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Thread: Choosing the fork oil

  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
    Stressed Member jmerlino's Avatar
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    Quote Originally Posted by sgborgstrom
    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
    Makes sense to me. Thank you.
    --Joe Merlino - Modified '82 R100RT

  9. #9
    Registered User boxerkuh's Avatar
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    BMW suggests the use of 7.5 as stated before. I find that way too plush and poor resistance. I am currently running 15 weight, by mixing a 10 weight and 20 weight together. It gives me a firm ride, excellent response and excellent feedback. I am very happy with it. I found this mix via trial and error. I would suggest that you start with 10 weight and if that is still too soft for you, you increase it from there. It does depend on what type and style of riding you do and what gives you the maximum level of compromise...
    Food for thought anyway.
    Good luck... BTW, the oil is cheap enough to change frequently, so if you don't like it, poor it out and change it, until you are happy....
    Keep the rubber side down!!
    1986 R 80 RS
    1992 R 100 R
    BMW MOA Life member; Ironbutt Member; Airhead Member

  10. #10
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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.

  11. #11
    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
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  12. #12
    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...........

  13. #13

    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

  14. #14
    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

  15. #15
    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.

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