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Coil Spring Basics

05 Sep 2019


There is a wide range of coil springs available on the market, each with their own benefits. The manufacturer may utilise a number of spring designs for the same model; sometimes even two different springs on the same axle.

Despite the variations, they all have something in common – being crucial to the comfort and safety of the occupants of a vehicle.

Types Of Spring

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 useful when there are space limitations in the suspension system. They generally 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 are also handy when space is limited, most frequently in the rear of the vehicle. They have adjusted coil diameters so that most of the coils fit inside each other when the spring is compressed which can reduce the block length close to twice the material diameter. 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.

Side Loaded Springs (Banana Springs)

Side load springs, also known as Banana Springs due to their curved shape, are becoming more commonplace in the suspension system of modern cars. These springs operate by exerting a lateral force on the shock absorber in the opposite direction thus creating a balance which allows the strut to operate at the optimum level and increase the life expectancy of the shock absorber.

Quality assurance

All coil springs manufactured by NAPA 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 epoxy powder coating which, together with powder paint, ensures galvanic protection against corrosion and a good cosmetic finish.

NAPA’s production complies with the ISO 9000 quality assurance system and is also certified to the environmental standard ISO14001.

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