444 series shocks: Comes with FST (Frequency Sensing Technology) which dynamically adjusts damping; according to road surface. Top mount threaded hand adjustable preload. What are Progressive Suspension''s ''Heavy Duty'' shocks? If you are a lightweight rider with a lightweight passenger; STD Duty is probably your best choice; heavy duty shocks will give you a harsh ride. For Big Guys & Gals with a lot of baggage; Heavy Duty springs are the way to go. Heavy Duty is not better; it''s different.Heavy Duty shocks or springs are recommended to be used on motorcycles that are operated at or near the manufacturers maximum load rating over 50% of the time. Super Heavy Duty shocks are for +350 lbs. loads.