Progressive Suspension 444 Shocks Progressive's 444 Series Shocks feature a proprietary patented Frequency Sensing Technology (Fst) that has the unique ability to dynamically adjust damping as you ride. FST allows the shocks to sense the frequency of a bump and automatically adjust damping for superior ride quality. A top mount threaded preload adjuster allows for fine-tuning with a simple twist of the cap. Every 444 shock is hand-built, tuned and dyno'd and include a Lifetime Warranty. Features Deflective disc valving Adjust spring pre-load by hand, no special tools needed Progressive rate spring Note spring rates below Standard duty for: 85-86 Honda 1100 Shadow, 84-87 700 Shadow, 83 750 Shadow, 98-03 750 Ace, 01-07 750 Spirit, 88 800 Shadow, 94-03 Vf750C Magna/Deluxe, 87-99 Kawasaki Vn1500 Vulcan, 87-09 Suzuki Vs1400 Intruder/Blvd.S83, 85-09 Vs700/750/800 Intruder/Blvd.S50, 84-99 Yamaha 1000/1100 Virago, 84-97 700/750 Virago, 85-07 1200 V-Max Heavy Duty for: 84-86 Honda V30, V45, Vf700C Magna, 85-06 Kawasaki VN 700/750 Vulcan, 87 Zl1000, 86-87 Zl600, 85-86 Zl900 Eliminator, 87-00 Yamaha 535 Virago