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Thread: 1985 K100RS front end clunking noise.

  1. #1

    1985 K100RS front end clunking noise.

    I took my bike out for the first time ever Saturday (yesterday) afternoon. I noticed a clunking noise coming from the front end on bumps. When I got home I investigated. When sitting on the bike, front brakes on and compressing the front shocks, there was a clunking noise on rebound, not when compressing. I didn't have to compress the front forks but an inch and when they rebounded I heard the noise and felt it through the handlebars. The loudness on rebound was directly related to how much I compressed (how hard I pushed) the front forks to begin with. I can't honestly remember if it was something I heard before or not.

    The only thing I noticed after removing the fork caps was the left fork tube was maybe 2-3mm lower than the right at the top of the upper fork bridge. I rectified that. That wasn't the source nor did I think it was going to be. I have 360ml (cc is the same) of fresh 5W in each fork leg-volume taken from Clymer manual.

    I jacked the bike up under the engine and re-checked how free the steering moved as I had checked this some time before. It moved easily and there wasn't any sign of a hitch or being loose. Is this pointing towards steering stem servicing of some kind? Not afraid to dive into it if I have to as it would appear that there is sufficient clearance for the forks to drop out after removing the wheel.

    I didn't really notice any issues while riding as it was quite windy and I was getting kicked around pretty good even though I did find a section of road and took it to 100mph.

    30,000 miles on bike and I have no idea when, or if, it ever had steering head servicing.

    Photo attached of bike as I was riding around the foothills of Southern Alberta southwest of Calgary.

    XA84GR.jpg

  2. #2
    I suspect you are hearing spring "slap". When you compress a coil spring in a fork it will curl so the center of the spring is against the inside of the fork tube. When the compression is released if there is very little preload the spring may straighten and "slap" the opposite side of the fork tube when doing so.

    If your steering head bearings were loose enough you could hear them you would barely be able to steer the bike while decelerating.
    Last edited by PGlaves; 05-04-2020 at 09:35 PM.
    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
    http://web.bigbend.net/~glaves/

  3. #3
    Registered User CABNFVR's Avatar
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    "They all do that". I've been waiting since 1985 to say that and mean it. Our mechanic had two phrases, " They all do that." and "Never saw that before." Used to believe him until my brother and I had identical issues on our '85 K100RS and the mechanic told each of us a different line. Oops.

    I forget the company but they used to sell a spring stiffening kit. A piece of sch 40 PVC pipe would also work. The clunk is especially noticeable on grass or dirt. It won't hurt a thing. Stiffening up the forks with the fore mentioned spacer will help the clunk and the handling. It's been too long for me to remember the specs but someone here surely has that written on a sheet of paper on their workbench and will chime in.
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    "Have BMW. Will Travel"

  4. #4
    Okay. So the engine whines and the forks clunk. Thanks, guys.

  5. #5
    Dances With Sheep GREGFEELER's Avatar
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    Quote Originally Posted by mcmlcccvrs View Post
    Okay. So the engine whines and the forks clunk. Thanks, guys.
    Yes, and it's a mighty fine whine!
    Greg Feeler
    Ambassador & amateur K-Bike collector, it seems
    1972 R75/5, 1990 K75, 1990 K1, 1992 K75S, 2003 K1200RS

  6. #6
    Quote Originally Posted by mcmlcccvrs View Post
    Okay. So the engine whines and the forks clunk. Thanks, guys.
    You can make the klunk go away by adding preload spacers atop the fork springs. Generally if the klunk is happening the forks are under sprung or the springs are tired.
    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
    http://web.bigbend.net/~glaves/

  7. #7
    Quote Originally Posted by GREGFEELER View Post
    Yes, and it's a mighty fine whine!
    I always enjoy a fine w(h)ine.

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