'91 K75S Fork oil changing.

[jconway607]

I know there has been a lot written on fork oil, but just to make sure I am getting it right, I'd like to ask a few questions. Mine is a 91 K75S. It has the "S" stamped on the top of the fork tube. I believe they are the pre 8/91 Fitchel Sachs Sport forks. Correct me if I'm wrong on that. Am I right that I should be putting in 290 cc of fork oil into BOTH sides of the forks? I thought only one side had a damper system in it.

Secondly, from what I read, it originally called for the old #7.5 wt fork oil. Now, I'll admit I'm frugal, but $25.05 a quart from BMW seems a bit steep. I had some 10wt oil, which I traded back for 5wt after doing some reading. Will the 5 be okay? I prefer a softer ride, and am not a canyon carver by any means. But I don't want to damage the forks either with them bottoming out.

Incidentally, I just called my local BMW shop, Cleveland BMW, and they tell me to use the 10wt oil. They claim the BMW fische calls that out. This seems more complicated that it has to be. I'm going to go by the experts here who actually work on and ride these bikes.

 

[98lee]

John,


In your previous thread, you said your bike was a '92 ???

 

[jconway607]

J,


In your previous thread, you said your bike was a '92 ???

That's true, Lee. I don't know why I thought it was a 92, since as I look at the title is says it's a 91.
I thought there were some changes made in the middle of 91, and I couldn't figure out which side of the line mine was built on. These forks seem to be one of the changes, if I'm following the readings properly. The last digits of my vin are ...152588. Maybe that helps.

 

[20774]

Looks like it's an early production '91...produced September 1990.

 

[98lee]

Yep, built 9/90.

That means they are indeed the Fitchel-Sachs Sport forks.

I use 290cc of BelRay 7wt fork oil (way cheaper, but just as good as BMW oil) in each fork with good results. But slight variations are warranted, depending on what rear shock you are running. Also what spring rate, settings, and condition of the rear shock. Afterall, the goal is to maintain a balance between the front and rear suspension.

I will change the model year in your previous post for you.

 

[pglaves]

I don't disagree with Lee about using 7Wt BelRay fork oil. Another possibility is a 50-50 mix of 5 wt and 10 wt oil from the same brand which will very closely approximate the 7.5 wt spec. I have done this with several K75 bikes we have owned/own.

 

[cycleman2]

10 wt is the stock weight. If you want to tune the forks, vary the quantity slightly up or down from the factory amounts. What you are doing is changing the air space inside the fork, as that is what gets compressed by the forks action. More air softer, less air harder. The oil doesn't compress.

The factory spec fork oil on a K 100 for example, varies from 330-360 cc, 330 for non faired bike and 360 for the RT/LT with fairing. In the early years they used the same internals, just changed the quantity of fork oil to accommodate the heavier faired bikes. Later years are different, so you have to make sure what fork you are dealing with as the quantities for each are different.

On my 86 K100RT I took the fairing off the bike and the front end is much stiffer with the 360 cc of fork oil, not in a bad way but there is definitely less initial sag. I'm changing the fork oil on my 86 K100, so have been doing some reading & research on this topic, so that I don't screw it up. As I'm not going to run the bike with the fairing, I plan on putting in the factory recommended 330 cc and go from there, much easier to add oil if I needed to. A pain to remove some oil.

 

[98lee]

To add to the confusion; the same wt from different manufacturers have different viscosities. So a 5 wt from one manufacturer could be thicker viscosity than a 7.5 wt from a different manufacturer. Why? I have no idea. I thought that was the whole point of the system of specifying the weight of oils. I guess they don't have tight standards for fork oil.

 

[gregfeeler]

To add to the confusion; the same wt from different manufacturers have different viscosities. So a 5 wt from one manufacturer could be thicker viscosity than a 7.5 wt from a different manufacturer. Why? I have no idea. I thought that was the whole point of the system of specifying the weight of oils. I guess they don't have tight standards for fork oil.

Here is a link to someone's research on "true" fork oil viscosity measurements.

http://mahonkin.com/~milktree/motorcycle/fork-oil.html