In this article, Bilstein explains why springs and dampers should be replaced in pairs

Bilstein says shock absorbers are a bit like teenagers: they feel most comfortable among their peers.
Therefore, if one of the components fails due to wear, defect or accident, the counterpart on the other side of the axle should always be replaced as well.
“Although this is not prescribed by law, technical expertise (says this) is required,” Head of the Bilstein Academy, Rainer Popiol, said.
Bilstein says the background is quite understandable for the layperson: just like tyres, shock absorbers wear out with every kilometre travelled; extremely slowly, but steadily. As a result, the damping force decreases more and more.
If a brand-new damper at the peak of its performance would be used together with an already aged specimen on the same axle, an imbalance would result.
This can vary according to the degree of wear of the “senior partner,” but even small differences are enough to disturb the finely balanced system that is a suspension.

“Even though the older damper may be quite good on its own, the decisive factor is that it no longer has the same damping force as the new part. And this is why agitation comes into the driving behaviour,” Rainer said.
The negative aspects of such a “marriage” characterised by age difference are particularly pronounced in extreme driving situations.
For example, if a quick evasive manoeuvre is necessary, many drivers are surprised at how unpredictable their car suddenly behaves. In the worst case, an accident occurs that could possibly have been avoided.
According to Rainer, the recommendation to exchange per axle applies equally to suspension springs.
Different degrees of wear can have an effect on driving behaviour also in this case. The combination of new springs and dampers on the one hand and relatively old components on the other would of course be particularly “dangerous.”
In the worst case, the effect even increases itself over time, since on the “weaker side” the periphery is also subject to more wear.
If the suspension geometry is adjusted in the course of this, even the tyres can ultimately wear out on one side. In the end, even initial savings are eaten up by the failure to replace shock absorbers or springs in pairs.

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