Harley-Davidson Forks – What’s Inside That Thing?

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November 12, 2010 | By: David Zemla

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.

Comments: 12 Comments | Categorized Under: Product Information

Comments (12)

Gerald, the Sporties are spring very soft and typically blow through the travel very quickly. A set of progressive rate springs and fresh oil can clean that up. As far as a noise specific to left turns, I suspect your kickstand could be hitting the ground.

i have 883 2001 forks are hitting bottom early plus i get a clanging noise when i bank left. is this a broken spring? or something in primary caseing ?

The 422 Series shocks are designed to ride at stock Softail chassis height or lower (within a 2″ range). Raising above the stock height is not doable or really recommended. A properly set up (preload and sag) High performance suspension will really clean up the handling of your bike.

Thank you for the response! One more question….. would I achieve better handling characteristics by raising the rear ride height, and is this achievable with the 422′s?

I had a 1974 CB750 Honda that had 4 inch pipe nipples put in the top of the forks. It raised the front end a considerable amount. Ride semed good to me. They were in there when I bought the bike used. Can I extend my xl1200cusom front end the same way? Dave from Gulfport

Tom,

You’ve picked a great setup. The 422 Series is an excellent damper and is both height and preload adjustable. The Monotube fork kit completely replaces the internals of your forks and provides true sealed dampers (like a high end shock) as well as a progressive rate spring tuned to the Softail. The first thing you’ll notice is a substantial reduction in fork dive at stop lights! Look for the monotube kits to be available early in January.

I ride a 2001 Dyna Wide Glide that has 4″ longer tubes and 7* triple trees. The ride is very smooth. For added support I have a crossover bar from Kuryakyn that keeps any high speed wobble out.

I have a 2005 Fatboy, and want to improve the handling characteristics. I’m 6’1“, 300 lbs., and mostly ride solo. I wouldn’t mind raising the rear an inch if possible. Am I looking at the right Progressive components; front: 31-2502, rear: 422-4037C?

Those new harley fork tubes are junk, I’ve seen them bend even with a low speed impact againt a curb and I mean real low speed, nothing like the older harleys these new ones scare me.
Griff

Sad but true , Mr. Proper….nothin’ much is made in America at all anymore….except poor people….more than ever before!

How many Harley owners know that many of the PARTS are made in CHINA and or JAPAN? Many of owners will put DOWN Japanese bike , but will failt to mentio that some of the parts in their HARDLEY DAvidson come from a foreign country.

Stretch the front end out about 12″ and the springs will last forever. I got my Suzuki so low it bottoms out for the shoulder to the pavement. It’s nothing like a Harley. I cannot imagine a good reason for changing the steering geometry, and making the handling so fast, but to each his own.

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