Progressive Heavy Duty Fork Spring Kit For Harley-Davidson 39mm ForkProgressive Heavy Duty Fork Spring Kit For Harley Davidson 39mm ForkSportster, FXR, and Dyna The 39mm fork is a wonderful upgrade from the 35mm unit it replaced, but it can display shortcomings when pushed hard, especially for loaded touring, when used on heavy Big Twins, or if routinely riding with a pillion. Progressive has a solution. Heavy duty version of Progressive Suspension's progressively wound fork springs Cost effective yet substantial