Fork Oil Change...

Discussion in 'Thumpers' started by neduro, Mar 28, 2005.

  1. neduro

    neduro Addict

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    Several folks have asked for this... so I took a few pics yesterday when I serviced the XR forks. These same instructions will work for upside down forks (like the WP's on KTM's) with a couple of small caveats, which I'll get to at the end. I accept no blame for my terminology. :lol3

    Start by loosening the upper triple clamp pinch bolts and breaking the fork caps free while the fork is securely held by the lower triple clamp. I don't have a picture of this, as I dont' torque my fork caps tightly enough to require doing so... but if you haven't done the job before, start here.

    In any case, once you have broken the fork caps free, drop the forks out of the triples and loosen the top caps until the slider drops away, like this:

    [​IMG]
    [​IMG]

    Now, with the slider down just a bit, compress the fork spring and put a wrench on the nut atop the damping rod. On the XR, it's a 17mm.

    [​IMG]

    Your goal here is to spin the topcap (silver) off the top of the damping rod (darker). To that end, put a wrench on the topcap, and pull against the wrench on the damping rod, and you should be there...

    [​IMG]

    With the topcap off, you can now pull the spring out (along with any washers or spacers- on the XR, there is a loose washer that sits atop the spring). Set it all aside.

    Push the damping rod into the forks, and the oil pressure beneath will force out the adjusting rod (made that term up! but the purpose of this rod is to put pressure on the rebound stack at the bottom of the damping rod, depending on how far the external adjustment is screwed in...)

    [​IMG]

    Pull these two parts out and set aside.

    Now you're ready to drain. Simply turning the fork upside down will get rid of most of the oil volume, but not the oil you most wish to swap out, which is trapped in the damper chamber. To get rid of it all, you'll have to pump the damper rod, turn things right way up and then upside down again, etc. You know you're done when the damper rod falls down into the fork without slowing as it enters oil...

    Again, even if the oil in the main chamber looks pretty good, it doesn't get exchanged much with the oil in the depths of the damping stack. It's worth the time and effort to get every last bit out. If it's really dirty, consider flushing it with some brake parts cleaner, but make sure you let it dry long enough that none of the cleaner remains to contaminate the new fluid.
    #1
  2. neduro

    neduro Addict

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    Now you're ready to fill.

    Drop the adjustment rod and "thingy" back into the damping rod.

    [​IMG]

    Push the slider all the way down into the fork, and let the damping rod drop as well.

    Fill the forks with oil.

    [​IMG]

    Pump the damping rod up and down until the damping is consistent from top to bottom. This indicates that you've filled the damping stack with oil... you can't set the oil height until this is done properly.

    A few words on fork operation: the damping stack controls how quickly oil can flow in and back out of the damping chamber, and thus, how quickly the forks can compress and extend. This "stack" is a bunch of flexible washers that flex to let oil past, and can be tuned to let a small volume of oil pass quickly but a large volume slowly (thus, a small amount of high speed compression damping but more low speed compression damping). The washer stack is sprung to let oil going the other way pass without resistance, so that there is a seperate stack for rebound and compression. The stack is the key variable to how a bike rides when high in the stroke.

    As the suspension compresses, though, a different variable comes into play- the volume outside the dampin chamber is decreasing, while the oil volume remains constant. At perhaps 1/2 to 2/3rds of the suspension travel, the increased pressure of the air trapped above the oil becomes a relevant force, and has a huge impact on bottoming resistance for the fork. More oil = less air to compress = greater bottoming resistance.

    So, the oil height (measured with the springs out and forks fully compressed) is absolutely critical to setup. Therefore: NEVER SET FORK OIL HEIGHT BASED ON OIL VOLUME!!! Always measure the air gap- a tiny difference in volume will make a huge difference in height and therefore bottoming resistance.

    On the XR, I like to run a bit more oil than factory recommended- they say 120mm if memory serves, I like about 100mm. Measure against the top of the compressed fork.

    [​IMG]
    [​IMG]

    Here is the key difference for inverted forks- some of them bleed oil to the area between the male and female, and some of them don't. To set the height correctly, you need make sure you understand what you are measuring- Oil height with oil between the walls, or with the walls dry, or etc. RTFM (Read the Manual).

    When you are satisfied that the damper is bled and the oil height is correct, it's time to reassemble.

    The first step is to get the preload where you want it- observe the relative position of the nut atop the damping rod in the following two photos- I set it in the middle for assembly:

    [​IMG]
    [​IMG]

    Now for the fun part. When you pull the damping rod up and drop the spring around it, the damping rod immediately starts to fall back into the fork, and you can't grab it because the spring is surrounding it. Here's my trick- slide an open end that has a smaller size than the nut on the end (but bigger than the rod, obviously) through the spring and then turn the spring while holding the open end still to pull the damping rod up. A few thousand words on the subject:

    [​IMG]
    [​IMG]

    Once the damping rod is poking out the top, drop any washers on and then spin the topcap down onto the damping rod until it contacts the damping rod nut. Compress the spring, fit the wrench again, and tighten:

    [​IMG]
    [​IMG]

    Pull the slider up, tighten the topcap onto it, and you're done!
    #2
    thirsty 1 and cptr like this.
  3. iillyyaa

    iillyyaa Adwrenchurer!

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    Ned, thanks for sharing!

    I have a question about the long steel rod in your picture. On my SV650 (damping rod forks) the "damping rod" is the tubular doohickey that is affixed at the bottom of the slider that slides tightly inside the inner tube. And there is no such long rod that goes up to the fork cap as in your pictures. What gives? Is there a piston with a shim stack at the end of the long steel rod?

    I can see that the forks on your XR are not "inverted", but is this setup called "damping rod" or "cartridge"? I'm a bit confused with terminology here...

    Thanks,
    --
    Ilya
    #3
  4. neduro

    neduro Addict

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    Hi Ilya!

    A good description of the difference between damping rod and cartridge forks can be found here.

    The upshot is that cartridge forks also have a rod running the length of the fork to control damping... but that the damping itself is controlled differently between the two types. Damping rod forks have a fixed sized orifice, Cartridge forks have flexibile shims. On the XR, there is indeed a shimstack at the bottom of that long rod, accessible from the bottom of the female fork slider.

    I'm trying to picture exactly what you're talking about, and google didn't help me. Can you find any pics?

    - Ned
    #4
  5. iillyyaa

    iillyyaa Adwrenchurer!

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    I just found the same article that you linked to. I read it (and a few others) to come to the following understanding:

    - "cartridge" forks (whether inverted or not) are ones that have a separate damping tube inside the fork, and they have two separate valves with shim stacks - one compression damping valve affixed at the bottom of the damping tube, and one rebound damping valve set in a piston attached to the top of the fork by a long rod and sliding inside the inner damping tube.

    Here's the exploded view of the XR650 fork:
    http://www.ronayers.com/fiche/200_0390/front_fork/front_fork.bmp


    Damping rod forks (like the ones on my SV) do NOT have this rebound piston sliding inside the inner tube, and hence they are not cartridge forks. The piece with holes that does the damping is a tubular stub (aka doohickey, aka damping rod) located at the bottom of the slider. That's what I was talking about.

    Here's the exploded view of the SV650 fork:
    http://www.ronayers.com/fiche/300_0305/front_fork_damper_model_x_y/front_fork_damper_model_x_y.bmp

    The doohickey that I was referring to is #13 in the SV650 diagram.

    So, it appears that your forks are in fact cartridge forks - which was my original question.

    I hope that my little diversion doesn't diminish the usefulness of your thread.

    Thanks for sharing!
    --
    Ilya
    #5
  6. neduro

    neduro Addict

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    Oh my god- not a diversion on advrider! :lol3 Didn't you read the rules!
    #6
  7. tirebiter

    tirebiter Been here awhile

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    I recently changed the fork oil in my 650R, and was wondering how to set the spring pre-load for the correct ride ht. I checked the service manual, owner's manual & the pigpen, no dice. Is there a specific ride ht. or sag measurement I should be looking for?

    Thanks,
    -tire-
    #7
  8. Loadedagain

    Loadedagain making chips

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    tanks ned. kinda universal info there meng :thumb
    #8
  9. Guzz

    Guzz Gutless wonder

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    Flushing out the all the old oil and crap is REALLY important. Don't skip it!

    Yes, dust and dirt will and does get inside the forks. No seal is perfect, and because they have worn they will let crap in.

    So don't forget when you have everything taken apart, start flushing and cleaning things. Brake cleaner can be used, if you have removed all the rubber seals and o-rings. Just remember to let the forks sit for a bit after the flush, to let the brake cleaner to evaporate.
    #9
  10. Cogswell

    Cogswell Trying to live the new normal.

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    Regular ATF (automatic transmission fluid) works great for flushing and cleaning the internals of the forks as it will not harm the o-rings and seals, and can even be used as a light weight fork oil if need.
    It's cheap too !

    Mike
    #10
  11. HellSickle

    HellSickle Scone Rider

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    I'll add my .02.

    Put a drop of blue loctite (242) on the threads of the damper rod before threading into the fork cap. It's not pretty with a damper rod comes loose.

    For feeding the damper rods thru the fork spring, I use a long piece of welding rod. I bend a hook to loop under the sleeve nut, thread it thru the spring, & drop the spring over the rod.

    -Jeff-
    #11
  12. alpiv

    alpiv Sr. nOOb

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    Many thanks for the instructions -- Used them myself last night with out any issues.......:clap:clap
    #12
  13. D_F

    D_F Meat servo

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    Thanks for the post. I'm going to give this a shot on my 650R this weekend. One stupid question? "More oil = less air to compress = greater bottoming resistance." From reading your post I kinda thought it would be the other way around???? Maybe it will make more sense after I get my hands dirty this weekend (always seems to help)
    Thanks again for the post and photos.
    #13
  14. Thorny

    Thorny Been here awhile

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    I'm bringing this back to the top, and hoping that someone can answer the below question. I will be changing shock oil soon and I am interested in how to determine how much preload to use as well.

    #14
  15. neduro

    neduro Addict

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    Shock Oil? That's an involved process, and it's critical to get everything bled properly...

    The way to set preload in the rear is with a tape measure- setting correct sag is covered lots of places. Generally somewhere just over 100mm of race sag, and hopefully 20-25mm of free sag.

    While I'm sure you could follow a similar process up front, my recommendation would be to note where it was set when you disassemble it, and if you felt they were too soft or too stiff, adjust accordingly.

    :dunno
    #15
  16. CFAmD7G

    CFAmD7G Adventurer

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    :rofl
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  17. slogger

    slogger Been here awhile

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    On photo #5, doesn't that 2 inch(approx.) piece that slides into the damping rod, on top of the adjusting rod, go in with the "valve" (cut out section) facing down? This is opposite of the way the photo looks? Maybe it doesn't make a difference?

    Also, from what I could tell from my manual, don't you measure the oil height with the damping rod down but without the adjusting rod and short valve thing out?
    Does that make any sense? If I'm wrong on these, it might explain why my forks have always felt awful.

    Thanks
    #17
  18. neduro

    neduro Addict

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    After doing what felt like hundreds but was probably only 20 oil changes on these forks, someone recently told me that I was putting that piece in wrong. I feel quite comfortable reporting that there is no functional difference between the two positions, but you are correct, I have it backwards. :D


    I always set the oil height with those two parts out, (and everything compressed) because when you pump the cardridge to fill it, you'll shoot them into the ceiling otherwise. I'm sure it introduces a tiny bit of variation, but since I was already raising the height by close to an inch, I didn't figure the .2cc's difference it made was probably all that consequential.

    Good eyes on both counts...
    #18
  19. slogger

    slogger Been here awhile

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    I'd like to ask you a question, if you don't mind.
    I just changed the oil in more XR650 forks over the weekend. The suspension tuner that did the valving suggested I put 2.5 weight at 110mm. His thought was that the lighter oil would improve the plushness over the 5 weight that he originally put in.

    Well, I turned the springs carefully to avoid losing to much oil out and thought I'd messure the oil height that I currrently had in them. Well, it was about 125-127 on each leg. Granted I lost a little oil when I removed the springs but I was surprised. The tuner told me he set them both at 110mm?

    So I followed the manual and set the level at 110mm/2.5 weight. I took a 200 miles desert loop yesterday and 20 feet from the truck it was obviously worse then before I changed the oil. All the sharp, fast hits we coming directly through the bars. Prior to "my" oil change the bike handled really good. Nice and cushy. I was 80% satisfied but managed to mess something up.

    Do you think a 15mm higher oil level would be this dramatic or did I screw something else up? Any idea's. My wrist, hands and arms are tingling today! Not good.
    #19
  20. slogger

    slogger Been here awhile

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    I reserviced my forks and learned the following. If you messure you fork oil height without the adjusting rod and 2inch "valve" thing on top, the oil level is about 10mm lower then with them in.
    So, I've been setting my oil level at 110 without these in, then inserting them which raised the level to 100mm. I suspect this is why my forks have always been really harsh of fast hits.
    Or at least that's what I'm telling myself.
    #20