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2004 CLK 240 Coupe
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Discussion Starter #1
In the UK, every car over 3 years old needs an annual comprehensive test at a certified MOT station. The tests check the vehicle for general road safety with figures to be met for brake efficiency and emissions. Visual checks are made of lighting, signalling, suspension, etc.

Last year my car failed its 2014 test when a broken front spring was discovered. I was unaware of this damage as the car sat and drove normally. The break was at the bottom end of the spring, the final 4 inches, where it sits in the cup. The broken ends were rusty, so it was not a recent break.

Last week my 2015 MOT test was done, and - would you believe it - another broken spring was found, this time at the rear. Again the ride is normal, the break is at the final 4 inches of the spring, and it is very rusty.

These are the only mechanical failures I've had on this 2004 CLK, so it is surprising that front and rear suspension springs seem to be a weakness on the car. I've never had a broken spring on any other car - over donkey's years!
 

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03 CLK55 Coupe
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162 Posts
Are the roads you drive on in really poor condition (pot holes, speed bumps)? I have had several different automobiles over the years and never had a broken spring, blown shocks, yes. The opposite usually happens, over time the coil springs loose their temper and start to sag. It is really odd that you have experienced 2 broken springs on the same car. Something in the manufacturing process must have gone wrong. Maybe during the tempering process the temperature was not maintained over the whole length causing them to carbonize. The springs would pass initial spring tests but the tempering hard spots would not show up until several thousands of suspension oscillations.

At least you have several good options. The specialty aftermarket offers so great products as well as purchasing from MB. Are you going to put a whole new set of coil springs?
 

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2004 CLK 240 Coupe
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11,369 Posts
Discussion Starter #3
Are the roads you drive on in really poor condition (pot holes, speed bumps)? I have had several different automobiles over the years and never had a broken spring....

As as well as purchasing from MB. Are you going to put a whole new set of coil springs?
My local roads are pretty good, and potholes are not an issue here. My near city of Norwich does have speed bumps on the approach roads, and they are very common in residential roads in London, where my daughters have their homes. I reckon that speed bumps have probably caused this damage, because they are not always visible when driving at night.

My local inde did the replacement of both springs. Actually I was over a barrel there as (both times) the car was being MOT tested. If the shop is allowed to fix the failure, you don't get a test failure fee. Also, it is illegal to drive here with a registered MOT failure.

I did consider replacing the springs in pairs, but as this 2004 car is merely 'transport' for me, and not a performance machine, I let that go..

Next time I am at my MB dealership I'll take the broken spring with me, and maybe they will admit a 'bad batch' situation in the past. Strange that both springs broke just before their ends, so possibly it was a tempering issue.
 

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2000 CLK 430 Cabriolet 2003 S500 4Matic 2004 S55 2005 CLK 500 Cabriolet 2008 S 450
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780 Posts
In my experience broken springs are a fairly common experience with both the 208 and 209. With my 208 I found a broken driver's front spring exactly as you describe. Recently with my 209 we found 2 broken springs, 1 front and 1 rear. I replaced all 4 springs. As you said it is always (in my experience) the bottom few inches that are broken off. I've never noticed any difference in height or performance other than a noise going over speed bumps at a very low speed. I would advise anyone owning one of these cars to very closely inspect their springs. The breaks are almost impossible to see, but I fear, are often there.
 

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2004 CLK 240 Coupe
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11,369 Posts
Discussion Starter #5
This is a photo of the broken spring. It's not a 'clean break'. The fracture is about 45 degrees through the material.

Any metallurgists here?
 

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I stumbled on this as I was searching for specific high hardness metal springs, I don't know if this still matters to you, I'm a metallurgist. I know exactly why and how the spring fractured. this a helical compression spring, that is constrained at both ends. During compression, the highest stress distribution is location on the inner surface of the spring, the smaller inner radius. try to visualize this, as stresses are distributed evenly on the inner surface of the spring the highest level of stresses build up at either constraint ends during the cyclic loading (that is moving up and down). Notice the crack initiation point is at the contact between spring coil to anther coil, this happens as a result of reaching the spring material fatigue limit. Then it simple crack. I don't see any corrosion pitting that may have initiated cracking. Rust confirms your statement that it may cracked and sat.
 

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2008 AMG CLK63 Conv., 2012 R350 4-Matic, Wife's 2015 C300 sedan
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9,488 Posts
^Cut back on the pork pies :devil
 

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2004 CLK 240 Coupe
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11,369 Posts
Discussion Starter #8
I stumbled on this as I was searching for specific high hardness metal springs, I don't know if this still matters to you, I'm a metallurgist. I know exactly why and how the spring fractured.
Many thanks for your technical comments on my 4-y/o post here.

It does make sense to me, although I would have expected the highest stress to be at the center of the spring and not at the ends where it is sitting in a 'cup'.

The replacement springs have both lasted for the past 4 years, so I have no complaints there. I do drive over speed bumps with more caution now. ...:smile
 

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2007 W211 E320 Bluetec
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539 Posts
It would appear at least to me that the issue at hand is the metal to metal contact of the spring and the seat. I dont see any parts of the spring broken where the spring sits in a rubber. For example, all brakes in the back are at the bottom portion of the spring where it sits in the lower control arm ON metal. Same in the front, all brakes are at the bottom where the spring sits in the lower metal perch.
Perhaps installing the new spring with some king of rubber "jacket" may help?
 

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2008 AMG CLK63 Conv., 2012 R350 4-Matic, Wife's 2015 C300 sedan
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It's the point of least for sure and probably highest fatigue.
 
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