“Why did my coils springs break?” It’s a question we hear often, especially on older vehicles. The answer is that its often not just one factor that leads to coil spring failure, but rather several. To answer this question, we’ve taken the time to do some research on coil spring failures, both common, and esoteric. Like most parts on your auto, coil springs have a finite service life, and when its up, it’s up, often in a very dramatic way. Broken coil springs are a safety issue, adversely affecting vehicle handling, tire wear and suspension alignment. A broken coil spring must be replaced at once, and because springs wear at the same rate, they should be replaced in pairs only. So without further delay, here are the main reasons for coil spring failure on your auto.

Coil Spring Fatigue
Most springs fail due to fatigue, meaning they have sustained many compression-extension cycles, and the metal becomes brittle and breaks. If the amplitude of these cycles is large, the fatiguing process is accelerated. Cars with continually overloaded trunks are candidates for early spring failure. While cars have coil springs on the front wheels too, large loads of passengers and a full trunk are assumed mostly by the rear wheels. Fatigue is also accelerated by continually driving over rough roads, such as cobblestones, as opposed to smooth asphalt. However, springs can be more susceptible to fatigue if they have not been properly annealed after the hardening step, which would lower the number of cycles they could sustain from the very outset.

Most of the load the spring carries is near the outer diameter of the spring wire’s cross-section, as this is the resilient outer case that receives most of the temper. As the metal starts to fatigue, micro cracks form in the surface, and the metal becomes susceptible to corrosion. Less and less of the resilient section of the coil is available to bear the spring’s load, and the spring’s weakening is further accelerated.

Decarburization is the gradual reduction in carbon content of the outer layer of fatiguing steel, as it is exposed to air. The right amount of carbon makes steel very strong. Its reduction in the outside circumference further weakens this outer region.

Impurity inclusion during the forming process can cause micro-voids containing these materials, which weaken steel. Over time, fatiguing and inclusion exacerbate each other. Many coil spring failures start at inclusions, similarly to sudden fractures of diamonds with trapped impurities.

Even perfectly-manufactured rear springs weaken over time from doing their job–flexing while bearing loads. Flaws in manufacturing and the fatiguing process greatly accelerate the normal process, causing rear coil springs to break.