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  1. #1
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    Choosing the fork oil

    All the parts are in for my '82 R65LS fork rebuild. I'm trying to determine the correct weight oil to use and haven't found clear direction. The owner's and shop manual do not appear to specify a weight. Clymers suggests 5W or 7.5W. I will be using Bel Ray oil. What is the most common choice amongst R owners?

    Rick T

  2. #2
    Retired and proud of it MOTORMAN's Avatar
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    Did you know what weight it had in it? Was it comfortable to you to ride? If so use the same. On my old airheads I found the front end with it's long travel was too soft. I went to a higher weight oil and it stiffened up so it didn't dive quite so much without getting a harsh ride. Oil is cheap and it doesn't take much. Try 10wt and see what ya think. If it's too harsh you can always change it fairly easily.

    IIRC the folks racing them said they used 15 wt oil but that memory is from WAAAAY back in time.

  3. #3
    James.A
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    I'm using 10w WolfsHead motor oil on my airheads front forks. I have several cans in my vast warehouse of stupid **** I have saved over the years. No trouble so far.

  4. #4
    On the Road bugtussle's Avatar
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    In my 84 R100 forks Im using synthetic Mercon V ATF fluid. If I remember right the viscosity is around 8.25. Its working good.

  5. #5
    Stressed Member jmerlino's Avatar
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    Can someone enlighten those of us who are ignorant on this subject - what effect does the weight have on the ride? Does a lower number indicate a lighter weight, and therefore a bouncier ride, or what?
    --Joe Merlino - Modified '82 R100RT

  6. #6
    Loose Cannon flash412's Avatar
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    Use 10wt. BMW says 7.5. Switching to 10wt will make the right slightly less plush. But you will feel MUCH more in control of the front end in the twisties.

    Hey, if you don't like it, spend another $8 and change it again.
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  7. #7
    Don't forget your towel
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    Quote Originally Posted by jmerlino
    Can someone enlighten those of us who are ignorant on this subject - what effect does the weight have on the ride? Does a lower number indicate a lighter weight, and therefore a bouncier ride, or what?
    Smaller numbers mean lower viscosity or as the dictionary says "resistance to flow". Oil in the forks provides damping to counteract the springs by flowing through a series of passages or valves in the internal fork assembly, the faster the oil can move the less damping (more spring) you'll feel.

    Steve

  8. #8
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    My experience is a little different, I have a '78 so the fork design is a little different. When I ran 10 wt I got a lot of "pack down" so I switched to 7.5 wt when I replaced the springs to get quicker recovery. Oil level can also affect damping, raising or lowering the oil level will slightly increase or decrease damping.

  9. #9
    Think in terms that the viscosity of the oil is only part of the equation. Viscosity primarily effects damping. The forks also contain a column of air above the oil which acts as an air spring. The air compresses when the forks shorten (dive) and decompresses when the forks extend. The volume of this column of air effects dive and the "plushness" of the ride. By increasing the volume of the oil you decrease the volume of the air - causing it to reach higher pressures with less volume change - to stiffen the ride and reduce dive.

    Then - since the damping needs to resist both the rebound of the metal spring and the expanding compressed air column you probably want to increase the viscosity of the fork oil to increase rebound damping..

    The Airheads came from the factory with relatively long travel forks. And BMW specs generally 7.5 wt fork oil. I like less dive and less compliant fork movement. I have had good luck installing 10wt oil and increasing the oil volume by 15 or 20 cc per fork leg. Install the spec'd amount plus 10cc and try it. Then add 5cc more. Maybe 5 cc more. I've never gone much over that because the fork action gets harsh when the air column gets too small.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://web.bigbend.net/~glaves/

  10. #10
    BUBBAZANETTI
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    Quote Originally Posted by PGlaves

    The Airheads came from the factory with relatively long travel forks. And BMW specs generally 7.5 wt fork oil. I like less dive and less compliant fork movement. I have had good luck installing 10wt oil and increasing the oil volume by 15 or 20 cc per fork leg. Install the spec'd amount plus 10cc and try it. Then add 5cc more. Maybe 5 cc more. I've never gone much over that because the fork action gets harsh when the air column gets too small.

    exactly!!!!!...........i found that "overfilling" slightly with fork oil a bit "softer" than i'd hoped for made for great results.............15W meant too stiff at required fill......10W. at required fill was a bit too "soft" that air at the top really does account for at lot of the "feel" in these telescopic forks...........

  11. #11

    fork oil drain plugs

    Years ago I had an R90/6 that had screws with rubber washers drilled
    on the bottom of the lower fork caps.
    Just behind the rubber caps.
    At the approximate low point of the forks.

    To change the fluid all I had to do was remove the screws and the fork oil drained out.

    I have never seen another like that and no one can remember a tech article about them.

    But they worked great and cut the change time to a fraction of doing it
    the recommended way.

    Any comments?

    Gerry

  12. #12
    airhead6
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    fork oil

    I have a 74 R90/6 and will be replacing the fork oil today. An oldtimer told me to use brake fluid as the oil? Any comments

    Gmac

  13. #13
    Liaison 20774's Avatar
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    I think you'd be smarter by using a fluid that was designed for the job. Use the BMW boutique oil (it won't break the bank) or try something from the HonKawaSuz shop. Anything else and you're inviting problems with compatibility, corrosion, foaming, flow rate, etc. Just doesn't seem worth it.

    Kurt in S.A.

  14. #14
    No, no, no, no no!!

    I would be pretty positive he did not mean brake fluid. He meant automatic transmission fluid. That was a common cheap shortcut back in the /5 days, and worked OK with the seals, and was approximately the right weight for those cushy long travel forks.

    DO NOT PUT BRAKE FLUID IN YOUR FORKS. You will create a corrosive mess. Brake fluid and air combine poorly, and moisture will quickly contaminate the mess and you will need new fork sliders in short order.

    You could get away with ATF, but I would recommend 10wt fork oil with 10cc extra in each fork.
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell
    http://web.bigbend.net/~glaves/

  15. #15
    Loose Cannon flash412's Avatar
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    Quote Originally Posted by airhead6
    I have a 74 R90/6 and will be replacing the fork oil today. An oldtimer told me to use brake fluid as the oil? Any comments
    By any chance did the same "friend" also tell you that brake fluid makes a great dessert topping?
    Click here for Tales of Motorcycles and Life
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