Sometimes, the most simple technical things need to be explained or re-explained. Progressive Suspension, the largest aftermarket suspension manufacturer in the United States as well as a top tier supplier of suspension components to domestic OE manufacturers and top custom builders, is dedicated to researching and producing the best suspension platforms for each brand and model, American or Metric. David Zemla, its Marketing Director, wrote this very interesting illustrated piece helping you understand what’s going on inside your motorcycle forks and ow they are lowered without affecting the suspension quality.
Motorcycle suspension tends to be something of a mystery with forks at the top of the “what the heck is 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? What 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. Beginning at the bottom of the lower tube and working 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, although its primary job is to meter the fork oil pushed through it during suspension stroke.
Although 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 (A wheelie is a good example of full extension). Sitting on top of the damper rod is the main spring. Generally very long, the main spring 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 so it has a softer rate spring in initial travel and then firms up as it is compressed. Most stock forks use straight, or dual, rate springs. Above the main spring a preload spacer is often found. 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. Next month we’ll walk through the next generation of aftermarket front suspension as well as lowering without traditional ride quality compromises. David Zemla, Progressive Suspension.