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Another K75S fork question.

My k75S was built in 9/90. It's titled as a 1991. According to my reading, it came with the older model Sachs-Fitchel type sport forks, although I'm not sure of that. They have the "S" stamped on the top of the fork tube. I gather that after 8/91 BMW put Showa brand forks on the S bikes. People seem to say the Showas were a better all around fork. Could be why BMW switched them.
I have the proper amount of oil in both legs, and it is 5 wt oil. But I would really prefer a softer dampening over normal road irregularities. The bike handles well, doesn't dive, but is a bit tiresome to ride. Stopped, and locking the front brake, I can rock the fork up and down maybe two inches, so I don't think it is a sticking problem. But on the road, I don't see much movement, and it doesn't fee like it.

I'm considering switching both fork tubes to a C fork. I'm sure they will dive more, but only under hard braking, and maybe the rest of the time make for a smoother ride. But the question is, should I try to find a post 91 set of Showa S forks instead? Are the tube diameters the same? Would that be better than getting a pre 91 C set? How do I know they are the post 91 Showas? Are they marked some way? People trying to unload these old bike parts often don't seem to know that there are subtle differences, just that they are " forks for a K75". My forks have the large, bulbous dust cover. Do the Showas have the more square dust covers? Thanks.
 
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The Showas dust cap looks like a regular fork seal (flat and square). Showa forks are a different diameter than the F-S forks.



:dance:dance:dance
 
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The Showas dust cap looks like a regular fork seal (flat and square). Showa forks are a different diameter than the F-S forks.:dance:dance:dance

So if I'm following this:

1) I cannot use any forks that were on bikes made after 8/91 because they will be the Japanese Showa forks and the tubes are larger. I must therefore stick with the German F-S forks, which according to most people, are inferior to the Showas. I guess BMW felt the same way.

2) The only workable switch would be to a C bike from 86-91. Any C bike after 91 will actually have the larger tube forks. So all these people on Ebay who claim that their forks will fit " anything from 1986 to 1996" have no idea what they are talking about. What a surprise.

3) The F-S "C" forks will probably dive more on hard braking, but I haven't seen anyone claim that they provide a smoother ride than the F-S "Sport" forks. So possibly there is nothing to be gained in spending another $200 and my time in changing them. Just live with the "sporty" ride.

4) I'm beginning to see a solution to this problem.
 
A cheap easy test might be to drain your 5WT fork oil and put ATF in there.
Thats what I did with my 1978 R80/7 and I like it.
Nick
1990 K75RT
 
John,

You can change to Showas by changing the triple clamps to the later clamps that came with the Showas.

You could possibly soften your F-S forks by replacing the springs with springs from Progressive Suspension.

You could also possibly soften you existing forks a bit by shortening your preload spacers.

But, I still think there is something wrong with your forks if you think they are too harsh.




:dance:dance:dance
 
jconway
Did you try the ATF fluid yet? Easy and cheap and WILL soften them up.
You can always save the 5Wt in a container.
Nick
1990 K75RT
 
The specification is 7.5 wt. ATF is approximately 3 wt. If you are experiencing harshness on compression with 5 wt oil it is not the oil. A significant overfill can add harshness as can any number of mechanical issues. But going to 3 wt will not solve anything except cause rebound damping to be minimal.
 
In a way, we may be forgetting that fork oil (viscosity and level) is only one of three things which affect your front suspension dampening, rebound, and dive. The other two are the spring rate & pre-load, and the dampening profile of the fork design.

When I go to setup a conventional dampener-rod forks (like we are talking about here), I first set the static sag of the forks by adjusting the pre-load to get the right amount of compression and rebound travel. I also set the static sag of the rear shock(s) since forks and shocks interact with each other to determine bike orientation and stability. Setting the sag is done by adjusting the spring pre-load which changes the effective spring rate of the next unit of movement - or "stiffness" (compression resistance at rest and rebound strength). That's not totally technically correct but for this example I think it works. If that "stiffness" is not to your liking, then you need to change spring rates - fork oil won't solve this problem.

Next, comes the type and quantity of fork oil. With dampener-rod forks like we have here (BMW or Showa - no matter) the viscosity determines the dampening and the quantity can increase the fork compression rate ("dive") by reducing the trapped air column above the fork oil level in the forks. I always start with the factory recommended band, weight, and quantity. This lets me see how that fluid dampens the forks over bumps and braking. It also lets me determine what I think of the native spring rate. Assuming I'm leaving the springs alone (at least for now) then I see if I like the compression and rebound performance. Do the forks follow the road (i.e. keep contact)? If I hit a washboard area at X speed do the forks follow it or start to chatter and pound me (i.e. skip from ridge to ridge)? I change the viscosity to get the dampening I'm looking for. This can take several tries, but I find it's essential as dampening is probably the single biggest characteristic that the average rider notices as "good" or "bad" suspension.

Finally, if I think the fork dive under aggressive braking is more than I like, then I will add small amounts of additional fork oil to reduce the size of the air column. Since it's easy to add precise amounts of oil but really hard to remove it accurately, I go for about 10cc at a time. Often I end up realizing I've gone over my preferred level by 10 or 20cc and then have to drain and refill to my "correct" level, but that that way I know my "correct" level.

At the end of the day, dampener-rod forks have a relatively narrow sweet spot where they are both firm and compliant enough to really track the road. They will always dive to some degree that can only be fixed by making them very firm (too firm?) for a comfortable and competent ride. The best fix that I know of is a set of cartridge emulators from Race Tech which completely changes the dampening technology to a velocity sensitive system. This is a whole topic in itself, but for about $300 to $350 and some relative easy shop time you can really bring your forks much closer to modern designs.


HOWEVER, I really started this post to address the question about the viscosity of ATF. I've been using Mobile1 Synthetic ATF for a number of years in both of my K75s with the BMW forks with great results. I started this as a test to see if I liked the performance, and since I prefer a bit firmer suspension I did like the ATF. Additionally, it is most likely a far superior lubricant than fork oil because of the much harsher environment in an automatic transmission than motorcycle forks, it is much less expensive than branded fork oil per unit, and it can be found in any auto parts house and will *always* be the same formulation and viscosity.

I think it's widely understood that there are no industry (i.e. SAE) standards for rating motorcycle fork oil viscosity. Brand X's "7.5W" may be a 10W or 15W - or even a 5W in another brand. But, many companies do publish data sheets, from which you can see the viscosity measures of their different formulations which should be a sufficient guide to cross reference to other brands.

Below is data I pulled together a couple of days ago about three branded fork oils (the weight ratings are what the brands used - not an SAE rating), and Mobile1 Synthetic ATF. As you can see, using the fork oil specs as a reference, the ATF is a solid "10W" and possibly up to 10% heavier.
 

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Greg,
I certainly appreciate your taking the time to reply to my question of the forks in such a detailed manner. Very nice of you.
However.....I have just added a new thread dealing with this bike and the fact that it seems to be trying to melt it's own muffler off. This is my fifth BMW motorcycle and frankly, every one seems to be worse than the last. They have become troublesome to the Nth degree, undependable, and staggeringly expensive to buy parts for. Spending hours taking a set of forks apart and carefully modifying various parts is way beyond my patience level for these bikes. Who does this? Nobody riding a Harley or a Japanese bike. I'm tired of working on my BMW and watching everyone else just get on their bikes and ride them. I guess if I wanted to drop $20,000 on a new one, it might stay together longer, but I wouldn't actually bet on it. My 76 R60/7 was never a problem. Every one since then has been. BMW has lost me.

But once again Greg, thanks for trying to help.
 
On the other hand...

Greg,
I certainly appreciate your taking the time to reply to my question of the forks in such a detailed manner. Very nice of you.
However.....I have just added a new thread dealing with this bike and the fact that it seems to be trying to melt it's own muffler off. This is my fifth BMW motorcycle and frankly, every one seems to be worse than the last. They have become troublesome to the Nth degree, undependable, and staggeringly expensive to buy parts for. Spending hours taking a set of forks apart and carefully modifying various parts is way beyond my patience level for these bikes. Who does this? Nobody riding a Harley or a Japanese bike. I'm tired of working on my BMW and watching everyone else just get on their bikes and ride them. I guess if I wanted to drop $20,000 on a new one, it might stay together longer, but I wouldn't actually bet on it. My 76 R60/7 was never a problem. Every one since then has been. BMW has lost me.

But once again Greg, thanks for trying to help.


I'm sorry to hear about your negative experiences, and you do have me curious about the "melt its own muffler" comment. We each have our opinions, and yours is no more "wrong" than mine is "right". However, I've been riding BMW's since 1972 and my experience seems to be much more positive about BMW's than yours.

This thread is about a bike that is pushing 30 years old. IMHO, it's just not reasonable to expect correct performance from equipment that old without doing whatever is required to bring it into factory specifications. That doesn't mean "as new", but it does mean to do all catch up maintenance. For me, that includes all factory scheduled services, plus anything the community has learned over the years that should be done due to mileage, age, or design shortcoming. I would also throw in a dash of modernization (i.e., better than stock shock(s), tires, brake pads, etc.). After all, fuel lines gum up, rubber gets hard and breaks or leaks, brake pads lose their coeffect of friction, bearing grease either thickens or gets runny, seals want to leak, and so on. It’s only fair to the bike to give it every chance to be what it was designed to be. I have two K75s which I’ve taken cross country without worry or problem.

You made the comment, "Spending hours taking a set of forks apart and carefully modifying various parts is way beyond my patience level for these bikes. Who does this? Nobody riding a Harley or a Japanese bike." I see a lot of the contrary. Many Harley riders spend cubic dollars on Screaming Eagle upgrades to brand new bikes. Look around the Internet and there are endless suspension mods available for brand new generation Japanese bikes. Again, a K75 is a 30+ year old design, and that shows especially in the suspension. The Race Tech fork mod I mentioned will bring a K-bike (or airhead) bike ahead by decades. Add in good tires & rear shock & brake pads, and you'll have something that will run with modern bikes and make you grin all day.

I think a key advantage of the classic BMW K-bikes is that the basic power train and frame are essentially modern even today. It was the first K100 which was the first bike in BMW history featuring a trellis frame with the engine as a stressed member. BMW was so proud of this they showed the “Compact Drive System” on the Owner’s Manual cover. This was at a time when most bikes - and all Airheads - used a double-loop cradle frame. The first oilhead R1100RS carried on the same trellis frame and stressed engine member design, which continues through the current boxer models.

If you google up "BMW K-bike custom" you will get endless pages of links to highly customized classic K-bikes. There are street fighters, bobbers, customs, old school "café racers", even dual-sport builds. Some are beautiful to my eye, and some are just weird, but I'd offer that there is no other bike series platform as popular today for custom bike builds as the classic BMW K-bikes.

These are my experiences and opinions, but at the end of the day may not address what you are wanting in a bike. I’ll be the first to say bike opinions are much like much like “proving” that chocolate tastes better (or worse) than vanilla. I hope you find the bike that makes you grin when you ride it. :dance
 
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