Progressive Harley-Davidson Tri-Glide 444 Series Shocks 13.5in - BlackThe 444 Series Shocks feature a proprietary patented Frequency Sensing Technology (FST) that has the unique ability to dynamically adjust damping as you ride; what this means is whether you're hitting the small or big bumps in the road, the shock distributes oil through ports accordingly to soak up the hit making for an ultimately comfortable ride. A top mount threaded preload adjuster allows for fine tuning with a simple twist of the cap. 444 Series