Progressive 444 Shocks For Harley Dyna After more than a decade of ruling the roads with their famous 440 Series shocks with Ias, Progressive Suspension has unveiled another breakthrough design! It is dubbed Frequency Sensing Technology (Fst) for its unique ability to dynamically adjust damping as you ride. Features: Frequency Sensing Technology allows the shocks to sense the frequency of a bump and automatically adjust damping for superior ride quality Deflective disc valving Hand adjustable spring pre-load (no special tools needed) Progressive rate springs High pressure gas charged for consistent performance Lifetime Limited Warranty Made in the USA Sold in pairs