I'm interested because there has been a plague of broken coil springs in Europe over the past 10 years. Affects all makes and models from BMWs and Mercedes down to Citroens and Fords. After a lot of research and some red herrings I think it's because European made springs are not properly finished at the ends. In the Far East, springs that don't have to go round a strut are 'pigtailed', and those that do have to go round a strut are either flattened or tapered at the ends. This helps them to resist corrosion.
But in Europe all too often a plain spring with an unfinished end sits in cup full of salty road dirt, so corrosion eats into it and the spring is then weakened by the proliferation of road humps and potholes. Though, happily, most of them seem to snap and stab a tyre first thing in the morning.
if anyone knows better, please post.
HJ
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It sounds like au unfinished spring is acting as a stress concentrator leading to fatigue failure possibly assisted by corrosion. Finishing the spring distriutes the forces more eavenly and so reduces the vulnerability to fatigue failure..
BIG
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What research has been done HJ? - is it published anywhere?
The fracture surfaces I've seen on broken coil springs haven't shown much evidence of corrosion.
Having said that, it's entirely possible that corrosion can help the fatigue crack to initiate, and in highly stressed items like coil springs, initiation is the longer process, and the fatigue crack growth to failure is a much quicker process.
Springs without any end features can be good design, if they sit in well formed spring seats.
There are a few items that will speed crack initiation;
- sharp features or marks in the surface of the wire
- poor coating, enable corrosion to grow at grain boudaries
- inherent defects or flaws in the spring material
>>This helps them to resist corrosion.
I would be a bit worried about this conclusion, and would need to have some firm data at hand before I would commit myself to it.
A more feasible mechanism might be that the expensive spring end features reduce the maximum stress level, hence the local surface strain, thus inhibiting fatigue crack initiation even if there is some corrosion.
Has any researcher compared the quality of the spring steels used by different manufacturers?
Has any researcher compared the quality of the coatings used by different manufacturers?
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I had a front spring break last week on my 2003 Corsa. Only 32k miles, and I've had broken spring, handbrake cable snap, gear linkage replaced, steering rack replaced and a engine managment issue. :-(
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I think I mentioned on another thread that both my broken springs were fatigue types of breaks - almost tears. The both went one turn up from the bottom and seemed to start where the 1st turn rested on the bottom end of the spring.
This end was straight cut and not tapered as some springs are. It definitely looked "wrong" with the new springs fitted, with a lot of potential force on a single point.
I got the angle grinder out and ground a taper on the ends of the springs.
These were the original 1990 Mazda springs.
Noted from the supplier of the replacements. www.mx5parts.co.uk
Replacement standard springs
These are exact copies of the standard Mazda spring, but made in Sheffield using superior spring wire and tempering so as not to suffer from the Mazda broken spring problem! These springs have the standard spring rate and give the standard ride height.
Edited by martint123 on 25/02/2009 at 21:33
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>>The fracture surfaces I've seen on broken coil springs haven't shown much evidence of corrosion.
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The cross section of my spring showed lots of clean shiny metal. there appeared to be just a little discolouring around the outer edge.
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In response to Number Cruncher I get an average of 2 - 4 reports a day of broken coil springs on European built cars. Not Far Eastern built cars. That's strangely high out of an average 100 - 150 non-spam e-mails a day, and to me, statistically, indicates a problem.
HJ
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Thanks for your reply HJ.
>>Not Far Eastern built cars
Great! - there's a starting point for the investigation - not the end point!
However, while there may be a visible difference in the spring design, it doesn't necessarily follow that the visible design difference is the important one, and unless there's more information available, I don't see how the conclusion you wrote above could actually be drawn. Are the springs the same material?, made using the same heat treatments?, are the protective coatings comparable?
I'm not writing this to snipe - it's just that I've been involved in quite a few engineering failure investigations, and more often than not, the simple, obvious, answer isn't the correct one - usually, you have to dig a bit deeper, keeping an open mind while you go. (Ever now and again, you do find a clearly and blatantly incompetent design - which is good fun!)
I would be surprised if all Western manufacturers had suddenly made such an obvious mistake on a well understood part all at the same time.
For example, un-modified coil spring ends have been used for many many years without a large failure rate (at least 30 years to my meagre knowledge). I think the answer is not quiteas simple as you suggest HJ, although I think you've got some useful statisitics to help you solve the problem.
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Get a look at this:
tinyurl.com/cwrym4
HJ
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