My '12 S550 4Matic with ~80k miles makes a HORRIBLE sound when I hit a pothole or extra recessed manhole cover. I guess it's a thunk but not really, it's just a loud boom like the world is ending. People in the car gasp when it happens. It sort of sounds like the suspension is reaching its max extension, which isn't possible given the holes I'm talking about.

I've read a few posts here suggesting sounds like that could be control arms, end links, etc. but I thought I'd ask the question directly to see if I can get more succinct suggestions.

 

My E550 and S550 both do the same thing. It is a sound like the suspension travel comes to end and I always thought it is the struts hitting the end of travel. Now after you mention that the travel is not enough to make struts to reach the and of travel and myself remembering how much the suspension has room to move I kind of think it is not the movement limit causing the noise.

I need to (or someone else can) check if the noise happens when both front wheels drop off or just one.

I remember it happening when one of the front wheels drops off and after seeing the sway bar bushing arrangement makes me think that the noise could be a result of the sway bar hitting the bushing bracket when one wheel drops off. In that situation the dropping wheel will pull sway bar down while the other wheel will equally pull the bar up. If rubber bushing is not thick enough, the bushing is not firm enough or the bracket is made to come too close to the bar at bushing ends the bar could hit the bracket during a sudden move that the dropping wheel causes. It would probably make the same noise if one wheel suddenly goes up but in that direction the shock limits how fast the wheel can move.

 

I had the clunk sound as well on 2012 w221 4 magic at 105k miles. Replaced left and right lower control arms. Clunks are gone.

 

I can confirm that it is on both the driver and passenger side of mine.

 

Did you do the work yourself?

 

But is it each side separately or is it both sides at the same time like if both wheels would drop from a curb at the same time?

Both at the same time would not cause excess forces at bushings but when just one wheel drops it does.

 

One wheel at a time.

 

Looking at the pictures in the document konigstiger posted it seems in the last picture that the sway bar is actually closest to the car bottom and not the bushing bracket itself.

Would be easy trial just to stick some rubber between the bar and car bottom at the narrowest gap to see if it changes the heavy CLUNK when wheel drops. If it does it means the rubber of the bushings is too soft and allows the bar to move enough to hit the bottom of the car that causes the noise.

Another interesting detail would be to know if this CLUNK happens the same way in winter time for patrons in states with cold weather? Depending on rubber material it can get very hard in cold and could prevent the compression to the point where the bar hits the car bottom.

If it turns out to be the bar hitting the car it would actually be a relief as I have been worried the struts will be pulled in two when this happens enough.

 

Can you link the document you're talking about?

And yeah, I live in MN so I know from cold. The sound is the same at -30 as it is at 101, well as near as my ears can tell anyway.

 

link rods from the struts to the anti-roll bar ?

mine doesn't make any noises

 

Sorry I thought the konigstiger post was in this thread. It is in the thread “Sway bar Bushings”.

 

No .My indie did the work.

 

Does anybody have dimensions for the sway bar and it's mounting in the car? I could do some calculations for the forces at the bushings when a wheel suddenly drops 4" for example. Would be interesting to see how much force the rubber bushing needs to carry...

 

Replying to my own post here to continue this "research" on the very annoying very loud "clunk" that feels like the car will fall apart.

I modeled the sway bar of my S550 (221) with a FEA program. It is not exactly correct shape as I don't have a lift to go under the car and measure it but it should be close enough to give an idea of the forces at the sway bar bushings when one wheel suddenly drops like from a curb. This is when my car makes the loud "bang" rather than a "clunk" but I think this is what the OP is talking about too.

There are lots of posts in the forums about the "clunking" caused by the worn sway bar links. I am not talking about this much softer "clunking" that you hear at any little bump on the road.

The result of my analysis is shown in the picture below. I made one end of the sway bar drop by 75 mm (3") like if you drive the car so that one front wheel drops off a higher surface like a curb. This movement in one end only cause tremendous forces at the bushings. By this analysis 7700N, which equals to 1730 lbs!!! This nears a ton of force and I'm questioning if the rubber in the bushing is capable of carrying this amount of force and prevent the bar from hitting the metal bracket around the rubber bushing. The second picture shows how little room there is between the top of the sway bar and hard metal bracket. I estimate this gap is only about 1/4" and this gap would close in the opposing side from where the wheel drops.

This analysis would be a lot more accurate if there were better dimensions for the sway bar available. I used a picture from a parts seller and rough measurements from the car. For the bar diameter I used 28 mm (1 1/8").

Also, the calculation shows forces at levels that the car's weight may not be able to do. If the weight cannot flex the bar 75 mm when the wheel drop would ask for that, then the wheel may not reach the bottom of the drop.

I seriously think the big noise these cars make when running one front wheel to a sudden drop comes from the sway bar hitting the bushing bracket. This would be a serious design flaw and MB should use a lot harder rubber for the bushings or better yet, change the bushings to use much thicker rubber that allows enough room for the sway bar to move without hitting the bracket.



221 Front Sway Bar Analysis

 

the bushings on the front of the lower aluminium arms (to control back and forth movement) often fail
and the damper to the lower track control arm have an interesting joint that can get play till the baby grub screw locks them together... (some aftermarket parts don't clamp up the taper fixing and rattle)

mine is silent in and out of bumps.... its not the design, there's something wrong / worn on yours

 

I don't think my car ahs any aluminum parts in it. Bushing brackets bolt in the bottom of the "sub frame" or whatever it is called under the engine and they are rock solid in place.

 

front suspension

steel track control arms
aluminium caster control arms, the bushes on mine were totally gone, hidden underneath the fore most rad / engine cover
and aluminium top wishbone arms too