412 series shocks: Best value Progressive Suspension shock, great as stock replacement. Nitrogen charged double steel wall body. Multi-staged velocity sensitive valving. Five position cam style preload adjustment. Note: Black 412 shocks may be supplied without lower cover. 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.