KONI Sport and FSD (Frequency Selective Damping)
During a typical mile of travel down the road, the damper will move in (compression) and out (rebound) millions of times. Break that mile into one second of travel, and the number of times the damper moves (stroke) defines the operating frequency (Hertz, Hz). Controlling body roll entering a corner induces relatively few strokes of the damper (low frequency), while chattering over broken pavement at highway speed results in a much higher quantity (high frequency) of very short strokes. Knowing this, damper frequency can also be used to tune handling and ride quality. Vehicle body roll control needed to aid handling occurs at a frequency of around 1Hz. Ride impact harshness is felt when damper frequency reaches approximately 10Hz.
Hit a bump without dampers, and the suspension would continue to bounce up and down uncontrollably like a bobble head doll. The damper's job is to reduce the size and/or speed of the suspension movement, preventing the never-ending bobble head scenario. We've all seen that older car going down a smooth highway with one of the rear tires visibly vibrating up and down very rapidly, which is caused by a worn-out damper that is no longer controlling the movement of the suspension. The lack of damper control allows the suspension to move uncontrolled.
The piston moving back and forth through the oil inside the damper creates the resistance needed to control (dampen) the suspension movement. How much resistance (force) develops for a given movement is determined by internal valves that control the flow of oil. Based on the force, the valves open or close to self adjust to each bump or any vehicle body roll. All dampers do this; but KONI's FSD units add a second valve system that responds to the damper's operating frequency, allowing ride and handling to be tuned more independently than with conventional dampers.
KONI's design theory for tuning dampers uses the compression (in) force to dictate the amount of axle movement and is directly linked to handling like steering response, road holding, etc. The rebound (out) force is used to control body movements (side-to-side body roll and/or pitch fore and aft) and is also directly linked to ride comfort.
KONI's traditional user-adjustable yellow sport dampers most applications have a fixed setting for the compression rate, but allow the user to manually adjust the amount of rebound control to tune the balance between ride comfort and handling. The FSD dampers also have a fixed compression setting, but instead of allowing the user to manually adjust the rebound, the internals of the FSD dampers automatically adjust the rate and shape of the force curve (when plotted on a graph) on the fly, responding immediately to the frequency of the damper’s movement. The unique secondary valve used inside the FSD units is engineered to provide large amounts of force to control vehicle body movement at relatively low frequencies, while generating less force at the higher frequencies that affect ride quality.