Spring preload is the amount of compression already present in the spring when the damper is at full extension.
The basic physics of preload is the same regardless of coilover design.
What changes is its relationship with ride height.
This article explains the fundamentals of spring preload and then compares two common coilover designs:
- coilovers with independent ride-height adjustment
- coilovers that adjust ride height through the spring perch
For simplicity, I will refer to them as independent-height coilovers and perch-adjusted coilovers.
The discussion assumes a linear-rate spring operating within its normal range, without the bump stop or other secondary spring elements coming into play.
In this article, the static condition means the vehicle at rest with its weight supported by the suspension.
What Is Spring Preload?
Spring preload is:
the amount by which the spring is already compressed when the damper is at full extension.
In other words, it is the difference between:
the spring’s free length and its installed length at full damper extension.
Here, preload refers to this amount of compression. Moving the spring perch is the adjustment used to change it.
For a linear spring, the basic relationship is:
F=kxF = kx
where:
- FF = force acting on the spring
- kk = spring rate
- xx = compression from the spring’s free length
Within its linear operating range, the spring rate kk remains constant.
A Linear Spring Keeps the Same Spring Rate
Consider a spring rated at 100 N/mm.
Compressing it by an additional 1 mm requires an additional 100 N of force.
The same remains true if the spring already has 10 mm of preload:
100 N/mm100\ {\rm N/mm}
is still its spring rate.
Changing preload changes sag, the suspension’s static position within its available stroke, and the portion of the stroke being used.
These changes can also affect how the suspension feels from the driver’s seat.
Under the Same Load, Total Spring Compression Is the Same
Suppose we have:
- spring rate: 100 N/mm
- spring load at static ride height: 3,000 N
The total spring compression from its free length is:
3,000÷100=30 mm3{,}000 ÷ 100 = 30\ {\rm mm}
With zero preload, the spring must compress an additional 30 mm between full damper extension and the static condition.
With 10 mm of preload, the spring is already compressed by 10 mm at full extension, so only another 20 mm of spring compression is required to reach the same load.
In both cases, the spring is compressed by a total of 30 mm from its free length under the same static load.
What changes is the additional spring displacement required to reach that point.
Increasing Preload Reduces Sag
Sag is the amount the suspension moves from full extension to its static loaded position.
Increasing preload means the spring is already carrying force at full damper extension.
Less additional suspension movement is therefore required to reach the static position, so sag decreases.
The suspension’s static position moves toward the extension side of its travel.
As a result:
- less rebound, or droop, travel remains
- more bump travel remains
A useful way to think about preload is:
it influences where the suspension sits within its available travel under the same static load.
The Relationship Between Preload and Ride Height
As preload increases and sag decreases, the suspension settles at a more extended position.
With the lower mount left unchanged, moving the spring perch changes ride height on both designs.
The difference is whether that ride-height change can then be corrected without undoing the preload change.
This is where the structural difference between the two coilover designs becomes important.
Independent-Height Coilovers
On an independent-height coilover, ride height can typically be changed by adjusting the lower mount or lower bracket, altering the overall installed length of the damper assembly.
This gives the suspension two separate adjustment mechanisms:
- the spring perch changes preload
- the lower mount changes ride height
For example, increasing preload at the spring perch will:
- reduce sag
- move the damper’s static position toward extension
- raise ride height
The lower mount can then be adjusted to return the vehicle to its previous ride height while retaining the new preload and static damper position.
This allows:
preload and ride height to be treated as separate setup variables.
Likewise, adjusting only the lower mount allows ride height to be changed while retaining the existing spring preload.
Independent Adjustment Also Changes Stroke Distribution
Changing preload shifts the damper’s static position within its available stroke.
After ride height is corrected through the lower mount, the vehicle can retain the same ride height while using a different distribution of:
- bump travel
- rebound or droop travel
An independent-height coilover therefore allows the tuner to consider two things separately:
the vehicle’s ride height and the damper’s static position within its stroke.
Perch-Adjusted Coilovers
A perch-adjusted coilover changes ride height by moving the spring perch itself.
As the spring perch moves, several things change together:
- preload
- sag
- the suspension’s static position
- ride height
Preload and ride height are therefore coupled setup variables.
Changing one through the spring perch changes the other at the same time.
The Essential Difference Between the Two Designs
The difference lies in how many aspects of the setup can be adjusted separately.
Independent-height coilovers
The spring perch and lower mount provide two separate adjustments.
This makes it possible to:
- change preload and then correct ride height
- change ride height while retaining preload
- retain ride height while changing the damper’s static position within its stroke
Perch-adjusted coilovers
The spring perch affects both preload and ride height.
As the perch position changes, ride height, sag, and the suspension’s static position change together.
Conclusion
Spring preload is:
the amount of spring compression present when the damper is at full extension.
With a linear spring, the spring rate remains constant. Changing preload changes sag, the suspension’s static position, the distribution between bump and rebound travel, and ride height.
With an independent-height coilover, a separate lower-mount adjustment allows:
preload and ride height to be treated as separate setup variables.
With a perch-adjusted coilover:
preload and ride height change together through the position of the spring perch.
The fundamental difference between the two designs is therefore not the physics of spring preload itself.
It is:
whether preload and ride height can be adjusted independently.
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