What would cause rear coil springs to break?
Coil compression springs are the most widely used automotive suspension springs. They are open compression springs, meaning that they spaces between their coils when they are unloaded.
They have been referred to as wound torsion bars because of the principle where twist is successively applied to a coil section by the sections next to it. While spring failure is infrequent, some do fail. Spring manufacturing will be explained first because many failures modes, including fatigue and corrosion, are tied in with some element of manufacturing.
Coil Spring Manufacturing
Automotive coil springs range in size from six to nine inches in diameter, and about a foot and a half in length. They are wound cold from ½ to 1-inch thick round carbon steel wire. They undergo several heat-cool operations during the manufacturing process, so that the steel's qualities are just right. The raw steel bar is first annealed where the metal is heated to cherry red, at about 815 degrees C (435 degrees C C). It cools slowly, which renders it dead soft (un-springy) and easy to work. It is machine-coiled soft. The spring is then hardened by again heating it to cherry red to relieve stresses from coiling. It is cooled rapidly by plunging into water, making it very hard. Then it is tempered by slowly reheating to a temperature of 260 to 371 degrees C, depending on the metallurgy, and then plunged into oil, which renders it springy, extremely tough and moderately hard. This is just right for automotive suspension springs.
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 boot 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.
Decarburisation 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.