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Thread: 1985 K100RS fork tube pitting?

  1. #1
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    1985 K100RS fork tube pitting?

    These marks you see in the black circle aren't scratches. They appear to be tiny pits about 40mm (to the lowest one) from the top of the slider. Corrosion? I can easily snag a tiny, flat-bladed screw driver on them which tells me it will scratch my stanchion and probably did, although I have dressed the stanchion with 600 grit emery cloth. I have tried removing them with 600 grit but it appears it's going to be more than a few passes with sandpaper to do this.

    I believe they are high enough in the slider so that suspension travel with scratch the stanchion and that area will then pass by the fork seal, possibly/probably nicking the seal and causing it to leak.

    I have no idea if they were there last winter when I upgraded my front forks with Race Tech emulators and springs but they could have easily been missed.

    What do you guys think I should do-sand some more and try and get them out or look for a set of used fork tubes/stanchions? The latter wouldn't be hard to locate at all.


  2. #2
    3 Red Bricks
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    A “pit” should not scratch the fork tube, as it is further away from the fork tube than the rest of the inner wall of the slider.

    A “burr” on the other hand WILL scratch the slider because it protrudes further in towards the fork tube.

    I would think that just enough light sanding to soften any sharp edges should be ok. Take a small straight edge and insure nothing sticks up from the wall of the slider.




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  3. #3
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    Thanks, Lee, I’ll proceed along those lines.

  4. #4
    Quote Originally Posted by mcmlcccvrs View Post
    These marks you see in the black circle aren't scratches. They appear to be tiny pits about 40mm (to the lowest one)



    when fork oil is whipped into a foam, the collapsing bubbles can etch metal

  5. #5
    Fortis Fortuna Adiuvat Omega Man's Avatar
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    Quote Originally Posted by gtgt_bangbang View Post
    when fork oil is whipped into a foam, the collapsing bubbles can etch metal
    I would be interested to hear more on this. The only time I have heard of this is regarding Diesel engines.

    https://www.brakeandfrontend.com/tec...rst-nightmare/

    This was the downfall of CAT 3208 engines (no liner) that didn’t have their coolant specifically maintained right from day one.

    The diesel “rattle” would cause “bubbles” and on a hot engine, that micro-second of the bubble created an air pocket that would “flash” rust. That continuing cycle would actually rust a hole in the cylinder from the wet side in.


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  6. #6
    MOA #24991 Pauls1150's Avatar
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    That's a new one on me, too - so just out of curiosity, I looked up a few MSDS's for fork oil.
    Ohlins and Spectro claim to have nothing hazardous, but the Bel-Ray includes less than 1% Phosphorodithioic Acid, O,o-di-c1-14-alkyl Esters, and Zinc Salts. Dunno if that could etch when whipped...
    Spectro Fork Oil.pdf
    Ohlins Fork Oil.pdf
    Bel-Ray Fork Oil.pdf

  7. #7

    cavitation

    the orifices in the damper rod are the source of the problem.

    First recall that Damper Rod suspensions dampen on compression strokes (ie react against the movement) due to the ORIFICES in the damper rod shaft.
    The orifices restrict the oil flow trying to cycle between the lower chambers & upper chamber inside fork tube.

    The orifice area ( size X number of holes) is a compromise and is dictated mostly by tuning the fork to respond best to fast abrupt movements - like striking sharp edged bumps in the road at high speeds.

    Unfortunately , the "better" orifice size for Compression Stroke dampening is a " not so great" orifice size for the very different situation during the Rebound Stroke .


    When the fork REBOUNDS ( re- extends after a compression stroke), oil is sucked back down through the damper rod & back out through the orifices, refilling the lowest chamber, because the lower chamber is growing in volume, creating a vacuum / low pressure .

    If rebound is trying to happen "too" fast (aka too forcefully) for all the variables, the suction/vacuum can reach some critical low pressure,
    a low enough pressure which can actually cause the fork oil to boil ! ; due to low pressure , the oil boils / vaporizes / bubbles, aka "cavitates",
    Microscopic bubbles form and then collapse. The collapsing bubbles cause tiny but numerous & extremely energetic shock waves ( think 'ultrasonic cleaner'...

    The orifice itself is the dividing line between the temporary higher and lower pressure chambers inside fork , so the cavitation/bubbles should form immediately at or barely DOWN STREAM of the orifices . ( remember that 'down stream" direction in the fork oil flow reverses during compression vs. rebound strokes ).

    (note - there is no 'vacuum' conditions generated during the compression stroke, so there is no cavitation during compression; cavitation only occurs on rebound stroke )

    I imagine the cavitation occurring inside the jets of oil emerging from the damper rod orifices and into the slider, and those jets are aimed smack into the soft aluminum slider wall, so the slider wall can become etched over time. For all I know , the bubbles are actually nucleating ON the slider wall, at some existing defect in just the right spot, and the defect grows into a pit as the process feeds-back on itself.

    While the etching voids are certainly not a Good Thing, & probably lead to a lot of additional wear issues, I also think that the voids on the wall only " leak" past the damper,
    exactly as the moment the damper ring is straddling the void. So the effect on "ride" , compression dampening and slider action is probably(?) small.

    The controllable variables affecting if cavitation occurs are orifice area, main spring rate (rebound force , controlling rebound speed ) and oil viscosity/characteristics.

    A worthy read on the damper rod suspensions + cavitation . They have more tuning info on their site that go into the Damper Rod design a bit more.
    https://www.racetech.com/page/title/...%20They%20Work

    cavitation - 1.jpeg

    It wasnt the cavitation / erosion problem that sold me, but I am installing these RT "cartridge emulators" in my airhead.

    The modifications required for the emulator install eliminate the cavitation problem;
    the emulator ( a spring-controlled disc valve ) takes over control of fluid flow to perform the compression dampening action ,
    and the orifice ( net area) is grossly enlarged ( drilled out larger and/or more orifices) which reduces the orifice effect so low that its insignificant for compression stroke dampening,

    Fortunately, the larger orifices ALSO greatly reduce the conditions present that can cause cavitation during rebound stroke
    (less restriction at orifice = less pressure difference at orfice = less tendency to boil )
    Last edited by gtgt_bangbang; 09-20-2022 at 05:05 PM.

  8. #8
    MOA #24991 Pauls1150's Avatar
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    A clarification of terms is in order here:
    "Cavitation" is caused by exceeding a pressure differential. (The same thing can happen behind boat and airplane propellers.)
    "Boiling" is caused by the addition of heat.

    I strongly doubt that the forks can move fast enough to generate the heat required to boil oil.

  9. #9
    Fortis Fortuna Adiuvat Omega Man's Avatar
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    Yeah. I always thought cavitation on a boat or plane was when the propeller(s) spun faster that the medium it was in- air or water.
    The oil doesn’t boil in an oil lubricated air compressor and that piston is a crankin’.
    OM
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  10. #10
    Quote Originally Posted by Pauls1150 View Post
    A clarification of terms is in order here:
    "Cavitation" is caused by exceeding a pressure differential. (The same thing can happen behind boat and airplane propellers.)
    "Boiling" is caused by the addition of heat. I strongly doubt that the forks can move fast enough to generate the heat required to boil oil.
    Kong Fhuz Us says ; There are Many Ways to Boil a Cat.

    to de-clarify then - the words are correct for the physics of liquids, and Im not posting about aviation or navyiation -

    but "cavitation" is ~ the formation of a bubble inside a liquid ( by ANY methods possible)
    A liquid with enough energy transforms into a bubble chock full of the vapor of the same liquid-
    It forms a cavity = thus , cavitation

    BOILING is the basic process of cavitation. Bubble form in a liquid , they rise due to denser liquid all around them , hit the surface and the liquid's VAPOR from inside the bubble wanders & floats away. Look up VAPOR PRESSURE if your curious-

    Yep you can boil water by adding heat - agitate the water molecules until they have enough energy to bust out into the air.
    on the microscopic scale, the process is; BUBBLES NUCLEATE , they actually commence , most easily seeded at some tiny surface defect on a hot spot on pan bottom.

    But another way you can boil water is by lowering the pressure sufficiently.
    This is actually what happens when 'pumping down' an AC system. The tiny bit of water / humidity inside the AC system will turn to " steam" . aka water vapor ,
    due to very low pressure inside the system, and the "steam" more or less finds it OWN way out, through the pump. ( yes a pump fan DOES hit the molecules, but the molecules wouldnt be there to get hit if they were not vapor state, bounding around )

    Thats the process better than I want to say it here


    The issue inside the forks is not HEAT -caused boiling, it is boiling caused by low pressure,
    the orifice is a choke point that causes a pressure differential ,
    its a momentary, intense low pressure "near" the orifice ,
    occurring only during an "excessive' rebound stroke ( excessive according to the design factors design... ,

    The bubbles wander off , to an extremely very short life, and they collapse back into the liquid,
    At final moment as a bubble collapse, there is a shock wave that can grind metal.

    It is known to happen in forks because of the various limitationsm problems & solutions inside a fork






    seems cavitation behind a propeller is not that different than through an orfice .... the air MISSING (a low pressure) in some region near the propeller , bashed out of the way by the propeller nanaoseconds before, simply cant rush back in fast enough , into the void, to eliminate the pressure difference 'smoothly', so it cavitates -
    it stretched " too thin" so to speak and pops back into place with some force, probably same shock wave afterward


    In a lot of processes, Liquids and Gases can be considered as basically the same darn thing, just different by density , with a more or less firm border between them.

    The liquid surface is a boundary. The gas basically has a boundary right at the liquid boundary the vapiors iquids and gases act very similar to each other; liquids an gases are basically the same thing

  11. #11
    Quote Originally Posted by Omega Man View Post
    Yeah. I always thought cavitation on a boat or plane was when the propeller(s) spun faster that the medium it was in- air or water.
    The oil doesn’t boil in an oil lubricated air compressor and that piston is a crankin’.
    OM
    ja but we arent discussin compressors...

    yeah I gonna bet that prop cavitation arises from being able to spin a propeller FASTER than the air can move to in & around it , after air has been pushed away.
    The tips are moving fastest



    the forces generated by road impacts occur extremely FAST.
    The guys that put up that linked info said he measured a 15 Meter per sec movement in the shock when a jump landed wrong side of a hill face

    Road forces casue movement faster than oil can smoothly flow through an orifice ,
    when that orifice was sized to control movement in the opposite direction ( and in "high " pressure direction) ,

    an orifice that was useful or " correct " for damping the compression stroke
    will be too constrictive trying to the flow the other way because of THE LOW PRESSURE isside lower shock chamber

    theres a lot good readin on that web site to focus on what matters here





    by the way , if you can smell it , there is " boiling" ooccuring at some scale . you are smelling the boiled off oil fractions using a very low sensitivity meter ( your nose)

  12. #12
    MOA #24991 Pauls1150's Avatar
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    Thanks for the lesson! I've always known that fluids and air flow in a similar fashion, but geeze, I never expected that bubbles could be so violent!

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