5/8" hole.

--So imagine my confusion when I opened the box with the new Bilsteins to see that, in addition to the split rubber bushing and the big flat metal washers, they have a pair of metal adapter plates with lips that are about 3/4". I now get it -- these are to adapt these newer-style split bushings to the original large 1 1/4" hole in the shock tower. For my car, I removed these metal adapter plates and just used the rubber bushings.

I've owned this car for 25 years. It has never been quieter or rode better.

Man, I LOVE it when you think something through and make the right choice.

 

[HB Chris]

Rob,

I agree you can compress a new shock if you push against the floor but once you let go it rebounds very quickly. This makes it a challenge to install once the plastic retaining strap is removed.