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Thread: '87 K75S installing progressive springs

  1. #1

    '87 K75S installing progressive springs


    I'm in the process of "refreshing" my 1987 K75S that I purchased last summer. Currently I'm focusing on the front end. I have the Clymer manual as a resource. This is my first motorbrick.

    I picked up a set of slightly bent fork tubes from a gent who had an unfortunate get-off (also a 1987 K75S). These fork tubes are rumored to have Progressive springs. I'm looking to install those springs on my bike.

    I'm taking the damaged fork tubes apart one at a time so I can learn as I go. I got the top cap off the first tube and found underneath it two plastic tubes (I think the "distance tubes" per Clymer). I then found a relatively short spring (maybe 4 inches) that is not progressively wound. Under that is a much longer spring that is progressively wound. I'm assuming this is the right order of things and have four questions, the last last of which is the big one:

    1) Is what I'm seeing in the correct order from top to bottom - top cap, distance tube 1, distance tube 2, short not-progressive spring, long progressive spring?
    2) Is the short spring and long spring part of the Progressive spring kit? Or just the longer one? If I disassembly my bike, for which I expect to find the stock setup, will I find two springs, neither of which are progressively wound that would simply be replaced by the springs from my donor forks?
    3) Is there a difference between the left and right fork? On my donor forks, I do not know which side is which. I don't have the lower tube for reference. Just the top (shiny) tube.
    4) Is it wise to attempt to replace my springs without removing my forks? I was thinking of the following procedure:
    a) Drain oil from the bottom
    b) Compress top cap (I'm worried this will be difficult as I'll be pushing against both the top and bottom as I attempt to compress the fork cap)
    c) Fish out the various distance tubes and springs, letting each fork compress slowly
    d) Reassembling reverse of removal
    e) Fill with oil
    f) Profit

    Or should I just punt on #4 and remove the fork tubes. I'm not sure which is the lazier, I mean better, approach.

    I'm a n00b on this bike but have been digging into all sorts of basic stuff to get it back in shape. The forks are totally foreign to me in my mechanical experience.

    Thanks much!

    Eville Rich
    '87 K75S
    '16 Super Tenere
    '15 WR250R

  2. #2
    My memory tell me that from bottom to top:

    longer progressive spring
    short linear wound spring
    top spacer
    Paul Glaves - "Big Bend", Texas U.S.A
    "The greatest challenge to any thinker is stating the problem in a way that will allow a solution." - Bertrand Russell

  3. #3
    Registered User drneo66's Avatar
    Join Date
    Jun 2009
    Rochester, MN
    Welcome to the wonderful and weird world of K-bike forks.

    If it was me, I'd remove the forks from the bike (you can do it one at a time). I think it's much easier to work on them on the bench. Plus you don't risk stripping out the drain plugs and you can really clean out all the crud from the bottom. Use a 4ft bar clamp to push down the fork caps for easier removal/install.

    IBMWR has a lot of correct(!!) info about how much fork oil to use for the job (I can't remember if it was Clymer or Haines that was wrong on the oil amount).

    I'd also add some fork gaiters on at the same time, since you're already in there.

    Here are the Works instructions for progressive springs.


    Current: 2007 BMW R1200RT, 2013 F800GS
    Former: 1995 BMW K75S, 2009 BMW G650GS
    MOA Member #:150400, IBA#: 37558

  4. #4
    Thank you for the links a pics. I'll do some reading, but a quick scan shows this to be very helpful.

    I do have a couple more questions, now that I've drained the oil from one of my fork legs (came out pretty dark).

    1) While I have a K75S, I do not have an "S" stamped on the top cap. I'm a little unclear on how much oil to use. realoem only links me to the "sport" diagrams. Do I, by default, have a "sport" suspension?

    2) The oil that came out totaled just shy of 300 ml. Clymer is showing the sport suspension getting 270 to 290 ml, which is right where I'm at after draining. The IBMWR article indicated an RT with over 500 ml, which is substantially more than the Clymer 320-330 cc for the "standard" suspension.

    Basically, I'm not quite clear on how to interpret the amounts given the definition of "sport" suspension or possible error in the Clymer manual.



  5. #5
    3 Red Bricks
    Join Date
    Jan 2007
    Pleasanton, Ca.

    All K75Ss up to 8/91 had the Sport Suspension. Ss after that had the Showa forks.

    Early Ss did not have the S stamped on the fork caps.

    The correct amount of oil for these forks is 280ml (or ccs, same thing). Belray 7.5 wt is recommended. Weights are not consistent. A 10wt fork oil in one brand can be significantly different viscosity than a 10wt from another brand. Why? Don't have a clue. . I just read a lab test of about 30 different weights and brands and it's so. The only thing that makes sense is that a higher weight oil of any brand is thicker than a lower weight of the same brand.

    The stock sport suspension has two springs per fork, a 3 3/8" x 1.342" OD with ,224" wire and a 10 1/2" x 1.328" OD with .184" wire progressively wound and one 4 3/16" preload spacer.

    The Progressive brand springs use one 18" long progrssively wound spring per side.

    The Sport Suspension only has dampening in the left fork. While they look similar inside, only the left damping rod has any valves in it.

    Last edited by 98lee; 04-10-2019 at 03:22 AM.

    Ride Safe, Ride Far, Ride Often

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

  6. #6
    Registered User
    Join Date
    May 2008
    Western Massachusetts
    Not sure about the need to remove the forks themselves, but I'd strongly recommend removing the sliders. I'm consistently amazed at the amount of crud that collects at the bottom of the sliders when I clean these out.

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