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Thread: Vibration at high speed/rpms in '90 K75S

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  1. #1

    Vibration at high speed/rpms in '90 K75S

    Hi All-

    Just picked up a new to me 1990 K75S and rode it home 200 miles from PA to DC yesterday. The bike has 55k miles on it and lots of major maintenance done. That being said, there was a crazy amount of vibration above 45 mph or so. Is there a common place to start when looking to fix this issue? I'm sure it could be a lot of different things, but any help for a K75 rookie like myself would be appreciated.

  2. #2
    rabid reader dbrick's Avatar
    Join Date
    Apr 2003
    Santa Cruz CA
    Welcome to K-land, cavisconage. The K75 is a great bike, I had a K75S from 1988 to 1997.

    The vibration you feel is definitely not normal; normal is really smooth at all rpms and speeds. As what you feel is speed-related, you should start with wheels and tires:

    Are the wheels straight and unbent? Put the bike on its centerstand, and spin each wheel slowly while holding a pencil or small piece of wood next to the rim. There should be little or no runout in either the radial or lateral direction. If there is, the wheel(s) should be straightened or replaced.

    Same issue for the tires. First, how old are they? Numbers molded into the sidewall will disclose the month and year of manufacture. The rubber compounds in tires degrade with age, and will generate less traction even if there's tread remaining. And there will be tread wear indicators molded into the tires, showing how much treadwear has occurred. If the tires are five years old or more, or are worn close to the wear bars, replace the tires.

    If the tires seem OK, make sure they're correctly inflated and inspect them carefully for flaws. Any significant out-of-roundness (checked in the same way you checked the wheels) or a sidewall bulge is immediately disqualifying; if either is found, replace the tire(s).

    Finally, each tire-and-wheel assembly must be balanced. An out-of-balance condition could easily generate the vibration you feel.

    Do some checking, and let us know what you discover.
    David Brick
    Santa Cruz CA
    2007 R1200R

  3. #3
    3 Red Bricks
    Join Date
    Jan 2007
    Pleasanton, Ca.
    Welcome to the forum!

    A K75S should have virtually NO vibration at virtually any speed. It is. One of the smoothest motors ever built.

    The first place to look would be the tires and wheels.

    Check for bent wheels.
    Checked for out of round or damaged tires.
    If neither of the above, have the tires rebalanced and ask the tech to double check for bent wheel or tire runout.

    Let us know the results. If that doesn't fix it, we can suggest some less likely and more involved possibilities.

    PS: I guess since David lives in Santa Cruz, just a quick ride from here, we both get up and check the Forum about the same time. I was typing while he was hitting send. But the advice is the same. It's most likely something wrong in the wheel/ tire area. These bikes don't vibrate. When the K75 first came out, one of the complaints from traditionalists was "These bikes don't have any soul" (read "vibration"). And, like David said, old hard tires are also possibly the problem. It's amazing how much smoother an already smooth bike feels with brand new tires.
    Last edited by 98lee; 07-14-2018 at 03:25 PM.

    Ride Safe, Ride Far, Ride Often

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

  4. #4
    cavisconage - I just got my 94 this Spring in Arlington, VA, somewhere near you!
    Yes, mine is smooth. In addition to checking tires and wheels you might want to check the bearings in each wheel. Are the forks straight? Anything going on in the steering head bearings? Any notches there?
    Fortunately for me, the previous owner was pretty maintainence savvy.
    Good luck getting yours smoothed out.

  5. #5
    Registered User mlytle's Avatar
    Join Date
    Oct 2015
    alexandria, va
    what they all said. K75's are super smooth at all speeds.

    welcome to the DC K75S crowd!
    92 K75s, 94 K75s, 96 K1100RS (caretaker), 09 K1300s

  6. #6
    Registered User
    Join Date
    Sep 2005
    Commerce Township, MI
    cavisconage Can you tell if the vibration is related to road speed or engine RPM?


    Richard Flood.

  7. #7
    Registered User drneo66's Avatar
    Join Date
    Jun 2009
    Rochester, MN
    Quote Originally Posted by richo2006 View Post
    cavisconage Can you tell if the vibration is related to road speed or engine RPM?


    Richard Flood.
    (Which you can do by running the bike up to speed and then pulling in the clutch... obviously done in a safe area)
    Current: 2007 BMW R1200RT, 2013 F800GS
    Former: 1995 BMW K75S, 2009 BMW G650GS
    MOA Member #:150400, IBA#: 37558

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