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Suspension

Side Load Coil Spring Technology

Side load springs, also known as ‘banana springs,’ are becoming commonplace in the suspension systems of modern cars. They work to solve a problem inherent in the design of the more traditional MacPherson strut and prolong the life of the shock absorber.

 

Problem Solving

The popularity of the MacPherson suspension system is due to the simplicity of its design, combined with its low manufacturing cost. These advantages have resulted in over 60 years of enduring popularity.

However, there is also a disadvantage in the MacPherson design. Lateral forces exist within the MacPherson systems which are exerted upon the shock absorber. These forces, unchecked, work to bend the shock absorber, pushing the piston rod against the cylinder seal, increasing the amount of friction.

This diagram shows a MacPherson system without a side load spring installed. The red arrow indicates the direction of lateral force caused by the geometry of the spring.

In practice, this additional friction increases the level of vibration transferred from the road, via the wheels into the car’s body, and impacts the smoothness of the ride while decreasing the life expectancy of the shock absorber.

These disadvantages can be reduced if the coil suspension spring is designed and assembled in such a way that it exerts a lateral force on the shock absorber in the opposite direction, thus creating a balance and allowing the strut to operate at the optimal level. These types of springs are called ‘Side Load Springs’, often nicknamed ‘Banana Springs’ due to their curved shape.

Several geometric options are open to the manufacturer to achieve this opposite lateral force:

• Position the end coils eccentric in relation to the centre of the shock absorber

• Incline the geometry of the end coils, usually with the aid of angled spring pans

• Design the spring with a bent centre line, known as a ‘C’ spring

 

 

Side load springs are becoming more popular as car designers embrace the benefits of such a simple change.

This diagram shows the side load principle as explained. The blue arrow shows the lateral force from the coil spring working in the opposite direction to the lateral force exerted by the MacPherson geometry.

 

 

 

Replacement Issues

As with any spring, Kilen recommends you replace side load springs in axle pairs. Failure to do so can result in a number of negative outcomes:

• Uneven ride height

• Less responsive steering

• Less secure road holding

• Increased braking distance

• Increased tyre wear

When fitting side load springs, it is important for mechanics to examine and make note of how the spring is positioned in relation to the spring pans before disassembly. Uncompressed, side load springs are banana-shaped. Installation compresses the springs causing them to straighten. If fitted incorrectly however, damage may be caused to the top mount and the spring may remain bent. This could lead to a spring touching the inner wing, creating noise, reducing effectiveness and perhaps even adding to the lateral force problem on the shock absorber.

It is not recommended that any attempt is made to retrofit side load springs onto a vehicle. They are intended for like-for-like replacement with the original equipment. Popular applications which utilise side load coil springs include the Fiat Punto, Citroen C3, Ford Mondeo, Vauxhall Astra and Volkswagen Polo.

Kilen is firmly committed to providing the highest quality products, and produces a side load spring design wherever that is used as the original. All Kilen springs come with a 3 year warranty and are certified OE ‘Matching Quality’, allowing fitment without warranty restrictions and giving you complete peace of mind.

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