OEM BMW 7.5 wt Fork Oil

[Bob_Roller]

I had a leaking fork seal on my oilhead, found it yesterday .
Stopped by the local BMW dealership for the seal and a bottle of fork oil .
It looks like the 7.5 wt oil is no longer being distributed, when what ever stock is still out there is gone, looks like it's gone for good .
I was going to purchase BMW 11.5 wt oil, the cost $26US for one quart, .95 l .
I decided against the new oil .
I did have an unopened bottle of 7.5 wt oil at home , have enough to replace the fork oil on one R65, after filling up the fork on the oilhead .
These oilheads have some good features, took 35 minutes to replace a fork seal .

[Barry]

Bob,

Is there a data sheet available for the BMW 7.5 fork oil (probably not) so that a similar substitute can be found.

[Tony Smith]

Bob,

Is there a data sheet available for the BMW 7.5 fork oil (probably not) so that a similar substitute can be found.

I have never seen a bottle of BMW fork fluid marked with a weight, but then it is decades since I've bothered looking for "genuine" fork fluid.

Back when I was passingly interested BMW used to specify an obscure MilSpec Aviation hydraulic fluid which I tracked down only to be told that it was no longer available and that the approved flush/replace fluid for it was Dexron II.

In any event the "weight" of the fluid is irrelevant as it has nothing whatsoever to do with viscosity which is the fluid property that actually does the damping.

If you require proof of the above statement consider this - cream is a far thicker fluid than milk, i.e. it has a higher viscosity than milk. Yet it floats on top of milk because its
"weight" is less.

Anyway, for what it is worth I think that Dexron II is a little too light in the damping department but then I live in a country with crap roads and I weigh a bit more than I should. I use a 50/50 mix of Castrol Fork 5 and Fork 10. For a person of average weight who lives where there are smooth roads - Fork 5 alone may well be adequate.

[Bob_Roller]

I'm going to use 10 wt oil from now on, readily available from  well known names, Bel-Ray, Motul, etc .. and the cost is under $10US for a one quart, .95 l bottle .
If this is too heavy of an oil, I'll consider mixing oils at that time .
My Guzzi uses ATF in the forks, but it is too light for the airhead bikes .

[Tony Smith]

I'm going to use 10 wt oil from now on, readily available from  well known names, Bel-Ray, Motul, etc .. and the cost is under $10US for a one quart, .95 l bottle .

The problem with selecting a fork fluid by weight is that they can have vastly different viscosity. And as viscosity is what does the damping, this leads to unexpected results when hopping brand names.

For what it is worth I think (based on the chart in the attached link) that you will find the fluids you mention a bit too heavy by far providing of course that your damer system hasn't become "leaky".

https://transmoto.com.au/comparative-oil-weights-table/

[Bob_Roller]

Don't have much choice in the matter, 5 wt or 10 wt, nothing available in between .

[BPT]

Hi Bob Roller - doesn't 5 + 10 = 7.5?  I'm needing to do this myself and it'll be the first time for me.  I'm not sure what is in mine now but I'd wanted to try at least 7.5.
Also, regarding the brands and what you said above - I was reading another thread recently that said BelRay should be avoided if you had a particular seal/spacer (I think it was red, maybe the original ones?).  Were you aware of that?

I guess I can just avoid the BelRay. But is there a simple way to know if I have these particular ones in my forks?

[Bob_Roller]

I've only used OEM BMW oil and it's been purple for the 37 years I've used it .
Other brands, no idea .
I've got the latest version of the washer that goes bad .

[Barry]

- doesn't 5 + 10 = 7.5? 

Purley academic and of little consequence but bizarrely it doesn't.

5  + 10 = 7   

      https://www.widman.biz/English/Calculators/Mixtures.html


That's assuming the originally oils are actually 5 cSt and 10 cSt @ 100 deg C which they won't be exactly and in some cases no where near the labelled figure. That's the real issue.   The only way of matching one fork oil manufacturer with another is to look at the data sheet and see what the viscosity is at 40 Deg C which even in Bob's climate is closer to the actual operating temperature.

Searching for "fork oil comparison chart"  will find many examples of where the actual viscosity in cSt has been compared for a large  number of different fork oils.
 

[BPT]

Sorry Barry, I realized as soon as I posted that I should have qualified it.  I realize after reading the posts and attachments from you and Tony that the weight is not what to look at.

I was just trying to point out that (I was told, and I think someone posted it here as well) you can mix 2 different ones together to customize if you can't find exactly what you need.  Is that still the case?

I've printed the list from the link that Tony posted and I'm studying it.  But I'll have to say that now I'm not really sure what to look for since I'm not sure what's in mine now.
Coincidentally, the last guy helping me out recommended exactly what Bob is looking for though......

[Tony Smith]

I was just trying to point out that (I was told, and I think someone posted it here as well) you can mix 2 different ones together to customize if you can't find exactly what you need.  Is that still the case?

I used to run a 50-50 mix of Castrol fork 5 and Fork 10 in my R65 when it still had r65 forks.

I did not call that "7.5" and the 50/50 formukation was the result of me starting with Fork 5 and trying it, drain, make a mix and reload until i achieved the level of damping I wanted - the mix being by happenstance 50/50.

I did a similar thing with the K100 forks and came undone. I started with a mix of Fork 10 and Fork 15 and am now running straight fork 10. The next time I am feeling enthusiastic I will adjust to 70/30 fork 10 and fork 5.

Please note that my K100 forks differ from standard in that they are fitted with adjustable cartridge emulators and all I need do is get them in the ball park with the emulators set at the mid-point and I can manually adjust from there (I hope). I was warned that the emulators even at their highest setting had less damping than stock, hence my starting with heavier fluid. I suspect that the new damper piston rings I fitted may be less "leaky" than the person who sold me the emulators had previously dealt with. Anyway, its been fun.

[BPT]

Thanks Tony.  I think what you're saying is what I intended - I just don't have the knowledge or terminology to express it properly!
It sounds like what the guy who was helping me was recommending to me.  Unfortunately he has since died so I can't go back to ask.  Mine needs to be done for a variety of reasons so it was a matter of starting with the oil and seeing if that too care of the squishiness I had before messing with springs and other more involved things.

But I do remember him telling me that we could experiment and mix some different ones together until it got to where it was comfortable and stiffer again.

Thanks.