The front forks on most Sportster models consists of two tubes - A slider tube that is mounted in the triple-trees and a lower leg tube into which the slider tube compresses. Each slider/leg assembly has an internal compression spring and damper tube. The spring supports the weight of the rider & bike, collapsing or expanding to cushion the ride over bumps and cavities in the road surface. The damper tube is submerged in oil and uses the passage of oil between internal chambers to help control the speed with which the slider/leg assembly compresses together & expands apart as it cushions the ride. Because the damper tube is attached to the lower leg, the length of the damper tube determines the amount of 'travel' the slider tube has within the lower leg.

In addition to the internal physical spring, there is an 'air spring' effect caused by the amount of air above the oil level in the fork assembly.

Functionally, there must be enough fork oil to fully submerge (engulf) the damper tube during its entire operational movement. The MoCo specified oil amount still leaves an air pocket above the level of the oil in the fork.

The air captured between the top of the oil level and the fork caps (which are sealed) creates an 'air spring' by which the compression of the air during fork compression helps the physical spring prevent total collapse of the fork, while also assisting the spring in expanding the fork after a compression event. The size of the air pocket, measured by determining the exact distance the top of the oil is from the top of the slider tube, can be varied (by adding or reducing oil) to produce a stronger or weaker 'air spring' effect (but the adjusted oil level must still allow the damper tube to be fully submerged during its entire operational movement).

Here's an example of the parts for that design (1994 Sportster XLH):

* . . . Future Subject

* . . . 2014 XL1200T Front Fork Testing w/ Spring Rate Charts

* . . . 1994 XLH Front Forks Disassembled - Measurements w/Pictures

* . . . 1200S Fork Rebuild & Hyperpro Spring Fitment

* . . . 1200S Suspension: Settings

* . . . Future Subject

Abbreviations:

• Std = 883 Standard Model & 1200 Standard Model
  
• Dlx = 883 Deluxe Model
  
• Hug = 883 Hugger Model
  
• Ctm = 883 Custom Model & 1200 Custom Model
  

Slider Tube, Damper Tube & Spring

Slider Lower Legs (Except 1200S)

See also Setting up the 1200S Suspension

Slider Lower Legs

• 1200S sliders don't have drain plugs. So, here is one way of doing so. ²⁴⁾
• Basically, once you have the forks out of the bike, remove the large top nut (loosen it slightly while you have the forks clamped in the lower triple tree, (as shown here Part 4 Dismantle the Forks ) while holding the slider. There is no spring pressure on this nut, so it's easy. ²⁵⁾
• The slider will drop away, but don't let it drop more than a couple of inches or you'll have oil pouring out and not know how much has come out. ²⁶⁾
• Carefully tip the fork upside-down and pour the oil into a clear measuring container (a wide funnel will help you to not lose any). ²⁷⁾
• Bring the fork upright and pull the slider to the very top again, pump the fork a few times to empty the damper tube, then turn it upside-down again and pour more oil into your measuring container. Repeat this a couple more times to get as much of the old oil out as possible. ²⁸⁾
• Check how much oil came out of the fork, make a note of it, then pour the old oil into a waste container. ²⁹⁾
• Measure the same quantity of fresh oil into the measuring container, then, with the slider still close to the top of its stroke, carefully pour the oil into it. You're pouring the oil between the coils of the spring, so you need to go slowly and keep the fork vertical to avoid losing any. If you can get your hands on a funnel and fit a narrow flexible hose to the end of it, it may make things easier. ³⁰⁾
• When all the fresh oil is in, thread the slider onto the top nut again; clamp the fork into the bottom triple tree only to hold it steady while you fully tighten the top nut; then push the fork all the way to the top again and clamp it in place. ³¹⁾
• Repeat on the other fork. ³²⁾

• Be careful, with the Low bikes, taking out the shock caps. There is about 2 1/2 “ of spring preload so the cap wants to come out in a hurry. I am about 200 lbs and fairly strong and it was about all I could do to reinstall the caps with a 1 3/8 socket on a 6” extension on a ratchet. ³³⁾

(Measurements for this section contributed by XLForum members)

Reference Additional Information on the 2004+ models in these XLForum Threads:
http://xlforum.net/vbportal/forums/showpost.php?p=3265511
http://xlforum.net/vbportal/forums/showthread.php?t=1827354
http://xlforum.net/vbportal/forums/showthread.php?t=1085901
http://xlforum.net/vbportal/forums/showthread.php?t=1971682

45790-80 - Stock Philips-head Drain Screw with Washer (Requires 1 for each fork)

• (Starting in 2008, some models do not have fork oil drain screws)

45815-03 - Chrome Fork Slider Drain Screw Kit¹⁰⁴⁾
Screw size is M4 (0.7mm) × 6mm - Includes a pair of chrome allen head drain screws and washers - Uses 2.5mm allen key