The Relationship Between Spacers and Spring Preload
Adding spacers within the shock absorber's strut mount (tower top) or upper rubber mount can-theoretically-influence the spring preload by altering the height of the spring's upper seat. Much like adding shims to sofa legs can adjust the firmness of the cushions, every 1mm increase in spacer thickness results in an approximate reduction of 0.8–1.2mm in spring preload (depending on the specific spring stiffness). However, the following points require careful attention:
Strut mount spacers act directly upon the spring seat.
Upper rubber mount spacers require accounting for the deformation of the rubber material.
The spring's free length must be remeasured following any modification.
Differences in Modification Effects Based on Location
Strut Mount Spacers: Provide a direct and immediate effect, though they may alter the suspension geometry parameters.
Upper Rubber Mount Spacers: Due to the elasticity of the rubber, the actual adjustment to the preload will incur a loss of 10–15%.
Combined Modifications: When utilizing both types of spacers simultaneously, the cumulative (superposition) effect must be calculated.
Three Major Issues to Watch Out for During Modification
This seemingly simple procedure can trigger a chain reaction of adverse effects:
The shock absorber's effective travel may be reduced, compromising its damping and buffering performance.
The risk of uneven spring wear (side-loading) increases by 30–40%.
The load distribution on the strut mount bearing becomes uneven, thereby reducing its service life.
It is strongly recommended to perform a four-wheel alignment check after every such modification.
