Coilsprings and their structural failure

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veteran
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Coilsprings and their structural failure

Post by veteran » Sat Jul 13, 2019 2:19 pm

A few days ago, in the topic called "Jacking Points?", RUM4MO introduced the possibility that a coilspring could break at almost any point in a car's life and that it happening while a keen DIYer were under their car and relying simply on their car ramps caused alarm bells to ring with me. For under-car work I've used my home-made wooden car ramps for some 40-plus years and during that time have glibly assumed that if the coilsprings appear to be sound they'll never choose to break while I'm under the car. I now stand better informed, and accordingly I'll now be taking steps to either change my setup or to add supplementary supports.

That sparked an interest in finding one or more published articles on the Web by professional engineers who'd investigated the longevity of automotive suspension coilsprings. I consequently turned up the international conference paper of 2013 entitled "Suspension Springs - Experimental Proof of Reliability under Complex Loading", by Decker, Rödling and Hück. For those in these forums interested in the subject at the R&D level, I'd recommend you read the paper. (Sorry I can't quote you the URL, but it was in PDF format, and if you google using the authors' names and the paper's title, you should easily find it). For those who don't want to plough through all the minutiae, here's a 6-points summary of its findings:

1. The higher the tensile strength, the more susceptible will be the spring to surface defects introduced in the production process, to grit impact, and to roadsalt and general wear and corrosion.
2. Steels such as 52CrMoV4 and 54SiCr6 tend to be used, which unfortunately result in very high levels of spring internal stresses.
3. Stresses and internal micro-cracking can quickly lead to serious failure of the spring. Product development and quality assurance become all-important.
4. Specialised coatings can reduce the frequency of cracking due to surface corrosion.
5. The spring's geometry and the sizing of the first and last winding can have a profound effect on outcomes. Linear springs are less prone to stress failures than non-linear.
6. Fatigue life is also strongly influenced by abrasive wear in the contact areas, particularly in the lower attachment (spring seat).

Perhaps quality control in the production process, both in the steel and then later the spring's manufacture, is not up to the necessary standards these days? Regular sampling of production lines is advocated. Specialised equipments exist for specimen testing of steel purity but it's not known to what degree, if any, such testing is done these days. Apparently, valid simulations, using a technique called Resonance Testing, can be acceptable substitutes for the employment of the more expensive equipments.

Other online discussions on this subject, among technically-minded consumers, suggest that car manufacturers may, in the last 10 years or so have been overdoing vehicle weight reduction so as to achieve better fuel consumption and emissions figures, and that this may have possibly included the reduction in cross-section of coilspring turns, leading to a greater tendency for failure. At local level, garage and MOT personnel are unsurprised by the now frequency of spring failures, particularly with the predominance of roadhumps in urban areas. And you may be able to get away with the odd pothole excursion, but repeated large spring movements such as happen with roadhumps will inevitably lead to gradual micro-cracking and ultimate shearing of a spring.

One thing I've noticed myself about the Polo 6R/6C is the lack of a substantial spring-to-lower seat buffer. For instance, on the rear suspension springs there's, at best, a very thin aluminium 'washer' included at the base of the spring. I've raised this point on the forums in the past; I accept that the longevity of any such buffer material placed in that position can be an issue, but the spring's end is more-or-less left to grind away against its seat. Incidentally, there is, you know, a guard or cover available from VW for the base of each rear coilspring. I believe it's listed as a 'shoe'. It's made of plastic and snap-fits to the spring seat and trailing arm. It strikes me that a pair of these could go some way to reducing the amount of grinding grit that would otherwise accumulate in the seat, grit thrown up by the back wheels. But perhaps these shoes introduce other problems?

Something more than just a thin powder-coating of paint on the typical spring is required, to stave off surface corrosion that could lead later on to more catastrophic failure of the spring. My own natural propensity for keeping corrosion at bay on my own Polo has been by judicious use of Waxoyl and it therefore seems to make good sense to keep the springs adequately coated with it and to keep a regular eye on the muck and grit that ends up in the spring seats, clearing it out whenever necessary. This includes the seats of the front springs too, those particular seats being fabricated as part of the front dampers.

Since first discovering how bouncy the suspension is on most of these 6R/6C Polos, I've wondered whether mine would benefit from changing the dampers to ones of a heavier-duty and adjustable type. That's what I did with my old Mk3 Golf, and that car lasted for over 24 years, with not a single coilspring breakage in that time.

Last but not least, maybe we should all take a bit more time and care when negotiating roadhumps and should resist being egged on by impatient motorists behind to take them at speed?
Last edited by veteran on Sat Jul 13, 2019 8:24 pm, edited 2 times in total.

2226
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Re: Coilsprings and their structural failure

Post by 2226 » Sat Jul 13, 2019 2:43 pm

Very interesting indeed.

Never had a spring failure in any cars I've driven for extended amounts of time. Wouldn't be surprised if modern springs have been cut down to the bare minimum required steel given that modern cars have such a tin-like feel to them. Seriously, my old mk1/citi golf (2006 model) might not be as comfy and pretty as the 2017 6C cross Polo but I swear it feels sturdier.

I'm wondering if the best thing to do is just swap to a set of proper aftermarket springs from the likes of Eibach or H&R. You know, a company whose bread and butter is suspension. Would companies like this be spending more time on quality control and engineering than what the OEMs are offering? Or are they further into the game of maximising profit?

RUM4MO
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Re: Coilsprings and their structural failure

Post by RUM4MO » Sat Jul 13, 2019 3:44 pm

2226 wrote:
Sat Jul 13, 2019 2:43 pm
Very interesting indeed.

Never had a spring failure in any cars I've driven for extended amounts of time. Wouldn't be surprised if modern springs have been cut down to the bare minimum required steel given that modern cars have such a tin-like feel to them. Seriously, my old mk1/citi golf (2006 model) might not be as comfy and pretty as the 2017 6C cross Polo but I swear it feels sturdier.

I'm wondering if the best thing to do is just swap to a set of proper aftermarket springs from the likes of Eibach or H&R. You know, a company whose bread and butter is suspension. Would companies like this be spending more time on quality control and engineering than what the OEMs are offering? Or are they further into the game of maximising profit?
As far as specialist springs/damper companies are concerned, you would hope that quality is higher, Lesjofors is a brand that I trust, but maybe read this https://www.ukpassats.co.uk/phpbb/viewtopic.php?t=86870 who knows what to believe!

Fitting these stone guards is probably not needed in most markets, and it will be stones and not grit that these guards are made to protect against.

Difficult to be sure that you are in a "win win" situation when trying to make things better, try walking into a VW Group and asking for these aluminium "inserts" and you will normally get faced with confusion as no one bothers about them and very few people know they exist, I did buy and fit a pair to my wife's previous Polo, just because I knew that it started life with them, so I wanted replacement springs (Sachs ECP) to have a "good life". Would I ever consider fitting new ones every 5 years, hummmmm - maybe, only maybe, they costs very little but will need ordering in from the VW Group parts warehouse.

2226
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Re: Coilsprings and their structural failure

Post by 2226 » Sat Jul 13, 2019 4:34 pm

Badge engineering seems to be the order of the day.

RUM4MO
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Re: Coilsprings and their structural failure

Post by RUM4MO » Sat Jul 13, 2019 5:25 pm

2226 wrote:
Sat Jul 13, 2019 4:34 pm
Badge engineering seems to be the order of the day.
Many similarities between the Lesjofors website pages and the Kilen website pages.

Have you removed factory fitted brake pads and worked out who actually manufactured them, can be very tricky, my wife's 2015 Polo 1.2TSI 110PS has ATE branded, Jurid branded, with the usual VW Group branding on the original front pads, so they may be Jurid manufactured for the front brake supplier who for that car is ATE and as usual will carry the VW Group branding and part numbers.

I'm not sure if I could any sense of the markings on the original rear pads.

joe6
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Re: Coilsprings and their structural failure

Post by joe6 » Sat Jul 13, 2019 5:50 pm

Good find veteran. I have had both car manufacture and good quality aftermarket front springs fail. The car manufacture ones were replaced free of charge although 7 years old as I found with some research that there had been a faulty batch in that period (quality control issue?). The aftermarket ones failed after some 120000 miles. My daughters 4 year old car (45000 miles) has had a spring fail recently and the paint/coating was still intact. It failed at the bottom end as suggested by the article. I think current road conditions are a significant factor (potholes) and where you live - cold with lots of road salt. The design these days tends to be progressive springs which the article also says is a factor.

2226
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Re: Coilsprings and their structural failure

Post by 2226 » Sat Jul 13, 2019 6:48 pm

RUM4MO wrote:
2226 wrote:
Sat Jul 13, 2019 4:34 pm
Badge engineering seems to be the order of the day.
Many similarities between the Lesjofors website pages and the Kilen website pages.

Have you removed factory fitted brake pads and worked out who actually manufactured them, can be very tricky, my wife's 2015 Polo 1.2TSI 110PS has ATE branded, Jurid branded, with the usual VW Group branding on the original front pads, so they may be Jurid manufactured for the front brake supplier who for that car is ATE and as usual will carry the VW Group branding and part numbers.

I'm not sure if I could any sense of the markings on the original rear pads.
Who ever makes the OEM pads for the front of the 6C makes them out of pure dust. My wheels coat up within a week of cleaning. Thinking of getting ebc, but I have this feeling they'll complain about that as being against warranty.

RUM4MO
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Re: Coilsprings and their structural failure

Post by RUM4MO » Sat Jul 13, 2019 10:45 pm

2226 wrote:
Sat Jul 13, 2019 6:48 pm
RUM4MO wrote:
2226 wrote:
Sat Jul 13, 2019 4:34 pm
Badge engineering seems to be the order of the day.
Many similarities between the Lesjofors website pages and the Kilen website pages.

Have you removed factory fitted brake pads and worked out who actually manufactured them, can be very tricky, my wife's 2015 Polo 1.2TSI 110PS has ATE branded, Jurid branded, with the usual VW Group branding on the original front pads, so they may be Jurid manufactured for the front brake supplier who for that car is ATE and as usual will carry the VW Group branding and part numbers.

I'm not sure if I could any sense of the markings on the original rear pads.
Who ever makes the OEM pads for the front of the 6C makes them out of pure dust. My wheels coat up within a week of cleaning. Thinking of getting ebc, but I have this feeling they'll complain about that as being against warranty.
Probably!

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