Harley Davidson Progressive fork spring kits The ‘Original’ Progressive Rate Fork Springs soak up the small road bumps, yet are firm enough to absorb the molar-rattling ones. Spring rates have been chosen to reduce front end “dive” during braking, yet still provide excellent ride comfort.
Progressive Rate Fork Springs have several advantages over straight rate springs. A straight rate spring only offers linear resistance to compression and is a compromise. For example, if you have a 20lb straight rate spring, it will take 20lbs of force to compress the spring one inch.
It will take 20 more lbs of force to compress it the next inch, and so on, until the end of the travel. A Progressive Rate Spring has the advantage of a rising rate resistance to compression.
For example, a 15lb - 25lb Progressive Rate Spring will take 15lbs to compress it the first inch, 17lbs the next inch, and so on, until the end of the travel, taking 25lbs to compress it the last inch. The benefit of this is that the spring can be soft enough at the start of the travel to offer a “plush” ride, yet be firm enough at the end of the travel to soak up the big bumps.