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Coil Spring Technology

The range and type of coil springs available has grown markedly in recent years, and the ease of availability of this product has become increasingly important.


The design and manufacture of coiled springs is crucial to the comfort and safety of the occupants of a vehicle, as well as affecting the lifespan of the vehicle itself.

Types of suspension spring

The most widely-used type of suspension spring is the coil spring, of which there are several different types that are described here. Differing market tastes may dictate that a manufacturer specifies a number of spring designs for the same model; there may even be two different springs on the same axle.

Cylindrical springs


A cylindrical spring design normally consists of end coils, transition coils and spring coils. It can be designed to provide a linear rate, or equally designed to give a progressive spring rate by varying the pitch. Cylindrical springs can also be bent in the axial direction.




Conical springs


Conical springs are normally chosen when there are space limitations for the spring in the suspension system. They usually have open ends and sometimes one pigtailed end. The springs can be designed with either a linear or progressive spring rate.




Mini block springs

Mini-block springs have adjusted coil diameters so that most of the coils fit inside each other when the spring is compressed to a short length. This design solution can reduce the block length close to twice the material diameter. The advantages of this are the very small block length of the spring and the space saved. This is often beneficial for fitment in the rear of the vehicle. The spring rate in mini-block designs can be controlled by four key variables: wire diameter, coil diameter, pitch and coil support from the spring pans.


Spring Design

The lifespan of the spring depends on the quality and mechanical properties of the material it is constructed from, as well as the designed stresses in the spring itself, and the manufacturing and surface treatment properties.

If a suspension spring is designed with stresses that are too high it will sag – a permanent deformation that is known as ‘relaxation’. Forming a spring with the wrong geometric dimensions or not using the correct manufacturing techniques can result in a spring with bad relaxation properties. Such springs would be very likely to shorten or sag in use, resulting in the vehicle ride-height dropping.

This is a problem with some suspension springs, and even if only one of the springs on an axle looks shortened, due to relaxation or sag, it is necessary to replace both. Failure to do this can result in an imbalance between the left and right wheel, adversely affecting safety through reduced braking performance and causing poor roadholding and handling.

Spring material

Today, almost all melting plants can produce steel with the specified spring steel chemical analysis but the requirements are much more stringent than that. The strength and hardness of the material are important quality factors.

The strength of the steel is the most important material property, and is the basis for the design of suspension springs. The steel used for suspension springs is of a grade designed for quenching and tempering to achieve the tensile strength required.

The quality of the surface of the material has a great impact on the performance of the spring. Cracks, flakes and other surface defects significantly reduce spring performance. Such surface defects would lower the spring fatigue strength and lead to premature breakage. The quality inside the material also influences its performance: decarburisation, macro and micro non-metallic inclusions being major factors. The type, size, distribution and quantity of such non-metallic inclusions can influence the fatigue strength and lead to premature breakage.

Wear and corrosion

Suspension systems are designed with the spring fully exposed to the road. This is a very harsh environment in which to operate, as the spring is continually under attack from water, stones and possibly salt during the winter months, and without adequate protection the steel will begin to rust immediately.

The endplates, especially the lower one, collect dust, gravel and water, making an excellent environment for heavy corrosion to begin. When the system deflects, wear occurs between the spring’s end coil and the end plate. Rubber or plastic sleeves are sometimes used on the end coils to delay this. Such wear is inevitable after time, as the lacquer layer will become damaged.

Corrosion, once the suspension system has been fitted to the car, is the most common cause of broken springs and there are different methods to protect the steel against this. A one-layer wet paint or one-paint coating is commonly used for aftermarket products. These are sufficient until a scratch or defect occurs in the paint layer. Then, because of the crevice effect under the paint surface, creep corrosion spreads rapidly.

Defects created by corrosion on the steel surface heavily increase the risk of spring breakage.

Defects in the layer of lacquer cannot be avoided, so it is clear from this that another form of protection between the paint and the steel should be used. The technique, specified by the car manufacturers, is normally a special zinc phosphate method. This technique effectively protects the steel against corrosion even if defects occur in the paint layer. To get the right protection effect, a multi-stage chemical process is needed. Spring manufacturer Kilen employ this process which also includes: activating, zinc phosphating, passivating and painting, and is not to be compared with iron phosphate or other phosphate methods that are less effective in corrosion protection. Kilen use a multi-stage surface treatment line, incorporating a zinc phosphating process, epoxy powder coating process and finally the ink-jet printing of the part number and production batch number onto each spring.

Quality assurance

All coil springs manufactured by Kilen are quality assured, using spring steel from suppliers that meet the most stringent standards for both internal and surface quality. The load and deflection parameters are assured by quality control throughout the production process. To verify that the result is within given tolerances, a final load testing of every batch is made before surface treatment. The load at rebound length, length at kerb weight, load at bump length and the spring rate are the most important measurements assessed.

Surface treatment is carried out using an advanced zinc phosphate system method which, together with powder paint, ensures galvanic protection against corrosion and a good cosmetic finish.

Kilen’s production complies with the ISO 9000 quality assurance system and is also certified to the environmental standard ISO14001. As a specialist spring manufacturer, Kilen are in control of the whole process – from spring calculation to the final product, therefore assuming full product liability in accordance with strict product safety regulations.

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