[keyhole]

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!

 

[m.e.g.]

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?

 

[keyhole]

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.

 

[mrbobby]

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.

 

[keyhole]

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?

 

[babak821]

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.

 

[503865]

^Cut back on the pork pies :devil

 

[keyhole]

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

 

[EFSiR]

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?

 

[503865]

It's the point of least for sure and probably highest fatigue.

 

[DavidWilson]

It's common with all modern cars - springs used to be made with a "closing coil" at each end (so that an unfitted spring would stand up by itself) That minimised the internal stresses in the spring itself - so the spring lasted longer. Sadly, it is MUCH cheaper to make a long length of evenly-coiled spring and then just cut off the right length. Guess what Mercedes does? !

 

[tony1963tony]

Please cite the engineering study to back up this claim. I'd like to read more.

 

[DavidWilson]

Well! That sent me down a rat hole - and up into the loft looking for my uni engineering notes (from the late 1960s.....) I can remember calculating the stresses in a closed end compression coil spring sat on a flat surface vs those in an open coil spring sat in a tilted pan (to make the spring stand vertical) . (See pictures in https://www.sciencedirect.com/topics/engineering/compression-spring) As the spring compresses, the stress in the open spring was sharply greater at the point of departure from the "floor" of the pan while the sresses in a closed spring are more even through the length of the spring. Explained why an open-ended spring always seems to fail inside the pan where it's hard to see. Sadly, after all those years, I can't find the maths behind it. Can any handy stress engineers out there remember how to work it out?
Cheers
David

 

[DavidWilson]

...and here's some more stuff on spring stresses: Compression Spring Design – What You Really Need to Know | Mentored Engineer
David