Question about fork oil change

Discussion in 'Airheads' started by r75/7, Sep 9, 2013.

  1. r75/7

    r75/7 n00b

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    Hi all!

    I recently bought an R75/7 1977, and last night i changed the oil in the forks.

    I drained the oil and filled it up with 5w fork oil, 235ml (in each leg) as stated in my book.
    After that was done i took it of the center stand and pumped it back and forth, holding the front brake.

    What makes me concerned is that i noticed the fork being much more compressed/lower then before and it feels softer.
    That is when it is standing on the ground off the center stand in resting position.

    Is it possible that the oil i drained was thicker?
    Or should i add oil to make it be less compressed in the normal resting position?

    Thanks for advices!
    Tommy

    (I am from Sweden so excuse my bad spelling)
    #1
  2. disston

    disston ShadeTreeExpert

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    Sounds like it had heavier oil in it. People do that a lot I think. They think it will make for a better performing ride but it doesn't really. See how you like it with the 5 wt oil. Not sure why it is sitting lower. Oil shouldn't effect the ride height but maybe it is just you think it's lower?

    I think the original oil was 4 wt. 5 should work.
    #2
  3. AliBaba

    AliBaba Been here awhile

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  4. Biebs

    Biebs BMW Airhead

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    I like using Dextron or ATF in my forks - weight is about 7 to 10 depending on brand. Dextron is about 7wt. It is hydralic fluid and is what was used back in the day -1970's:deal
    #4
  5. blaine.hale

    blaine.hale Long timer

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    5wt is pretty darn thin. I run 10w in both of mine and I'm a very thin dude.
    I believe the stock is 7wt. Just rebuilt my r75 forks and filled them with 10w40 motorcycle engine oil. Seems to be doing pretty darn well, and it's cheap.
    #5
  6. Stan_R80/7

    Stan_R80/7 Beastly Gnarly

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    I use Mercon V ATF in my '78 forks. It's 7.5 wt hydraulic oil and very similar to the Dextron ATF. I use a 265 ml volume for replacing oil (even though it has been considered obsolete) because I believe my 1978 Clymers manual more than the internet (and I like the results more than the 235 ml volume). YMMV.
    #6
  7. boxerboy81

    boxerboy81 Stay Horizontal

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    My factory manual says 250mls and mentions Shell Aero 4.

    Clymer mentions Belray SAE 5 and Shell Aero 4 amongst others.
    #7
  8. supershaft

    supershaft because I can

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    You did absolutely nothing to change your sag. Fork oil weight does not change sag. Or alter your spring in any matter. Do not fall into the misconception that it does. Fork oil weight alters your rebound and compression DAMPING. That is all. If you want to adjust your sag, adjust your spring preload.

    Fork oil weight varies. I have been using Spectro fork oil for forty years so keep that in mind when I recommend weights. Those bikes came stock with 5 weight or 2.5. IMO, heavier than 7.5 in your forks gives you too much compression damping and that gives you too harsh of a ride even with springs that are too soft. If we had just rebound in mind? I would run around 15 or 20 weight but we don't have just rebound in mind. The oil also effects compression damping and too much compression damping is WAY worse than too little rebound. It's an ill handling confliction. What you need is more preload, NOT heavier oil.
    #8
  9. SculptD

    SculptD Smells like tech

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    In my perfect world we would abandon all this 5/7/10/15 talk. The different brands are all over the place in their definition of what viscosity that actually corresponds to. Let's use centistokes. It's all in this handy chart:

    http://www.peterverdone.com/wiki/index.php?title=Suspension_Fluid

    ATF fluid? Quality stuff, but at 34 cSt it's way heavier than the traditional BMW stuff. Snowbum claims that was 13.2 cSt at 40C. I find the forks work best for me in roughly that range. Certainly below 20 cSt.
    #9
  10. supershaft

    supershaft because I can

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    No experience with that talk but that doesn't change the fact that what the OP described has nothing to do with his fork oil weight. ATF or 80/90 hypoid wouldn't matter.
    #10
  11. Stan_R80/7

    Stan_R80/7 Beastly Gnarly

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    Yes, the SAE 5W, 10W, etc. weights are not so comparable between fork oils. Maybe the intention is not that the numbers are SAE, but are just a 'weight'? Here is (yet another) thread talking about fork oils and weights (viscosities): http://www.advrider.com/forums/showthread.php?t=748254

    FWIW, Ford's specs on their Mercon V show 33 cSt at 40C and 7.5 at 100C: https://www.fcsdchemicalsandlubricants.com/main/additionalinfo/Product Data Sheet MERCON V.pdf

    I tried some lower weight fork oil and liked the ATF better. But then, I don't consider myself a fork oil connoisseur and don't race the bike. So, as usual, YMMV.
    #11
  12. SculptD

    SculptD Smells like tech

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    Quite so, YMMV. And riders have different priorities. Some will tolerate performance compromise in one area and not another. Many just cant abide fork dive under braking. Well, with light oil, these forks just do that. So be it.

    I will say that I think ATF is a quite reasonable fluid for forks—antifoam properties, corrosion inhibition, good on seals, etc. I would need some convincing about the use of straight motor oil.
    #12
  13. supershaft

    supershaft because I can

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    Fork dive under braking? Unless you are grabbing all the brake you can as quickly and as hard as you can, fork oil weight has very little to do with how much our forks dive under braking. Once again, that is best compensated by fork spring preload. Real slow speed damping? Sure, I am sure our forks have a little of it but most all of any brake dive is best handled by spring rate. If you think you can grab some brake as quickly as you can hit a bump at 100mph, you think too much of your forearms. :D That and you crash too much! :lol3
    #13
  14. fallingoff

    fallingoff Banned

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    has atf changed its weight over the years?.
    better oils etc
    I was told this the other day
    oh shit I asked a question in a oil thread.
    sorry/lol
    cheers
    #14
  15. SculptD

    SculptD Smells like tech

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    It would be hard to imagine that ATF hasn't been changed given everything involved. There may be high-end manufacturers specifying non-standard fluid. But the basic stuff has to be that 34 cSt viscosity, as the transmissions are literally designed around that fact.

    As for brake dive, I'll stir the hornets nest. Over some viscosity range, you must agree that fluid damping will affect this. Compare water and peanut butter. Water will dive, peanut butter will not compress at all, even without springs. The question is whether the viscosity range we are talking about overlaps with compression rates during braking. I say that when comparing that original light BMW oil to something as thick as ATF, there is a noticeable difference.

    Yes, you are right about springs as usual, but there are overlapping forces at work, and it's not one or the other.

    Did this noob just start something with SS over suspension? I must be nuts.
    #15
  16. pommie john

    pommie john Long timer

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    Tommy

    This thread has got diverted so to give you a short answer:
    No. Adding more fork oil will make no major difference to the ride height. That is affected by the springs and the pre load spacers ( if any).

    The air in the fork can act as a spring, so less air ( more oil) = harder forks, but the effect is so small I doubt if any of us would notice unless it is a long way out.
    #16
  17. Plaka

    Plaka Brevis illi vita est

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    I have noticed that when Swedes come to the USA for spelling competitions, they do poorly. English incorporates words from many languages, including the germanic languages underlying the nordic ones. You'd think the kids would do OK. But the spelling competitions all have words derived from french, italian, and even british (just to eliminate the australians). So they often go home disappointed. USA kids don't go to spelling competitions is Sweden. They know better.
    \
    -----------------

    When you drain oil, you do some bouncing to get all the old oil out. When you fill, you use the fill volumes for "oil change", not "fork rebuild".

    Do a simple fork alignment. Loosen pinch bolt and axle, fork brace at fender and lower triple clamp. Then do the bouncy-bouncy tightening each set of bolts from the top down. Do a bouncy-bouncy before tightening the next set. Tighten the lower triple, then the fender brace. End up at the axle.

    Look for sticking anywhere. Then go for a ride and see how things do. With the legs aligned and the new oil moved around, the legs should spring up to the proper height.
    #17
  18. supershaft

    supershaft because I can

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    Start something, I am just offering some good advise based on 46 years of hard riding. Brake dive is not fast enough for most all damping to have any effect on it. Fixing your fork springs with oil is such a common misconception it deserves some time to try to fix. It isn't one or the other. With around a quarter million miles experience with our forks riding them harder and faster than anyone I have ever ridden with, I would say it's around 90 or 95% one and 10 or 5% the other. Don't try to fix your springs with your oil. It makes for an under spring and harsh ride all in one and will get you into a lot of shatter and that can get you on your face. I am just trying to help. :norton
    #18
  19. r75/7

    r75/7 n00b

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    Thanks for all your input!
    I could not imagine this much response!

    I used the Haynes manual as instruction and for volymes!

    As i can understand from all this answers is this:
    The oil viscosity do affect the forks feeling softer.
    (I am ok with that.)

    The oil viscosity or volyme does not affect the hight of the forks in resting position.
    (It is in normal resting position standing on the ground (not on the central stand), that i notice the fork to be more compressed then before.)

    The manual said nothing about doing any adjustments after the oil change!

    Is it common to adjust "spring preload" as someone mentioned after an oil change?
    I have no idea how that is done!

    Thanks again for all your input!
    Regards
    Tommy
    #19
  20. r75/7

    r75/7 n00b

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    I forgot to mention that i did a test drive last night, and i could not notice any big differenties from before the oil change!
    It was maybe a little softer.
    So i guess it is maybe nothing to be hung up about!

    Tommy
    #20