Racer20's deep dive into the strut clunk.

[NORAGRETS]

Originally Posted by Racer20

Possibly significant update:

I drove the car for about an hour and a half last night in all modes on all types of roads, and it was generally pretty quiet. Only during the first few minutes and last few minutes did I hear the clunking.

The lack of consistency led me to take another look at the test data from my ZCP struts from before I installed them. I found a minor issue that MIGHT be the smoking gun (further testing required):



This graph shows the damping force produced over one complete cycle (75mm rebound stroke then a 75mm compression stroke) at high damper speed and at a very soft amperage setting (as it would be when driving at low speed or on a reasonably smooth road)

Ideally, this curve would be smooth and rounded with no discontinuities, like the black line (my passenger side strut). However, the driver's side strut (the noisy side) exhibits a little bit of damper lag in compression.

That means that if the impact is large/sharp enough to reach high piston speeds, the damper is not building force properly during the initial portion of the stroke. That means energy is not being dissipated, and suspension loads and velocities can be much higher than intended.

Damper lag is a bit of a "chaotic" phenomenon . . . once it occurs a little bit, it tends to amplify itself and become very unpredictable. It's also highly dependent on the road profile, temperature, and other suspension parameters (ride height, jounce bumpers, etc.) Considering that my damper test is done at "standard conditions," it's not clear if this lag would be better or worse during actual use under the conditions that create the noise on vehicle.

It's also not clear that this lag will actually cause noise on the vehicle, as all dampers will exhibit lag under some conditions, and some vehicles are not very sensitive to it.

This data is from when I bought the ZCP dampers used with ~6k miles on them. I need to take them off again and test them under more dynamic conditions to see if the lag has deteriorated in the ~2000 miles that I've put on them.

The challenge here is that there are many possible causes for lag. Some dampers inherently lag when new due to the length or valve setup. Some dampers can lag if the tolerance stack up of the internals is over some threshold, (meaning some will and some won't), and some dampers won't lag unless something is worn or damage.

It's also possible that ride height, alignment, or the different loading of aftermarket springs contributes to the lag to some extent, but I have no way to know that at the moment.

So, with the data in front of me, I can't say whether buying a new strut will solve the problem, or for how long. It's possible that a brand new strut will show the same amount of lag.

I'm hesitant to spend $1200 on a new strut just to have the noise come right back.

Maybe a long shot, but if someone has some ZCP shocks/struts laying around and would be kind enough to ship them to me, I could test them to get a larger sample size of this problem. Then we could see if the clunk and the lag were correlated. Of course, I'd ship them back to you as soon as I'm done testing.

bro, are you an engineer? lol this is crazy data you're pulling, its impressive

[irritable_vowels]

another note of huge thanks for the problem solving in this thread, you've saved me from the teeth-clenching I was doing every time i went over a bump with the driver's side.

[Tetsu69]

Hi guys, this is very first post in this forum for me and I have been having this clunk issue since I changed to coilover (Aragosta type s). I asked the mechanic who has changed my suspension to tighten this steering rack bolt but he said he couldn’t because bolt is too sticky to turn.
Question to the guys who has done this fix: Did you need very high torque to rotate this bolt? I found that it is 70Nm for F30 repair kit so I am expecting similar but in that case mechanic could have rotated…

Thank you very much for your support in advance!