2100-4112 The Koni FSD (frequency selective damping) shock combines excellent road holding and handing characteristics with no-compromise comfort levels. It's a two-in-one solution from koni that doesn't require the installation of sophisticated electronics, cables or sensors. Firmness for sporty driving on even road surfaces. Smoothness for a comfortable ride on uneven road surfaces. 2 x front struts & 2 x rear shocks, fits all 993 including turbo's. In standard shock absorbers the main damping characteristic is defined by the oil flow going through the piston assembly (1). Combining it with the FSD feature (2), KONI added a special valve that controls an oil flow parallel to the one going through the piston rod (3). This parallel oil flow is closed by the FSD feature, giving a rise in damping force almost linear to the time that the piston is moving in one direction. Put simply: the FSD feature is a hydraulic amplifier that delays the build up of pressure. In some ways an extra tuning option has been created in order to get the best possible combination of handling and comfort. Since it is an integrated part of the hydraulic valve system inside the damper, no additional cables, sensors or any other electronic devices are needed to operate an FSD damper.