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Old 09-10-2013, 12:40 AM   #1
r75/7 OP
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Question Question about fork oil change

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)
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Old 09-10-2013, 01:00 AM   #2
disston
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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.
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Old 09-10-2013, 06:01 AM   #3
AliBaba
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Try SAE10: http://realoem.com/bmw/showparts.do?model=0302&mospid=51656&btnr=31_0301&hg=31&fg=42
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Old 09-10-2013, 08:07 AM   #4
Biebs
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Dextron ATF

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
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Old 09-10-2013, 08:17 AM   #5
blaine.hale
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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.
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Old 09-10-2013, 08:35 AM   #6
Stan_R80/7
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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.
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Old 09-10-2013, 08:54 AM   #7
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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.
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Old 09-10-2013, 12:13 PM   #8
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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.
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Old 09-10-2013, 02:31 PM   #9
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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/ind...spension_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.
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Old 09-10-2013, 02:36 PM   #10
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Quote:
Originally Posted by SculptD View Post
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/ind...spension_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.
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.
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Old 09-10-2013, 03:25 PM   #11
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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.fcsdchemicalsandlubrican...MERCON%20V.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.
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Old 09-10-2013, 04:36 PM   #12
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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.
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Old 09-10-2013, 05:44 PM   #13
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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. That and you crash too much!
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Old 09-10-2013, 05:50 PM   #14
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Quote:
Originally Posted by Biebs View Post
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
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
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Old 09-10-2013, 06:10 PM   #15
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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.
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