Editor’s Note: This post was written David Zemla who works at Progressive Suspension. This is our fifth guest blog post from David and we want to thank him for sharing some of his expertise with us. His earlier posts can be found here.
Motorcycle suspension tends to be something of a mystery, with forks being at the top of the “what the heck’s in there?” list.
Sure, there’s a spring and some oil, but did you know there are two springs in each fork? Is the oil just a lube or does it have a bigger job? And what exactly happens when I lower my forks? All good questions.
Harley forks generally consist of upper tubes that mount to the triple trees, and lower tubes that slide over them and connect to each other via the front wheel. If we begin at the bottom of the lower tube and work our way up, we find a bottoming cup and damper rod. The damper rod and bottoming cup are bolted into the lower tube and serve to keep the two tubes from separating. However, its primary job is to meter the fork oil that is pushed through during suspension stroke. While it is a rather unsophisticated method, it damps the suspension travel and keeps your front end from bouncing back like a pogo stick after each stroke. The bottoming cup also adds additional compression damping to resist fork bottoming.
Captured under the damper rod is a top out spring. This small but critical spring keeps the upper and lower tubes from colliding during full extension, with a wheelie being a good example of full extension. Sitting on top of the damper rod is the main coil spring, which holds the front end up and compresses as the front wheel comes into contact with a bump. Higher-quality springs will generally be progressive rate, meaning the coils are wound tighter at one end than the other. This translates to a softer rate spring in initial travel, firming up as it is compressed. Most stock forks use straight or dual rate springs. Often found above the main spring is a preload spacer . This spacer is used to fine tune the ride quality and ride height by adding a little preload to the spring.
Simple enough, but what about lowering? Take a look at the illustration to better understand the components as well as the changes that occur to lower a traditional HD fork. In December I’ll walk you through the next generation of aftermarket front suspension as well as lowering without traditional ride quality compromises.