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Thread: K-100/K-75 Sport forks - what about them?

  1. #1
    Dances With Sheep GREGFEELER's Avatar
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    K-100/K-75 Sport forks - what about them?

    I have two K75's: an 1992 "S" which came with the standard BMW forks (not the Showa or the "S" Sport forks), and another "standard" 1990 K75 which came with the Sport forks. As far as I know the K75S should have had either the "S" forks or the Showa's. The guy I bought it from got it from the original owner, and I'm pretty sure it's never been wrecked. The "standard" K75 might have started life as an "S", but I don't think so, as it came to me with a Pichler fairing, low seat, and belly pan with crash bars. So why it has the "S" forks I don't know.

    I have managed to get the standard forks on my K75S working very nicely with changes in fork oil type and amount. They are now a good combination of initial "soft" dampening followed by excellent firmness, and very little dive on hard braking. I think the next step with them with them would be the Racetech Cartridge Emulators. But now my interest is with the Sport forks on the other K75.

    These Sport forks are noticeably stiffer than the forks on the K75S, and apparently offer compression dampening in one leg and rebound on the other (but I don't know which is which). That brings to mind the idea of possibly using different oil weights and/or amounts to tune compression and rebound individually. Also, BMW offers a number of different thicknesses for a shim which implies the possibility of another kind of tuning.

    It looks like they went to some trouble to engineer what they thought was "better" fork for aggressive riding, but I don't recall seeing any real commentary on whether the K-bike community agrees, and if there are things to be done to them which could make them better, or if you are just better off to get the later Showa forks or get the Racetech Cartridge Emulators?
    Greg Feeler
    Ambassador & amateur K-Bike collector, it seems
    1972 R75/5, 1990 K75, 1990 K1, 1992 K75S, 2003 K1200RS

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    3 Red Bricks
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    Greg,

    The only K75 that came with the "standard" Fichtel Sachs forks were the K75 C and K75T. These had 7" of travel and (poor) dampening in both legs.

    Prior to 8/91 all K75Ss and K75RTs came with the Fichtel Sachs Sport forks with dampening in only the left leg and 5" of travel. The later (89-90) Cs may have had the Sort forks

    After 8/91 ALL K75s came with the Showa forks with dampening in both legs.

    The Fichtel Sachs forks were 41.35mm (1.628") in diameter with a bulbous dust cover on top of the slider.

    The Showa forks were 40.95mm (1.612") in diameter with a standard lip seal at the top of the slider.






    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

    Lee Fulton Forum Moderator
    3 Marakesh Red K75Ss
    Mine, Hers, Spare

  3. #3
    Dances With Sheep GREGFEELER's Avatar
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    Quote Originally Posted by 98lee View Post
    Greg,

    The only K75 that came with the "standard" Fichtel Sachs forks were the K75 C and K75T. These had 7" of travel and (poor) dampening in both legs.

    Prior to 8/91 all K75Ss and K75RTs came with the Fichtel Sachs Sport forks with dampening in only the left leg and 5" of travel. The later (89-90) Cs may have had the Sort forks

    After 8/91 ALL K75s came with the Showa forks with dampening in both legs.

    The Fichtel Sachs forks were 41.35mm (1.628") in diameter with a bulbous dust cover on top of the slider.

    The Showa forks were 40.95mm (1.612") in diameter with a standard lip seal at the top of the slider.


    Lee,
    That is pretty much how I understood it. However, my "1992" K75S has the Fichtel Sachs forks - but not the Sport versions. The VIN on the other K75 comes back saying it's a 1990 (March production) "C", but it has the Sport forks and a disk rear brake. Your comments about some C's coming with the Sport forks explains that part, but I thought all C's came with the drum rear brake? So here are a couple of mysteries. But, back to my original question, what is your estimation of this Sport fork, and between the three flavors (including the Showa) which has the most potential for a reasonable amount of work? Thanks.
    Greg Feeler
    Ambassador & amateur K-Bike collector, it seems
    1972 R75/5, 1990 K75, 1990 K1, 1992 K75S, 2003 K1200RS

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    Greg,

    Yes, I was also under the impresion that the late Cs had a disc rear.

    What's the last four of the VIN on your S?

    Did you already look inside to confirm that they have damping parts in both forks?

    VERY early Ss do not have the S stamped on the fill cap end, I have two of them.




    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

    Lee Fulton Forum Moderator
    3 Marakesh Red K75Ss
    Mine, Hers, Spare

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    Dances With Sheep GREGFEELER's Avatar
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    Quote Originally Posted by 98lee View Post
    Greg,

    Yes, I was also under the impresion that the late Cs had a disc rear.

    What's the last four of the VIN on your S?

    Did you already look inside to confirm that they have damping parts in both forks?

    VERY early Ss do not have the S stamped on the fill cap end, I have two of them.


    Now that I have traced the VIN on my "standard" K75 and determined that it is a "C" model, then your information about the later ones shipping with a rear disk brake explains that mystery.

    The last four digits of the VIN on my "S" are "3117", and looking it up says it was manufactured in 7/91, so one month before the change over to the Showa forks. I got curious last night and popped off the plastic fork trim covers, and now can see that they are stamped with the "S" for Sport forks. I honestly would have sworn that they were not so stamped, but somehow I had missed that. So, now I know that both bikes have the forks they are supposed to have, but am left with why one set is far stiffer than the other. Since I've never pulled the springs, or dissembled them any further then to remove the sliders for fork seals, sounds like where I need to go next if I want to improve them.

    Lee, how would you rate these options in terms of getting the "best" peformance: a) working on these forks with new springs and a rebuild (oil & levels, etc.), b) installing the RaceTech Cartridge Emulators (and springs), or c) find some Showa forks, rebuild, and run them as "stock"? I know there is some money swing in there, but the Cartridge Emulators and eBay Showa forks are not far different in price - about $200.00. Thanks.
    Greg Feeler
    Ambassador & amateur K-Bike collector, it seems
    1972 R75/5, 1990 K75, 1990 K1, 1992 K75S, 2003 K1200RS

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    Greg,

    I have not done any experimenting with the front of my S or the Ss that I work on. My bike supposedly had Progressive front springs installed by the previous owner (I have never verified this). After changing the oil to 290cc of 7wt (stock spec.), I've been reasonably happy with it. I don't know what was in it when I got it, but stock spec. was an improvement.

    Most Ss that I work on have different weight or amount in the forks. Since there are 3 different oil specs. for K75 fork oil, there is room for confusion. I change them back to stock and the owners have always noticed an improvement. The bikes that I work on usually are ones that have had some neglect and need everything brought back to original specs. to establish a baseline.

    The fact that you believed your S had the standard forks instead of the sport forks, could you have put too much oil in them?




    LONG MAY YOUR BRICK FLY!

    Ride Safe, Ride Far, Ride Often

    Lee Fulton Forum Moderator
    3 Marakesh Red K75Ss
    Mine, Hers, Spare

  7. #7
    Dances With Sheep GREGFEELER's Avatar
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    Quote Originally Posted by 98lee View Post
    Greg,

    I have not done any experimenting with the front of my S or the Ss that I work on. My bike supposedly had Progressive front springs installed by the previous owner (I have never verified this). After changing the oil to 290cc of 7wt (stock spec.), I've been reasonably happy with it. I don't know what was in it when I got it, but stock spec. was an improvement.

    Most Ss that I work on have different weight or amount in the forks. Since there are 3 different oil specs. for K75 fork oil, there is room for confusion. I change them back to stock and the owners have always noticed an improvement. The bikes that I work on usually are ones that have had some neglect and need everything brought back to original specs. to establish a baseline.

    The fact that you believed your S had the standard forks instead of the sport forks, could you have put too much oil in them?


    Several years ago I started using Mobile1 Synthetic ATF in all my dampener-rod BMW bikes. The reasoning was two-fold. First, my assessment is that ATF has to meet much higher standards than fork oil (for which there are no standards I'm aware of) - greater heat, pressure, sheer forces, and service life. Secondly, the actual viscosity of different brands of fork oil vs. the claimed viscosity is literally all over the place. This link is to a chart (one of a number I've seen with similar data) which proves this point: http://mahonkin.com/~milktree/motorcycle/fork-oil.html

    So, if you find a brand and weight of fork oil you like in a given bike, it seems that it's a total crap shoot to try a different brand labeled as the same weight. Alternatively, Mobile1 ATF can be found everywhere and will always meet the same specifications. The only remaining question was if the viscosity was correct. In practice, I have really liked how it's worked, and from the linked chart, and others I've seen, it appears to be between a "real" 7.5w and 10w, but closer to 10w.

    I say all that to say that I am running ATF in both K75's and like it. I added additional oil to the K75S - a lot - and they work really well in my opinion. While I was under the impression that they were *not* "S" forks and therefore specked at 330ccs, I added another 45ccs through some experimenting for a total of 375. But, since they are really "S" forks, and should have 280ccs, that brings the real overage to 95ccs! Sounds like a lot, and brings to mind the fear of blowing out fork seals, but I've been using that amount for at about four years, over a wide range of roads with very aggressive riding, and I would call them very, very good. Some inital softness to take the edge off of bumps, but then firming up nicely and dampening well. Very little dive under hard braking. However, now that I have seen the light, as it were, I'm going to go back to the 280cc baseline and experiment to see where I end up.

    It has only been the forks on the other K75 that I wanted to change, and now that I know they are the same forks, the logical explanation for their greater firmness is that a previous owner either added a spacer, or stiffer springs, to compensate for the weight of the Pichler fairing.

    Running the rally this summer meant that I gave up riding for the nine months preceding, so now I have time to play in the garage. Both K75's need a full service, so I'm going to pull the fork springs and see what I find. But, from all I've heard and read, the RaceTech Cartridge Emulators are a real transformation, so I think I will give those a try come spring.
    Greg Feeler
    Ambassador & amateur K-Bike collector, it seems
    1972 R75/5, 1990 K75, 1990 K1, 1992 K75S, 2003 K1200RS

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