Harley-Davidson Forks. Do You Really Know What’s Inside? How They Work?

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.

Zipper's

11 Responses to “Harley-Davidson Forks. Do You Really Know What’s Inside? How They Work?”


  1. 1 Kirk Perry Jul 7th, 2011 at 8:32 am

    Anyone using ’49-59 front forks should note, that where the dampening rod exits the bottoming cup is where V-Twin’s forks can leak. Dampening rod protrusion, coupled with an uneven milled bottoming cup, and the thin paper washers that ship with forks can gang-up on you and create a leak.
    The fix is to install thicker replacement washers (46111-48) and (46125-48) available at Kick-Start M/C Parts (616) 245-8991.
    That’s all from the past, now back to the future.

  2. 2 Chris A Jul 7th, 2011 at 12:10 pm

    @Kirk

    Dampening Rod? Why would there be a rod that makes things wet? I think you would want a Damping Rod

  3. 3 Kirk Perry Jul 7th, 2011 at 1:29 pm

    “that where the dampening rod exits the bottoming cup is where V-Twin’s forks can leak. Dampening rod protrusion, coupled with an uneven milled bottoming cup”
    ••••••••••••••••••••••••••••
    The dampening rod goes through the bottoming cup (’49-59 and later, but my knowledge stops at 1959). Having the damper rod pierce the bottoming cup is necessary, but the set-length of damper rods end consist of a shoulder above a flat-sided damper rod protrusion. If the bottom of the slider (fork leg) was counter-bored “uneven” (counter-BORE has a flat bottom. Counter-SINK has a cone bottom), then the damper rod will protrude too far into atmosphere, so far that the leading edge of the shoulder (behind the tip-end threaded section) will make metal contact with the bottom of the unevenly bored bottoming cups top surface and wick-oil so to speak, until the slider is empty of it.
    If you disassemble the slider by removing the top seal(s) then you can place (1) of the thicker paper washers INSIDE the “lower damper bushing” and (2) thick paper washers UNDER the lower damper bushing and thusly pull the damper stud shoulder out of atmosphere and back into the slider leg. Then when you tighten the damper stud nut w/ it’s “crush washer”, the stud shoulder WON’T bottom out on the bottoming cup and the nut WILLl tighten against the compressed paper washers to 20 ft. lbs. No more leak.
    The whole 1941-1959 thing is about overcoming the bare wires of repop, figuring stuff out and making a sound reliable machine out of ill-fitting parts. Sound like fun?
    They say thinking staves-off Alzheimer’s. 🙂

  4. 4 Kirk Perry Jul 7th, 2011 at 2:25 pm

    “the nut WILL tighten against the compressed paper washers to 20 ft. lbs. No more leak.”
    •••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••
    Correction: Tighten the damper stud nut to 15 ft. lbs., NOT 20 ft. lbs.

  5. 5 Luis Jul 7th, 2011 at 8:58 pm

    I still don’t get why HD can’t come w/ cartridge forks.

  6. 6 Hungouver Jul 8th, 2011 at 5:10 am

    Hey Luis, why cant HD make a non-pushrod motor o any host of modern features? LOL

  7. 7 Progressive Suspension Jul 8th, 2011 at 9:12 am

    Kirk, wow, that’s a lot of info on a very vintage fork. Thanks for sharing!

    Luis, we wondered about that cartridge thing as well and decided to do something about it ourselves.

    http://www.progressivesuspension.com/monotube/index.html

    Sealed, high pressure monotube damper, using deflective disc damping and a progressive rate spring (with some real rate, to deal with the brake dive big bikes suffer from).

    Don’t tell Harley.

  8. 8 Jc Jul 8th, 2011 at 12:49 pm

    I always found humor in explaining to people that you add spring to lower your front end.
    When you explain it to them they always have the “light bulb” moment.

  9. 9 Rob Campbell Jul 10th, 2011 at 1:41 pm

    This is great. I’ve had a progressive lowering kit sitting on my bench for a while. I recently (two days ago) got my shovelhead running again but the 2000 wide glide I put on it makes it sit too high up front. This article, coupled with the Progressive instructions, make it a bit easier to approach the job (at least in my head).

    Thanks.

  10. 10 MEL HARVEY Aug 30th, 2011 at 6:15 pm

    I HAVE A 1999 SPORTSTER HUGER. THE FRONT FORKS BOTTOM OUT WHEN I HIT A BUMP. I HAVE CHANGED OIL TO A HEAVER TYPE AND STILL NO HELP. CAN YOU HELP?

  11. 11 Progressive Suspension Aug 30th, 2011 at 8:11 pm

    The Hugger is cursed with a very small amount of suspension travel. A set of Progressive rate springs will certainly help, though. Springs will increase rate as they moves through the stroke and better handle bottoming control. Inexpensive and easy install on the Sporty.

    http://www.progressivesuspension.com/forksprings/index.html

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