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Re:rotating mass question

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  • GeronimoPFudgemuffin@gmail.com
    ... Good luck, John... I ve been asking this for years (along with the Bore/Stroke Vs. Ring Friction question) and, like you, have not got it answered.
    Message 1 of 8 , Dec 1, 2006
      > settle a bet dept.
      > anybody seen any figures for rotating mass-
      <SNIP>

      Good luck, John...

      I've been asking this for years (along with the Bore/Stroke Vs. Ring
      Friction question) and, like you, have not got it answered.

      The weight of the crank assembly is not tied directly to the
      rotational inertia (is "RI" the right term here?), but it's at least
      indicative of same. And, like you, I feel the gyroscopic effect is a
      combination of the rotational inertia (I usually call this "flywheel
      effect") AS WELL AS the RPMs the crank turns at... and I never see
      this being taken into account when commenting on the concept.

      I keep reading in the mainstream mags about how (for instance) "the
      GSXR-1000s heavier crank (longer stroke) being responsible for the
      much greater gyro effect and, therefore, the difficulty in getting the
      bike to turn-in when compared to its smaller 750cc brother."

      Well, in downshifting for a given turn, is not the 1000's crank
      spinning 8-grand, while the 750's is tach'ing more like 12,000?

      Me? I dunno the difference, but it seems any crank-weight inequalities
      would be near-meaningless if one did not also figure the RPM's DURING
      USE. And say; isn't this something that could actually be measured,
      quantified, AND STATED.

      Sorry for the caps, but I get excited when things like this and the
      "longer stroke = more torque" monster rears its ugly head in print.
      And yes, I still see that on occasion in the mainstream mags.

      But not from Kevin Cameron.

      jrc in SC

      PS: I wrote Cameron an e-mail (through Cycle World) asking this very
      question. No reply as of yet.
      ~j
    • Ian
      ... The rotational inertia is what you need, the mass itself is not so important. You could draw it up in CAD and hit the right button to spit out the R.I., or
      Message 2 of 8 , Dec 3, 2006
        >The weight of the crank assembly is not tied directly to the
        >rotational inertia (is "RI" the right term here?), but it's at least
        >indicative of same.


        The rotational inertia is what you need, the mass itself is
        not so important.

        You could draw it up in CAD and hit the right button to
        spit out the R.I., or measure the R.I. physically ( pulley &
        weight method is best ).

        I did have a complete ( but damaged ) R6 crank here, but
        I cut the last web off for the alternator taper, so I can't
        offer to measure it. You would need to add the R.I. of
        the conrod BE's, but you could just plug in their mass
        ( or rotating component of ) and radius into the appropriate
        formula.


        Cheers IAN


        See www.drysdalev8.com for :
        - Drysdale 750-V8 Sports & 1000-V8 Cruiser
        - DRYVTECH 2x2x2 Experimental
        - Carberry Enfield 1000cc V-Twin
        - Drysdale Hillclimb Open Wheeler


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      • john fisher
        OK I contacted the daughter-unit who walked me through the math. She really is learning something in college! At a first guess, using numbers I made up, the
        Message 3 of 8 , Dec 6, 2006
          OK I contacted the daughter-unit who walked me through the math. She really is learning something in college!

          At a first guess, using numbers I made up, the big crank does have a lot more angular momentum.

          Observations:
          the radius of the crank is very important because it gets squared
          a twin might have more because the crank has a large diameter.

          Assumptions:
          a crank is represented by a solid cylinder
          ignore the torque required to change angular momentum, since its the same angle and leverage on all race bikes, or close
          enough.
          add some arbitrary number to the radius to represent the big ends.

          ---> If somebody gives me the mass/weight and diameter of two cranks, ideally from same manufacturer and very similar,
          i.e. the 1000cc one is scaled up from the 600 or vice versa, I can calculate the angular momentum difference. <--

          If anybody wants, I'll post up the math and the online references.

          Better yet somebody use their CAD and FEA programs to find out the real number and compare to the solid cylinder model.


          > >
          > > anybody seen any figures for rotating mass-
          > > 4 cyl 600 at 14,000 rpm
          > > 4 cyl 1000 at 11,000 rpm
          > > 2 cyl Ducati 1098 or 999 at ?
          > >
          > > pick any rpm that makes sense, I am just saying that there isn't much
          > difference in rotational inertia between the
          > > supposedly nimbler 600 and the big bikes, once you factor in the
          > higher rpm.
          > >
        • Otto Nikolaus
          But John, We knew the math(s). It s the comparative figures we need. ;) Otto
          Message 4 of 8 , Dec 6, 2006
            But John,

            We knew the math(s). It's the comparative figures we need. ;)

            Otto

            On 06/12/06, john fisher <john@...> wrote:
            > OK I contacted the daughter-unit who walked me through the math. She really is learning something in college!
            >
            > At a first guess, using numbers I made up, the big crank does have a lot more angular momentum.
            >
            > Observations:
            > the radius of the crank is very important because it gets squared
            > a twin might have more because the crank has a large diameter.
            >
            > Assumptions:
            > a crank is represented by a solid cylinder
            > ignore the torque required to change angular momentum, since its the same angle and leverage on all race bikes, or close
            > enough.
            > add some arbitrary number to the radius to represent the big ends.
            >
            > ---> If somebody gives me the mass/weight and diameter of two cranks, ideally from same manufacturer and very similar,
            > i.e. the 1000cc one is scaled up from the 600 or vice versa, I can calculate the angular momentum difference. <--
            >
            > If anybody wants, I'll post up the math and the online references.
            >
            > Better yet somebody use their CAD and FEA programs to find out the real number and compare to the solid cylinder model.
          • john fisher
            But Otto, nobody told me! ; )
            Message 5 of 8 , Dec 6, 2006
              But Otto, nobody told me!
              ;>)

              Otto Nikolaus wrote:
              >
              >
              > But John,
              >
              > We knew the math(s). It's the comparative figures we need. ;)
              >
              > Otto
              >
            • Otto Nikolaus
              Yes, you re right. Should have made that obvious. :) Otto
              Message 6 of 8 , Dec 6, 2006
                Yes, you're right. Should have made that obvious. :)

                Otto

                On 06/12/06, john fisher <john@...> wrote:
                > But Otto, nobody told me!
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