Coilsprings and their structural failure
Posted: 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?
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?